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FEATURE ARTICLE Soft lithographic methods for nano-fabrication Xiao-Mei Zhao, Younan Xia and George M. Whitesides* Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA 02138, USA Soft lithography is a low-cost, non-photolithographic strategy for carrying out micro- and nano-fabrication. This unconventional approach consists of techniques based on self-assembly and replica molding of organic molecules and polymeric materials.Four such techniques, microcontact printing (mCP), replica molding, micromolding in capillaries (MIMIC), and microtransfer molding (mTM), have been demonstrated for the fabrication of patterns and structures of a variety of materials with dimension30 nm. This review describes these techniques and their applications in fabrication and manufacturing at the sub-100 nm scale.The demand for fabrication techniques that are capable of of functionalized alkanes onto the surfaces of appropriate substrates.16,17 The thickness of a SAM is usually 2–3 nm, and forming nanometre-sized structures rapidly and economically is a major driving force in the development of nanoscience can be tuned with an accuracy of ca. 0.1 nm by varying the number of carbon atoms in the alkyl chain. The interfacial and nanotechnology.1 A wide range of techniques have been and are being developed for nano-fabrication: e.g., deep UV properties of a SAM-covered substrate can be easily modified by the incorporation of organic and inorganic functional (l=200–290 nm) and extreme UV (l<200 nm) photolithography, 2,3 phase-shift photolithography,4 electron-beam writ- groups into and/or at the end of the alkyl chain.SAMs of long-chain alkanethiolates [in particular, hexadecanethiolate, ing,5 focused ion beam (FIB) lithography,6 X-ray lithography,7 scanning probe lithography,8 and others.9 Despite the extra- CH3(CH2 )15S-] on gold represent one of the most developed and best characterized systems.They have been used as model ordinary success of these technologies, new strategies are still desired for manufacturing nanostructures; a major hurdle to systems for studying the properties of SAMs, such as structures, 18–20 wettabilities,21,22 and densities of defects.23–25 cross in the development of future technologies for nanofabrication is the enormous expense (both capital expense and We10,26–35 and others36–43 have demonstrated and developed SAMs of long-chain alkanethiolates and alkylsiloxanes as operating expense) of these technologies, and of the clean rooms and specialized reagents they usually require.ultrathin (2–3 nm) resists in lithography at the nanometre scale (100 nm). The formation of patterned SAMs is the key to This paper discusses soft lithography, a collective name for techniques based on self-assembly and molding,as a convenient such applications.Table 2 lists techniques that have been demonstrated for the generation of patterned SAMs with and low-cost approach to micro- and nano-fabrication (Table 1). Soft lithography uses soft, organic materials (e.g., feature sizes 100 nm. Other lithographic techniques (for example, photochemical oxidation,44,45 cross-linking,46 and functionalized alkanes and polymeric materials) to generate patterns and structures without the use of light or other high- generationof reactive groups47–50 ) are generally less convenient than mCP and have not been used with sub-mm structures, energy particles.Its strengths and weaknesses are very dierent from other microlithographic techniques.Although it is at an although they may still find some applications. Microcontact printing (mCP) is perhaps the most versatile very early stage of development, soft lithography has been shown to be a rapid and inexpensive way of forming and and cost-eective method for the generation of patterned SAMs with lateral dimension 100 nm. Fig. 1 shows the transferring patterns and structures (30 nm in dimension) onto or into other materials. This review focuses primarily on schematic procedure for mCP. It uses an elastomeric stamp (usually made from polydimethylsiloxane, PDMS) with a relief the procedures for four soft lithographic techniques, microcontact printing (mCP), replica molding, micromolding in capillaries (MIMIC), and microtransfer molding (mTM), as Table 2 Techniques for patterning SAMs at sub-100 nm scales well as their potential applications in the fabrication of patterns and structures having at least one dimension 100 nm.technique smallest feature size/nm ref. microcontact printing ca. 100 10, 26–29 Self-assembled monolayers and microcontact micromachining ca. 100 30–32 printing neutral atom lithography ca. 70 33–35 electron-beam writing ca. 5–6 36–38 Self-assembled monolayers (SAMs) are highly ordered molecu- STM writing ca. 10 39–42 lar assemblies that form spontaneously by the chemisorption Table 1 Four soft lithographic techniques smaller feature/nm largest patterned area technique (lateral dimension) (for this feature size) ref. microcontact printing (mCP) ca. 100 ca. 50 cm2 (ca. 0.5 mm) 10, 11 replica molding ca. 30 ca. 1 cm2 (ca. 0.2 mm) 12, 13 micromolding in capillaries (MIMIC) ca. 1000 ca. 1 cm2 (ca. 1 mm) 14 microtransfer molding (mTM) ca. 700 ca. 2 cm2 (ca. 1 mm) 15 J. Mater. Chem., 1997, 7(7), 1069–1074 1069Fig. 2 (A) Scanning electron micrograph (SEM) of an array of 2.5 mm wide lines of Au generated using the standard procedure of mCP, followed by chemical etching in a basic cyanide solution. (B) SEM of a gold pattern that was produced using mCP under water with the same PDMS stamp as in (A).The inked stamp was allowed to remain in contact with the gold surface for ca. 5 min. Fig. 1 Schematic illustration of the procedure for mCP. An elastomeric decanethiol from the edges of the pattern on the surface of the stamp is made by casting a prepolymer of PDMS against a master stamp.The resulting 100 nm wide lines were transferred into that is usually made by microlithographic techniques. The stamp is the thin film of gold by selective etching in an oxygen-saturated inked with a solution of hexadecanethiol in ethanol, dried in a stream cyanide solution. of N2, and then brought into contact with the gold surface.The Microcontact printing followed by selective chemical etching patterned SAMs can be used as resists in wet chemical etching to transfer patterns to the Au film. is capable of generating arrays of micro- and nano-structures of a variety of materials with controlled shapes and dimensions. This capability has direct applications in the fabrication of structure on its surface to transfer alkanethiol molecules (the custom-designed structures for studies of cell attachment,64–66 ‘ink’) to the surface of gold by contact.51 It is experimentally sensors,67 and other electrochemical and optical devices,68–70 simple and inherently parallel: it can form patterned sub-mm as well as in the fundamental studies of tribology such as features over an area of ca. 50cm2 in a single impression wetting and adhesion.71–74 The quality of the final products, within 30 s.11 Microcontact printing has been used to form however, has not yet met the requirements for the fabrication patterned SAMs of alkanethiolates on Au,10,26–29 Ag,52 Cu,53,54 of microelectronic devices for several reasons. First, the best and GaAs;55 and of alkylsiloxanes on Si/SiO2, glass and plasma- SAMs are formed on Au and Ag, and these metals are not treated polymer films.56–60 These processes are best understood acceptable as masks in the manufacturing of microelectronic for Au and Ag substrates, where the quality of SAMs is higher devices.Second, even high-quality SAMs have a relatively high relative to those on the other substrates. SAMs of long-chain density (5 pits mm-2) of defects.23 Third, the technology needed alkanethiolates [CH3(CH2)nS-, n12] with hydrophobic ter- to register patterns in multilevel fabrication has not yet been minal groups can eectively protect the underlying substrates developed.Nevertheless, the microlithographic techniques from dissolution in certain types of aqueous etchants.61–63 The based on mCP have attracted broad attention, and their patterns in printed SAMs can, therefore, be transferred into development is proceeding rapidly.We believe mCP will the underlying layers (e.g., Au, Ag and Cu) by selective chemical become an alternative method to conventional techniques for etching. The patterned structures of metals obtained this way micro- and nano-fabrication in the future.can be further used as secondary masks for the etching of underlying substrates of SiO2, Si, or GaAs. Patterns of SAMs with dimensions >200 nm are routinely Replica molding and related techniques for nano- generated using mCP. Smaller features (ca. 100 nm in dimen- fabrication sion) can also be generated using mCP by modifying the stamp and/or the printing procedure.10,26–29 For example, mechanical Photolithography,75 replica molding,76–78 embossing (or imprinting),79 and related techniques80–82 have been demon- compression of the stamp,28 controlled reactive spreading of hexadecanethiol under water,10 and casting stamps from blazed strated for the fabrication of micropatterns and microstructures of polymeric materials.Deep UV photolithography, electron- diraction gratings29 or masters prepared by anisotropic etching of Si(100)27 have been used to successfully fabricate features beam writing and X-ray lithography are the techniques commonly used in producing nano-structures (100 nm in with dimensions in the range of ca. 100 nm. Fig. 2 shows an example in which a reduction in feature size from ca. 2.5 mm dimension) with reasonably high wafer throughput. Cost of ownership issues and the requirements for fabrication space, to ca. 100 nm was accomplished by carrying out mCP under water, leaving the stamp in contact with the gold surface for however, have so far limited their applications, research and development. ca. 5 min. The reduction in dimension for the underivatized regions was caused by the reactive spreading of the hexa- Molding and embossing have been applied in generating 1070 J.Mater. Chem., 1997, 7(7), 1069–1074polymeric structures with feature sizes 100 nm. Functional making multiple (30) copies. This result demonstrates that replica molding against an elastomeric mold is capable of microstructures such as diraction gratings,77,83 compact discs (CDs),76,84 and microtools,85 are routinely manufactured at providing multiple copies of nano-structures from a single master: that is, it is capable of manufacturing nano-structures.the mass-production scale. Fabrication in the nanometre scale using these techniques has begun to attract attention. In Replica molding against an elastomeric mold is an extended form of the conventional technique based on rigid molds.The particular, we have introduced a range of techniques, replica molding against an elastomeric master,13 micromolding use of elastomeric molds allows the sizes and shapes of the features on the final replicas to be controlled by using mechan- in capillaries (MIMIC),14,86,87 and microtransfer molding (mTM),15,88 for forming micro- and nano-structures of polymers ical compression, bending, stretching, or a combination of these techniques, and thus, adds flexibility to the replica and sol–gel materials, and Chou et al.89,90 have demonstrated excellent results with embossing.Replica molding, MIMIC, molding technique. Replica molding against a deformed elastomeric mold provides a unique new route to fabricate complex and mTM were initially developed to make microstructures of polymers with controlled shapes on planar and/or contoured micro- and nano-structures with shapes, sizes, and periodicities that are significantly dierent from those on the original surfaces; they are now being extended to the fabrication and manufacturing of nano-structures.12 master.Fig. 4D gives a representative AFM image of an array of 30 nm wide lines fabricated by replica molding against a bent PDMS mold.Comparison of the nano-features on the Replica molding replica (ca. 30 nm wide) to those on the original Au master The form of replica molding which we have developed diers (Fig. 4C) establishes that the dimensions of certain features on from the conventional molding techniques in the use of an elastomeric PDMS mold.Fig. 3 illustrates the general procedure we used in the replication.12,13 The use of an elastomeric (rather than rigid) mold simplifies the separation between the replica and the mold, and greatly reduces the possible damage to the mold and the fragile structures on the surface of the replica. Fig. 4A and B show AFM images of a master having an array of ca. 60 nm wide and ca. 50 nm high lines, and one of its replicas of polyurethane (PU). It is evident that replica molding against an elastomeric mold faithfully replicates the original master having delicate features. We have also monitored the changes in quality of the nano-structures on the original master and the PU replicas vs. the number of replications conducted. No observable reduction in quality was found either on the original Au master or on the replicas after Fig. 3 Schematic procedure for carrying out replica molding against an elastomeric PDMS mold. The PDMS mold is fabricated by casting against nanometre-sized relief structures fabricated using X-ray lith- Fig. 4 (A, B) AFM images of a master with an array of 60 nm wide ography or electron-beam writing. The test pattern shown here is an array of ca. 50 nm lines. Replica molding can also be conducted while lines of Au on Si/SiO2 and a PU replica generated from the PDMS mold cast from this Au master; (C, D) AFM images of another Au the PDMS mold is deformed, for example, by mechanical bending (B). The dimensions of the lines were reduced from ca. 50 nm to ca. 30 nm master having an array of 50 nm wide lines and a PU replica generated from a bent PDMS mold cast from this Au master in this process while the spacings between the lines increased slightly.J. Mater. Chem., 1997, 7(7), 1069–1074 1071the master have been reduced significantly from ca. 50 nm to (Fig. 7).15 It is also capable of generating isolated structures. Fig. 8 shows an array of submicron-wide pyramids having ca.ca. 30 nm by casting against a mold deformed by mechanical bending. We have also demonstrated that replica molding 100 nm size tips made using mTM. A disadvantage of mTM is that the features that are formed usually rest on a continuous, against a PDMS mold is capable of generating microstructures on curved surfaces,13 and producing functional microdevices thin (100 nm) film of the polymer.Micromolding in capillaries and microtransfer molding are with changing periodicities (e.g., chirped diraction gratings).13 the two new techniques capable of generating microstructures of polymers, inorganic salts, and sol–gel materials on Micromolding in capillaries and microtransfer molding substrates of completely dierent materials. Fabrication of We recently developed a new technique, micromolding in free-standing polymeric webs (using MIMIC),86 multilayer capillaries (MIMIC), for the fabrication of microstructures of structures (using mTM),15 and functional devices (e.g., polymers and other materials (Fig. 5).14,86–88 The PDMS polymeric waveguides,15,91 waveguide couplers,92,93 and master used in MIMIC is cast from an original master (for interdigitated carbon capacitors and suspended carbon micro- example, a photoresist master made using photolithography). resonators94,95) have also been demonstrated.MIMIC and Fig. 6 shows a test pattern having regions which are <100 nm mTM attract attention because of their abilities to fabricate high but ca. 2 mm wide.14 We have not applied MIMIC to complex topologies and structures with a broad range of smaller structures.Although it should, in principle, be appli- materials and to accept non-planar surfaces as substrates. cable to such structures, in practice, the very slow filling of Until now, MIMIC and mTM have primarily been applied to very small capillaries may limit its usefulness. the fabrication of features at micrometre scales. Their utility Microtransfer molding (mTM) oers a procedure, for rep- in forming nano-structures has begun to be explored with licating microstructures from an elastomeric mold, that is more promising initial results (Fig. 6 and 8). rapid than MIMIC, and applicable to larger areas; it has been applied successfully to both planar and contoured surfaces Conclusions and future work Nano-structures (100 nm in dimension) are an important set of targets in materials science.In the past, they have been fabricated mainly using electron-beam and ion-beam writing; deep UV, X-ray, and scanning probe lithographies. Although these technologies are very capable of generating a broad range of structures, they have a number of disadvantages that may limit their applications in manufacturing: for example, they are restricted in the types of materials that can be used as resists; they are not easily applicable to curved surfaces; and most importantly, they require high capital and operating costs.Soft lithography, in contrast, represents a class of largely unexplored, non-photolithographic techniques that oer a cost-eective strategy for fabricating and manufacturing nano- Fig. 5 Schematic procedure for MIMIC. This technique relies on a conformal contact formed between a support and an elastomeric (PDMS) mold with relief features on its surface to create a network Fig. 6 SEM (A) and AFM (B) images of patterned microstructures of of microchannels.A low-viscosity, liquidprepolymer fillsthese channels by capillary action. Solidification of the precursor in situ, followed by PU on a Si/SiO2 surface generated using MIMIC.The arrow in (A) indicates a line that is <100 nm in height. (C) A cross-sectional SEM removal of the PDMS mold, results in the formation of polymeric structures on the surface of the support. image of the fractured sample. 1072 J. Mater. Chem., 1997, 7(7), 1069–1074conditions, and we have, in fact, been able to generate patterned features 30 nm using a number of soft lithographic techniques, in a chemical laboratory, without using clean room facilities.Several issues remain to be solved before soft lithographic techniques find applications in the fabrication of complex, functional nano-structures. For example, the densities of defects23 in the structures formed by chemical etching using printed SAMs as resists are still too high to be used for the fabrication of microelectronic devices.A lack of tools for registration with nanometre accuracy limits its use in multilayer fabrication. Replica molding, MIMIC, and mTM may suer from artifacts due to deformation of the molds.97 The levels of defects in these structures (and in structures produced using techniques such as embossing) have only begun to be characterized.All these techniques for soft lithography are still in their early stages of development. Their opportunities and limitations in nano-fabrication and nanomanufacturing are still being defined. It is clear, however, that they oer exceptional convenience and economy in making certain kinds of structures, and the most probable strategy for their use will be to produce copies of master structures prepared by conventional but more expensive techniques (for example, X-ray lithography and electron-beam writing).The applicability of soft lithography to more complex structures will be defined as it is developed further. Fig. 7 Schematic diagram for mTM. A drop of prepolymer is applied This work was supported in part by the Oce of Naval on the patterned surface of a PDMS mold.The excess prepolymer is Research, the Advanced Research Projects Agency, and the scraped away using a piece of flat PDMS, leaving a filled PDMS National Science Foundation (PHY 9312572). This work made mold. The filled mold is then brought into contact with a substrate use of MRSEC Shared Facilities supported by the National and the prepolymer is allowed to solidify in situ.Patterned microstructures are obtained after the PDMS mold is removed. The process can Science Foundation under Award Number DMR-9400396. We be repeated on a substrate whose surface has already been patterned thank Dr. Hans Biebuyck for helpful discussions, and Dr. Je with a layer (or layers) of relief structures to build multilayer structures Carbeck, Andrew Black, and Joe Tien for their help in editing layer by layer.this manuscript. References 1 F. Cerrina and C. 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Xia, E. Kim, M. Mrksich and G. M. Whitesides, Chem. Mater., Whitesides, Chem. Mater., 1997, in press. 1996, 8, 601. 96 G. M. Whitesides, Sci. Am., 1995, 273, 146. 54 T. P. Moat and H. Yang, J. Electrochem. Soc., 1995, 142, L220. 97 E. Delamarche, H. Schmid, B. Michel and H. Biebuyck, Adv. 55 E. Kim, G. M. Whitesides, M. B. Freiler, M. Levy, J. L. Lin and Mater., 1997, in press. R. M. Osgood Jr., Nanotechnology, 1996, 7, 266. 56 Y. Xia, M. Mrksich, E. Kim and G. M. Whitesides, J. Am. Chem. Soc., 1995, 117, 9576. Paper 7/00145B; Received 7th January, 1997 1074 J. Mater. Chem., 1997, 7(7), 1069–1074

ISSN:0959-9428    

DOI:10.1039/a700145b

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

FEATURE ARTICLE A general template-based method for the preparation of nanomaterials John C. Hulteen and Charles R. Martin*† Department of Chemistry, Colorado State University, Fort Collins, CO 80523, USA This article reviews a general template-based approach for the preparation of nanomaterials. The method involves the synthesis of a desired material within the pores of a nanoporous membrane.We have termed this approach ‘template synthesis’ because the pores within these nanoporous membranes act as templates for the synthesis of nanostructures of the desired material.Because the pores within these membranes are cylindrical and of uniform diameter, monodisperse nanocylinders of the desired material are obtained. Depending on the chemistry of the pore wall and material, these nanocylinders may be either hollow (a tubule) or solid (a fibril or nanowire). This template process will be shown to be a very general approach in the fabrication of nanotubes and fibrils composed of a variety of materials including polymers, metals, semiconductors, carbons, and other materials.While there has been a significant amount of research in the Introduction area of template synthesis of conductive polymer nanostruc- Many methods for the fabrication of nanoparticles have been tures, this has been reviewed elsewhere recently.32,33 developed, ranging from lithographic techniques to chemical methods.1,2 This research group has been exploring a fabri- Membranes used cation method termed template synthesis for the preparation of a variety of micro- and nano-materials.3–30 This process Most of the work in template synthesis, to date, has entailed involves synthesizing a desired material within the pores of a the use of two types of nanoporous membranes, ‘track-etch’ porous membrane.Because the membranes that are used have polymeric membranes and porous alumina membranes. cylindrical pores of uniform diameter, a nanocylinder of the However, there are a variety of other templates that could desired material is obtained in each pore.Depending on the be utilized. properties of the material and the chemistry of the pore wall, this nanocylinder may be solid (a nanofibril) or hollow (a Track-etch nanotubule). A number of companies (such as Nucleopore and Poretics) sell There are a variety of interesting and useful characteristics microporous and nanoporous polymeric filtration membranes associated with template synthesis.Probably the most useful that have been prepared by the track-etch method.34 This feature of this method is that it is extremely general with method entails bombarding a non-porous sheet of the desired regard to the types of materials that can be prepared.For material (standard thickness range from 6 to 20 mm) with example, we have used this method to prepare both nanotub- nuclear fission fragments to create damage tracks in the ules and nanofibrils composed of conductive polymers,4–13 material, and then chemically etching these tracks into pores. metals,14–25 semiconductors,26,27 carbon28–30 and other mate- The resulting membranes contain randomly distributed cylin- rials. Next, both tubular and fibrillar nanostructures with very drical pores of uniform diameter, Fig. 1(A,B). The commercial small diameters can be prepared. For example, conductive membranes are available with pore diameters as small as polymer nanowires with diameters as small as 3 nm have been 10 nm (pore density ca. 109 pores cm-2). These commercial prepared using this method.31 Also, because the membranes membranes are prepared from polycarbonate or polyester; employed contain cylindrical pores of uniform diameter,mono- however, a number of other materials are also amenable to disperse nanocylinders of the desired material, whose dimen- the track-etch process.34 sions can be carefully controlled, are obtained.Finally, these Owing to the random nature of the pore-production process, tubular or fibrillar nanostructures can be assembled into a the angle of the pores with respect to the surface normal can variety of architectures. The nanostructures can remain inside be as large as 34°.35 Therefore, depending on the specific pore the pores of the template membrane or they can be freed from diameter and pore density of the track-etched membrane, a the template membrane and collected as an ensemble of free number of pores may actually intersect within the membrane.nanoparticles. Alternatively, if the nanostructure-containing This is a problem when theoretically modelling the optical membrane is attached to a surface and the membrane is properties of template-synthesized nanometals, a topic of great removed, an ensemble of micro- or nano-structures that pro- interest to our group.18–20 For example, theory predicts a trude from the surface like the bristles of a brush can be specific wavelength maximum in the absorption band of iso- obtained.lated metal nanoparticles.18–20 However, physical contact The intent of this article is to provide an overview of the between the metal nanoparticles synthesized within the pores template method.We will start with a brief description of can shift this absorption maximum by 200 nm or more.36 the types of membranes used for template synthesis. Next, the dierent types of chemistries that have been used to prepare Porous alumina template-synthesized nanostructures will be reviewed. Finally, we will discuss fundamental properties and applications of Porous alumina membranes are prepared via the anodization template-synthesized metal and semiconductor nanostructures.of aluminium metal in an acidic solution.37 These membranes contain cylindrical pores of uniform diameter arranged in a hexagonal array, Fig. 1(C,D). However, unlike the track-etch † E-mail: crmartin@lamar.colostate.edu J.Mater. Chem., 1997, 7(7), 1075–1087 1075Other nanoporous materials Tonucci et al. have recently described a nanochannel array glass with pore diameters as small as 33 nm and pore densities as high as 3×1010 pores cm-2.39 Beck et al. have prepared a new class of mesoporous zeolites with large pore diameters.40 Douglas et al. have shown that the nanoscopic pores in a protein derived from a bacterium can be used to transfer an image of these pores to an underlying substrate.41 Clark and Ghadiri have prepared arrays of polypeptide tubules.42 Finally, both Ozin1 and Schollhorn43 have discussed a wide variety of nanoporous solids that could be used as template materials.Template synthetic strategies The limits to which materials can be used in template synthesis are defined by the chemistry required to synthesize the material.Nearly any material can in principle be synthesized within these nanoporous membranes, provided a suitable chemical pathway can be developed. Typical concerns that need to be addressed when developing new template synthetic methods include the following: (1) will the precursor solutions used to prepare the material ‘wet’ the pore (i.e., hydrophobic/hydrophilic considerations); (2) will the deposition reaction proceed too fast resulting in pore blockage at the membrane surface before tubule/fibre growth can occur within the pores; (3) will the host membrane be stable (i.e., thermally and chemically) with respect to the reaction conditions? The following is a general outline of five representative chemical strategies that have been used in our laboratory to conduct template synthesis within the alumina and polymeric template membranes.Electrochemical deposition Electrochemical deposition of a material within the pores is accomplished by coating one face of the membrane with a metal film (usually via either ion sputtering or thermal evaporation) and using this metal film as a cathode for electroplating. 17–22,44,45 This method has been used to prepare a variety of metal nanowires including copper, platinum, gold, silver, and nickel in both track-etch and alumina templates. Typical gold nanowires are shown in Fig. 2(A). The lengths of these nanowires can be controlled by varying the amount of metal deposited. By depositing a small amount of metal, short wires Fig. 1 Electron micrographs of polycarbonate (A and B) and alumina can be obtained; alternatively, by depositing large quantities (C and D) template membranes. For each type of membrane an image of metal, long needle-like wires can be prepared.18–20 This of a larger pore membrane is presented (A and C) so that the characteristics of the pores can be clearly seen.Images of a membrane ability to control the length or aspect ratio (length to diameter) with extremely small pores are also presented (B and D). A, SEM of of the metal nanowires is especially important in our optical the surface of a polycarbonate membrane with 1 mm diameter pores. investigations because the optical properties of nanometals are B, TEM of a graphite replica of the surface of a polycarbonate dependent on the aspect ratio.18–20,24 membrane with 30 nm diameter pores.The pores appear ‘ragged’ Hollow metal tubules can also be prepared via this method, owing to the artifact of using a graphite replica. C and D, TEMs of Fig. 2(B).17,22 To obtain tubules, one must typically chemically microtomed sections of alumina membranes with 70 nm (C) and 10 nm (D) diameter pores.derivatize the pore walls so that the electrodeposited metal preferentially deposits on the pore wall; that is, a molecular anchor must be applied. For example, gold tubules have been prepared by attaching a cyanosilane to the walls of the alumina membranes, the pores in these membranes have little or no tilt template membrane prior to metal depositions.17,22,46 Owing with respect to the surface normal resulting in an isolating, to the large number of commercially available silanes, this non-connecting pore structure.Although such membranes are method can provide a general route for tailoring the pore walls sold commercially (Whatman), a very limited number of pore in the alumina membranes. diameters are available. We have, however, prepared mem- Electrochemical deposition can also be used to synthesize branes of this type with a broad range of pore diameters.18,20 conductive polymers [such as polypyrrole, polyaniline, or We have made membranes with pore diameters as large as poly(3-methylthiophene)] within the pores of these template 200 nm and as small as 5 nm, and we believe that even smaller membranes.10,13 When these polymers are synthesized within pores can be prepared. Pore densities as high as 1011 the pores of track-etched polycarbonate membranes, the polypores cm-2 can be achieved,38 and typical membrane thickness mer preferentially nucleates and grows on the pore walls, can range from 10 to 100 mm.The higher pore density is resulting in polymeric tubules at short polymerization times, important if one wanted to mass-produce a nanomaterial by Fig. 2(C).By controlling the polymerization time, we can the template method. Membranes with high pore density would produce thin-walled tubules, thick-walled tubules or solid allow a greater number of nanostructures to be produced per fibrils. The reason why the polymer preferentially nucleates and unit area of template membrane. 1076 J. Mater. Chem., 1997, 7(7), 1075–1087Fig. 2 Electron micrographs of tubules and fibrils. A, TEM of a microtomed section of an alumina template membrane showing Au nanofibrils that are 70 nm in diameter within the pores. B, SEM of an array of Au microtubules. C, TEM of three polypyrrole nanotubules. The outer diameter is ca. 90 nm; the inner diameter is ca. 20–30 nm.grows on the pore walls is straightforward.8 Although the membrane is immersed into a Au plating bath containing AuI and a reducing agent, which results in Au plating on the monomers are soluble,the polycationic forms of these polymers are completely insoluble. Hence, there is a solvophobic compo- membrane faces and pore walls. The key feature of the electroless deposition process is that nent to the interaction between the polymer and the pore wall.There is also an electrostatic component because the polymers metal deposition in the pores starts at the pore wall. Therefore, after short deposition times, a hollow metal tubule (Fig. 3) is are cationic and there are anionic sites on the pore walls.8 obtained within each pore while long deposition times result in solid metal nanowires.Unlike the electrochemical deposition Electroless deposition method where the length of the metal nanowire can be Electroless metal deposition involves the use of a chemical controlled at will, electroless deposition yields structures that reducing agent to plate a metal from solution onto a surface.47 run the complete thickness of the template membrane.This method diers from electrochemical deposition in that However, the inside diameter of the tubules formed via electro- the surface to be coated need not be electronically conductive. less deposition can be controlled at will by varying the metal We have developed methods by which gold and other metals deposition time.15,16 Of course the outside diameter is deter- can be plated from solution onto the surfaces of both the mined by the diameter of the pores in the template membrane.plastic and alumina membranes.15 This method involves applying a sensitizer (typically Sn2+) to the membrane surfaces Chemical polymerization (pore walls and faces). The sensitizer binds to the surfaces via complexation with surface amine, carbonyl, and hydroxy Chemical template synthesis of a polymer can be accomplished by simply immersing the membrane into a solution containing groups.This sensitized membrane is then activated by exposure to Ag+ resulting in the formation of discrete nanoscopic Ag the desired monomer and a polymerization reagent. This process has been used to synthesize a variety of conductive particles on the membrane’s surfaces.Finally, the Ag-coated J. Mater. Chem., 1997, 7(7), 1075–1087 1077Fig. 3 TEM showing a microtomed section of a Au nanotubulecontaining membrane.The Au tubules are the black rings. The elliptical appearance is caused by the microtoming process. Pore diameter 50 nm; plating time 10 min. polymers within the pores of various template membranes. 6,9,48,49 As with electrochemical polymerization, the polymer preferentially nucleates and grows on the pore walls, resulting in tubules at short deposition times and fibres at long times.Conventional (electronically insulating) plastics can also be Fig. 4 SEM images of carbon tubules (A) and fibrils (B) with an outer chemically synthesized within the pores of these template diameter of 200 nm prepared in an alumina template membrane; membranes.For example, polyacrylonitrile tubules can be membrane was removed for imaging purposes prepared by immersing an alumina template membrane into a solution containing acrylonitrile and a polymerization Chemical vapour deposition initiator.28,29 The inside diameter of the resulting polyacrylon- A major hurdle in applying chemical vapour deposition (CVD) itrile (PAN) tubules is varied by controlling the time the techniques to template synthesis has been that deposition rates membrane remains in the polymerization bath.These PAN are often too fast. As a result, the surface of the pores becomes tubules have been further processed to create conducting blocked before the chemical vapour can traverse the length of graphitic carbon tubules and fibrils in alumina membranes, the pore.We have, however, developed two template-based Fig. 4.28,29 This is accomplished by heating the PAN tubules– CVD syntheses that circumvent this problem. The first entails alumina membrane composite to 700 °C under an argon flow the CVD of carbon within porous alumina membranes which or under vacuum. has been achieved by our group30 and others.50 This involves placing an alumina membrane in a high-temperature furnace Sol–gel deposition (ca. 700 °C) and passing a gas such as ethene or propene Sol–gel chemistry typically involves hydrolysis of a solution of through the membrane. Thermal decomposition of the gas a precursor molecule to obtain first a suspension of colloidal occurs throughout the pores, resulting in the deposition of particles (the sol) and then a gel composed of aggregated sol carbon films along the length of the pore walls (i.e., carbon particles.The gel is then thermally treated to yield the desired tubules are obtained within the pores). The thickness of the product. We have recently conducted various sol–gel syntheses walls of the carbon tubes is again dependent on total reaction within the pores of the alumina membranes to create both time and precursor pressure.tubules and fibres of a variety of inorganic semiconducting The second CVD technique utilizes a template-synthesized materials including TiO2, ZnO and WO3.26 First, an alumina structure as a substrate for CVD deposition.51 For example, template membrane is immersed into a sol for a given period we have used a CVD method to coat an ensemble of gold of time, and the sol deposits on the pore walls.After thermal nanotubules with concentric TiS2 outer nanotubules. The first treatment, either a tubule or fibril of the desired semiconductor step of this process requires the electroless plating of Au tubules is formed within the pores, Fig. 5. As with other template or fibrils into the pores of a template membrane. The Au synthesis techniques, longer immersion times yield fibres while surface layer is removed from one face of the plated membrane, brief immersion times produce tubules. and the membrane is dissolved away. The resulting structure The formation of tubules after short immersion times indi- is an ensemble of Au tubules or fibrils protruding from the cates that the sol particles adsorb to the alumina membrane’s remaining Au surface layer like the bristles of a brush, Fig. 6(A). pore walls. This is expected because the pore walls are nega- This structure is exposed to the precursor gases used to carry tively charged while the sol particles used to date26 are out the CVD synthesis of TiS2. As indicated in Fig. 6(B) the positively charged (a similar situation to what was described Au tubules become coated with outer TiS2 tubules. for conductive polymers). It has also been found that the rate of gelation is faster within the pore than in bulk solution.26 Composite nanostructures This is most likely due to the enhancement in the local concentration of the sol particles due to adsorption on the We have shown that a large number of dierent chemical techniques can be used to prepare tubules or fibrils that are pore walls. 1078 J. Mater. Chem., 1997, 7(7), 1075–1087Fig. 5 SEM images of TiO2 tubules and fibrils prepared in an alumina membrane with 200 nm diameter pores. The sol was maintained at 15 °C, and the immersion time varied from 5 to 60 s. A, Immersion time=5 s; remnants of the TiO2 surface layer can be seen in this image.B, Immersion time=25 s. C, Immersion time=60 s. alumina membrane via the sol–gel process discussed above, Fig. 7(A). After thermal treatment of the TiO2 tubules, conductive polypyrrole nanowires were grown using the chemical polymerization method inside the semiconductor tubules, Fig. 7(B). TiO2 is a promising material for photoelectrochemical energy production, and it has been shown that high surface area TiO2 has a higher photoeciency.52 Therefore, these TiO2–polypyrrole nanocomposites should be excellent photocatalysts because these template-synthesized structures have very high surface area.One problem in using high surface area TiO2 as a photocatalyst is the low electrical conductivity of the material.However, this tubular nanocomposite structure should solve this problem because each TiO2 tubule has its own current-collecting electrode inside. Another method for the construction of a two-component Fig. 6 SEM images of an ensemble of Au tubules before (A) and after (B) CV deposition of the outer TiS2 tubules. The tubules are protruding from the substrate Au surface layer.composed of a single material. However, one can imagine a host of applications where composite tubular nanostructures would be necessary. Examples might include concentric nanocapacitor or nanobattery tubules. We have recently developed chemical strategies for preparing such concentric tubular nanostructures.51 These composites have very high interfacial surface areas between concentric layers of materials.High interfacial areas are obtained because the interfaces are parallel to the long axis of the composite tubular nanostructure. The fabrication of a semiconductor–conductor tubular nanocomposite will introduce this concept of sequential tubular synthesis.51 This composite was prepared in a 60 mm thick Fig. 7 SEM images of TiO2 nanotubules prepared by sol–gel methods alumina template membrane with 200 nm diameter pores.before (A) and after (B) filling with the polypyrrole nanowires. Outer diameter of tubular composite is 200 nm. First, TiO2 tubules are synthesized within the pores of the J. Mater. Chem., 1997, 7(7), 1075–1087 1079concentric composite has already been described, Fig. 6.51 Gold the capacitors are connected in parallel from the surfaces of the template membrane. This will require that all of the tubules are electrolessly synthesized within the template membrane pores.The membrane is dissolved away, and a thin film electronically conductive outer tubules are electronically insulated from the conductive inner nanowires. of TiS2 is synthesized on the surface of the Au tubules via CVD.TiS2 is a Li+-intercalation material for Li-based Finally, self-assembly chemistry54 can also be used as a synthetic step to prepare tubular composites. For example, Au rechargeable batteries. We have recently shown that templatesynthesized Li+-intercalating materials can provide higher tubules were synthesized within the 1 mm pores of a polycarbonate template membrane via the electroless deposition discharge capacities than conventional electrodes made from the same material.53 As with the photoconductor materials, method.The inside diameter of these tubules was ca. 500 nm, and the length of the tubules was 1.0 mm. The Au tubule- many Li+-intercalation materials have low electrical conductivities. However, the current-collecting Au electrode inside containing membrane was then immersed in a solution of hexadecyl thiol causing the thiol to self-assemble onto the each TiS2 tubule should again solve this problem. We have shown that the TiS2–polypyrrole composite nanostructures inner surfaces of the Au tubules.The template membrane was dissolved away and the freed tubules were collected by reversibly intercalate and deintercalate Li+, and we are currently investigating the charge–discharge kinetics and capacit- filtration. When these gold–thiol tubules were placed in water, they ies of these tubular composite battery electrode materials.An alternative set of chemistries was used to fabricate floated at the air/water interface owing to the presence of the hydrophobic thiol on the inside of the tubule.In contrast, a conductor–insulator–conductor composite consisting of carbon–polyacrylonitrile–gold concentric tubules, Fig. 8.51 tubules that were not treated with the thiol filled with water and sank.51 Because self-assembly provides a general way to Initially, polyacrylonitrile (PAN) tubules were chemically polymerized within the pores of an alumina membrane followed apply a large number of dierent chemical functionalities to the inner (and outer) surfaces of such tubules, composite by thermal carbonization resulting in conductive carbon tubules, Fig. 8(B). The PAN polymerization step was then tubules with diverse inner and outer chemistries should be possible. repeated creating insulating PAN tubules within the carbon tubules, Fig. 8(C). A gold film was then sputtered onto one These have been just afew examplesof the types of composite structures that can be fabricated with template synthesis.face of the membrane. Using this film, Au nanowires were electroplated within the inner PAN tubules resulting in the Composites composed of a variety of dierent conducting, insulating, semiconducting, photoconducting and electroactive desired concentric tubular C–PAN–Au composite structures, Fig. 8(D). We are currently using this synthetic strategy and materials have been prepared. The limits as to how many dierent components each composite can contain is limited others to prepare ensembles of nanocapacitors, where all of Fig. 8 A, SEM image of the surface of the alumina template membrane. B, The carbon tubules obtained after dissolution of the template membrane; C, as per B but after polymerization of a PAN tubule within each carbon tubule.D, After electrodeposition of a Au nanowire within each PAN tubule. As noted, the carbon–PAN–Au composites were prepared by performing the appropriate chemistries in sequence leaving the alumina membrane intact; however, it is easier to image these extremely small structures by dissolving the membrane. 1080 J.Mater. Chem., 1997, 7(7), 1075–1087only by the initial diameter of the template pore and the rate Structural characterization of material deposition. The diameter of the electroplated gold nanoparticles is equivalent to the pore diameter of the alumina template membrane. Thus, Au nanoparticles with dierent diameters can be fabri- Optical properties of gold nanoparticles cated in alumina membranes containing dierent pore diameters.The aspect ratio is controlled by changing the amount We18–20 and others38,55 have been investigating the properties of Au electrochemically deposited into the pores. However, we of nanometals prepared within the pores of alumina have found that it is not possible to quantitatively predict the membranes.Through confinement of metals to a nano-sized aspect ratio of the Au nanoparticles because the plating current dimension, a variety of changes occur in the optical,18–20,55 eciency varies from membrane to membrane.24 Hence, it is electronic56 and magnetic38,57 properties. The first demon- not possible to calculate the aspect ratio of the Au nanoparticle stration of template synthesis for the creation of nanometal obtained from the known quantity of Au deposited and the fibrils was by Possin in 1970.58 Earlier work in which nanomet- pore diameter and density.Therefore, transmission electron als were used to colour alumina is also of interest.59 Nanometal- microscopy (TEM) analysis of the Au nanoparticles synthe- containing membranes of this type have also been used as sized in each membrane is necessary to determine the lengths selective solar absorbers.60 Finally, magnetic metals have been (and aspect ratios) of the nanoparticles.24 A TEM image of a deposited within the pores of such membranes to make vertical transverse section of a Au nanoparticle–alumina composite is magnetic recording media.61 shown in Fig. 2(A). When dierent amounts of Au are elec- This research group18–20 and others55 have been primarily trodeposited within the pores of the template membrane, we interested in the fundamental optical properties of nano- can produce Au nanoparticle shapes that are prolate, spheroid cylinders of Au imbedded into alumina membranes. The or oblate.24 colours of colloidal suspensions of Au can range from red to purple to blue depending on the diameter of the particles,62 Optical characterization and we have been able to demonstrate analogous colours for Au particles electroplated into the alumina template mem- The dierences in the shapes of the Au nanoparticles result in brane.18–20 These colours result from shape-induced changes changes in the optical absorption properties of the comin the plasmon resonance band of the Au nanoparticle which posite.18–20,24 Such changes are clearly visible as a membrane’s corresponds to the wavelength of light that induces the largest colour can vary from a bright red to deep blue to turquoise electric field on the nanoparticles.depending on the particle shape.18–20,24 The alumina membranes are optically transparent, so the colours are predominantly due to the Au nanoparticles.It should also be noted Fabrication that the parallel orientation of the pores in the alumina membrane confines the Au particles to a single dimensional The gold nanoparticles are prepared using the electrodepos- alignment. Correspondingly, there is no ambiguity in particle ition method discussed above, Fig. 9.18–20 First, Ag is deposited orientation which is a necessary feature for theoretical model- onto one face of an alumina template membrane to provide a ling of the absorption spectrum. conductive film for electrodeposition, Fig. 9(A). The membrane Fig. 10 shows the experimental absorption spectra for a is placed, Ag film side down, on a glass plate and covered with variety of Au nanoparticle–alumina composites.The Au par- a Ag plating solution. Then, short Ag ‘plugs’ or ‘posts’ are ticle aspect ratio (length/diameter) varies from 7.7 to 0.38, and electrochemically grown into the pores, Fig. 9(B). These Ag nanoposts are used as foundations onto which the Au nanoparticles are electrochemically grown, Fig. 9(C). Finally, the Ag foundations are removed with a nitric acid wash resulting in an array of Au nanoparticles imbedded within the pores of the alumina membrane, Fig. 9(D). Fig. 9 Fabrication procedure for Au nanoparticle–alumina composite. A, Ag is sputtered on one side of the host alumina membrane. Fig. 10 Experimental absorption spectra for the Au nanoparticle con- B, Membrane is placed sputtered side down onto a glass plate, and a Ag foundation is deposited electrochemically.C, Au is electrochemi- taining membranes. The spectrum with the highest absorbance maximum is for the membrane containing the aspect ratio (length/diameter) cally deposited onto the Ag foundations. D, Ag is removed with nitric acid. of 7.7, followed by 2.7, 1.3, 0.77, 0.54, 0.46 and 0.38 respectively. J. Mater. Chem., 1997, 7(7), 1075–1087 1081the diameter of each Au particle is constant at ca. 52 nm. The higher values of the background or double-layer charging currents. In the case of the NEE, the polycarbonate is stretch- reduction in absorption intensity with decreasing aspect ratio is expected due to the decrease in the metal volume fraction oriented during fabrication to improve mechanical properties. Upon heating above the glass transition temperature of the composites.The shift in the absorption maximum from 518 nm (aspect ratio 7.7) to 738 nm (aspect ratio 0.38) is (ca. 150 °C), the membrane relaxes, shrinks, and seals the junction between the Au nanowires and the polymer predicted from simulated spectra obtained using a dynamic Maxwell–Garnett theory.24 membrane.14,15 This article shows that the template method can be used to fabricate Au nanoparticles with various diameters and aspect Current response of the NEE ratios.Shifts in the absorption maximum and changes in the Two dierent electrochemical response limiting cases can be absorption intensity of the Au nanoparticle–alumina com- observed at an NEE, the ‘total overlap’ and ‘radial’ response.14 posites have been studied as a function of both particle Which limiting case is achieved depends strongly upon the diameter and aspect ratio.Current work involves determining distance between the electrode elements and the timescale (e.g., the eects of heating the Au nanoparticle–alumina composite. scan rate) of the electrochemical experiment. When the elec- Changes in the Au nanoparticles aspect ratios, optical proper- trode elements are in close proximity and the scan rate is ties and crystal structure have been observed.relatively low, the diusion layers at each electrode element overlap, Fig. 12(A). This overlap results in a single diusion Nanoelectrode ensembles layer that covers the total geometric area of the NEE. Linear diusion occurs to the entire NEE surface, and conventional One very exciting application of template synthesis is in the peak-shaped voltammograms are obtained. Also, the total area of electrochemistry.Nanoelectrodes oer opportunities to faradaic current is equivalent to that obtained at an electrode perform electrochemistry in highly resistive media63,64 and to of equivalent geometric area whose entire surface area is gold.investigate the kinetics of redox processes that are too fast If the electrode elements are located far apart and the to measure at conventional macroscopic electrodes.65–68 (By timescale of the experiment is relatively fast, the diusion macroscopic electrodes we mean disk-shaped electrodes with layers at each electrode act independently resulting in a radial diameters of the order of 1 mm.) We have used the template diusion field at each individual electrode element, Fig. 12(B). method to prepare ensembles of Au nanodisk electrodes where The voltammogram in this case has a sigmoidal shape, and the diameters of the Au disks are as small as 10 nm. the predicted total faradaic current is equivalent to the sum of the current generated at each individual electrode element Fabrication within the NEE.Fig. 13 shows a series of SEM images of NEEs with varying Using the electroless Au deposition procedure, Au nanowires are synthesized within the pores of a polycarbonate track-etch average distances between the electrode elements.14 The NEEs were fabricated from polymer template membranes with dier- membrane. In addition, both faces of the membrane become coated with thin Au films.If one of these surface Au films is ent pore densities but similar pore diameters. Fig. 14 presents the faradaic response of an electroactive species [trimethylami- removed, the disk-shaped ends of the Au nanowires traversing the membrane are exposed. These nanodisks can be used as nomethylferrocene (TMAFc+)] at each of these NEEs.14 The NEE with the highest electrode element density [Fig. 13(A), active elements in an ensemble of nanoelectrodes. Fig. 11 shows a schematic of such a nanoelectrode ensemble (NEE).14 Fig. 14(A)] shows a peak-shaped voltammogram indicative of the total overlap response. In contrast, the NEE with the Electrical contact is made to the remaining surface layer which acts as a common current collector for all the nanoelectrode lowest electrode element density [Fig. 13(D), Fig. 14(D)] shows the expected sigmoidal voltammogram. The other elements. A consistent problem associated with micro- and nano- two NEEs have an intermediate nanoelectrode density [Fig. 13(B,C), Fig. 14(B,C)] and show an intermediate electrodes is achieving an ecient seal between the conductive element and the host material.If a good seal is not achieved, response. We can demonstrate quantitatively that the NEEs in solution can creep into this junction resulting in significantly Fig. 13(A,D) are operating in the total overlap and radial Fig. 11 Schematic of an edge view of a nanoelectrode ensemble. The nanometal fibrils running through the pores of the template membrane are shown. The lower ends of the fibrils define nanodisks which are Fig. 12 Schematic of a side view of NEEs and the corresponding the electrodes. The opposite (upper) ends of the nanofibrils are connected to a common metal film which is used to make electrical diusion fields for the total overlap (A) and radial (B) limiting electrochemical response contact to the nanodisks. 1082 J.Mater. Chem., 1997, 7(7), 1075–1087Fig. 13 SEM images of the surfaces of NEEs showing the disk-like electrode elements prepared from membranes with varying pore densities. Average distance between pores are: A, 0.25 mm; B, 1.1 mm; C, 3.5 mm; D, 17.5 mm. Diameters of electrode elements are 100 nm (A,D) and 200 nm (B,C). response modes by comparing experimental and simulated voltammograns.Such a comparison is shown in Fig. 15. The simulated voltammogram in Fig. 15(A) is based on the reversible total overlap limiting case, and the experimental voltammogram is the same as Fig. 14(A).14 The quantitative Fig. 14 Cyclic voltammograms (50 mV s-1) for 50 mm TMAFc+ in Fig. 15 Simulated and experimental voltammograms for NEEs prepared from Fig. 13A and D.Scan rate and solution same as Fig. 14. 5 mMNaNO3 forNEEs prepared from the membranes shown in Fig. 13 J. Mater. Chem., 1997, 7(7), 1075–1087 1083agreement between the simulated and experimental voltammo- elements and a geometric area equivalent to that of the macroelectrode at various low concentrations of TMAFc+. grams confirms that the NEEs at this pore density and scan rate are in the total overlap electrochemical response.It is While the voltammograms essentially look identical to those obtained at the macroelectrode, the concentrations of the important to point out that there are no adjustable parameters in this simulation. electroactive species at the NEE are three orders of magnitude lower than those for the macroelectrode. The detection limit The simulated voltammogram in Fig. 15(B) assumes a single 100 nm diameter disk electrode, but the total current is multi- at the macroelectrode was determined to be ca. 2 mM while the detection limit at the NEE was ca. 2 nM.15 plied by the number of electrodes within the geometric area of the NEE. The experimental voltammogram is equivalent to Template synthesis has been shown to provide a simple means of creating ensembles of nanoelectrode ensembles.These Fig. 14(D). The quantitative agreement between the simulated and experimental voltammograms proves that the radial elec- NEEs can achieve electroanalytical detection limits that are three orders of magnitude lower than detection limits obtained trochemical response has been achieved at this NEE. Again, there are no adjustable parameters in this simulation. at conventional macroelectrodes.We are currently investigating fabrication processes that allow the use of NEEs in nonaqueous solvents. Detection limits A possible application of these NEEs is the ultra trace detection Metal nanotube membranes of electroactive species. We have recently shown that NEEs with 10 nm diameter disks operating in the total overlap mode We close our discussion of metal nanostructures with an show electroanalytical detection limits that are three orders of interesting new type of membrane consisting of Au nanotubes magnitude lower than detection limits obtained at macroscopic that span the complete thickness of the membrane.We have Au disk electrodes of comparable geometric area.15 This occurs previously mentioned that by controlling the electroless Au because in the total overlap mode, the total faradaic signal deposition time, the inside diameters of these tubes can be generated at the NEE is equivalent to that obtained at the controlled at will.We recently asked the question, can tubes conventional macroelectrode of equivalent geometric area. with inside diameters that approach the sizes of molecules be However, the background double-layer charging current is prepared, and if so, what applications might exist for such significantly less because these currents are proportional only nanotubule containing membranes? to the active Au area.The ratio of active area to geometric area for a 10 nm NEE is approximately 0.001.15 As a result, Fabrication the background current is reduced by three orders of magnitude, and detection limits can be improved by three orders of Typical templates used to prepare the metal nanotubule membranes were 6 mm thick polycarbonate membranes with 50 nm magnitude.An example of this enhancement in detection limits at an pore diameters and 6×108 pores cm-2. Gold was electrolessly plated onto the walls of the pores yielding a Au nanotube NEE is shown in Fig. 16.15 Fig. 16(A) shows voltammograms at a conventional Au macroelectrode at various low within each pore. Variation in the plating time has been shown to produce Au tubules with internal diameters ranging from concentrations of TMAFc+. As expected, the faradaic signal eventually vanishes into the double-layer charging currents as 34 to 1.4 nm.16 The diameter of these Au tubules was determined from measurements of gas (He) flux across the mem- the concentration of TMAFc+ decreases. Fig. 16(B) shows voltammograms at a NEE with 10 nm diameter electrode brane.16 Because the electroless processplates on the membrane surface as well as within the pores, electrical contact with the surfaceallows electrical control of the potentialinside the pores.Ion-selective membranes The ion transport properties of these Au nanotubule-containing membranes were studied using a U-tube concentration cell where the membrane separates two diering aqueous solutions, Fig. 17.16 In an initial experiment, diering concentrations of KCl were placed on each side of the membrane, and reference electrodes were inserted into each solution to measure the membrane potential (Em).When the diameters of the Au nanotubules approached 2 nm or less, the membranes dis- Fig. 16 Cyclic voltammograms at 100 mV s-1 in aqueous TMAFc+ at (A) a gold macrodisk electrode in 50 mM NaNO3 [TMAFc+]=0.5 (a), 7.8 (b), 15.6 (c), 31.2 mM (d ); (B) a 10 nm NEE in 1 mM NaNO3 Fig. 17 Schematic of a U-tube concentration cell [TMAFc+]=0.5 (a), 7.8 (b), 15.6 (c), 31.2 nM (d). 1084 J. Mater. Chem., 1997, 7(7), 1075–1087played near ideal cation-permselective behaviour, i.e., these cations in both fundamental and applied electrochemistry. Because the Au tubules have dimensions of the order of membranes transport cations but reject anions.16 This behaviour occurs because Cl- adsorbs strongly to Au, and as a molecular sizes and are quite monodisperse, we have been exploring the possibility of separating molecules based upon result, the Au tubules have an excess of negative charge (Cl-) on their inner surfaces.This causes anions to be excluded from dierences in their physical dimensions. the pores. Ion permselectivity can also be controlled by directly chang- Semiconductor nanotubules and nanofibres ing the potential applied to the Au nanotubules.For this work, it was essential to use an anion that does not adsorb to Au Electrochemical methods have been used previously as a means of depositing semiconductor materials into the pores of a because we wanted to control the charge in the Au tubes and not have it predetermined due to excess charge from counter- template membrane.27 However, this section will discuss the properties of semiconductor tubules and fibrils synthesized by ion adsorption.Because F- does not adsorb to Au, KF was chosen as the electrolyte. The U-tube assembly was used again, a much more versatile deposition method, sol–gel chemistry.26 but this time the membrane was connected to the working electrode lead of a potentiostat.The potential applied to the Structural characterization Au nanotube membrane varied from -0.5 to +0.5 V vs. Upon the confinement of a semiconductor to nanoscopic Ag/AgCl. The membrane was placed between solutions of dimensions, the first two questions that arise are: can we see 10 mM and 1 mM KF, and Em values were measured at each evidence for quantum confinement, and what is the crystal applied potential. structure of the material? TiO2 fibrils have been synthesized The dashed lines at the top and bottom of Fig. 18 are the within the pores of both 200 nm and 22 nm pore diameter Em values that would be achieved if the nanotubule membrane alumina membranes.26 The sol–gel fabrication of TiO2 fibrils showed ideal cation and ideal anion permselectivity, respect- within the pores of alumina membranes was described earlier.ively. At negative applied potentials, the nanotubule membrane An absorption spectrum of the template alumina membrane shows ideal cation permselectivity, whereas at positive applied containing these fibres showed an abrupt increase in potentials the membrane shows ideal anion permselectivity.absorbance at an approximate wavelength of 389 nm. This This selectivity occurs because at negative applied potentials, corresponds to the bandgap of bulk TiO2.70 This suggests that an excess negative charge is present on the walls of the Au the diameter of these fibrils is too large to see evidence for tubes. This results in the exclusion of anions from the tubes. quantum confinement in the absorption spectrum.We are At positive applied potentials, the opposite situation occurs: capable of preparing alumina template membranes with pore cations are excluded and anions are transported. diameters approaching 5 nm or smaller. Correspondingly, we For any combination of metal and electrolyte, there is a are currently attempting to prepare fibrils small enough to potential called the potential of zero charge (p.z.c.) where there provide evidence for quantum confinement.is no excess charge on the metal. At this potential the nano- Electron diraction has been employed to determine the tubule membranes should show neither cation nor anion crystal structure of the template-synthesized TiO2 fibrils.26 permselectivity, and Em should approach 0 mV.Em for the Fig. 19(A) shows a TEM image of 15 nm diameter TiO2 tubule-containing membrane does go from the ideal cation nanofibres with the membrane dissolved away. The small fibres permselective value, through zero, to the ideal anion permselec- are arranged in bundles which can contain anywhere from 2 tive value. Furthermore, the potential at which Em approaches to 10 or more fibres.Fig. 19(B) shows the indexed electron zero is close to the reported p.z.c. (-4 mV).69 diraction pattern obtained from the centre of the fibril bundle We have demonstrated that these Au nanotubule-containing on the left side of the main feature in Fig. 19(A). The orien- membranes can be cation permselective, anion permselective, tations of the images are the same, i.e., the c* axis in Fig. 19(B) or non-selective depending on the potential applied to the is parallel to the fibril bundle axis in Fig. 19(A). These data membrane. These membranes can be as permselective as the show that the fibrils are highly crystalline anatase-phase TiO2, commercially available Nafion polymer and should have appli- with the c* axis of the anatase oriented along the long axis of the fibril.Small fibril bundles throughout the sample display the same crystalline orientation; i.e., the reciprocal lattice direction [110] is almost always parallel to the electron beam, and the c* axis is along the fibril axis. We have concluded that these fibrils crystallize as long, prismatic crystals with the rare, Fig. 18 Variation of Em with potential applied to the membrane [1 mM KF on the low concentration (l) side, and 10 mM KF on the high concentration (h) side of the membrane; tubule radius ca. 1.1 nm]. The potential of the membrane was controlled with a potentiostat vs. a Fig. 19 A, TEM image of a bundle of 15 nm diameter TiO2 fibrils. Ag/AgCl reference electrode immersed in the side-h solution. Em was measured with the membrane under potentiostatic control. B, Corresponding electron diraction pattern.J. Mater. Chem., 1997, 7(7), 1075–1087 1085and metastable, anatase mineralogical orientation [001] with The decomposition data can be used to determine quantitatively the rate of photodecomposition. If a pseudo-first-order {100}.71 rate law with respect to the salicylic acid concentration is plotted versus reaction time, rate constants for the decompo- Photocatalysis sition of salicylic acid can be determined, Fig. 20(B).73,75–77 A standard application of TiO2 has been as a photocatalyst The slope of these lines provides the decomposition rate for the decomposition of organic molecules.72–76 This is a constant. The thin film catalyst has a rate constant of surface reaction that is thought to involve absorption of a UV 0.003 min-1 while the fibrillar catalyst shows an increased rate photon by TiO2 to produce an electron–hole pair which reacts constant of 0.03 min-1.This order of magnitude increase in with water to yield hydroxyl and superoxide radicals. These reaction rate is much smaller than the 315 times enhancement radicals can then oxidize the organic molecule. Template- predicted.This is not surprising because the thin film TiO2 synthesized TiO2 structures should increase the TiO2 surface undoubtedly has some degree of surface roughness resulting area and correspondingly increase the decomposition reaction in higher surface areas and higher decomposition rates than rates. For example, TiO2 fibrils can be synthesized within the predicted.Also, scanning electron microscopy (SEM) analysis pores of a 60 mm thick alumina membrane with 200 nm of the fibrillar TiO2, Fig. 5(C), shows that the fibres ‘lean’ diameter pores.26 The TiO2 fibril-containing membrane is against each other, possibly shading large portions of the attached to an epoxy surface, and the membrane is dissolved surface from the sunlight resulting in lower decomposition away.The calculated surface area of the immobilized fibrils rates than predicted. is 315 cm2 of TiO2 surface area per cm2 of planar geometric Single-crystal TiO2 fibrils can be fabricated via template area. This suggests that, in principle, an enhancement of 315 synthesis and sol–gel chemistry. Also, owing to the increased in the catalytic rate of organic decomposition on template- surface area of the TiO2 fibril array, the decomposition rate of synthesized TiO2 fibres is possible versus a thin film TiO2 an organic molecule increases.However, this prototype fibrillar catalyst. Through the use of tubular structures and/or template catalyst is not optimal. We are currently working on processes membranes with smaller diameter pores (with correspondingly to optimize the fibril arrays by varying the fibril diameter and higher pore densities and surface area) even larger increases in aspect ratio and the distance between the fibrils.We are also the rate would be predicted. exploring additional applications of these TiO2 nanofibres We have studied the decomposition of salicylic acid over including electrochemistry, battery research, photoelectro- time on an array of immobilized TiO2 fibres, Fig. 5(C), with chemistry and enzyme immobilization. exposure to sunlight, Fig. 20(A).26 The upper curve follows the concentration of salicylic acid for a solution containing no TiO2 catalyst, and no significant decomposition is observed. Conclusions The small increase in salicylic acid concentration has been The template method has become a very simple yet powerful ascribed to the evaporation of water during the exposure to process for the synthesis of nanomaterials. This article has sunlight.The middle curve follows salicylic acid decomposition described a host of chemistries that are now available for the on a thin film of TiO2, and the bottom curve shows a marked template synthesis of a wide variety of nanomaterials including increase in decomposition of salicylic acid for the template- metals, polymers, carbon, and semiconductors. Applications synthesized TiO2 fibres.have ranged from fundamental optical studies to ultra trace molecular detection to high surface area catalysis. What does the future hold for template synthesis? From a fundamental viewpoint, our group is interested in fabricating nanostructures with significantly smaller diameters in order to explore further the eects of size on the properties of materials. We are also developing new chemistries so that tubules and fibrils composed of an even larger variety of materials are available.New applications for template-synthesized nanomaterials are also being developed. We are exploring applications in photocatalysis, chemical analysis, bioencapsulation, biosensors, bioreactors, molecular separations, and electronic and electrooptical devices.Finally, it is clear that if practical applications are to be realized, methods for mass producing template-synthesized nanostructures will be required. This work would not have been possible without the eorts of a number of hardworking and highly motivated graduate students and postdocs.They include Vinod P. Menon, Zhihua Cai, Junting Lei, Wenbin Liang, Ranjani V. Parthasarathy, Charles J. Brumlik, Gabor L. Hornyak, Leon S. Van Dyke, Colby Foss, Matsuhiko Nishizawa, Reginald M. Penner, Charles J. Patrissi, Veronica M. Cepak, Brinda B. Lakshmi, Guangli Che and Kshama B. Jirage. Financial support from the National Science Foundation, the Oce of Naval Research and the Department of Energy is also gratefully acknowledged.We also wish to thank the Colorado State University Electron Microscopy Center. Fig. 20 A, Photodecomposition of salicylic acid in sunlight. Data for References no photocatalyst, the thin film TiO2 photocatalyst, and the fibrillar (200 nm) TiO2 photocatalyst are shown.B, First-order kinetics of the 1 G. A. Ozin, Adv. Mater., 1992, 4, 612. 2 Engineering a Small World: From Atomic Manipulation to photodecomposition of salicylic acid with both the thin film and fibrillar TiO2 photocatalyst. Microfabrication, special section of Science, 1991, 254, 1300–1342. 1086 J. Mater. Chem., 1997, 7(7), 1075–10873 C. R. Martin, Chem.Mater., 1996, 8, 1739.S. B. McCullen, J. B. Higgins and J. 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ISSN:0959-9428    

DOI:10.1039/a700027h

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

FEATURE ARTICLE Crystallisation inside fullerene related structures Jeremy Sloan,a,b Jessica Cook,a† Malcolm L. H. Green,*a John L. Hutchisonb and Reshef Tennec aInorganic Chemistry L aboratory, University of Oxford, South Parks Road, Oxford, UK OX1 3QR bDepartment ofMaterials, University of Oxford, Parks Road, Oxford, UK OX1 3PH cDepartment ofMaterials and Interfaces, Weizmann Institute, Rehovot 76100, Israel The dierent methods for encapsulating crystalline materials inside fullerene related structures are reviewed.The relationships between the mode of encapsulation and the crystallisation behaviour obtained in each case are described. In particular, the mechanisms of morphological and orientational control of crystallite growth inside carbon nanotubes and the comparative encapsulation behaviours of materials encapsulated by physical and catalytic methods are described and discussed.The encapsulation of defect tungsten oxide structures within inorganic fullerene-like structures are also described. Since the discovery of carbon nanotubes in 1991,1 a number opened carbon nanotubes.2–4 Once encapsulated, the materials can be further modified in situ to give reduced,33 oxidised34 or of researchers have encapsulated both crystalline and noncrystalline materials inside their cavities using either chemical otherwise chemically modified encapsulates.The second strategy involves formation of the encapsulating medium around or physical methods.2–5 Carbon encapsulation has also been achieved in situ by the arc-evaporation of composite carbon the included material.This may be achieved catalytically, either in situ in a carbon arc or by gas-phase deposition onto catalytic electrodes resulting in the formation of sealed carbon coated species contained either within the cores of carbon nanopart- metal particles; or chemically, via formation of the encapsulating material from the outside surface of the encapsulated icles6–12 or as continuous filling, or ‘nanowires’, formed along the internal bore of nanotubes.13–15 Encapsulation has also material.A third strategy can also be identified, although this is a special case restricted to the encapsulation of diamond been achieved via gas-phase deposition of carbon containing species onto catalytic metal particles.16–19 In related develop- only.Banhart and Ajayan36 recently demonstrated that the cores of carbon onions can behave like high pressure cells ments, encapsulates contained within fullerene-like cage structures of the general form MX2 (M=W, Mo; X=S, Se) have in which diamond formation can be induced by irradiation with a high energy (1.25 MeV) electron beam at elevated been synthesised in which the encapsulating material is grown chemically inwards from the surface of finely divided and temperatures.partially reduced material.20–25 Encapsulation research has been directed mainly towards Chemical insertion of materials inside carbon nanotubes the enhancement of the electrical2,6–8,13,15,25,26 and mag- Chemical insertion involves the selective opening of the carbon netic10,27,28 properties of both the encapsulated and encapsulat- nanotubes at their tips, either by refluxing in concentrated ing materials although they are also being considered for nitric acid or, alternatively, by heating in O23 or CO2,25 and applications in fields as diverse as biotechnology,29 and cataly- then precipitating from solution the encapsulated material sis.4,29–32 Benefits arising from this researchhave so far included inside the opened cavities.This can be achieved via a one-step the ability to observe the in situ chemistry of encapsulated procedure4 in which the opening reagent contains a solubilised materials33 and also size limited crystallisation behaviour on metal nitrate that precipitates upon calcination to form an an approaching atomic scale.11,34,35 In this article, it is the encapsulated metal oxide.Alternatively, a two-step pro- latter with which we shall be most concerned. We will attempt cedure37 may be utilised in which the closed nanotubes are to elucidate some of the relationships between modes of first treated with nitric acid and then heated in air to remove encapsulation and the crystallisation behaviour obtained in surface acid groups known to be present on nanotubes opened each case.Some of the more interesting and unusual crystallis- in this way.37 The nanotubes can subsequently be filled by ation behaviour exhibited by encapsulated species will also be stirring with a solution of a metal nitrate or the metal halide, described and discussed. It is hoped that these phenomena will followed by calcination.This technique is useful for encapsulat- contribute to an understanding of how materials formation ing materials that can interact unfavourably with surface acid can be manipulated at the most intimatescale and, additionally, groups, such as the metal halides. how new types of materials with hybrid physical properties Oxides of the metals Ni,4 U,4 Co,4 Fe,4 Nd,38 Sm,38 Eu,38 can be created.La,38 Ce,38 Y38 and Cd38 have all been encapsulated via the one-step procedure. The mixed-metal oxide FeBiO3 has been Methods of encapsulating materials inside similarly encapsulated38 by calcining a mixture containing an fullerene related materials equimolar solution of iron and bismuth nitrates, concentrated HNO3 and closed nanotubes.Pd,37 Ag,39 Au39 and AuCl39 Two main strategies can be identified for encapsulating mate- have all been encapsulated using the two-step procedure using, rials inside fullerene related structures. The first involves in the case of the latter three, their respective metal halides. In inserting the materials, either physically or chemically, into a modification to the two-step procedure,34 polycrystalline SnO has been encapsulated by mixing opened nanotubes with SnCl2 in acidic solution to which was added a weak base, † Present address: M.I.T., 77 Massachusetts Ave., Room 6-329, Cambridge, MA 02139, USA.resulting in precipitation at a pH of 10.2. J. Mater. Chem., 1997, 7(7), 1089–1095 1089Encapsulated metal oxides can be reduced to their respective Dai et al.61 have deliberately prepared encapsulated nanowires containing copper and germanium by pyrolysis of polycyclic metals by reduction with hydrogen gas at elevated temperatures. Encapsulated Ni metal has been prepared in this aromatic hydrocarbons over finely divided copper and germanium, respectively.fashion.27 Similarly, hydrogen reduction of tubes filled with precipitated KReO4 gives encapsulated crystallites of rhenium metal.38 Encapsulated metal halides can be further modified in Chemical encapsulation situ by treatment with H2S gas at elevated temperatures to MX2–fullerene-like materials, with M=W or Mo and X=S give their respective sulfides.Encapsulated CdS38 and Au2S338 or Se, consist of a network of 2H-MX2 prisms that, in have both been prepared in this way.projection, resemble the graphitic network common to carbon fullerene related structures such as nanotubes (2H refers to the Physical insertion of materials into nanotubes hexagonal symmetry, which repeats every two layers). The relationship between the two types of structures is shown Opened nanotubes can be filled via capillary action using schematically in Fig. 1. Like nanotubes and other fullerene either a low melting, low surface tension metal salt30 or, derived structures, the interlinked 2H-MX2 hexagonal net- alternatively, a eutectic or low melting mixture34 of two works are capable of incorporating dierent alternative components with a resulting surface tension lower than the polyhedra which, depending on the type, impart either positive threshold value of 100–200 mN m-1.30 When a single molten or negative curvature into the structures resulting ultimately component is used, continuous and preferentially orientated in the formation of nanotube and nanoparticle-like structures.filling of the nanotube cavities is invariably obtained, as has Based on these relationships, such materials are often referred been observed for the oxide phases of lead,2,3 vanadium30 and to as the inorganic fullerenes.molybdenum.40 In the case of the latter, subsequent treatment There are now a number of specific and general methods by of the nanotubes filled by MoO3 with hydrogen at 450–500 °C which inorganic fullerenes can be prepared, including: spon- causes reduction to pure and continuously orientated MoO2 taneous room temperature growth from reduced WS3 soot;22,23 filling.40 When a eutectic or low melting mixture of two gas-phase growth from ion beam sputtered W or Mo films;62 components is employed, as in the case of UCl4/KCl,34 continu- pulsed laser evaporation from non-fullerenic MoS2 films;63,64 ous polycrystalline filling is obtained.Encapsulated UCl4 STM induced growth from amorphous finely divided MoS3 formed in this way hydrolyses slowly in air to give continuous particles;65 and gas-phase growth from partially reduced Mo filling with an oxidised product, U(Cl,O)x .34 or W oxides.20–24 Only in the last two cases are filled or partially filled encapsulates obtained whereas in the other Arc encapsulation instances, hollow inorganic fullerene cage structures are pro- The in situ arc encapsulation technique consists of packing a duced.STM induced MoS2 growth produces encapsulates hollow graphite anode with an element to be encapsulated and containing amorphous MoS3 filling, while gas-phase growth then proceeding with the conventional Kra�tschmer–Human from the reduced Mo and W oxides produces encapsulates arc deposition experiment. During the arcing process, the tip with partially reduced oxide filling. of the anode and its contents are rendered into the vapour phase and the carbon shell then grows catalytically from the Relationships between mode of formation and condensing species.This technique has now been applied to crystallisation behaviour inside fullerene related nearly half of the elements in the Periodic Table.All of the lanthanides, except Pm, Sm and Eu;7,8,10–12,41–45 all the first- structures row transition metals;11,44–49 the platinum group metals Ru,19 Control over crystallite morphology and orientation in carbon Rh,19 Pd,19,50 Os,19 Ir19 and Pt;19 selected other transition nanotubes metals, such as Y,6,12,45 Au,8 Ta,11,26 Mo,11,45,48 W,11,45,51 Nb,45 Zr45,49 and Re;51 the main-group elements Ge,14 Sn,14 Pb,14 The behaviour of crystalline materials formed by precipitation Sb,14 Bi,14 S,145 Se,14 Te,14,45 B,14,45 Si14,45 and Al;45 and the inside carbon nanotubes allows for the study of their morpho- actinides Th52 and U52 have all been encapsulated by the arc logical control over crystallite formation.Initially, crystallis- method, either as their respective carbides or in elemental ation will proceed according to conventional nucleation and form.With the exception of Cr, Dy, Ni and Gd, which are growth mechanisms but once the size of the encapsulated obtained as continuous filling inside nanotubes,11 all of the crystallite approaches that of the internal diameter of the encapsulates are obtained as single crystals encased inside carbon nanoparticles.However, a mixture of the latter type of filling with continuous filling is also observed in the case of the encapsulated metals or carbides of Gd,10 Y12 and Mn.48 Encapsulation via catalytic growth from solid particles Arc encapsulation, as described above, is a very specific mode of catalytic growth that occurs during co-condensation after both the encapsulated and encapsulating materials have been rendered into the vapour phase.The type of catalytic growth described here pertains to gas-phase53–56 carbon deposition onto catalytic particles. The carbon carrier can vary from hydrocarbon gases, such as methane55 and acetylene,57 to aromatics, including benzene,58 to polymers, such as polyethylene, 59 and even more complex organic species.60,61 Only certain metals eciently promote carbon growth from the gas phase and these include Co,16,18,53 Fe,53 Ni,53–55,57–60 Pd,54 Pt,54 Ti,57 Fig. 1 Structural relationship between graphite and 2H-MX2 struc- W,57 Cu61 or Ge.61 In nearly all cases, the goal has been to tures. In both cases, the fullerene derived structures form networks of produce either nanotubes or modified nanotubes, including interlinked hexagonal units which can form positive or negative single walled nanotubes (SWTs), rather than encapsulated curvature based on the number and type of incorporated polyhedra other than hexagons.species which are essentially a by-product. Recently, however, 1090 J. Mater. Chem., 1997, 7(7), 1089–1095nanotube cavity, then the walls will start to exert control over directions must come to an end once it meets the carbon walls.By contrast, crystallite I may continue growing along its any future growth. The precise nature of the control at this juncture raises some interesting questions. How the capillary current axis until either it meets an obstruction or until the crystallisation process is terminated.In Fig. 2(c) a micrograph walls influence the orientational behaviour of growing crystallites and, secondly, how they interact at the atomic level with of a nanotube with a more regular packing of SnO crystallites can be seen.35 In this case the smaller crystallites observe encapsulated crystalline materialsare problems that are worthy of investigation. apparently random orientations while the larger crystallites (denoted IV), all have their d101 lattice fringes orientated at Polycrystalline SnO forms spherical or ellipsoidal encapsulates with diameters in the range 20–60 A° 35 inside carbon 90° with respect to the nanotube wall.The nanotubescapillaries shown in Fig. 2(b) and (c) clearly exert influence over both the nanotubes with internal diameters in the range 20–90 A° .The packing of such crystallites in nanotubes provides some direct morphology and orientation of their encapsulated SnO crystallites. The type of morphological control exhibited by SnO can insight into the mechanisms of control over crystallisation. Fig. 2(a) shows a high-resolution transmission electron micro- sometimes lead to some interesting behaviour inside the capillaries of carbon nanotubes.Fig. 3(a) and (b) show a micrograph graph (HRTEM) of an agglomeration of randomly orientated SnO crystallites inside a nanotube with a large internal diam- and schematic representation of a crystal spiral formed from a chain of single SnO crystallites of approximately equal size eter (ca. 90 A° ).In this instance, the nanotube exerts morphological control over the agglomeration but not over the individual and observed to form inside the capillary of a carbon nanotube. 35 The mode of formation of such a spiral can be explained crystallites. In Fig. 2(b), another micrograph showing a nanotube with a smaller diameter (ca. 35 A° ) can be seen in which completely in terms of the mechanisms of morphological control described above. two well resolved and slightly elongated crystallites I and II reside in the central cavity.A third, less well resolved crystallite Other examples of polycrystalline filling show similar types of morphological and orientational control to SnO. Fig. 4(a) (III) is also visible. Whereas crystallite I has its growth axis (arrowed) aligned parallel to the nanotube axis, crystallite III and (b) show examples of filling with polycrystalline SnO266 and ZrO2,66 respectively. Whereas polycrystalline SnO forms has eectively two growth axes (arrowed) that are both at an angle to the tube axis.Regardless of where the nucleation of small crystallites with diameters approximately equal to the internal diameter and which apparently randomly orientate crystallite II started, the crystal growth in either of the two along the nanotube capillary, polycrystalline ZrO2 crystallites Fig. 3 (a) Spiralling crystal growth, induced by morphological control, observed inside a carbon nanotube capillary. (b) Schematic representation of spiralling crystal growth. Fig. 2 (a) Micrograph illustrating nanotube capillary morphological control over agglomerated SnO crystallites.The crystallites clump together and eectively form a blockage inside the capillary. (b) Micrograph showing oentational control over individual crystallites. I is free to grow along its crystallite axis while the growth of II Fig. 4 (a) Randomly orientated SnO2 crystallites observed inside a is constrained by the nanotube capillary. (c) Random and preferentially orientated SnO crystallites (IV) inside a densely packed carbon nanotube capillary.(b) Preferentially orientated ZrO2 crystallites (arrowed) observed inside a nanotube capillary. nanotube. J. Mater. Chem., 1997, 7(7), 1089–1095 1091are elongated and several are aligned with their lattice fringes et al.30 have noted for V2O5 and Chen et al.40 have similarly noted for MoO3, continuously orientated behaviour is almost parallel to the nanotube axis.In order to attempt to answer the question of how crystalline invariably obtained. The one exception to this is when the filling is achieved using a low melting mixture. In the case of materials interact with carbon nanotube walls, it is instructive to look at well resolved HRTEM images of continuous crystal- UCl4/KCl, continuous polycrystalline filling is obtained.Fig. 6(a) shows an example of continuously orientated MoO3 line filling. Fig. 5(a) shows an elongated Sm2O3 crystallite, that completely fills the internal volume of a nanotube for a distance filling. The spacing of the observed lattice fringes is 3.7 A° , which corresponds to the c lattice repeat of MoO3.40 This of ca. 600 A° .Beneath this image are two enlargements [Fig. 5(b) and (c)] of regions at intervals along the capillary distance is also incommensurate with the atomic periodicity of the carbon nanotube wall along the tube axis (2.3 A° ). Hence where the image contrast reveals the same precise arrangement of the Sm3+ cations (which image much more strongly than crystal growth along the nanotube in this instance is again a function of orientational and morphological control.Fig. 6(b) O2- and resolve as dots) at the interface of the Sm2O3 crystallite with the nanotube wall. The cations closest to the shows an example of polycrystalline filling obtained inside nanotubes filled using a low melting mixture.34 The filling in nanotube wall can be seen to be arranged in a triangular motif that extends along the wall of the carbon nanotube. Due to a this case is polycrystalline with several crystallites (denoted V in the micrograph) apparently observing the same preferred slight tilt in the crystal, the lattice image in Fig. 5(b) images more strongly as lines which actually represent the (400) planes orientation, analogously to the case of SnO and ZrO2, described above.of Sm2O3, although the motif is still just visible. This motif actually represents the point at which parallel Sm2O3 (222) lattice planes terminate along the nanotube wall and this Formation of products inside catalytically formed carbon cages occurs at precise intervals of 5.46 A° (equivalent to d200 of There is still much uncertainty concerning the mechanism of Sm2O3).The precise arrangement is depicted schematically in formation of materials encapsulated by the arc method. Fig. 5(d). If we now look at a two-dimensional projection of Giuerret-Pie�cort et al.11 claim that the propensity for the a carbon nanotube wall [Fig. 5(e)], we see that the unit cell of formation of long metallic ‘nanowires’ rather than encapsulates hexagonal graphite repeats every 2.13 A° along the axis of the inside nanocapsules is correlated with the existence of an carbon nanotube.This is incommensurate with the periodicity incomplete shell in the most stable ionic state of the element. of the (222) lattice plane terminations [shown schematically Saito et al.67 have indicated that there is an additional corre- in Fig. 5(d)]. Thus, in the case of this Sm2O3 crystallite, the lation for lanthanoids between their volatility and their ability orientation and periodical arrangement must be a function of to form encapsulates.Recently however, Seraphin et al.45 have the gross morphological influence of the nanotube capillary indicated that neither of these models are wholly without during crystallisation rather than any influence due to the exceptions and have advanced their own model, defined in atomic arrangement of the nanotube wall.terms of the interfacial compatibility of the carbide with the When materials are inserted into nanotubes by capillary encapsulating graphitic network. The phenomenon of arc action, the orientational behaviour of the encapsulated crystal- encapsulation is further complicated by the fact that, in some line material is much the same as described above.As Ajayan cases, mixed products are often obtained. In the case of encapsulated manganese, for example, Liu and Cowley47 have observed no fewer than four dierent encapsulated carbides, Mn3C, Mn5C2, Mn7C3 and Mn23C6 , some of which are incommensurate and presumably metastable. Similarly Sloan et al.51 have observed the formation of rhenium metal, hexagonal ReC and an unknown metastable RexCy product inside carbon nanocapsules.A micrograph obtained from the latter Fig. 5 (a) 600 A° long Sm2O3 crystallites observed inside the bore of a Fig. 6 (a) Micrograph showing continuous MoO3 filling formed by carbon nanotube. (b), (c) Enlargements of Sm2O3/carbon interface showing periodic stacking of Sm3+ cations. (d) Schematic represen- the capillary method.The lattice fringes repeat at regular intervals of 3.7 A° which is incompatible with the repeat of the graphitic network tation of termination of Sm2O3 (222) lattice planes on carbon wall. (e) Schematic projection of graphitic nanotube wall showing periodicity (2.3 A° ; see Fig. 5). (b) Polycrystalline UCl4 arranged along the bore of a carbon nanotube obtained via eutectic filling.Some crystals show of graphitic network (repeats every 2.3 A° ). This is incommensurate with the repeat of (222) lattice plane terminations. clear preferred orientations (V). 1092 J. Mater. Chem., 1997, 7(7), 1089–1095product is reproduced here [Fig. 7(a)]. Giuerret-Pie�cort et al.11 proceed via a solution–precipitation mechanism,56,68 which involves absorption of carbon onto the surface of the catalytic have observed the formation of both microcrystalline Yb and ‘spiral’ Dy products in their nanowires, both of which are also particle resulting in the formation of a small amount of interfacial carbide, thus leaving the remainder of the encapsu- indicative of a complex formation process.In view of the complexity of the products obtained in these and other cases late in its native elemental state. The carbon then grows progressively from the element/carbide interface. An example cited in this article, it seems unlikely that one model or explanation alone will suce to account for all of the encapsul- of an encapsulated Ni particle formed in the presence of a Ni- Harshaw catalyst at 780 °C, according to conditions specified ation behaviour observed in arc deposited products.A particular practical diculty is the fact that there is at present no by Tsang et al.,55 is shown in Fig. 7(b). eective way of observing in situ the encapsulation process. Perhaps a better approach is to consider each case individually, Novel layered and defect structures observed inside inorganic taking into consideration the complex kinetic and thermo- fullerenes dynamic mechanisms that can be obtained within a particular system.The encapsulation mechanism of sulfide and oxide particles The natures of the encapsulated products obtained in the encapsulated inside inorganic fullerenes is relatively easy to case of gas-phase deposition onto catalytic particles are much interpret for the simple reason that the encapsulating material simpler to interpret than those of species formed by arc co- is formed by consumption of the outside of the encapsulated deposition.In general, encapsulation has been proposed to material. Thus, the growth mechanisms can be interpreted in terms of progressive growth from the exterior.The three dierent mechanisms for STM induced MoS2 growth from MoS3, gas-phase growth of MoS2 from condensing MoO3-d vapour and gas-phase reaction with solid WO3-d are depicted schematically in Fig. 8((b) and (c), respectively. In the case of MoS2 induced growth in the STM, encapsulation results in the formation of amorphous MoS3 encapsulates only. In this instance, the morphology of the resulting encapsulates will be influenced by the morphology of the amorphous precursors.In the case of the formation of Mo and W oxide encapsulates, the first occurs wholly in the gas phase whereas the second occurs as a gas–solid reaction with the oxide remaining in the solid phase. The encapsulation mechanism of MoO3-d will presumably be similar to that exhibited by arc deposited encapsulates (see above), with the whole process occurring during condensation from the vapour phase.The morphology of the encapsulated products will therefore be Fig. 7 (a) Microstructure of the encapsulated metastable RexCy prod- determined by the extent of reaction and the conditions of uct formed via arc co-deposition. (b) Micrograph showing Ni encapsulated by catalytic formation of carbon from the Ni-Harshaw catalyst.condensation. Feldman and coworkers24,69 have observed a Fig. 8 Schemes showing: (a) mechanism of formation of MoS2 induced by STM from MoS3, resulting in the formation of amorphous MoS3 encapsulates; (b) mechanism of gas-phase reaction of H2S with MoO3-d to form encapsulated oxide and empty MoS2 inorganic fullerenes; (c) mechanism of gas–solid reaction of H2S with solid WO3-d to form encapsulated oxide and empty WS2 inorganic fullerenes.All mechanisms eventually produce empty nested inorganic fullerenes (adapted from Feldman et al.69). J. Mater. Chem., 1997, 7(7), 1089–1095 1093variety of products, from MoS2 fullerenes, to MoS2 nanotubes, to MoS2 encapsulated MoO3-d, obtained precisely by varying these conditions. In the case of the formation of WS2 encapsulated WO3-d, however, the situation is made more interesting by the fact that encapsulation occurs via reaction of H2S gas with the reduced WO3 solid in a situation analogous to the gas-phase carbon deposition onto solid catalytic particles, described above.Under these conditions, the overall morphology of the encapsulated species will be determined by the morphology of the reduced WO3-d precursor particles.An example of this phenomenon is shown in the micrograph in Fig. 9(a). The encapsulate in this case is a ‘bent’ crystallite of WO3-d encapsulated by a ‘skin’ of WS2 fullerenic material. Closer inspection of the crystallite reveals that the bend in the crystallite is due to several grain boundaries (arrowed) that occur as defects within the crystal.A further interesting feature of these encapsulates is that, as partially reduced WO3, there Fig. 10 Layered and lamellar reduced tungsten oxide structure is always the potential that other types of defects and structural observed inside WS2 nested fullerene features may be observed inside encapsulates. This is in fact now the case.70 In Fig. 9(b), an example of an encapsulated structure of the Wadsley defect type is shown. The shear planes sharing WO3 octahedra network partially collapsing to form observed in this encapsulate arise as a result of the corner- shear planes comprised of edge-sharing WO3 octahedra. A schematic representation of the encapsulated defect structure is shown in Fig. 9(c). Previously, this type of defect structure has only been observed in reduced single crystals of reduced WO3.71 Another new type of encapsulated structure is shown in Fig. 10. Inside a bilayer of WS2 material, a novel layered tungsten oxide can clearly be seen. This is the first example of a complex layered structure formed inside a fullerene related structure. A more detailed analysis of the structure of this encapsulate will appear elsewhere.70 Concluding remarks The types of encapsulation behaviour discussed in this article raise the prospect of manipulating materials formation at the most intimate scale.The mechanisms of morphological control of nanotube capillaries over crystallite formation contribute to an understanding of how encapsulates such as nanowires can form inside nanotubes.The main benefit of catalytic encapsulation of materials, either in situ or by gas-phase deposition onto catalytic particles, is in the coating of reactive or airsensitive species. In the case of in situ formed materials, an additional benefit lies in the fact that many of the encapsulated products are metastable and are often not readily obtainable by other methods.The observation of defect and layered materials inside inorganic fullerenes presents a wholly new perspective in encapsulation technology. These materials represent, for the first time, the first realistic possibility for incorporating complex layered or defect structures, with one or more useful properties, inside a wholly dierent type of structure, with completely dierent properties, thus generating a new class of hybrid materials. The authors are indebted to Dr.Edman S. C. Tsang, of Reading University, for supplying the encapsulated MoO3 specimen, to Dr. Jens Hammer and Rufus Heesom, of Oxford University, for preparing the encapsulated rhenium carbide specimen, to Dr. Andrew P. E. York, of the Inorganic Chemistry Laboratory, for the provision of the Ni-Harshaw specimen and also to Moshe Homyonfer and Yishai Feldman, of the Weizmann Institute, who prepared the WO3-d encapsulated WS2 specimen.Fig. 9 (a) External morphology of encapsulate induced by defect structure of reduced oxide encapsulate. WS2 skin follows the bend in the WO3-d crystal induced by grain boundaries (arrowed). (b)Wadsley References defect structure incorporated inside WS2 nested fullerene.(c) Schematic representation of (b) showing how the partial collapse of WO3 network 1 S. Iijima, Nature (L ondon), 1991, 354, 56. 2 P.M. Ajayan and S. Iijima, Nature (L ondon), 1993, 361, 333. leads to the formation of shear planes. The total number of shear planes depicted is 4, but there are no less than 18 inside the imaged 3 P.M. Ajayan, T. W. Ebbesen, T. Ichihashi, S. Iijima, K. Tanigaki and H. Hiura, Nature (L ondon), 1993, 362, 522. structure. 1094 J. Mater. Chem., 1997, 7(7), 1089–10954 S. C. Tsang, Y. 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Commun., 1995, 1355. 71 R. J. D. Tilley, Chem. Scr., 1979, 14, 147. 38 Y. K. Chen, A. Chu, J. Cook, M. L. H. Green, P. J. F. Harris, R. Heesom, M. Humphries, J. Sloan, S. C. Tsang and J. C. F. Turner, J.Mater. Chem., 1997, in press. Paper 7/00035I; Received 2nd January, 1997 J. Mater. Chem., 1997, 7(7), 1089–1095 1095

ISSN:0959-9428    

DOI:10.1039/a700035i

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

FEATURE ARTICLE [60]Fullerene chemistry for materials science applications Maurizio Prato Dipartimento di Scienze Farmaceutiche, Universita` di T rieste, Piazzale Europa 1, 34127 T rieste, Italy Since their first detection and bulk production, the fullerenes have gained a primary role on the scientific scene, reaching their climax when the 1996 Nobel Prize for Chemistry was awarded to Kroto, Curl and Smalley for their seminal discovery.The unique physical and chemical properties of these new forms of carbon led many scientists to predict several technological applications. This created a heavy disappointment when it was clear that fullerene-based materials would not soon be ready for the market. However, the fullerenes have so far delighted several dozens of researchers who found that C60 and its relatives undergo a variety of chemical reactions. In most cases, the new derivatives retain the main properties of the original fullerene, and it is now not unlikely that some functionalized fullerenes may find useful applications in the field of materials science and technology. In this Article we summarize the basic principles of the organic chemistry of fullerenes, together with a description of the physicochemical properties that have made these carbon cages popular in materials science, and review the most recent achievements in the functionalization of fullerenes aimed at the production of new molecular materials.[60]Fullerene, the most abundant representative of the fuller- Initially hypothesized as a ‘super aromatic’ molecule, C60 was ene family,1 was produced for the first time on a preparative rather found to possess a polyenic structure, with all the double scale in 1990, by resistive heating of graphite.2 The availability bonds inside the six-membered rings.31 X-Ray crystal structure of milligram quantities of C60 generated an extraordinary determinations on C60 and on some of its derivatives have outburst of academic and industrial research that led to the proved the existence of two dierent types of bonds: ‘short discovery of several interesting physical properties, along with bonds’ or 6,6 junctions, shared by two adjacent hexagons (ca. a careful definition of the chemical reactivity of the fuller- 1.38 A° long) and ‘long bonds’, or 5,6 junctions, fusing a enes.3–10 Among the most spectacular findings, C60 was found pentagon and a hexagon (ca. 1.45 A° long). The geometric to become a superconductor in M3C60 species (M=alkali demand of the spherical cage is such that all the double bonds metal),11–14 an organic soft ferromagnet in TDAE+VC60-V in C60 deviate from planarity.13 This pyramidalization of the (TDAE=tetrakisdiethylaminoethylene),15 a relatively stable sp2-hybridized carbon atoms confers an excess of strain to C60 hexaanion in cyclic voltammetry,16,17 and an interesting mate- which is responsible for the enhanced reactivity of the fullerene.rial with non-linear optical properties.18,19 A release of strain is in fact associated with the change of It was immediately clear that a new molecular material had hybridization from sp2 to sp3 that accompanies most chemical been discovered with enormous potential in several dierent reactions.32 disciplines.Especially in materials science, the rich electronic The chemical reactivity of C60 is typical of an electron- and electrochemical behaviour generated great expectations. deficient olefin. C60, in fact, reacts readily with nucleophiles However, the dicult processibility of the fullerenes has rep- and is a reactive 2p component in cycloadditions.33 The vast resented a major problem in the hectic search for practical majority of reactants will attack the 6,6 ring junctions of C60, applications.C60, in fact, is insoluble or only sparingly soluble which possess more electron density. Insertions into 5,6 bonds in most solvents and aggregates very easily, becoming even have been reported only as rearrangements following a 6,6 less soluble.20 This serious obstacle could be, at least in part, junction attack (see below).surmounted with the help of the ‘functionalization chemistry The main objective of fullerene chemistry is the production of the fullerenes’.21–29 The organic derivatization of C60 has of well-defined, stable and characterizable adducts. In this put forth an increasingly high number of compounds which, respect, several dierent approaches have given excellent while retaining most of the original properties of the fullerene, results.The reaction types can be of widely dierent nature, become much easier to handle. but the single-addition products can be classified into a few In this review we will focus on the use of fullerene chemistry broad categories, based on the structure which is obtained. In to produce compounds useful in materials science and technol- particular, with relation to the geometrical shape built on a ogy.An exhaustive review of all the literature produced so far 6,6 ring junction of C60, there can be: an open structure; a on this argument is beyond the scope of the present work.An three-membered ring, which also includes carbon or nitrogen attempt will be made to give an idea of the potential of the insertion into a 5,6 ring junction; a four-membered ring; a five- fullerene materials in practical applications. membered ring; a six-membered ring (Fig. 1). First, we will briefly review the basic principles of the In general, the word dihydrofullerenes has been coined to chemical reactivity of C60, as the chemistry of fullerenes has specifically indicate a monofunctionalized fullerenes, or else already been reviewed in detail by several authors.21–29 We the word organofullerene more widely indicates a fullerene shall then illustrate the main physicochemical properties that derivative containing an organic appendage.have made fullerenes popular materials. We will finally address the main subject of this review, considering only materials derived from functionalized C60. For unmodified C60-based Open structures thin films and materials, the reader is referred to a recent, Adducts can be obtained by careful hydrogenation (Nu=E= excellent review.30 H),34–38 or by addition of a nucleophile followed by quenching with acid or an electrophile (Scheme 1).Basic Principles of C60 Chemistry Usually a 1,2-addition is observed, but 1,4-additions have been reported in a few cases where hindrance between sterically The C60 surface contains 20 hexagons and 12 pentagons. All the rings are fused, all the double bonds are conjugated. demanding addends becomes relevant.Nucleophiles success- J. Mater. Chem., 1997, 7(7), 1097–1109 1097Fig. 1 Geometrical shapes built onto a 6,6 ring junction of C60: (a) open, (b) three-membered ring, (c) four-membered ring, (d) five-membered ring and (e) six-membered ring Nu Nu E Nu– E+ – 1 Scheme 1 fully employed include Grignardreagents,33,39,40 organolithium lead to azafulleroids.75–77 If nitrenes are generated instead, azamethanofullerenes are formed.77–83 derivatives,39–41 cyanide ion,42 etc.43–55 A case which illustrates the utility of this approach to produce interesting materials is It is interesting to note that, among the many families of organofullerenes, fulleroids and azafulleroids are the only reported in Scheme 1.Cyanide addition to C60, followed by quenching with toluene-p-sulfonyl cyanide, led to the synthesis derivatives which maintain the 60p electron configuration typical of C60.of a dinitrile derivative 1 (Nu=E=CN).42 As detected by cyclic voltammetry, 1 and also other monocyano-dihydrofullerenes display interesting properties, allowing a fine-tuning of Four-membered rings the electron-accepting capacity of cyanodihydrofullerenes.Cyclobutanofullerene derivatives are typically obtained by [2+2] cycloadditions. Benzyne addition was reported Three-membered rings first,84,85 followed by addition of electron-poor alkenes,86–88 electron-rich alkenes and alkynes.89–92 [2+2+2] Cyclo- This category represents one of the most fascinating and thoroughly investigated classes of functionalized fullerenes.addition of quadricyclane to C60 gave rise to a norbornene derivative93 which was used for polymer preparation.94 The addition of diazomethane derivatives to C60, pioneered by the Wudl group,21,25,56 can lead to two dierent structures, commonly called fulleroids 3 and methanofullerenes 4. Five-membered rings These are usually prepared by [3+2] cycloadditions. A variety of carbocyclic or heterocyclic systems have been reported, which include cyclopentane derivatives,95,96 pyrrolidines, 81,97–101 isoxazolines,102–104 pyrazolines,105 furans,106 etc.107,108 The addition of azomethine ylide to C60, leading to fulleropyrrolidines, is becoming increasingly popular (see below).The reason for such success is probably due to the simple approach, as starting materials are usually commercially available or easily prepared, and a single product of monoaddition across a 6,6 junction of the fullerene is obtained The first step of the reaction is a 1,3-dipolar cycloaddition (Scheme 2).of diazomethane to C60, yielding a pyrazoline derivative 2, isolated only in the case of diazomethane,57 but not in other Six-membered rings cases. Extrusion of nitrogen leads typically to a mixture of The classical [4+2] cycloaddition to C60 produces six-mem- fulleroids and methanofullerenes.58,59 Conversion of fulleroids bered rings fused to 6,6 junctions.95,109–122 This is also a very to methanofullerenes can be achieved (in most cases, but not for R=R¾=H60) thermally,25,58 electrochemically61 or photochemically. 62 It can also be acid-catalysed.63 A wide variety of diaryl, aryl-alkyl and dialkyl fulleroids and methanofullerenes21,25 have been prepared so far, providing materials for potential applications in many fields (see below).Three-membered rings fused on 6,6 junctions of C60 can be produced cleanly (without formation of fulleroids) electrochemically64 or by addition of nucleophiles,65–67 diazirines,68 carbenes,69–72 sulfonium ylides.73 Azide additions to C60 follow closely the reaction course of diazo compounds.In this case, triazoline derivatives can be R1 N+ CH2 HC N R1 R2 R2 –CO2 –H2O C60 – R1NHCH2CO2H + R2CHO heat Scheme 2 isolated and characterized,74 which, after extrusion of nitrogen, 1098 J. Mater. Chem., 1997, 7(7), 1097–1109popular reaction, utilized by many groups, which has oered bonds, all sharing the same reactivity.Typically, the addition of a nucleophile to a 6,6 bond of C60 in a stoichiometric entries to a wide variety of functionalized fullerenes (see below). amount leads to a complex mixture containing one product of monoaddition together with several multiple addition prod- Holes ucts. Usually, the monoadduct is separated by chromatography Ever since the fullerenes were discovered, the idea of trapping and the multiple adducts discarded.This is because a mixture atoms, molecules or ions inside the carbon cage has fascinated of diadducts (from a symmetrical reagent) can contain up to the scientific community.1,123 It is in fact believed that novel eight dierent positional isomers, with the number of possible materials with peculiar properties may be produced.Whereas isomers increasing with the number of additions. Isomers some fullerenes containing transition metals inside have been inside each family of adducts (monoadducts, diadducts, triad- isolated,124 the preparation of bulk quantities of inclusion ducts, etc.) tend to possess the same chromatographic proper- assemblies would require a chemical modification of C60 such ties, and this makes the separations a very complex operation.that a hole is opened on the fullerene surface by breaking one Therefore, addition conditions are usually optimized for the or more double bonds. Then the atom, molecule or ion must maximum yield of the monoaddition product, with little atten- be forced inside the cage and trapped by restoring the carbon– tion to more highly functionalized fullerenes.carbon bonds. Conceptually very simple, the opening of a hole Recently, the chemistry of multiple additions to the fullerene on the fullerene cage proved very dicult. The first orifice core has become a fundamental issue in the design of useful generated was obtained by oxidative light incision.125 The C60 derivatives, and several research groups have taken the sample, however, was prepared on an analytical scale, and was challenge of isolating and characterizing diadducts as well as only characterized by IR spectroscopy.An eleven-membered higher adducts. Investigations of polyadditions have been ring hole was produced on the fullerene surface by means of carried out using osmylation,139 g2-metal complexation,140–142 regioselective diaddition of azides.76 Other holey spheres have hydrogenation,143,144 cyclopropanation,145–150 azide been obtained on preparative scales, and the resulting com- addition,76,77,151,152 [4+2] cycloaddition,113,148,149,153,154 pounds fully characterized.126–128 A very interesting result has [3+2] cycloaddition,155 azomethine ylide cycloaddition,156 been disclosed recently by Hirsch and collaborators, who silylation,157 epoxidation158,159 and amine addition.50,160 reported the first example of chemical modification of fullerenes In particular, the Hirsch group and the Diederich group that allows the synthesis of open and closed valence isomers have engaged in a systematic study aimed at determining the with the same addition pattern.They found that diadducts 5 factors that govern the regiochemistry of these additions and and 6 formed by addition of azides to C60 possess dierent at gaining control over multiple additions. The German team, structures, depending on the substituent on the nitrogen atom. after isolating and characterizing all diadducts formed in the Unsubstituted diadduct 5 is locked in the ring-closed form, base-catalysed addition of bromomalonates,145 reached the whereas a carbamate functionality gives rise to the open form, conclusion that the addition of a second nucleophile does not which has a relatively large hole.occur randomly, but is controlled by the frontier molecular orbitals of the monofunctionalized fullerene.146 Furthermore, in a decisive step toward the synthesis of hexakis adducts, the same group employed the reversible addition of 9,10-dimethyl anthracene to produce the Th-symmetrical hexakis adduct of cyclopropanation of C60 with an octahedral addition pattern in an astonishing 48% yield.161 On the other hand, the strategy used by the ETH group has been the tether-directed functionalization of C60, which produces exclusively adducts derived by equatorial addition.29,148 This approach has allowed the Swiss group to achieve outstanding results, like two Saunders and collaborators have demonstrated that at very solubilized representatives of a new class of carbon allotropes, high temperatures (650 °C) and pressures (3000 atm) the noble C195 and C260.162 A unique case of topochemically controlled gases helium, neon, argon, krypton and xenon can be intro- fullerene difunctionalization has been recently reported.153 In duced inside the cage in one in every 1000 molecules of C60.129 the solid state, when heated at 180 °C for 10 min, the crystalline A temporary bond breaking of the cage has been proposed to monoadduct of C60 and anthracene evolved to the antipodal explain the process.For obvious reasons this methodology diadduct in a quantitative way. cannot be extended easily to other guests but noble gases. In Polyhydroxylated C60 derivatives, fullerenols C60(OH)10–12, addition, Saunders warned that conventional chemical syn- can be obtained by dierent methods.163–166 The hydroxy thesis at ambient pressures, though elegant, may not succeed, groups are randomly distributed on the fullerene surface, due to the small free volume inside the fullerene sphere.generating mixtures of isomeric structures, but providing a high density of reactive sites useful for practical applications Heterofullerenes (see below). Although complete control over the addition chemistry of Another objective of fullerene chemistry relates to the possi- the fullerenes has yet to be reached, the encouraging results bility of substituting one or more carbon atoms of the cage reported so far give way to the hope that, in the future, the with heteroatoms.130–132 This substitution leads to hetero- fullerenes, and in particular C60, may be used as building fullerenes, which may possess properties dierent from the blocks in the construction of very complex molecular parent fullerenes.The most popular heteroatom so far incor- assemblies. porated in fullerenes is nitrogen,133,134 and the C60 homologue has been isolated in bulk quantities as its dimer is (C59N)2.135,136 The chemical and physical properties of these Physicochemical properties of fullerene derivatives new compounds are the object of intense current investigations.137,138 Electrochemical properties From the early days of the functionalization chemistry, the Multiple additions electrochemistryof fullerene derivativeshas been systematically studied by the Wudl group at the University of California at Without taking into account 5,6 bonds (which can undergo insertions, see above), C60 possesses 30 equivalent double Santa Barbara.In fact, a striking feature of C60, as shown by J. Mater. Chem., 1997, 7(7), 1097–1109 1099cyclic voltammetry, is that, in solution, this fullerene can accept the visible region up to 650 nm, are retained in most derivatives. In addition, dihydrofullerenes extend their absorptions reversibly up to six electrons. The UCSB group found that both fulleroids and methanofullerenes essentially retain the throughout the entire visible region, with a weak maximum at ca. 700 nm. This additional feature makes excitation possible electronic properties of C60.21,56,61,75,95,167–171 The same behaviour has been observed for most C60 derivatives, whose cyclic by means of irradiation at very low energy. Analogously to C60, dihydrofullerenes are excited to a short- voltammograms are typically characterized by a small shift to more negative values of the reduction potentials. This is lived singlet which converts rapidly into a long-lived triplet, with quantum yields slightly lower than C60.179,180 Whereas expected on considering that saturation of a double bond in C60 causes a partial loss of conjugation. Due to this eect, at C60 exhibits a triplet–triplet absorption at 750 nm, the same peak is shifted to ca. 700 nm in C60 derivatives.180–185 This most five reduction peaks for the C60 moiety in fullerene derivatives have been detected so far in the accessible potential triplet–triplet transition is characterized by a higher absorption coecient than the ground state, and may be responsible for range.172–176 An extensive investigation of the redox properties of several its non-linear behaviour.Accordingly, solutions of C60 and C70 exhibit optical limiting (OL) properties, which compare very variously functionalized organofullerenes has been reported by Suzuki et al., who studied the influence of the groups attached well with those of materials currently in use.18 This feature holds great promise for practical applications, such as incorpor- directly to C60 on CV potentials.177 A small inductive eect was found, revealed by changes in the reduction and oxidation ation of fullerene derivatives in proper transparent matrices for protection against high-energy laser pulses. As compared (where possible) potentials and mainly related to the electronegativity of the atoms attached.to C60, fullerene derivatives show a lower singlet–triplet quantum yield,179,180 so that they are expected to exhibit a lower A more incisive control of the electronic properties of the fullerenes might still be a relevant issue. The conjugated p- OL eciency. However, it has been demonstrated that the optical limiting properties of C60 and its derivatives depend system of C60 seems ideally suited for non-linear optical (NLO) applications.18 Molecules with large NLO properties are often on the excitation wavelength.186,187 When the latter is closer to the triplet–triplet absorption maximum of the fullerene characterized by an electron-donating group and an electronwithdrawing group at opposite ends of a conjugated p system.derivative (700 nm) than to the equivalent transition of C60 (750 nm), the OL performance of the organofullerene becomes In principle, attachment of donors and acceptorsin conjugation with the fullerene p system should result in an interesting more ecient.Dierences in the ground-state absorption can also play a role. push–pull assembly. However, most reactions of the fullerenes lead to derivatives in which the addends are attached to C60 via sp3 carbons, an event that breaks the conjugation.This Multiple adducts problem was ingeniously faced by Wudl and co-workers, who The physicochemical properties of the multiple adducts are used fluorenyl systems spiro-linked to a methanofullerene largely dependent on the number of addends.56,146,188 In a moiety. An interaction through a ‘periconjugation’ mechanism systematic electrochemical investigation, it was found that the between the fluorene group and the fullerene spheroid was reduction and oxidation characteristics depend very heavily detected by cyclic voltammetry.168,171 It was found that the on the number and pattern of the addends in fullerene deriva- electrochemical behaviour of compounds 7 is relatively sensitives. As a general trend, the fullerene derivatives become tive to the presence of substituents on the fluorene moiety.harder to reduce going from mono to hexakis adducts. This With electron-donating groups (7b) the reduction potentials has been attributed to the reduced conjugation occurring in are shifted to more negative values. On the other hand, the multiply functionalized fullerene compounds, which leads to a first reduction potential becomes less negative if strong eleccorresponding increase of the energy of the LUMO.188 The tron-attracting groups are placed in the 9-fluorenyl moiety same trend is observed in the study of UV–VIS absorption (7c).The electronic properties of spiromethanofullerenes 7 can features, where changes due to loss of conjugation are be attributed to their peculiar geometry, as the fluorenyl planar observed.146,188 skeleton is held rigidly perpendicular to the surface of the spheroid.This unique arrangement may be responsible for ‘through-space’ interactions between the fluorenyl moiety and Spin-labelled derivatives the spheroid, which thus becomes sensitive to electronic A series of C60 derivatives incorporating a nitroxide unit has changes in the fluorenyl counterpart.The promising NLO been synthesized (Fig. 2).189–192 These compounds possess a molecules 8 were also synthesized as a mixture of diadducts paramagnetic probe useful for investigating the electronic (R=electron-withdrawing and electron-donating groups). A properties of the fullerenes. Indeed, they were successfully linear free energy relationship of the reduction potentials on employed for the study of the anions192 and the excited triplet the Hammett sm of the substituent inside a family of methanostates189 of the C60 moiety.fullerenes was also reported by Wudl and collaborators.178 Applications Polymers As we have already seen, the fullerenes possess several outstanding properties.The incorporation of fullerenes in polymers would potentially endow the polymer of most of the fullerene properties.193 Thus, electroactive polymers can be obtained, or polymers with optical limiting properties.194 On the other hand, fullerenes embedded in polymers become more easily processible. The resulting materials can be used for surface coating, photoconducting devices, and also to create new molecular networks.Optical properties There may be several ways to combine polymers with fullerenes. The simplest way is the plain mixing of the two The ground state absorption properties of C60, characterized by strong bands in the UV region and weaker absorptions in components, either as a solid mixture, or as a solution in a 1100 J. Mater. Chem., 1997, 7(7), 1097–1109N x y N3 x y C60, heat –N2 Scheme 4 (d) A dendrimer can be built on a fullerene nucleus (Fig. 3).211,212 In addition, three-dimensional, starburst polyurethane networks have been prepared using fullerenols as molecular cores and condensing them with isocyanate prepolymers. Highperformance elastomers with enhanced thermal stability are thus obtained.213,214 A comparison between dierent ways of producing C60 polymers has been reported recently.215 Free radical polymeriz- Fig. 2 A series of C60 derivatives incorporating a nitroxide unit ation of methyl methacrylate (MMA) was carried out in the absence (PMMA) and in the presence (PMMA-9) of derivatives 9, and compared to simple embedding compounds 9 in common solvent which is then evaporated.The latter mixing preformed PMMA (emb-9). It was found that samples from produces more homogeneous samples. This practice usually dierent preparations dier significantly. PMMA-9 clearly leads to non-covalent interactions between the two shows cross linking of the polymer chains, which leads to an components. increase in Tg of ca. 8°C with respect to plain PMMA. In The chemical linking of polymers and fullerenes can be addition, the cross linked material did not dissolve in chloro- obtained by four main ways.form, a solvent in which PMMA and emb-9 were readily (a) Fullerenes that are present during the polymerization of a soluble. Cross linking was not observed when C60 was used in monomer can react and be attached to the polymer place of 9 during the polymerization process.This was attri- chain.194–201 Typically, this happens in anionic and free radical buted to the lower solubility of C60 in MMA as compared polymerizations, where species are generated that react ran- to 9. domly with the double bonds of the fullerene. In this case there is no chemical control: multiple additions to the fullerene double bonds occur, so that the fullerene structure is not welldefined.Cross linked materials are usually obtained (Scheme 3). (b) A preformed polymer is treated under conditions that favour the chemical linking to the fullerenes.202–209 This is generally obtained by generating nucleophilic polymeric species. Also in this case the chemical attack to the fullerene double bonds is indiscriminate, and mixtures of isomeric fullerene species can be obtained (Scheme 4).(c) A monomer containing a fullerene unit is polymerized or Information on the structure of the cross linked species was co-polymerized.94,210 In this case, if the fullerene monomer is obtained from the analysis of the EPR spectra recorded for a well-defined monoaddition product and if the conditions the lowest excited triplet of PMMA-9.An unusually large employed are chemically inert to the fullerene double bonds, electron dipolar splitting D parameter of positive sign was the final polymer contains a fullerene species with an almost observed. The spectrum was simulated using a simple model intact electronic configuration, in which only one double bond calculation which considers C60 and derivatives as a collection (or none, in the case of fulleroids or azafulleroids) of the of fully localized double bonds. A positive sign of D is expected pristine fullerene has been saturated (Scheme 5).The electronic for PMMA-9 in which cross linking has occurred in the and electrochemical properties of C60 were shown to be equatorial belt of the molecule.215 retained in the polymers.94,210 A peculiar type of fullerene polymers (all-carbon polymers) has been obtained by irradiation of oxygen-free films or solutions of C60216,217 as well as by heating AC60 crystals (A= K, Rb, Cs).14 A quasi-linear structure, derived from [2+2] cycloadditions of C60 double bonds leading to four-membered rings, has been proposed for these polymers.218,219 Fullerenes, and C60 in particular, show very limited solubility in any medium, especially polar solvents.For any molecule that contains more than one C60 sphere, this experimental problem is amplified. Therefore, when preparing fullerene- Scheme 3 based polymers, one of the main issues that needs to be J. Mater. Chem., 1997, 7(7), 1097–1109 1101N Mo Ar But RO RO m [Mo] = [Mo] + n Scheme 5 Fig. 3 A dendrimer built on a fullerene nucleus addressed is the solubility of the material.The typical result is to the diminished solubility of 10 in most solvents, Wudl warned that polymers containing C60 should be expected to that only oligomers are obtained with relatively low molecular weights. Wudl and co-workers reported the synthesis of deriva- be insoluble and intractable, unless a solubilizing group is attached.The same conclusions were reached by other groups, tives containing two C60 units (10),167 which were shown to retain the original electronic fullerene properties. But, owing during the synthesis of compounds 11 and 12.110,220 1102 J. Mater. Chem., 1997, 7(7), 1097–1109polymerization degree, the authors reported that both components, namely the conjugated polymer and C60, retain their original electrochemical properties, and that some new properties may be expected from their interactions. Thin films Thin films containing fullerenes are of current high interest, owing to the possibility of transferring the interesting fullerene properties to bulk materials by simple surface coating.In this respect, self-assembled monolayers (SAM)30,229,230 and Langmuir films are being increasingly used, as controlled organized structures can be obtained.In a very stimulating experiment, Echegoyen and Kaifer used molecular recognition to induce the formation of molecular monolayers of the 18-crown-6 functionalized fullerene 13.231 A gold surface was modified using a thiol-terminated ammonium salt (Scheme 6). When the modified gold layer was immersed into a CH2Cl2 solution of 13, surface coverage was obtained which corresponds to a compact monolayer of C60, as found by OSWV measurements.The attachment of 13 to the ammonium salt, and thus to the gold surface, was demonstrated to be reversible in a CH2Cl2 solution. A major problem encountered during the preparation of Langmuir films of fullerenes is related to the high hydrophobicity of the carbon cage compounds.Eorts have been aimed at the preparation of fullerene derivatives which present a hydrophilic end.111,232–244 In these cases, monomolecular layers with an area per molecule of approximately 10 A° 2 have 10 11 12 been often obtained. Langmuir–Blodgett transfers to solid substrates, however, proved very dicult, and only a few However, even when solubilizing hexyloxy chains were intro- successful cases have been reported.Only two recent represen- duced in the polymer, the number of fullerene moieties involved tative examples of successful transfers will be discussed here was still low, and precipitation of oligomers (n=0–5) occurred (for a more detailed discussion on Langmuir films of fullerenes, due to cross linking.221 The facile cross linking of C60-contain- see ref. 30). ing polymers represents, in fact, another, strictly connected An extensive investigation on the Langmuir behaviour of problem. The number of reactive double bonds in C60 is such several C60 derivatives was reported recently.235 The amphi- that up to eight or ten chains can radiate from a fullerene philic fullerene monoadducts studied include carboxylic acid nucleus.This leads to a very tight, cross linked structure, and amine derivatives, a bis-phenol, a crown ether and a whose solubility and processibility become problematic. A cryptate, together with some protected and deprotected sugars. higher Tg is usually observed upon addition of C60 or organof- Monomolecular layers were obtained for the cryptate deriva- ullerenes to a polymer,94,207,215 which suggests that fullerenes tive, but were not very stable as assessed by compression– may be used as additives for increasing the thermal stability expansion cycles.Langmuir–Blodgett transfer of films derived of a material. Eventually, when cross linking was avoided, a from some sugar derivatives was only possible using highly soluble polymer with Mw of ca. 80000 was prepared. The hydrophobic, phenyl-functionalized glass or quartz substrates. improvement was obtained using a mixture of two dierent o- Spreading behaviour independent of concentration in the quinodimethanes, one of which helped avoid cross linking range 0.1–2.0 mM and area/molecule of 96 A° 2 , with thickness while increasing the solubility.221 of 7±3 A°, in excellent agreement with theory, was obtained An interesting example of a C60 end-capped polystyrene star for methanofullerene 14.233 The monolayers were transferred has been recently reported (Fig. 4). The attachment to C60 was to solid substrates (quartz or mica) with transfer ratios close obtained via azide addition, and the resulting polymer was to unity.shown to retain the basic C60 electrochemical properties.222 Rotello and co-workers took advantage of the reversibility of the addition of cyclopentadiene to C60 for the temporary attachment of the fullerene to a modified Merrifield resin.223 Addition of the cyclopentadiene-modified resin to C60 was achieved at room temperature, whereas the fullerene was released at 180°C upon addition of maleic anhydride as a CO2CH2CH2OCH2CH2OCH2CH2OCH3 CO2CH2CH2OCH2CH2OCH2CH2OCH3 14 cyclopentadiene trap.The authors proposed the methodology for a non-chromatographic purification of the fullerenes.224 Thin films useful for laser protection can be obtained by A few examples of electrochemical polymerization of C60 incorporation or covalent attachment of fullerenes to transderivatives have been reported. Starting from a dialkynylated parent solid matrices.The optical limiting properties of C60, methanofullerene, Diederich et al. observed formation of an originally detected in toluene solutions (see above),18 can be electrically conducting film on the surface of the platinum transferred to solid substrates without significant activity cathode.225 A redox-active fullerene polymer with interesting loss.245 mechanical and electrical properties was also obtained by Whereas polymeric substrates are damaged by high power electrochemically polymerizing the fullerene oxide C60O.226,227 laser pulses,245 glasses show very high damage thresholds, A monomer unit, having a cyclopentadithiophene moiety which makes them ideal for OL purposes.Sol–gel processing attached to C60 was electrochemically polymerized, leading to provides an excellent means for the preparation of glassy a conjugated polymer that contains C60 covalently attached.228 matrices at reasonable temperatures, compatible with the stability of most organic compounds.246–250 However, C60 has Although some solubility problems arose, leading to a low J.Mater. Chem., 1997, 7(7), 1097–1109 1103Fig. 4 A C60 end-capped polystyrene star attach the monofunctionalized fullerene to the silicon matrix. This can be achieved by introducing a silicon alkoxide functionality in the diene or the 1,3-dipole that will add to C60. To this aim, the derivatives shown in Fig. 5 were synthesized, whose OL properties in solution have been reported.186 Their chemical attachment to silicon matrices has been obtained and the OL properties of the resulting materials are under investigation.252 Electrooptical devices The combination of the rich electronic and electrochemical properties of C60 with those of other electroactive species is currently a field under intense investigation.It is in fact believedthat chemically modified fullerenes may play a relevant role in the design of novel molecular electronic devices, and in particular for applications in artificial photosynthesis.To this end, a number of electron-rich groups have been covalently attached to C60, which acts as an electron acceptor, for the O O O O O O S NH3 + S NH3 + O O O O O O Au + Au 13 creation of a large variety of dyads. Donor units used to this Scheme 6 end include aromatics,119,182,183,253–255 porphyrins181,256–259 and phthalocyanines,260 a rotaxane,261 tetrathiafulvalene,176,262 a carotene unit,263 Ru–bipy264 and Ru–terpy265 complexes, no or very low solubility in the polar solvents typically used as well as ferrocene.172,176 Some of these dyads have been during the sol–gel process.In addition, C60 has a high tendency studied with respect to photoinduced charge separ- to form clusters, thus making it very dicult to prepare opticalation. 181,182,263,266–268 quality films. When solubilized in the form of organofullerene, Both energy and electron transfer processes between the the optical properties of C60 can be transferred to sol–gel donor and the acceptor (C60 moiety) have been reported. For materials.251 The best way to pursue the preparation of homoinstance, intramolecular quenching of C60 singlet excited state geneous thin films of optical quality, with tunable amounts of dihydrofullerene for applications in theOL field is to covalently was detected, from electron transfer by the ferrocene moiety 1104 J.Mater. Chem., 1997, 7(7), 1097–1109blends is the high tendency of the fullerene to form clusters and to crystallize.This results in poor homogeneity and low optical quality of the films. These problems have been partially overcome with the use of soluble fullerene derivatives, such as the methanofullerenes 20 and 21. Ecient charge transfer in composite films of poly(bis-2,5-epi-cholestanoxy-1,4-phenylene vinylene) and 20 or 21 showed that the increased miscibility of the functionalized fullerene with the conjugated polymer can represent an important prerequisite for the construction of electrooptical devices.273–276 Fig. 5 Silicon-functionalised fullerene derivatives in dyads 15–19.269 The nature of the spacer was found to play a role: through bond electron transfer was shown for dyads 15–17, whereas formation of a transient intramolecular exciplex was observed for compounds 18 and 19.While in 15–17 fast charge recombination probably prevents sucient stabilization, the saturated hydrocarbon bridge in dyads 18 and 19 is able to avoid charge recombination and long-lived charge separated states are detected in polar solvents (t1/2=1.8 and 2.5 ms in benzonitrile). In a real step toward the manufacturing of ecient photovoltaic devices, photoinduced electron transfer from p-conjugated polymers to C60 has been reported by several groups.270–272 The electron transfer is very fast and the photoluminescence of the polymer is heavily quenched, which impliesa competition O O O O O O 20 21 between radiative emission and electron transfer from the excited polymer to C60.Composite films made by simple Liquid crystals mixing of p-conjugated polymers and C60 in dierent molar ratios have been employed during these investigations. The first thermotropic liquid crystal containing two cholesterol units attached to a methanofullerene has been synthesized and However, a major drawback in the use of C60 in these polymer N CH3 Fe N CH3 O O Fe N CH3 O O Fe N CH3 Fe N CH3 Fe 19 17 18 15 16 J.Mater. Chem., 1997, 7(7), 1097–1109 110531 N. Matsuzawa, D. A. Dixon and T. Fukunaga, J. Phys. 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Akiyama, S. Taniguchi, T. Okada and Y. Sakata, Chem. L ett., 1995, 265. Paper 7/00080D; Received 8th January, 1997 J. Mater. Chem., 1997, 7(7), 1097–1109

ISSN:0959-9428    

DOI:10.1039/a700080d

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

C–H,O Hydrogen bonded multi-point recognition in molecular assemblies of dibenzylidene ketones and 1,3,5-trinitrobenzenes Kumar Biradha,a Ashwini Nangia,*a Gautam R. Desiraju,*a C. J. Carrellb and H. L. Carrell*b aSchool of Chemistry, University of Hyderabad, Hyderabad 500 046, India bT he Institute for Cancer Research, Fox Chase Cancer Center, 7701 Burholme Avenue, Philadelphia, PA 19111, USA Dibenzylideneacetone 1a, 2,5-dibenzylidenecyclopentanone 1b, 2,6-dibenzylidenecyclohexanone 1c and 2,5-dibenzylidenecyclopent-3-enone 1d form crystalline stoichiometric complexes with 1,3,5-trinitrobenzene 2a, picryl chloride 2b and picric acid 2c.The structures of these complexes are mediated by multi-point C–H,O hydrogen bonds. Some of these patterns of molecular recognition also contain stronger O–H,Ohydrogen bonds.The C–H,Ohydrogen bonds within these multi-point supramolecular synthons are generally shorter and more linear than the other C–H,O hydrogen bonds found in these complexes. The assembly of molecules into nanosize aggregates has emerged as a major endeavour in modern chemistry.1 It has been recognised that nanoscale systems represent a meeting point of the chiselling down by technologists of macrosize precursors and of the building up by chemists from molecular size precursors. Supramolecular systems formed by self-organisation principles are good examples of nanostructures.Supramolecular chemistry emphasises the collective properties of molecules and in this regard, the physical and chemical properties of molecular aggregates are often significantly dierent from those of the constituent molecules.2 A crystal is a supermolecule par excellence3 and the recognition patterns that are formed in crystals may be termed supramolecular synthons if a crystal is viewed as a retrosynthetic target.4 Crystal engineering, the premeditated assembly of molecules in the solid state, then becomes the supramolecular equivalent of organic synthesis and accordingly, a supramolecular synthon may be defined as a structural unit within a supermolecule which can be formed and/or assembled by known or conceivable synthetic operations involving intermolecular interactions.Crystal engineering with conventional (or strong) O–H,O and N–H,O hydrogen bonds may appear suciently reliable, but it is in reality incomplete if weak intermolecular interactions are not considered.5 The advantage of using weak intermolecular interactions in crystal engineering is that the repertoire of compounds that can be used to construct supramolecular motifs and patterns is significantly increased.Among the weak intermolecular interactions, C–H,O hydrogen bonds have attracted considerable attention.The ability of the C–H group to form various types of hydrogen bonds, such as C–H,O,6a–c C–H,N,6d C–H,F6e and C–H,M,6f is well-established and is comparable to that of the N–H and O–H groups. Cambridge Structural Database (CSD) studies on several aspects of C–H,O hydrogen bonds have resulted in a better understanding of the nature of these interactions7 and this in turn has led to the utilisation of these interactions in the construction of supramolecular synthons through a consideration of the complementarity of functional groups.8 Recently, we have shown that the C–H group in organometallic cluster compounds also forms C–H,O hydrogen bonds with the CO ligand and that the stability of these interactions depends on the basicity of the CO ligand.9 These studies also reveal that C–H,ONC hydrogen bonds are directional with the preferred CNO,H angle being around 140°.All these studies suggest that C–H,O hydrogen bonds show properties similar to those of strong hydrogen bonds. To summarise, C–H,O hydrogen bonds can be used quite eciently in the design of supramolecular synthons and crystal structures.However, owing to the inherent weakness of these interactions, multi-point recognition rather than single-point recognition is the preferred strategy. Here we aim to design and analyse the robustness of the three-point C–H,O recognition synthon I which is a mimic of the well-known synthon II that is constructed purely with strong hydrogen bonds. For this purpose, the crystal structures of complexes 3a–g have been solved and analysed.10 Synthon I is found to occur in complexes 3a–e but not in 3f and 3g.The CSD was used to analyse the patterns observed in these structures and in some other a,b-unsaturated carbonyl compounds. 1111 Experimental Preparation of materials 1,3,5-Trinitrobenzene 2a was prepared in three steps from 2,4-dinitrotoluene.Nitration of 2,4-dinitrotoluene gave 2,4,6- J. Mater. Chem., 1997, 7(7), 1111–11223 4 trinitrotoluene, which was oxidised with K2Cr2O7 to provide 2,4,6-trinitrobenzoic acid.11a,b This was decarboxylated in the presence of NaOH to give compound 2a.11c Picric acid 2c was purchased and picryl chloride 2b was prepared by the reaction of 2c with POCl and N,N-diethylaniline.12 The dibenzylideneketones were prepared by the condensation reaction of 2 equiv.of benzaldehyde with 1 equiv. of the corresponding ketones. Compound 1d was prepared by allylic dibromination of 1b with N-bromosuccinimide (NBS)–CCl followed by debromination with Zn–MeOH.13 Pentacenedione was prepared by the condensation reaction of cyclohexane-1,4-dione and phthalaldehyde.14 Preparation of crystals Yellow crystals of the 251 complex 3c were obtained from an equimolar solution of 2a and 1c in 151 dichloromethane– hexane.Similarly, yellow crystals of the complexes 3d–g were obtained from an equimolar solution of the molecular components in 151 chloroform–hexane.The preparation of complexes 3a and 3b has been described by us previously.10 X-Ray crystallographic studies Data were collected for all the complexes on an Enraf-Nonius FAST area detector with a rotating anode X-ray source. The crystal structures of complexes 3a and 3b have been reported already10 and only the essential features are given here. The crystal structures of complexes 3c–g are presented here.The solution of the structures for all the complexes were carried out with the SHELXS8615 program and the refinements were carried out with the SHELXL93 program.16 Complex 3c crystallises in the triclinic space group P1�. Most of the sample consisted of twinned crystals and the data were collected on a solitary untwinned sample.The solution for its crystal structure was obtained in the space group P1 since it failed to solve in P1�; the structure was then refined in the space group P1�. In complexes 3d and 3e, the dibenzylideneketone moiety and one of the nitro groups are disordered. It may be noted that this disorder could not be fully modelled in the refinements and that the attendant lack of precision in the atomic positions is unavoidable.This means that the finer details of the hydrogen bonding cannot be discussed in detail. However, these structures have been included here for the sake of completeness. All the non H-atoms were refined anisotropically. All the H-atoms, except in complex 3f, were located from dierence Fourier maps and refined isotropically in the final stages of the refinement because this is a study of the C–H,O hydrogen bonds.The hydrogen atoms in complex 3f were fixed geometrically and refined with the riding model. Salient crystallographic information for the complexes in this study is given in Table 1.† CSD studies Data were retrieved from the CSD (ver. 5.08).17 Screens -28, 34, 85 and 88 were used to eliminate organometallic entries and unmatched chemical and crystallographic connectivities. Entries with R-factor greater than 0.10 and disordered structures were also excluded. A C–H,O geometry was considered a bona fide hydrogen bond when the C,O distance is less than 4.0 A°and the C–H,O angle is between 110 and 180°.Geometrical calculations were performed using QUEST3DGSTAT, an automatic graphical non-bonded search program of the CSD. † Atomic coordinates, thermal parameters, and bogths and angles have been deposited at the Cambridge Crystallographic Data Centre (CCDC). See Information for Authors, J. Mater. Chem., 1997, Issue 1. Any request to the CCDC for this material should quote the full literature citation and the reference number 1145/32.1112 J. Mater. Chem., 1997, 7(7), 1111–1122 Results and Discussion Synthon I in complexes of 2a The choice of trinitrobenzene 2a as one of the components to form the supramolecular synthon I is due to the high acidity of its aromatic C–H and the ability of nitro groups to form C–H,O hydrogen bonds.The choice of dibenzylideneketones as the second supramolecular component is made by matching complementary groups. Our earlier observation18 that a,bunsaturated carbonyl compounds form C–H,O hydrogen bonded patterns III and IV further strengthened our idea for the choice of dibenzylideneketones. Compound 2a forms a 251 crystalline complex 3a with dibenzylideneketone 1a.The expected synthon I is observed in 3a and the overall crystal structure is fortified by additional C–H,O hydrogen bonds on the other side of the carbonyl group [Fig. 1(a)]. In order to avoid this alternate C–H,O hydrogen bonded pattern and also to assess the robustness of synthon I, we considered complexes of 2a with 1b and 1c instead of 1a. Compound 2a also forms 251 crystalline complexes 3b and 3c with 1b and 1c, respectively [Fig. 1(b) and (c)]. Synthon I is found in both these complexes and the alternate C–H,O hydrogen bonded pattern which involves the hydrocarbon side of the molecule is found in complex 3b but not in 3c. The presence of the alternate motif on the hydrocarbon side in complexes of 1a and 1b is attributed to the higher acidity of vinylic and allylic C–H groups, respectively, which enables them to make the three-centre motif consisting of vinylic/allylic C–H, aromatic C–H and nitro oxygen atom.Curiously, the 1b molecules in complex 3b are disordered about a line that roughly bisects the molecular length because of the alternate C–H,O hydrogen bonded pattern. The observation that the occupancies of two orientations of 1b in this complex are constrained by crystallographic symmetry to be 0.5 and 0.5 indicates that synthon I and the alternate C–H,O hydrogen bonded pattern are of comparable significance. The C–H,O hydrogen bond recognition pattern V is found in complexes 3b and 3c but not in 3a.The presence of synthon I and pattern V in the complexes of 3b and 3c are shown in Fig. 1(b) and (c), respectively. To analyse the C–H,O hydrogen bonds in the complexes of 2a, plots of the C,O distances versus the C–H,O angles were obtained (Fig. 2). Circles, triangles and squares represent the C–H,O hydrogen bonds in complexes 3a, 3b and 3c respectively. The C–H,O hydrogen bonds which are part of synthon I are shown as filled symbols. From this plot, one notes that the C–H,O hydrogen bonds within synthon I are shorter and more linear and that they constitute the essence of these crystal structures.This observation strengthens the idea that the significance of a C–H,O hydrogen bond increases if it is part of a multi-point synthon. The C–H,O hydrogen bonds of synthon I are strongest in complex 3b but somewhat weaker in 3a/3c.compound formula P21 Mcrystal system space group /A° b/A° c/A°ac(°) V/A°3 T/K 680 1.47 0.7107 3a C29H20N6O13 660.52 monoclinic 7.092(2) 27.927(4) 7.569(2) 96.67(2) 1488.9(5) Mo-Ka 1–27.5 -7 to 7 0.038 0.041a -0.19 0.13 a(°) b(°) (000) c/g cm-3 ZFlD/A° m/mm-1 crystal size radiation h range (°) hkls-level 0 to 34 0 to 8 total reflections 2931 unique reflections 1828 3 min.electron density/e A°-3 max. electron density/e A°-3 R1 wR2 aR values w 3109(2) 2248(2) 293 293 120 293 293 293 293 2 2 4 4 4 4 2 708 1688 788 768 1016 480 1.484 1.754 0.7107 0.7107 0.76 0.76 0.134 0.23×0.31×0.30 0.22×0.25×0.33 0.1×0.1×0.2 Mo-Ka Mo-Ka � Pn Pn P222 1� 1 P 2.56–28.69 0 to 11 -19 to 18 -21 to 21 11741 6695 20.049 0.12 -0.404 1.826 9.722(6) 39.097(4) 15.287(2) 14.435(2) 15.140(4) 14.394(7) 96.42(2) 95.806(10) 11.343(14) 11.68(2) 76.63(6) 84.39(6) 68.84(9) 1079(2) 1638.9(4) 1621.9(10) 1.426 0.7107 0.110 1.446 0.7107 0.110 1.527 0.7107 0.276 1.580 0.7107 0.283 0.24×0.22×0.35 0.26×0.20×0.38 0.20×0.15×0.3 0.15×0.20×0.3 Mo-Ka Mo-Ka Mo-Ka Mo-Ka 2.69–25.35 0 to 9 0 to 18 -17 to 17 2.162–28.96 0 to 8 0 to 13 0 to 15 3171 1867 2.37–23.50 0 to 10 -11 to 12 -12 to 13 3062 1857 2 2.94–27.30 0 to 9 0 to 19 -18 to 18 3571 2359 2 2956 1634 20.08 0.23 0.238 20.04 0.12 -0.204 0.05 0.11 -0.173 0.16 0.47 -0.806 0.195 0.163 0.524 0.282 allows it to mimic a molecule of pentacenedione, as shown in Scheme 1.Therefore, it was anticipated that pentacenedione should cocrystallise with compound 2b to yield I. However, this could not be realised experimentally because pentacenedione failed to cocrystallise with 2b due to a mismatch of solubilities. 1113 Synthon I in complexes of 2b To test the robustness of synthon I in the presence of other functional groups like Cl and OH, we have prepared complexes of 2b with 1b and 1d and of 2c with 1a and 1b.It is well known that 2c forms stable complexes with various aromatic compounds through p–p interactions19 and it has been used in crystal engineering experiments to design a three-point synthon VI that contains two C–H,O and one N–H,O hydrogen bonds.20 From the examination of the crystal structures of pure 2b and 2c, it was found that the three nitro groups are coplanar with the aromatic ring in 2c but not in 2b.In 2b the ortho nitro group which is not involved in intramolecular O–H,O hydrogen bond is out of the aromatic ring plane. Therefore, both 2b and 2c were expected to form complexes with dibenzylideneketones. Compound 2b forms 251 crystalline complexes 3d and 3e with ketones 1b and 1d, respectively [Fig. 3(a) and (b)]. Even though compounds 1b and 1d are chemically dierent, they form isostructural complexes with 2b due to the disorder in the dibenzylidene moiety.The presence of C–H,O hydrogen bonded synthon I in complexes 3d and 3e indicates that the Cl group does not interfere in the formation of I. These crystal structures are almost reminiscent of 3a, 3b and 3c except that O,Cl interactions are formed as shown in pattern VII. The nitro group oxygen atoms which participate in VII are disordered. The disorder of molecule 1b in these two complexes Table 1 Crystallographic data of complexes 3a–g 3c C64H48N12O26 1641.34 triclinic 13.721(3) 14.374(4) 16.349(7) 92.13(2) 102.67(2) 97.90(2) 3b 31H22N6O12 C686.56 monoclinic P21 /c P1 7.493(2) 27.384(6) 7.491(2) 92.39(2) 1535.8(6) 1–27.5 -7 to 7 0 to 33 0 to 8 3532 1328 30.048 0.050a -0.13 0.13 3f orthorhombic 3d 376.71 monoclinic 2/ 3g C23H17N3O8 463.40 triclinic 5.91511(2) 8.979(10) 3e C15.5H9ClN3O6.5 C15.5H9ClN3O6.5 C25H19N3O8 376.71 489.43 monoclinic 2/ 7.481 1 1(2) 7.474(1) Absence of synthon I in complexes of 2c Compound 2c forms 151 molecular complexes 3f and 3g with compounds 1b and 1a, respectively.In both complexes, the C–H,O hydrogen bonded synthon I is absent.Interestingly, in complex 3f the O–H group forms an intermolecular hydrogen bond with the keto group of molecule 1b (O,O, H,O, O–H,O; 2.90 and 2.06 A°, 123°). It also forms two C–H,O hydrogen bonds (C,O, H,O, C–H,O; 3.18 and 2.98 A°, 164° and 3.42 and 2.23 A°, 156°) to form a supermolecule of 1b and 2c that involves one O–H,O and two C–H,O hydrogen bonds [Fig. 4(a)]. The three-dimensional packing of these supermolecules is shown in Fig. 4(b). These results show that there is a limit to the robustness of synthon I in this family of crystal structures. Picric acid 2c which contains a strongly hydrogen bonding OH group is capable of disrupting the recognition motif I of trinitrobenzenes with dibenzylideneketones.In complex 3g, the situation is entirely dierent from the other complexes in that e is no intermolecular O–H,O hydrogen bond with a keto group as in complex 3f. Instead, J. Mater. Chem., 1997, 7(7), 1111–1122Fig. 1 (a) Crystal structure of complex 3a (1a52a in 152 ratio) to show synthon I (highlighted) and the alternate C–H,O hydrogen bonded patterns. (b) Crystal structure of complex 3b (1b52a in 152 ratio) to show synthons I and V and the alternate C–H,O hydrogen bonded patterns.(c) Crystal structure of complex 3c (1c52a in 152 ratio) to show synthons I and V. 1114 J. Mater. Chem., 1997, 7(7), 1111–1122one finds that centrosymmetric dimers of 1a and 2c are connected by C–H,O hydrogen bonds (Fig. 5). The dimer of 2c is engaged in tandem hydrogen bonding21 whereas that of 1a is involved in C–H,O hydrogen bonded pattern VIII, which is another example of three-point recognition. 1 , A2, NIPMAT plots of complexes 3f and 3g A2, Am, An) and the matrix element AmAn which is defined The dierences in the packing of 3f and 3g, the two molecular complexes of 2c with 1b and 1a respectively, have been examined by looking at their NIPMAT plots.6c A pictorial matrix is formed using the atoms of a molecular skeleton (A1, by the shortest intermolecular contact Am,An and shown in terms of a grey scale.The shorter the contact, the greyer the square which represents that particular contact. This greyness is scaled at the bottom of the figure.The dark line in this scale indicates the sum of the van der Waals radii of any two atoms. If there are two dierent molecular skeletons (A Am,An) and (B1 , B2, Bi, Bj) in the supramolecular structure, then their interactions are shown in four sections. The upper left and lower right rectangles indicate the A,B interactions while the lower left and upper right squares indicate A,A and B,B interactions. Hence, the plot obtained is a simultaneous visual representation of all the intermolecular interactions.Fig. 6(a) and (b) are the NIPMAT plots of complexes 3f and 3g, respectively. In Fig. 6(b), the overall greyness in the upper left and lower right rectangles, which represent the C–H,O interactions and stacking interactions between molecules 2c and 1a, is more when compared with the corresponding areas of Fig. 6(a) (2c and 1b). This implies that in complex 3g the C–H,O hydrogen bonds and stacking interactions dominate the packing of the crystal. In complex 3f, compound 1b has relatively less acidic sp3 C–H groups compared to the sp2 C–H moiety of 1a and so the intermolecular O–H,O hydrogen bonds take the lead with the assistance of some weaker C–H,O bonds.p–p Stacking interactions The formation of 152 molecular complexes 3a–e from solutions containing equimolar amounts of 1 and 2 can be justified by considering p–p stacking. A molecule of 1 contains two phenyl rings and can accommodate two molecules of 2. It is well known that aryl groups prefer to interact in either an edge-toface or an oset face-to-face orientation22 and compound 2a forms charge-transfer complexes with aromatic compounds.23 Recently, compound 2a has been used as a guest for chiral molecular tweezers through these interactions.24 All the molecular complexes of 2a and 2b form these interactions with slightly oset stacking.Fig. 7 shows a superposition of these Fig. 2 Scatter plot of C–H,O interactions in the complexes (#) 3a, (') 3b and (%) 3c. Filled symbols are the C–H,O hydrogen bonds that contribute to synthon I. Notice that all these filled symbols are in the strong hydrogen bonded region. Fig. 3 (a) Crystal structure of 3d (1b52b in 152 ratio) to show synthons I, V and VII. (b) Crystal structure of 3e (1d52b in 152 ratio) to show synthons I, V and VII.Both disordered positions of the nitro group are shown for molecule 2b. interactions in the above complexes. The centroid to centroid distance and plane to plane angles in these complexes range from 3.64 to 4.83 A°and 0.81 to 10.4°. However, the stacking interactions are dierent in complexes 3f and 3g. Complex 3f is stabilised by p–p and herringbone interactions [Fig. 4(b) and 7( f )] while complex 3g is stabilised by p–p interactions alone [Fig. 7(g)]. The variation in the donor–acceptor ratios of complexes 3f and 3g when compared with that in 3a–e is a consequence of the change in interactions between aromatic rings in these complexes. CSD studies The CSD was searched for the C–H,O hydrogen bonded patterns III, IV and VIII and O,Cl interaction VII to understand their nature and to ascertain their frequency of occurrence, which would indicate their robustness.There are 1115 J. Mater. Chem., 1997, 7(7), 1111–1122Scheme 1 Disorder of molecule 1b and 1d in complexes 3d and 3e Fig. 4 (a) Crystal structure of complex 3f (1b52c in 151 ratio) to show the supermolecule that involves one intermolecular O–H,O and two C–H,O hydrogen bonds (oxygen atoms are shaded).(b) Packing diagram of the crystal structure of complex 3f. Note the herringbone and stacking interactions. 189, 159, 44 and 18 crystal structures present for synthons III, IV, VIIIa and VIIIb, respectively. Synthon III is present 205 times in 189 crystal structures. Fig. 8(a) is the scattergram of C(7),O(1) versus C(3),O(6) distances and it shows the centrosymmetric nature of III. The o-diagonal points are from structures that have two molecules in the asymmetric unit, in other words these interactions occur between symmetry independent molecules. Fig. 8(b) is the scattergram of C–H,O angle versus C,O distances for III. 1116 J. Mater. Chem., 1997, 7(7), 1111–1122 Fig. 5 Packing diagram of the crystal structure of complex 3g (1a52c in 151 ratio) to show the dimers of molecules 1a and 2c which are formed by C–H,O hydrogen bonded pattern VIIIa and O–H,O tandem hydrogen bonds, respectively. From this plot it can be seen that many of the C–H,O hydrogen bonds are clustered in the strong hydrogen bonds region, i.e. in a C,O range of 3.35 to 3.65 A°and a C–H,O range of 155–175°.There are 173 hits in 159 crystal structures for synthon IV. Fig. 9(a) is the scattergram of C(9),O(1) versus C(4),O(6) and indicates the centrosymmetric nature of IV. Fig. 9(b) is the scattergram of C–H,O angles versus C,O distances in IV. Here the C–H,O hydrogen bonds are clustered between the C,O distance of 3.25 to 3.55 A°and C–H,O angle of 140 to 160°.Halogen to nitro oxygen atom contacts are well known and have been used in the design of target crystal structures.25 A total of 19 crystal structures is present in the CSD for orthosubstituted chloro–nitro aromatics and if O,Cl distances only in the range 2.8 to 4.0 A°were considered, 17 of these were found to contain VII, indicating robustness of this synthon.There is a total 82 hits from 17 crystal structures for the alternate O,Cl interaction displayed in synthon IX. That the number of hits per structure is higher than expected is an artefact of the unsymmetrical O,Cl interaction IXa being a subset of the larger synthon IXb. From the scattergram of Cl(10),O(1) versus Cl(5),O(6) distances (Fig. 10), it is clear that the O,Cl interactions exist as symmetrical VII and unsymmetrical IX variations.Fig. 6 NIPMAT plots of complexes (a) 3f and (b) 3g. Note that there are more dark grey squares in the upper left and lower right rectangles in (b) when compared with (a). This indicates stronger and more numerous C–H,O hydrogen bonds and stacking interactions in 3g compared to 3f.1117 J. Mater. Chem., 1997, 7(7), 1111–1122Fig. 6 (continued). Motif VIIIa occurs in the molecular complex 3g and was found 47 times in 44 crystal structures, whereas VIIIb was found 18 times in 18 crystal structures. Fig. 11(a) is the scattergram of C(15),O(1) versus C(7),O(9) and shows the centrosymmetric nature of VIIIa. Fig. 11(b) is the scattergram of C,O distances versus C–H,Oangles.Open circles 1118 J. Mater. Chem., 1997, 7(7), 1111–1122 represent the C–H,O motifs of VIIIa and filled circles represent the C–H,O motifs of VIIIb. The C–H,O hydrogen bonds and angles of VIIIa are clustered between 3.35 and 3.50 A°and 145 and 155°, and for VIIIb they are clustered in the C,O range of 3.15 and 3.35 A°and C–H,O angle range of 135 and 145°. These distance and angle distributions indicateFig. 7 Stacking interactions of complexes (a) 3a, (b) 3b, (c) 3c, (d) 3d, (e) 3e, ( f ) 3f and (g) 3g. The phenyl rings of trinitrobenzene derivatives are shaded for clarity. that many of the C–H,O motifs involved in VIIIb are shorter but less linear compared to the hydrogen bonds in synthon VIIIa.When Fig. 8(b) and 9(b) are compared with Fig. 11(b) it is clear that the C–H,O bonds involved in synthon VIIIa,b are less linear than the bonds in synthons I and III. 1119 Supramolecular synthons III, IV, VII and VIII in crystal engineering We now discuss a few occurrences of the synthons III, IV, VII and VIII to highlight dierent structural aspects. For this exercise, we have chosen molecules 4–7.Synthon III is the C–H,O counterpart of the N–H,O hydrogen bonded recognition motif found in cis-amides. The crystal structure of benzoquinone is composed of synthon III which gives it a sheet-like structure. In 4, the two quinonoid halves of the molecule are tetrahedrally disposed because of the spiro ring junction. In the crystal structure of 4 (Fig. 12), there are two symmetry-independent molecules and these form a ribbon pattern constituted with successive synthons III. Only one of the two quinonoid halves of any molecule participates in this pattern and four distinct C,O distances result because two symmetry-independent molecules are involved.26 The C–H,O dimeric motif and the zig-zag chain arrangement of molecules of 4 resemble the structure found in secondary amides.The crystal structure of 5 (Fig. 13) shows the expected linear chain with synthon IV.27 The crystal structure of 6 (Fig. 14) is interesting as it maintains the three-fold symmetry with the three nitro groups nearly perpendicular to the plane of the phenyl rings.28 This arrangement leads to the formation of a rosette-like structure J.Mater. Chem., 1997, 7(7), 1111–1122Fig. 8 (a) Scatter plot of C,O distances to show the centrosymmetric nature of synthon III. (b) Scatter plot of C,O distances versus C–H,O angles in synthon III. Notice that the points are clustered in the C,O range 3.4–3.6 A°and C–H,O range 155–175°. Fig. 9 (a) Scatter plot of C,O distances to show the centrosymmetric nature of synthon IV.(b) Scatter plot of C,O distances versus C–H,O angles in synthon IV. Notice that the points are clustered in the C,O range 3.25–3.55 A°and in the C–H,O range 140–160°. 1120 J. Mater. Chem., 1997, 7(7), 1111–1122 Fig. 10 Scatter plot of Cl(10),O(1) versus Cl(5),O(6) distances. Notice the centrosymmetric nature of synthons VII and IXb.Fig. 11 (a) Scatter plot of C(7),O(9) and C(15),O(1) distances to show the centrosymmetric nature of synthon VIIIa. (b) Scatter plot of C,O distances versus C–H,O angles in synthons VIIIa and VIIIb. Open circles represent the C–H,O hydrogen bonds of synthon VIIIa and closed circles represent the bonds in VIIIb with synthon VII. The crystal structure of 7 (Fig. 15) contains two molecular components and leads to the anticipated chain structure through C–H,O hydrogen bonds as in synthon VIII.29 These studies suggest that one can utilise III, IV, VII and VIII as supramolecular synthons in crystal engineering experiments.Fig. 12 Crystal structure of 4 (SPUNDD20) to show the zig-zag chain of molecules linked by C–H,O hydrogen bonded synthon III Fig. 13 Crystal structure of 5 (MIMOSA10) to show the linear chain of molecules linked by synthon IV Conclusions This work shows that C–H,O hydrogen bonds can be profitably utilised to design robust three-point supramolecular synthons. The C–H,O hydrogen bonds involved in multipoint synthons are stronger than the other isolated C–H,O hydrogen bonds in the same and related structures.The presence of strong hydrogen bonding functional groups usually influences C–H,O bonded recognition, but if the strong Fig. 14 Crystal structure of 6 (WANMON) to show the hexagonal network of molecules linked through O,Cl interactions of synthonVII Fig. 15 Crystal structure of 7 (BERGAG) to show the linear arrangement of molecules linked through C–H,O hydrogen bonded synthon VIIIa hydrogen bonds are optimised, the recognition through the weak interactions would be just as eective.Furthermore, the retroanalysis of a supramolecular synthon leads to complementary molecules which assemble in a predictable fashion and form the target motif. Such an approach to the construction of molecular assemblies is analogous to the synthesis of complex molecules from simpler substrates.Here we have discussed the formation of the C–H,O hydrogen bonded synthon I where donors and acceptors are arranged in alternate fashion (ADA5DAD). The related AAD5DDA supramolecular synthon made up of stronger N–H,O and O–H,O hydrogen bonds has been identi- fied recently in the crystal structure of 2¾-deoxycytidine 1121 J.Mater. Chem., 1997, 7(7), 1111–1122hemidihydrogen phosphate.30 Such observations lead to the idea that it should also be possible to design related AAA5DDD and AAD5DDA supramolecular synthons with C–H,O hydrogen bonds. Finally, this work also shows that p–p interactions are important in determining the stoichiometry of molecular components and in turn in governing crystal packing.Financial assistance from the University Grants Commission (K.B.), Department of Science and Technology, Project SP/S1/G19/94 (G.R.D. and A.N.) and Grant CA-10925 from the National Institutes of Health (C.J.C. and H.L.C.) is gratefully acknowledged. References 1 (a) See the organic solid state chemistry special issue of Chem.Mater., 1994, 6, 1087 (ed. M. D. Ward and M. D. Hollingsworth); (b) See the nanostructured materials special issue of Chem. Mater., 1996, 8, 1571 (ed. T. Bein and G. D. Stucky). 2 J.-M. Lehn, Supramolecular Chemistry: Concepts and Perspectives, VCH,Weinheim, 1995. 3 J. D. Dunitz, in Perspectives in Supramolecular Chemistry: T he Crystal as a Supramolecular Entity, ed.G. R. Desiraju, Wiley, Chichester, 1996, vol. 2, pp. 1–32. 4 G. R. Desiraju, Angew. Chem., Int. Ed. Engl., 1995, 34, 2311. 5 (a) G. R. Desiraju, in Perspectives in Supramolecular Chemistry: T he Crystal as a Supramolecular Entity, ed. G. R. Desiraju, Wiley, Chichester, 1996, vol. 2, pp. 31–61; (b) G. R. Desiraju, in Comprehensive Supramolecular Chemistry, ed. D.D. MacNicol, F. Toda and R. Bishop, Pergamon, Oxford, 1996, vol. 6, pp. 1–16. 6 (a) J. A. R. P. Sarma and G. R. Desiraju, Acc. Chem. Res., 1986, 19, 222; (b) G. R. Desiraju, Acc. Chem. Res., 1991, 24, 290; (c) G. R. Desiraju, Acc. Chem. Res., 1996, 29, 441; (d) D. S. Reddy, B. S. Goud, K. Panneerselvam and G. R. Desiraju, J. Chem. Soc., Chem. Commun., 1993, 663; (e) L.Shimoni, H. L. Carrell, J. P. Glusker and M. M. Coombs, J. Am. Chem. Soc., 1994, 116, 8162; (f ) D. Braga, F. Grepioni, K. Biradha and G. R. Desiraju, J. Chem. Soc., Dalton T rans., 1996, 3925. 7 (a) V. R. Pedireddi and G. R. Desiraju, J. Chem. Soc., Chem., Commun., 1992, 988; (b) T. Steiner and W. Saenger, J. Am. Chem. Soc., 1993, 115, 4540; (c) T. Steiner and W. Saenger, J.Am. Chem. Soc., 1992, 114, 10146. 8 (a) V. R. Thalladi, K. Panneerselvam, C. J. Carrell, H. L. Carrell and G. R. Desiraju, J. Chem. Soc., Chem. Commun., 1995, 341; (b) C. V. K. Sharma and G. R. Desiraju, J. Chem. Soc., Perkin T rans. 2, 1994, 2345; (c) T. Suzuki, H. Fujii and T. Miyashi, J. Org. Chem., 1992, 57, 6744. 9 D. Braga, F. Grepioni, K. Biradha, V.R. Pedireddi and G. R. Desiraju, J. Am. Chem. Soc., 1995, 117, 3156. 1122 J. Mater. Chem., 1997, 7(7), 1111–1122 10 A preliminary account of this work has appeared: K. Biradha, C. V. K. Sharma, K. Panneerselvam, L. Shimoni, H. L. Carrell, D. E. Zacharias and G. R. Desiraju, J. Chem. Soc., Chem. Commun., 1993, 1473. 11 (a) H. D. William, H. D. Rosenblatt, W. G. B. Rosenblatt and L.C. Cliord, J. Chem. Eng. Data, 1975, 20, 202; (b) A. I. Vogel, A T ext-Book of Practical Organic Chemistry, 3rd edn., English Language Book Society, 1975, p. 758; (c) A. I. Vogel, A T ext-Book of Practical Organic Chemistry, 3rd edn., E.L.B.S, 1975, p. 965. 12 E. T. Borrows, J. C. Clayton, B. A. Hems and A. G. Long, J. Chem. Soc., 1949, S190. 13 G. R.Desiraju and K. V. R. Kishan, Indian J. Chem., Sect. B, 1988, 27, 953. 14 W. Ried and F. Anthofer, Angew. Chem., 1953, 65, 601. 15 G. M. Sheldrick, SHELXS86, in Crystallographic Computing 3, ed. G. M. Sheldrick, C. Kruger and R. Goddard, Oxford University Press, Oxford, UK, 1985, pp. 175–189. 16 G. M. Sheldrick, SHELXL, An Integrated System for Solving, Refining and Displaying Crystal Structures from Diraction Data, University of Go�ttingen, Germany, 1993. 17 F. H. Allen, J. E. Davies, J. J. Galloy, O. Johnson, O. Kennard, C. F. Macrae and D. G. Watson, J. Chem. Inf. Comput. Sci., 1991, 31, 204. 18 S. K. Kearsley and G. R. Desiraju, Proc. R. Soc. L ondon, Ser. A, 1985, 397, 157. 19 H. Nagata, Y. In, M. Doi and T. Ishida, Acta Crystallogr., Sect. B, 1995, 51, 1051. 20 V. Agafonov, P. Dubois, F. Moussa, J. M. Cense and S. Toscani, J. Chem. Soc., Perkin T rans. 2, 1994, 2007. 21 G. A. Jerey and W. Saenger, Hydrogen Bonding in Biological Structures, Springer-Verlag, Berlin, 1991, p. 24. 22 C. A. Hunter and J. K. M. Sanders, J. Am. Chem. Soc., 1990, 112, 5525. 23 R. Foster, Organic Charge-T ransfer Complexes, Academic Press, London, 1969, p. 113. 24 (a) M. Harmata and C.L. Barnes, J. Am. Chem. Soc., 1990, 112, 5655; (b) M. Harmata, C. L. Barnes, S. R. Karra and S. Elahmad, J. Am. Chem. Soc., 1994, 116, 8392. 25 (a) V. R. Thalladi, B. S. Goud, V. J. Hoy, F. H. Allen, J. A. K. Howard and G. R. Desiraju, Chem. Commun., 1996, 401; (b) F. H. Allen, B. S. Goud, V. J. Hoy, J. A. K. Howard and G. R. Desiraju, J. Chem. Soc. Chem. Commun., 1994, 2729. 26 D. L. Cullen, B. Hass, D. G. Klunk, T. V. Willoughby, C. N. Morimoto, E. F. M. Junior, G. Farges and A. Dreiding, Acta Crystallogr., Sect. B, 1976, 32, 555. 27 T. Hata, H. Fukumi, S. Sato, K. Aiba and C. Tamura, Acta. Crystallogr., Sect. B, 1978, 34, 2899. 28 F. Gerard, A. Hardy and A. Becuwe, Acta Crystallogr., Sect. C, 1993, 49, 1215. 29 B. M. Gatehouse, Cryst. Struct. Commun., 1982, 11, 365. 30 M. Jasko�lski, M. Gdaniec, M. Gilski, M. Alejska and M. D. Bratek-Wiewio�rowska, J. Biomol. Struct. Dyn., 1994, 11, 1287. Paper 6/07106F; Received 18th October

ISSN:0959-9428    

DOI:10.1039/a607106f

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Two-dimensional hydrogen-bonded assemblies: the influence of sterics and competitive hydrogen bonding on the structures of guanidinium arenesulfonate networks Victoria A. Russell and Michael D.Ward* Department of Chemical Engineering and Materials Science, University ofMinnesota, Amundson Hall, 421 Washington Ave. SE,Minneapolis, MN 55455, USA Guanidinium and organosulfonate ions self-assemble into crystalline lattices described by robust two-dimensional hydrogenbonded networks with the general formula [C(NH2)3]+RSO3-.These networks, which typically have quasihexagonal symmetry due to favourable hydrogen bonding between six guanidinium proton donors and six sulfonate electron lone pair acceptors, assemble in the third dimension by stacking in a manner which maximizes van derWaals interactions between R groups.The steric requirements of the R groups dictate whether this assembly results in interdigitated bilayer stacking in which all the R groups are orientated to one side of a given sheet or interdigitated single layer stacking in which R groups are orientated to both sides of a given hydrogen-bonded sheet. The two-dimensional network tolerates very dierent steric requirements of the R groups due to the ability to form either of these stacking motifs and to the inherent flexibility of the hydrogen-bonded network about onedimensional hydrogen-bonding ‘hinges’.This flexibility allows the sheets to pucker in order to accommodate steric strain between R groups within the layers. We describe here the influence of substituents on the R groups whose steric and hydrogen bonding capacity influence the puckering of the two-dimensional guanidinium sulfonate network.In particular, we examine the X-ray crystal structures of the guanidinium salts of ferrocenesulfonate and methyl- and nitro-substituted benzenesulfonates. The retention of the hydrogen-bonding motif in spite of steric and hydrogen bonding interference by the R group substituents illustrates the robustness of the guanidinium sulfonate network. However, additional competing hydrogen bonding and sterics influence the crystal packing, and in the case of multiple substituents on the R groups, these factors may disrupt the guanidinium sulfonate network.Overall, this work demonstrates that the use of robust two-dimensional supramolecular modules can reduce the crystal engineering problem to the last remaining dimension, which can simplify the design of functional molecular materials.materials, or are susceptible to dramatic changes in crystal Introduction packing upon such changes. A reasonable strategy for sur- Crucial to the design and synthesis of molecular materials is a mounting these obstacles is to use robust supramolecular thorough understanding, and ultimately control, of the ‘modules’11,12 or ‘synthons’,13 where robust is defined as the assembly of constituent molecules into the supramolecular ability of the module to maintain its dimensionality and general motif that defines the solid state structure.It is instructive to structural features upon changes in ancillary functional groups consider the constituent molecules of a material as the funda- or other molecular species in the lattice.Robust n-dimensional mental building blocks of the solid state. The formation of modules can reduce the crystal engineering problem to 3-n ordered solid-state networks with a desired arrangement and dimensions, thereby simplifying materials design.dimensionality relies on an appropriate ‘topological director’, Recently, we reported molecular layered materials based on that is, a module having a well-defined functional group that a two-dimensional hydrogen-bonded (HB) network composed can recognize complementary functional groups on other like of guanidinium cations (G) and the sulfonate groups of alkane- molecules (homomeric assembly) or dierent molecules (heter- and arene-substituted monosulfonate anions (S).14–17 The omeric assembly).A crucial property of a director is its ability topological equivalence of the guanidinium ions and sulfon- to participate in intermolecular interactions which are strong ate groups and strong (guanidinium)NMH,O(sulfonate) and highly directional relative to competing ones.Formation hydrogen bonds favoured the formation of quasihexagonal of extended networks also requires ‘polyvalent’ modules, that two-dimensional GS networks in over 30 dierent crystalline is, molecules having more than one bonding functionality. phases containing various sulfonate functionalities (Fig. 1). All These capabilities are provided by molecules containing hydro- the hydrogen bonding capacity is fulfilled within this network, gen bonding functionalities.which is important in forming robust networks. The networks Several examples of ordered, extended hydrogen bonding assembled in the third dimension via van derWaals interactions networks have been reported that illustrate the important between sulfonate R groups extending from the GS sheets, influence of this interaction on directing the organization of either as densely packed bilayers or continuous stacks of molecules in the crystallization of solid-state materials.These interdigitated single layers. The pervasiveness of the GS sheets reports have demonstrated that the local supramolecular was attributed to their ability to form ‘accordion’ or ‘pleated’ organization about each module can be predicted with reason- sheets by puckering about (guanidinium)NMH,O(sulfonate) able confidence based on molecular topology.Flat molecules HB ‘hinges’ joining adjacent one-dimensional hydrogen- having one-dimensional hydrogen-bonding topologies form bonded ribbons. This puckering, which can be defined by inter- ‘ribbon’ or ‘tape’ networks,1–5 while tetrahedral-like hydrogen- ribbon dihedral angles of hIR<180°, enables the sheets to adapt bonding topologies have aorded diamondoid networks.6–10 to the steric demands of dierent R groups.In a few cases However, control of packing in three dimensions can be elusive these steric demands were also accommodated by the formation owing to the contribution of numerous intermolecular inter- of a shifted ribbon HB motif.Although considered to be less actions in the crystal, many of which are nondirectional, optimal than the quasihexagonal motif because of the loss of resulting in a multiplicity of structural possibilities. Further- one strong hydrogen bond, two-dimensional sheet formation more, most of the aforementioned systems either do not in the shifted motif was enforced by the one remaining strong provide for the systematic introduction of ancillary molecu- inter-ribbon hydrogen bond.Such motifs oer unique opportunities for new layered materials based on the GS network lar functionality that is required for the synthesis of functional J. Mater. Chem., 1997, 7(7), 1123–1133 1123least as a first approximation, from the gross steric requirements of the R group extending from the GS sheets (Fig. 2). If the R group is described by either spheres or cylinders it can be shown that interdigitation in a bilayer motif is possible only if the diameter of the R group, as viewed normal to the hydrogen-bonded sheet, is less than dS-S /Ó3, where dS-S is the centre-to-centre distance between nearest sulfonate residues (typically ca. 7.5 A° ). If the diameter exceeds this value of ca. 4.4 A° ,interdigitation is not possible in the bilayer motif. Rather, the networks resort to the continuously stacked single layer motif in which the interdigitation is possible because the R groups of adjacent ribbons are orientated to opposite sides of the GS sheet. Previously, we illustrated several examples that conformed to this model (although in most cases the sulfonate residueis not rigorously cylindrical).For example,guanidinium naphthalene-2-sulfonate, (G)(1), assembles into the bilayer stacking motif (with hIR=146°), whereas the sterically more demanding naphthalene-1-sulfonate homologue (G)(2) assembles into the single layer stacking motif (with hIR=77°).14 The eect of steric demand is also evident in the structure of the guanidinium salt of ferrocenesulfonate (3), which exhibits the continuously stacked single layer stacking motif (Fig. 3). Although the steric ‘footprint’ of 3 (43 A° 2) is larger than that of 2 (31 A° 2), the puckering of the GS sheet is less severe. This reflects the need for the network to pucker more in (G)(2) in order to recover the dense packing lost by forming the single layer motif.The dense packing in (G)(2) is achieved through Fig. 1 (Top) Schematic representation of the sheet-like HB networks p–p stacking interactions between neighbouring naphthyl resi- formed from guanidinium cations and alkane- and arene-substituted dues (distance between neighbouring ring planes ca. 4.1 A° ). In monosulfonates and disulfonates. The most commonly observed net- contrast, the cross-sectional area of the ferrocene residue is work is quasihexagonal, in which every sulfonate oxygen atom is comparable to the molecular area of a guanidinium sulfonate hydrogen bonded to two guanidinium protons (typical dO H=2.0 A° ) so that all HB capacity is fulfilled.These sheets can be considered as assembling from one-dimensional HB ribbons (shaded) via hydrogen bonds.In some compounds these hydrogen bonds behave as hinges, resulting in a pleated GS network that can adapt to the steric requirements of the R groups. The inter-ribbon puckering angle is described by hIR. (Bottom) Schematic representations of layered materials synthesized from guanidinium cations and alkane- and arenesubstituted monosulfonates, as viewed along the long axis of the HB ribbons contained in the nominally planar GS networks.The white and shaded rectangles represent the narrow edge of the ribbons. Bilayer motifs (left) are observed for R groups which are small enough to allow interdigitation of R groups in the non-polar region separating the GS sheets. If the alkane or arene groups are too large, the R groups of adjacent ribbons are orientated to opposite sides of each sheet, which provides room for interdigitation and the continuous single layer stacking of the GS sheets (right).The sheets can adapt further to the steric requirements of the R groups in either layering motif by puckering about (guanidinium)N-H,O(sulfonate) HB ‘hinges’ between adjacent ribbons (hIR). with optical, magnetic, or conducting properties that will depend upon the choice of molecules in the region spanning the layers.However, a precise understanding of the influence of R group substituents, specifically the proximity of these substituents to and their ability to interact with the HB network, is required for rational design of such materials. This prompted us to examine systematically the influence of substituents on arenesulfonates whose steric and hydrogen bonding capacity influence the puckering of the two-dimensional GS network. The layered motif is retained in spite of steric interference and competing hydrogen bonding interactions in a majority of cases, illustrating the robustness of this network.Most importantly, our results demonstrate that the use of robust two-dimensional supramolecular Fig. 2 Schematic representation illustrating the steric influence of the R group on the layering motif in guanidinium sulfonate salts. modules can reduce the crystal engineering problem to the last Interdigitation of R groups all arranged on the same side of the remaining dimension. hydrogen-bonded sheet is possible if the R group projected diameter <dS-S /Ó3 (ca. 4.4 A° ) where dS-S is the distance between nearest sulfonate Results and Discussion groups. This results in the bilayer structure (top). If the diameter >dSS/ Ó3, interdigitation is not possible (centre) and the single layer motif Substituent sterics and hydrogen bonding (bottom), in which R groups on adjacent ribbons are orientated to The occurrence of either the interdigitated bilayer or the opposite sides, is formed as this allows interdigitation and ecient packing between the hydrogen-bonded sheets.continuously stacked single layer motif can be predicted, at 1124 J. Mater. Chem., 1997, 7(7), 1123–1133and the nitro group in p-nitrobenzenesulfonate (6), whose structures exhibited the single layer motif with substantial corrugation of the GS sheet (hIR=51° and 72°, respectively).In contrast, the guanidinium salt of p-toluenesulfonate (7), which has a size similar to that of 5 and 6 but no hydrogen bonding capability, exhibited the bilayer motif with modest puckering (hIR=151°). This demonstrated that the layering motif adopted by these materials was influenced by both steric and hydrogen bonding eects.Although the bilayer motif should have been accessible to 5 and 6 based on steric eects alone, a highly puckered single layer motif was formed owing to modest hydrogen bonding competition by the hydroxy and nitro groups for the sulfonate oxygens and guanidinium protons, respectively. In order to elucidate the relative contributions of steric and hydrogen-bonding competition eects from substituents forced to be in close proximity to the GS sheet, we have examined the guanidinium salts of various ortho- and meta-substituted methyl- and nitro-benzenesulfonates (8–16).An ortho substituent may sterically block the sulfonate oxygen acceptor sites by hindering the approach of the potential guanidinium donor (Scheme 1). The oxygen atoms of the nitro groups, as hydrogen- bonding acceptors, may compete for the guanidinium protons in the GS sheet.Nitro groups generally are not strong hydrogen-bond acceptors, particularly when compared to the sulfonate oxygen atoms, suggesting that the perturbation of the GS network may not be so severe that its formation is prohibited. Our previous observation that the p-nitrobenzenesulfonate compound (G)(6) possesses the quasihexagonal Fig. 3 (Top) Crystal structure of guanidinium ferrocenesulfonate GS network, in contrast to (G)(4) which contains the stronger (G)(3), as viewed along the hydrogen-bonded ribbon direction, which hydrogen-bonding carboxylic acid group, supports this conten- extends out of the plane of the page.This view illustrates the tion. Investigation of the ortho- and meta-substituted phases segregation of the non-polar ferrocene-containing regions and the allows comparison between residues with identical volumes polar hydrogen-bonding regions into a puckered interdigitated single but with substituents in dierent positions.The conformational layer motif. The filled circles denote the hydrogen-bonded quasihexagonal GS sheet. (Bottom) Space-filling representation of the packing freedom of the ortho substituents also can provide steric relief of two adjacent ferrocene residues contained within the (100) galleries and minimize the perturbation of the GS sheet.of (G)(3). The C–H dipole of one of the residues projects into the centre of the cyclopentadiene ring of the neighbouring ferrocene, Synthesis of guanidinium arenesulfonates suggesting Cd-–Hd+,p-electron interactions. Guanidinium salts of variously substituted methyl- and nitrobenzenesulfonates were prepared by slow evaporation crys- unit (ca. 45A° 2), therefore requiring less puckering than (G)(2) tallization techniques. Several of these compounds appeared to recover lost packing density. The CMH dipole of each to form unstable solvated crystalline phases, as evidenced by ferrocene projects into the centre of the cyclopentadiene ring the physical transformation of transparent crystals to opaque of a neighbouring ferrocene, suggesting a role for solids soon after their removal from the mother liquor.Low Cd-MHd+,p-electron interactions in the ordering of these temperature broad endotherms observed by dierential scan- residues (see Fig. 3). The ferrocene containing phase introduces ning calorimetry (DSC) of samples characterized immediately redox centres into ‘galleries’ between the robust two-dimen- after their removal from solution also suggested the loss of sional layers, suggesting interesting possibilities for charge solvent from the crystals, and IR spectroscopy confirmed the trapping and electron transport.The structure of this salt presence of solvent molecules in the solids. IR spectroscopy resembles recently reported materials which are based on two- also revealed that several of these phases did not contain the dimensional zirconium phosphonate (ZrP) networks with desired quasihexagonal HB sheet motif. Consequently, we redox centres within the galleries defined by ZrP layers.19–21 pursued characterization of phases which were stable under While the aforementioned model has been useful in the ambient conditions and/or that contained the two-dimensional design and synthesis of over 30 crystalline GS salts containing HB motif.Although unstable phases were sometimes isolated, the two-dimensional hydrogen-bonded network,12 it does not stable, high quality crystals suitable for single crystal X-ray address the more subtle eects of positional substitution.diraction were obtained readily for most of the substituted Proximity of functional groups to the GS sheet may perturb arenesulfonates depicted above. Experimental details of the X- significantly the planarity of these networks, and in severe ray structural determinations are given in Table 1.All of the cases, may actually prohibit formation of the two-dimensional phases described here for which crystal structures have been network. If these functional groups are hydrogen-bond donors determined exhibit typical molecular geometries, including the or acceptors, competition with complementary sites of the GS guanidinium ions and MSO3 groups.Therefore, detailed sheet may perturb the hydrogen bonding and geometry of the descriptions of the molecular structures are not presented here. GS network. Previously, we discovered that the guanidinium salt of p-carboxybenzenesulfonate (4) did not form layered Methyl-substituted sulfonates networks because of hydrogen-bonding competition of the carboxylic acid group for guanidinium proton donor and Crystal structures were determined for guanidinium salts of toluene-3-sulfonate (G)(9), and mesitylenesulfonate (G)(11), sulfonate oxygen acceptor sites.15 However, the two-dimensional layer structure was preserved for guanidinium salts of which crystallize in orthorhombic space group Pnma (Fig. 4). The salt (G)(9) crystallizes with quasihexagonal GS sheets benzenesulfonates with weaker hydrogen-bonding substituents such as the phenolic group in p-hydroxybenzenesulfonate (5) which are extremely puckered (hIR=88°) and assemble into the J.Mater. Chem., 1997, 7(7), 1123–1133 1125Scheme 1 interdigitated single layer stacking motif. The structural details (dstk=8.71 and 10.51 A° for (G)(9) and (G)(11), respectively). The arene–arene interactions in the organic region are best of the layering motifs are summarized for these compounds, and for the other layered materials described below, in Table 2.described as oset p-stacking interactions in both salts, whereas in the (G)(7) and other guanidinium arenesulfonate salts, The packing of (G)(9) into a puckered single layer rather than a bilayer motif is somewhat surprising, as the para-tolyl herringbone motifs are present.The crystal structures of the 2-methylbenzenesulfonate (tolu- compound (G)(7) crystallizes into a bilayer structure with a herringbone arrangement of adjacent arene rings within the ene-2-sulfonate) (G)(8) and its 2,4-dimethylbenzenesulfonate homologue (G)(10) could not be determined because of poor bilayer galleries. The meta-methyl substituent does not block the approach of the guanidinium protons to the sulfonate crystal quality.In the case of (G)(8), the fine needles obtained were not large enough and attempts to crystallize an unsolvated acceptor sites, so a similar bilayer motif may be expected. However, the interactions between neighbouring inversion- form always led to opaque solids with poor crystallinity.Crystallographic data for (G)(10) could not be refined satisfac- related arene rings in the region between the GS sheets in (G)(9) appear to dier from the herringbone orientations torily. However, the IR spectral features for (G)(8) and (G)(10), particularly in the nN-H region, are essentially identical to those observed in (G)(7) and other guanidinium arenesulfonates, with each methyl group in (G)(9) lying over the p-system of a of (G)(9), (G)(11) and (G)(7) (Fig. 5). Correlation of IR spectral data and X-ray crystal structures for over 30 GS salts neighbouring arene ring (see Fig. 4). Highly puckered quasihexagonal GS sheets (hIR=86°) and in our laboratory has demonstrated that a particular absorption band profile in the nN-H region from 3500–3100 cm-1 (as the interdigitated single layer stacking motif also are observed in (G)(11).The presence of the quasihexagonal topology, in in Fig. 5) is highly diagnostic of the quasihexagonal HB sheet motif. Consequently, we surmise from examination of the IR which all six sulfonate oxygen lone electron pairs participate in hydrogen bonding to the guanidinium protons with typical absorption band structures observed for the unsolvated forms of (G)(8) and (G)(10) that these compounds also form layered hydrogen bond distances, indicates that the ortho methyl substituents in (G)(11) do not prohibit the formation of this structures containing the quasihexagonal HB sheet.network. However, the mesitylenesulfonate ion does not exceed the steric limit of ca. 4.4 A° which is considered the threshold Nitro-substituted sulfonates of stability for the interdigitated bilayer structure.Steric interference by the ortho-methyl substituents hinders coplanar Each guanidinium salt of the variously substituted nitrobenzenesulfonates, guanidinium 2-nitrobenzenesulfonate approach of the hydrogen-bonded ribbons to form a twodimensional sheet if the mesitylene groups are orientated to (G)(12), 3-nitrobenzenesulfonate (G)(13), 2,4-dinitrobenzenesulfonate (G)(14), 2,4-dinitrobenzenesulfonate mono- the same side of the HB sheet (see Scheme 1).Thus, the ribbons are forced to approach each other nearly orthogonally, with hydrate (G)(14) H2O, and picrylsulfonate (G)(15) crystallizes in space group P1� with one ion pair per asymmetric unit.The the mesitylene groups of neighbouring ribbons orientated to opposite sides of the hydrogen-bonding plane, resulting in nitro NMO bond geometries compare well with those determined from a search of the Cambridge Crystallographic substantial puckering. The nearly identical packing of (G)(11) and (G)(9) is reflected in the similarities of the b and c Database, which revealed a mean dN-O of 1.217±0.011 for 1116 aromatic nitro compounds.18 The twisting of the ortho nitro crystallographic lattice constants in these phases.The repeat distances parallel and perpendicular to the hydrogen bonding groups out of the arene ring plane in compounds (G)(12), (G)(14), (G)(14) H2O and (G)(15) is 50–60°. In contrast, the ribbon in the GS network, denoted as ddrib and d)rib, respectively, are nearly identical in (G)(9) and (G)(11).The stacking para nitro group is nearly coplanar in (G)(6), (G)(14), (G)(14) H2O and (G)(15). A value of 16° is observed for the repeat distance, dstk, is larger for (G)(11) than for (G)(9) due to the steric demand of the para-methyl substituent in (G)(11) meta nitro group in (G)(13). The severe twisting of the ortho 1126 J.Mater. Chem., 1997, 7(7), 1123–1133Table 1 Crystallographic data for guanidinium sulfonates compound (G)(3) (G)(9) (G)(11) (G)(12) (G)(13) (G)(14) (G)(14) H2O (G)(15) formula C11H15N3O3SFe C8H13N3O3S C10H17N3O3S C7H10N4O5S C7H10N4O5S C7H9N5O7S C7H11N5O8S C7H8N6O9S MW 325.16 231.27 259.33 262.25 262.24 307.24 324.24 352.23 crystal size/mm3 0.67×0.57×0.05 0.57×0.47×0.27 0.50×0.30×0.08 0.50×0.38×0.25 0.60×0.30×0.25 0.55×0.50×0.42 0.55×0.50×0.42 0.55×0.50×0.42 crystal system orthorhombic orthorhombic orthorhombic triclinic triclinic triclinic triclinic triclinic space group Pna21 Pnma Pnma P1� P1� P1� P1� P1� a/A° 17.160(6) 17.410(8) 21.021(2) 7.2416(8) 7.196(8) 7.761(3) 7.715(2) 7.782(6) b/A° 7.693(4) 7.595(6) 7.5584(6) 7.596) 7.842(4) 8.314(4) 8.187(2) c/A° 10.740(4) 8.613(4) 8.3710(6) 12.1475(14) 11.694(5) 11.591(3) 10.987(5) 10.813(2) a (°) 90 90 90 88.257(2) 76.45(5) 97.87(3) 82.73(4) 100.83(2) b (°) 90 90 90 78.135(2) 78.32(8) 95.33(3) 72.25(3) 92.32(4) c (°) 90 90 90 62.150(1) 62.94(9) 118.32(4) 77.88(2) 99.20(6) V/A° 3 1418(2) 1139(2) 1330.0(2) 576.12(11) 552(2) 605(1) 655(1) 666(1) Z 4 4 4 2 2 2 2 2 Dcalc/g cm-3 1.523 1.349 1.295 1.512 1.577 1.686 1.645 1.756 F (000) 672 488 552 272 272 316 334 360 m (Mo-Ka) (cm-1) 12.09 2.64 2.45 2.98 2.97 2.97 2.84 2.93 T /°C 24 24 25 24 24 24 24 24 diractometer type Enraf-Nonius Enraf-Nonius Siemens Siemens Enraf-Nonius Enraf-Nonius Enraf-Nonius Enraf-Nonius scan mode v v — — v-2h v-2h v v-2h scan speed (deg/min in v) 1.8–16.5 16.5 — — 8.2 8.2 8.2–16.5 16.5 2hmax (°) 52.0 47.9 48.2 48.2 56.0 55.9 55.9 63.9 range of hkl -21,±9,±13 ±8,-9,±19 -24 to 22,-4 ±8,±8,-5 +9,±9,±14 ±10,±10,±15 ±10,±11,±14 ±10,±11,±15 to 8,±9 to+13 no.refl. collected 4087 3727 5066 2470 5310 2989 3256 6019 no. unique refl. 2595 1077 1134 1742 2655 2919 3138 4613 Rint 0.054 0.048 0.045 0.0313 0.040 0.022 0.024 0.023 corrections applieda abs, 2 ext abs, 2 ext abs abs abs, 2 ext abs abs, 2 ext 2 ext Rb 0.045 0.060 0.050 0.0405 0.042 0.052 0.070 0.047 Rwb 0.046 0.075 0.111 0.1052 0.055 0.056 0.067 0.058 D(r) e/A°-3 0.39 0.94 0.154 0.281 0.42 0.47 0.46 0.48 no.indep refl obs I>2s(I) 1571 735 1134 1742 2287 2218 2018 3544 No/Nv 9.13 8.65 11.57 8.98 12.78 12.25 10.35 15.21 GOF 1.12 2.26 1.098 1.073 2.01 1.74 2.05 1.55 aAll structures were corrected for Lorentz and polarization eects. abs=empirical absorption using DIFABS (N.Walker and D. Stuart, Acta Crystallogr. Sect. A, 1983, 39, 158.); 2 ext=secondary extinction. bR(F)=S||Fo|-|Fc ||/S|Fo |. cR(wF)=[(Sw(|Fo|-|Fc |)2 /SwFo2)]1/2; w=4Fo2/s2(Fo2). J. Mater. Chem., 1997, 7(7), 1123–1133 1127Fig. 5 Comparison of the solid-state IR spectra (Nujol mulls) of guanidinium toluenesulfonates (G)(8) (ortho-substituted), (G)(9) (metasubstituted), and (G)(7) (para-substituted).The structure of the nN-H absorption bands, which is diagnostic of the quasihexagonal GS network, is essentially identical in these spectra. This argues that (G)(8), for which a single crystal structure could not be obtained, possesses a quasihexagonal hydrogen-bonding motif.nitro groups out of the ring planes most likely arises from the need to relieve steric crowding with the ortho sulfonate groups and to alleviate repulsive interaction with the negatively chargedsulfonate groups. A database study of ortho-substituted nitro groups found an average twist angle of 27±1° for nitro groups with one substituent in the ortho position (n=392, n= number of observations).24 When the substituent is a sulfonate group (a substantially smaller sample, n=7), both steric hindrance and a negative charge are important factors, resulting in a much larger mean twist angle of 65±3°, very close to that observed in our guanidinium nitrobenzenesulfonate compounds. The database study also revealed that for nitro groups Fig. 4 Crystal structures of guanidinium toluene-3-sulfonate (G)(9), with two sterically undemanding hydrogen atoms in the guanidinium mesitylenesulfonate (G)(11), and guanidinium toluene-4- positions ortho to the nitro group (as in the 4-, 2,4-, and sulfonate (G)(7) as viewed along the hydrogen-bonded ribbon direc- 2,4,6-dinitrobenzenesulfonate compounds), a nearly coplanar tion, which extends out of the plane of the page.These views illustrate arrangement with the benzene rings is favoured (average twist the segregation of the non-polar arene-containing regions and the angle of 7.3±0.3°, n=270). Steric eects clearly play a role in polar hydrogen-bonding regions into bilayers for (G)(7), and severely puckered interdigitated single layers for (G)(9) and (G)(11). The filled determining the geometry of the ortho nitro substituents.circles denote the hydrogen-bonded quasihexagonal GS sheet. The guanidinium salts of mono-substituted nitrobenzenesulfonates crystallize with structures similar to those of other Table 2 Summary of key structural parameters and layering motifs for layered guanidinium sulfonates compound (G)(3) (G)(9) (G)(11) (G)(12) (G)(13) repeat distance d ribbon, ddrib/A° b=7.69 b=7.60 b=7.56 b=7.59 b=7.63 repeat distance ~) ribbon, d)rib/A° c=10.74 c=8.61 c=8.37 a=7.24 a=7.20 HB plane (100) (100) (100) (001) (001) layering motif single layer single layer single layer bilayer bilayer interribbon dihedral angle, hIR/° 153 88 86 180 180 stacking repeat distance, dstk/A° 8.58 8.71 10.51 11.89 11.45 1128 J.Mater.Chem., 1997, 7(7), 1123–1133guanidinium organosulfonates, with the nitro groups influenc- 3.34 A° between neighbouring ring planes in (G)(12) and (G)(13), respectively. These structures dier from those of ing the crystal packings in subtle ways (Fig. 6). Guanidinium 2- and 3-nitrobenzenesulfonates (G)(12) and (G)(13) possess other guanidinium arenesulfonates, in which the arene rings in the bilayer galleries adopt the herringbone (edge-to-face) planar quasihexagonal hydrogen-bonded sheets (hIR=180°) arranged in a bilayer stacking motif (the latter contrasts with arrangement. The inversion symmetry within the layers results in a favourable configuration in which nitrobenzene dipoles the single layer motif observed for (G)(9), the meta methyl substituted analogue).The planarity of these networks is are opposed. As in many organic crystals, the nitro groups in (G)(12) and (G)(13) do not participate in strong hydrogen unusual when compared to other guanidinium arenesulfonates with bilayer structures, which exhibit hIR values of 150–165°. bonds. While the major driving force controlling the crystal packing in both (G)(12) and (G)(13) is the hydrogen bonding Inspection of the structures of (G)(12) and (G)(13) reveals p–p stacking between arene rings in the bilayer galleries, in within the GS sheets, inspection of the structures suggests that secondary CMH,O interactions25–29 may play a role in which the rings are laterally oset in a manner commonly observed for p–p stacks.These interactions appear to be influencing the orientations of the molecules in the van der Waals interlayer regions.Additionally, one short contact of a significant, as indicated by the very short distances of 3.46 and nitro oxygen to a guanidinium ion is present in (G)(12), with a bifurcated nitrogen oxygen acceptor forming a four-membered ring with one guanidinium NH2 group. The ddrib and d)rib values are nearly identical in (G)(12) and (G)(13).However, the dstk values dier due to the dierent steric demands along the layer stacking direction imposed by the dierent position of the nitro groups (Table 2). We note that the bilayer structures of (G)(12) and (G)(13) dier markedly from the single layer motif found in guanidinium 4- nitrobenzenesulfonate (G)(6). This dierence can be attributed to (guanidinium)NMH,O(nitro) hydrogen-bonding interactions in (G)(6), in which a nitro group extending from a GS sheet hydrogen bonds to two guanidinium protons on an opposing GS sheet.These examples illustrate that weak electrostatic interactions, steric eects, and hydrogen-bonding all contribute to the solid state packing in these salts. Guanidinium 2,4-dinitrobenzenesulfonate (G)(14), its monohydrate (G)(14) H2O, and picrylsulfonate (G)(15) salts do not exhibit quasihexagonal layered GS sheets (Fig. 7 and 8). Rather, these compounds form complex hydrogen bonding networks that organize the hydrophobic arene-containing regions into galleries separated by two-dimensional polar regions containing the hydrogen bonding guanidinium ions, sulfonate groups and nitro groups.The major dierence between the structures of (G)(14) and (G)(15) arises from the orientation of guanidinium ions with respect to the arene ring planes. Hydrogen-bonding between guanidinium protons and sulfonate and nitro acceptors is extensive in these compounds. The orientations of the guanidinium ions allow them to participate in multiple hydrogen bonds with both sulfonate and nitro acceptor sites of neighbouring anion sheets.The strongest hydrogen bonding occurs for (guanidinium) NMH,O(sulfonate) hydrogen bonds as expected, but many (guanidinium)NMH,O(nitro) CMH,O intermolecular contacts are also observed. Although weak attractive nitro,nitro intermolecular N,O contacts have been suggested to direct crystal packing in complexes of N,N-dipicrylamine,30 this type of interaction is not present in the guanidinium nitrobenzenesulfonate salts described here.The nitro group substituents in (G)(14), (G)(14) H2O and (G)(15) so severely perturb the GS HB network that even the GS hydrogen-bonded ribbon motif, which is pervasive and has been observed in all previously determined structures of guanidinium alkane- and arene-sulfonates, is absent. However, six-membered GS ring motifs are present that dier from the eight-membered ring dimers in the GS sheets, but are similar to those found in guanidinium carboxylates and phosphates.31,32 These structures reveal that the presence of numerous weak hydrogen bonding interactions can steer the crystal packing away from the quasihexagonal Fig. 6 Crystal structures of guanidinium 2-nitrobenzenesulfonate (G)(12), guanidinium 3-nitrobenzenesulfonate (G)(13), and guanidin- GS motif.A more detailed description of these complex ium 4-nitrobenzenesulfonate (G)(6) as viewed along the hydrogen- hydrogen-bonding motifs has been reported previously.33 bonded ribbon direction, which extends out of the plane of the paper. These views illustrate the segregation of the non-polar arene-containing regions and the polar hydrogen-bonding regions into bilayers for Comparison of methyl and nitro substitution (G)(12) and (G)(13), and severely puckered interdigitated single layers A comparison of the layering structures of the guanidinium for (G)(6).The filled circles denote the hydrogen-bonded quasihexagonal GS sheet. methyl- and nitro-benzenesulfonates reveals that bilayer motifs J.Mater. Chem., 1997, 7(7), 1123–1133 1129are observed for guanidinium tosylate (G)(7) and 2- and 3- nitrobenzenesulfonates (G)(12) and (G)(13), while puckered single layer motifs are observed for guanidinium toluene-3- (G)(9), mesitylene- (G)(11), and 4-nitrobenzene-sulfonates (G)(6). The van der Waals volume of the nitro group is significantly larger than its methyl counterpart, with volumes of 23.5 and 15.3 A° 3 , respectively.34 However, the shape of the planar nitro group may provide some relief from steric crowding around the sulfonate group as it can twist out of the arene ring plane, whereas the geometry of the methyl group is more isotropic.However, twisting of the nitro group may not have a large eect, as the steric bulk of the nitro group is still larger than the methyl group.The substitution of methyl for nitro in the case of the meta-substituted salts (G)(9) and (G)(13) results in an unexpected change in the layering motif, with the former crystallizing in an extremely puckered single layer structure and the latter crystallizing in the bilayer motif.This is counterintuitive as the smaller volume of 9 should make the bilayer structure more favourable for this compound. These structures reveal that steric eects are quite subtle, particularly when comparing substituents with diering substitutional position or hydrogen bonding ability. The presence of two or more nitro groups on an arenesulfonate so severely perturbs hydrogen bonding that the GS sheet network, so pervasive in these materials, is completely absent in compounds derived from these anions.Conclusion This work demonstrates that crystal packing can be controlled through use of the guanidinium-sulfonate module as a topological director of crystal packing. Guanidinium salts of benzenesulfonates containing a single methyl or nitro substituent or multiple methyl substituents crystallize with the predicted quasihexagonal GS network, with layers assembling into either bilayer or single layer motifs.The robustness and prevalence of the GS network suggests that this module can be used in the design and synthesis of new crystalline materials. Its twodimensional nature reduces crystal engineering to the last remaining dimension.However, unanticipated dierences in layering motif, i.e. bilayer versus single layer packing, for analogous methyl versus nitro substituted benzenesulfonate salts shows that subtle steric and hydrogen-bonding eects can have a dramatic eect in determining crystal packing in the third dimension. In the cases of multiple substitution of nitro groups, the quasihexagonal HB sheets and layering structures are completely disrupted in order to form multiple weak hydrogen bonds to nitro groups.These studies illustrate that even if robust modules are employed, the presence of ancillary intermolecular interactions can limit the predictability of the entire 3D structure. However, we anticipate that restricting ancillary residues to galleries within the robust 2D HB networks, thereby limiting the degrees of freedom available for crystal packing, will facilitate computational predictions of these structures.Fig. 7 Crystal structures of guanidinium 2,4-dinitrobenzenesulfonate (G)(14), guanidinium 2,4-dinitrobenzenesulfonate monohydrate (middle) (G)(14) H2O, and guanidinium 2,4,6-trinitrobenzenesulfonate Experimental (G)(15) as viewed normal to their (100) planes.These views Materials illustrate the segregation, on (001) planes, of the non-polar arenecontaining regions and the polar hydrogen-bonding regions (indicated Guanidine chloride and guanidine carbonate were purchased by the open squares) containing the guanidinium ions, nitro and from Aldrich Chemical Co. All other starting materials were MSO3 groups. The severe tilting of guanidinium ions and the presence purchased from the companies indicated and used as received. of nitro groups in the polar region prohibit the formation of the quasihexagonal GS sheet.The (100) planes of (G)(14 ) and (G)(15) Spectroscopic-grade solvents and/or deionized water were used consist of arenesulfonate layers in which the arene rings lie in the for all crystallizations.plane. The arenesulfonate layers in (G)(14) H2O actually lie in the (102) plane. These layers are evident in the views depicted in Fig. 8. Characterization Melting points were determined by dierential scanning calorimetry (DSC) with a Mettler FP80/FP84 system (100 mV, 1°C min-1). Solid-state IR spectra were recorded on a Nicolet 1130 J. Mater. Chem., 1997, 7(7), 1123–1133Fig. 8 Crystal structures of guanidinium 2,4-dinitrobenzenesulfonate (G)(14), guanidinium 2,4-dinitrobenzenesulfonate monohydrate (G)(14) H2O, and guanidinium 2,4,6-trinitrobenzenesulfonate (G)(15) as viewed normal to their (010) planes. These views illustrate the segregation, on (001) planes, of the non-polar arene-containing regions and the polar hydrogen-bonding regions (indicated by the open squares) containing the guanidinium ions, nitro and MSO3 groups.The (100) planes of (G)(14) and (G)(15) consist of arenesulfonate layers in which the arene rings lie in the plane. The arenesulfonate layers in (G)(14) H2O actually lie in the (102) plane. Because of its severe tilt, the guanidinium cation bridges these layers by hydrogen bonding in all three compounds, resulting in a three-dimensional hydrogen-bonding network. 510M spectrometer (4 cm-1 resolution) as Nujol mulls. 1H {s, 6 H, [C(NH2 )3 ]}, 2.52 (s, 3 H, Ar-CH3), 2.08 (s, 3 H, CH3CN). The presence of acetonitrile is confirmed by IR (nCN NMR spectra were recorded on an IBM NR200AF spectrometer (200MHz) in (CD3)2SO unless stated otherwise at 2252 cm-1) and by observation of its methyl protons in the 1H NMR spectrum at 2.08 ppm, as well as a broad desolvation (Cambridge Isotope Laboratories) relative to internal standard SiMe4; J in Hz.Experimental details of the X-ray structural endotherm in the DSC. The crystals desolvate soon after their removal from solution, resulting in an opaque solid having an determinations are given in Table 1, and atomic coordinates are available as supplementary material or at our World Wide IR spectrum identical to (G)(8).Web site (http://www.cems.umn.edu/research/ward). Structures (G)(3), (G)(9), (G)(13), (G)(14), (G)(14) H2O and (G)(15) Guanidinium toluene-2-sulfonate, [C(NH2)3]+ were determined using an Enraf-Nonius CAD4 diractometer 2-CH3(C6H4)SO3-, (G)(8) with graphite monochromated Mo-Ka radiation at l This phase was isolated from a 151 methanol–acetonitrile 0.71069 A° .Structures (G)(11) and (G)(12) were determined solution containing equimolar quantities of guanidine hydro- using a Siemens SMART system diractometer with graphite chloride and toluene-2-sulfonic acid (Aldrich). This compound monochromated Mo-Ka radiation at l 0.71069 A° . All data formed as an opaque solid on the sides of the crystallization were collected at room temp.(24°C). vessel or after desolvation of solvated crystals (G)(8) MeCN. Atomic coordinates, thermal parameters, and bond lengths Attempts to isolate single crystals of (G)(8) from solution were and angles have been deposited at the Cambridge dicult, but clear thin needles of unsolvated (G)(8) were Crystallographic Data Centre (CCDC).See Information for isolated together with opaque solid (presumably, desolvated Authors, J. Mater. Chem., 1997, Issue 1. Any request to the (G)(8) H2O or (G)(8) EtOH) from 20% aqueous ethanol. CCDC for this material should quote the full literature citation However, these needles were not large enough for single crystal and the reference number 1145/37. X-ray diraction. DSC mp 220–222°C; n/cm-1 3365 (s), 3332 (s), 3259 (m), 3186 (s), 1683 (s), 1588 (m), 1463 (s), 1378 (s), Guanidinium toluene-2-sulfonate acetonitrile solvate, 1302 (w), 1281 (w), 1208 (m), 1169 (s), 1146 (s), 1094 (m), 1052 [C(NH2)3]+ 2-CH3(C6H4)SO3- CH3CN, (G) (8) MeCN (vw), 1036 (vw), 1017 (s), 808 (w), 751 (m), 708 (s); d 7.73 (~d, This phase was recrystallized as colourless plates from a 151 1 H, ortho to SO3-), 7.21–7.12 (m, 3 H, meta/para to SO3-), methanol–acetonitrile solution containingequimolar quantities 6.95 {s, 6 H, [C(NH2)3]}, 2.52 (s, 3 H, Ar-CH3).of guanidine hydrochloride and toluene-2-sulfonic acid (Aldrich). The following characterization was performed Guanidinium toluene-3-sulfonate, [C(NH2)3]+ immediately after removal of the crystals from solution. DSC 3-CH3(C6H4)SO3-, (G)(9) 30–52°C (broad endotherm, loss MeCN), mp 222–224 °C; n/cm-1 3363 (s), 3330 (s), 3255 (m), 3190 (s), 2252 (m, sharp), This phase was crystallized from methanol or 10% aqueous acetonitrile solutions containing equimolar quantities of guani- 1677 (s), 1582 (m), 1463 (s), 1378 (m), 1283 (w), 1208 (s), 1187 (s), 1171 (s), 1144 (s), 1094 (s), 1050 (m), 1038 (m), 1017 (s), 918 dine hydrochloride and toluene-3-sulfonic acid monohydrate (Lancaster) or from aqueous solutions containing 152 molar (vw), 808 (w), 768 (m), 741 (m), 708 (s), 614 (s); d 7.73 (~d, 1 H, ortho to SO3-), 7.21–7.12 (m, 3 H, meta/para to SO3-), 6.95 quantities of guanidine carbonate and toluene-3-sulfonic acid J.Mater. Chem., 1997, 7(7), 1123–1133 1131monohydrate as colourless needles: DSC with concurrent Guanidinium 2-nitrobenzenesulfonate hydrate, [C(NH2)3]+ 2-NO2(C6H4)SO3- H2O, (G) (12) xH2O visual observation 152–156 (slightly broad endotherm, crystals fracture, turn somewhat cloudy), mp 215–216 °C; visual obser- This phase was crystallized from 10% aqueous acetonitrile vation of single crystals on a Fisher–Johns hot stage: solution containing equimolar quantities of guanidine hydro- 155–160 °C: very slight clouding, but crystal remained some- chloride and 2-nitrobenzenesulfonic acid (Pfaltz and Bauer) or what clear, possibly melting and resolidifying; 218–219 °C: from aqueous or 10% aqueous methanol solutions containing melting; n/cm-1 3371 (s), 3328 (s), 3257 (m-s), 3190 (s), 1677 152 molar quantities of guanidine carbonate and 2-nitroben- (s), 1586 (m), 1463 (s), 1378 (s), 1304 (w), 1225 (m, sh), 1194 (s, zenesulfonic acid as colourless needles:DSC endotherms: 72–76 sh), 1169 (s), 1115 (s), 1090 (m), 1038 (s), 996 (m), 783 (m), 741 (br), 90–95 (br), mp 134–136°C; n/cm-1 3656 (m), 3558 (m), (w), 708 (m), 681 (s), 627 (s); d 7.43 (~d, 2 H, Ar-H ortho to 3440 (sh, s), 3367 (s), 3274 (s), 3205 (s), 3095 (m), 1675 (s), 1613 SO3-), 7.22–7.15 (m, 2 H, J 7.9, Ar-H meta and para to SO3-), (w), 1596 (w), 1582 (m), 1540 (s, nN-O asym), 1530 (s, nN-O asym), 6.96 {s, 6 H, [C(NH2)3]+}, 2.32 (s, 3 H, Ar-CH3).The X-ray 1465 (s), 1374 (s, nN-O sym), 1364 (s, nN-O sym), 1300 (w), 1214 crystal structure of this compound was solved. (s), 1164 (m), 1142 (s), 1079 (s), 1040 (m), 1025 (s), 855 (m), 780 (m), 743 (s), 733 (s), 702 (m), 664 (s), 646 (s), 614 (s), 581 (s); d 7.85 (~d, 1 H, 6-Ar-H), 7.60–7.55 (m, 3 H, 3,4,5-Ar-H), 6.93 Guanidinium 2,4-dimethylbenzenesulfonate [C(NH2 )3 ]+ 2,4- {s, 6 H, [C(NH2)3]+}, 3.41 (s, H2O, ~5 H, hydrate and (CH3)2(C6H3)SO3-, (G)(10) exchange with water in Me2SO).The stoichiometric amount of hydrated water in the crystal was not determined.However, This phase was crystallized from 351 methanol–toluene solu- this salt may be a dihydrate, as indicated by the two broad tion containing equimolar quantities of guanidine hydro- endotherms in the DSC. The integration of the water peak in chloride and sodium 2,4-dimethylbenzenesulfonate (Kodak) as the NMR to a value corresponding to nearly four hydrogens colourless needles; DSC mp 281 °C; n/cm-1 3373 (s), 3330 (s), also suggests that the complex may be a dihydrate.The sharp 3263 (m), 3188 (s), 1677 (s), 1588 (m), 1463 (s), 1378 (s), 1189 IR nO-H at high wavenumber positions indicate that the water (m), 1158 (s), 1092 (m), 1017 (s), 822 (w), 816 (w), 745 (w), 726 is not strongly associated by hydrogen bonding in the lattice.(w), 685 (m); d 7.60 (d, 1 H, J=7.6), 6.95 with 6.92 side peak The existence of split IR nN–O bands at 1540/1530 cm-1 and {s, 8 H, [C(NH2 )3 ]+, Ar-H ortho to SO3-}, 2.48 (s, 3 H, Ar- 1374/1364 cm-1 suggests two dierent solid-state environments CH3), 2.25 (s, 3 H, Ar-CH3). An attempt was made to solve for the nitro group and possibly two ion pairs in the asymmet- the X-ray crystal structure, but refinement was not successful.ric unit. The structure determination was not pursued further. Guanidinium 3-nitrobenzenesulfonate, [C(NH2)3]+ 3-NO2(C6H4)SO3-, (G)(13) Guanidinium mesitylenesulfonate (guanidinium 2,4,6- This phase was crystallized from 25% aqueous acetonitrile or trimethylbenzenesulfonate), [C(NH2)3]+ 2,4,6- 35351 methanol–ethyl acetate–water solutions containing equi- (CH3)3(C6H2)SO3-, (G)(11) molar quantities of guanidine hydrochloride and sodium 3- This phase was crystallized from methanol or 30% aqueous nitrobenzenesulfonate (Kodak) as light-yellow elongated acetonitrile solutions containing equimolar quantities of guani- diamonds/parallelograms: DSC endotherm 178–180, dine hydrochloride and mesitylenesulfonic acid dihydrate mp 184–187 °C; visual observation of a single crystal on a (Aldrich) or from aqueous or methanol solutions containing Fisher–Johns hot stage showed no obvious change at 180 °C 152 molar quantities of guanidine carbonate and mesitylene- and melting at 185–190 °C; n/cm-1 3400 (s), 3371 (s), 3249 (m), sulfonic acid dihydrate as aggregates of colourless rectangular, 3207 (s), 3105 (w), 1673 (s), 1580 (m), 1573 (m), 1532 (s, nN-O flat plates: DSC mp 270–300 (decomp.)°C; n/cm-1 3375 (s), asym), 1465 (s, Nujol), 1378 (s, Nujol), 1356 (s, nN-O sym), 1277 3323 (s), 3259 (m-s), 3186 (s), 1675 (s), 1605 (w), 1586 (m), 1569 (w), 1208 (s), 1150 (m), 1096 (m), 1079 (m), 1038 (m-s), 1001 (w), 1461 (s), 1378 (s), 1252 (w), 1191 (m), 1183 (m), 1158 (s), (w), 934 (w), 907 (w), 882 (w), 812 (m), 762 (m), 737 (m), 671 1092 (s), 1013 (s), 841 (m), 743 (m), 689 (s); d 6.97 {s, 6 H, (s); d 8.34 (m, 1 H, 2-Ar-H), 8.22 (d, 1 H, J 9.1, 6-Ar-H), 8.03 [C(NH2)3]+}, 6.77 (s, 2 H, arene ring H), 2.50 (s, ~6 H, 2,6- (d, 1 H, J 7.7, 4-Ar-H), 7.67 (t, 1 H, J 7.9, 5-Ar-H), 6.93 {s, 6 CH3, overlaps with (CH3)2SO solvent peak), 2.18 (s, 3 H, 4- H, [C(NH2)3]+}.The X-ray crystal structureof this compound CH3); d (D2O) 7.05 (s, 2 H, arene ring H), 4.91 {s, [C(NH2 )3 ]+, was solved. overlaps with H2O solvent impurity peak}, 2.57 (s, 6 H, 2,6- Guanidinium 2,4-dinitrobenzenesulfonate, [C(NH2)3]+ CH3), 2.29 (s, 3 H, 4-CH3). The X-ray crystal structure of this 2,4-(NO2)2(C6H3)SO3-, (G)(14) compound was solved. This phase was crystallized from 151 methanol–ethyl acetate solution containing equimolar quantities of guanidine hydro- Guanidinium 2-nitrobenzenesulfonate, [C(NH2)3]+ chloride and 2,4-dinitrobenzenesulfonic acid (Eastman) as 2-NO2(C6H4)SO3-, (G)(12) hard, light tan needles or from 10% aqueous acetonitrile solution as opaque cream-coloured powder.The opaque This phase was crystallized from methanol solution containing material is probably a dehydrated form of (G)(14) H2O, as equimolar quantities of guanidine hydrochloride and 2-nitro- dehydration of (G)(14) H2O yields an identical solid based on benzenesulfonic acid (Pfaltz and Bauer) as colourless thick IR spectroscopy.DSC mp 176°C; n/cm-1 3477, 3433, 3365, hexagonal plates and wide needles or from methanol–toluene 3272, 3199, 3095, 1675, 1663, 1605, 1551 (s, nN-O asym), 1542, solution as colourless needles: DSC endotherm 117–118, 1465, 1378, 1368, 1356 (s, nN-O sym), 1227, 1136, 1119, 1069, mp 129–135°C; n/cm-1 3406 (s), 3381 (s), 3284 (m), 3255 (m), 1028, 906, 849, 834, 750, 739, 724, 662, 635; d 8.58 (d, 1 H, 3213 (s), 1679 (s), 1598 (w), 1578 (m), 1538 (s, nN-O asym), 1463 J 2.3), 8.42 (dd, 1 H, J1 8.6, J2 2.3), 8.11 (d, 1 H, J 8.6), 6.92 (s, Nujol), 1378 (s, Nujol, overlapping with nN-O sym), 1302 (s, 6 H).The X-ray crystal structure of this compound was (w), 1270 (w), 1208 (s), 1171 (m), 1146 (m), 1079 (m), 1042 (w), solved. 1025 (s), 857 (w-m), 776 (m), 743 (m), 733 (m), 662 (s), 614 (s); d 7.85 (~d, 1 H, 6-Ar-H), 7.63–7.55 (m, 3 H, 3,4,5-Ar-H), 6.94 Guanidinium 2,4-dinitrobenzenesulfonate monohydrate, {s, 6 H, [C(NH2)3]+}; d (D2O) d 8.02–8.00 (m, 1 H, 6-Ar-H), [C(NH2)3]+ 2,4-(NO2)2(C6H3)SO3- H2O, (G) (14) H2O 7.77–7.71 (m, 3 H, 3,4,5-Ar-H), 4.80 {s, ~10 H, [C(NH2 )3 ]+, also contains HDO peak}.The X-ray crystal structure of this This phase was crystallized from aqueous or 10% aqueous acetonitrile solutions containing equimolar quantities of compound was solved. 1132 J. Mater. Chem., 1997, 7(7), 1123–1133guanidine hydrochloride and 2,4-dinitrobenzenesulfonic acid (~d, 1 H, 3-Ar-H), 8.09 (dd, 1 H, 5-Ar-H), 7.47 (d, 1 H, J 8.4, 6-Ar-H), 6.94 {s, 6 H, [C(NH2)3]+}, 2.66 (s, 3 H, Ar-CH3). (Eastman) as light tan parallelograms/plates: DSC endotherm 60–69 (br, determined to be loss of H2O by comparison of IR The authors gratefully acknowledge the National Science spectra), mp 173–175 °C.Note that the dehydration endotherm Foundation and the Oce of Naval Research for financial position may vary depending upon sample, but occurs at support, and Professor J. Doyle Britton and Dr Victor Young 50–90° and within a 10–15° range; n/cm-1 3587, 3494, 3452, for crystallographic services. 3404, 3365, 3265, 3203, 3107, 3099, 1675, 1636, 1605, 1574, 1547 (s, nN-O asym), 1536, 1465, 1378, 1364 (s, nN-O sym), 1312, 1239, 1227, 1154, 1138, 1119, 1067, 1032, 974, 920, 905, 859, References 837, 754, 745, 718, 641, 594; d 8.57 (d, 1 H, J 2.2), 8.42 (dd, 1 1 J.-M.Lehn, M. Mascal, A. DeCian and J. J. Fisher, J. Chem. Soc., H, J1 8.6, J2 2.3), 8.10 (d, 1 H, J 8.6), 6.92 (s, 6 H), 3.37 [s, ~3 Perkin T rans. 2, 1992, 461.H: 2 H from hydrated water of the crystal, ~1 H from H2O 2 E. Fan, L. Yang, S. J. Geib, T. C. Stoner, M. D. Hopkins and A. D. impurity in (CD3)2SO]. Crystals of (G)(12) H2O are stable Hamilton, J. Chem. Soc., Chem. Commun., 1995, 1251. for days to weeks after removal from solution, but eventually 3 J. C. MacDonald and G. M. Whitesides, Chem.Rev., 1994, 94, 2383. dehydrate, as suggested by a loss of crystal transparency and 4 J. A. Zerkowski, J. C. MacDonald, C. T. Seto, D. A. Wierda and confirmed by IR spectroscopy. The low temperature of dehy- G. M. Whitesides, J. Am. Chem. Soc., 1994, 116, 4305. dration characterized by DSC suggests that the water molecules 5 J.-M. Lehn, M. Mascal, A. DeCian and J. J. Fisher, J.Chem. Soc., are loosely bound in the lattice, as also suggested by the high Chem. Commun., 1990, 479. wavenumber nO-H IR band at 3587 cm-1. The X-ray crystal 6 X. Wang, M. Simard and J. D. Wuest, J. Am. Chem. Soc., 1994, structure of this compound was solved. No hydrogens were 116, 12119. 7 M. Simard, D. Su and J. D. Wuest, J. Am. Chem. Soc., 1991, 113, refined in the structure determination except one water proton, 4696.H10, which was identified on the dierence map and refined. 8 O. Ermer and L. A. Lindenberg, Helv. Chim. Acta, 1991, 74, 825. The other water proton could not be located on the dierence 9 O. Ermer, J. Am. Chem. Soc., 1988, 111, 3747. map and was left out. 10 M. J. Zaworotko, Chem. Soc. Rev., 1994, 283. 11 V. A. Russell, C. C. Evans, W.Li and M. D.Ward, Science, in press. 12 V. A. Russell and M. D. Ward, Chem.Mater., 1996, 8, 1654. Guanidinium picrylsulfonate (guanidinium 13 G. R. Desiraju, Angew. Chem., Int. Ed. Engl., 1995, 34, 2311. 2,4,6-trinitrobenzenesulfonate), [C(NH2)3]+ 14 V. A. Russell, M. C. Etter and M. D. Ward, J. Am. Chem. Soc., 2,4,6-(NO2)3(C6H2)SO3-, (G)(15) 1994, 116, 1941. 15 V. A. Russell, M.C. Etter and M. D. Ward, Chem. Mater., 1994, This phase was crystallized from a methanol–toluene solution 6, 1206. containing equimolar quantities of guanidine hydrochloride 16 V. A. Russell and M. D. Ward, Acta Crystallogr. Sect. B, 1996, and picrylsulfonic acid trihydrate (Kodak) as very high quality 52, 209. light-yellow, thick plates with well-developed faces: DSC 17 V. A. Russell and M.D. Ward, Proceedings of the NATO Advanced ResearchWorkshop on Modular Chemistry, September exotherms 230–233, 239–241 °C (decomp.); visual observation 9–12, Estes Park, Colorado, in press. of single crystals on a Fisher–Johns hot stage confirmed the 18 M. C. Etter, J. Phys. Chem., 1991, 95, 4601. decomposition: crystals begin to turn brown at 225 °C and 19 M. E. Thompson, Chem.Mater., 1994, 6, 1168.eventually turn black with bubbling occurring from 20 K. P. Reis, V. K. Joshi and M. E. Thompson, J. Catal., 1996, 236–240 °C; n/cm-1 3492 (m), 3456 (m-s), 3377 (m), 3290 (m), 161, 62. 21 G. Cao and T. E. Mallouk, J. Solid State Chem., 1991, 94, 59. 3213 (w), 3095 (w), 3083 (m), 1661 (s, sl br), 1609 (m), 1559 (s, 22 F. H. Allen, O. Kennard, D. G. Watson, L. Brammer, A. G. Orpen nN-O asym), 1546 (s,` nN-O asym), 1465 (s), 1378 (s, possibly nN-O and R. Taylor, J. Chem. Soc., Perkin T rans. 2, 1987, S1. sym), 1356 (s, nN-O sym), 1252 (s), 1233 (s), 1131 (m), 1073 (m), 23 G. R. Desiraju, Crystal Engineering: T he Design of Organic Solids, 1030 (m), 974 (w), 926 (w), 907 (w), 753 (m), 735 (m), 724 (m), Elsevier, New York, 1989, p. 92. 631 (m); d 8.85 (s, 2 H, Ar-H), 6.90 {s, 6H, [C(NH2)3]+}. The 24 D. J. A. De Ridder and H. Schenk, Acta Crystallogr. Sect. B, 1995, X-ray crystal structure of this compound was solved. 51, 221. 25 G. R. Desiraju, Acc. Chem. Res., 1996, 29, 441. 26 G. R. Desiraju, Acc. Chem. Res., 1991, 24, 290. Guanidinium 4-nitrotoluene-2-sulfonate, [C(NH2)3]+ 27 J. A. R. P. Sarma and G. R. Desiraju, Acc. Chem. Res., 1986, 4-(NO2)-2-(CH3 )-(C6H3)SO3-, (G)(16) 19, 222. 28 J. A. R. P. Sarma and G. R. Desiraju, J. Chem. Soc., Perkin T rans. This phase was crystallized from methanol or 10% aqueous 2, 1987, 1195. acetonitrile solutions containing equimolar quantities of guani- 29 Z. Berkovitch-Yellin and L. Leiserowitz, Acta Crystallogr. Sect. B, dine hydrochloride and 4-nitrotoluene-2-sulfonic acid dihy- 1984, 40, 159. 30 K. Wozniak, H. He, J. Klinowski, W. Jones and E. Grech, J. Phys. drate (Pfaltz and Bauer) or from aqueous or methanol solutions Chem., 1994, 98, 13755. containing 152 molar quantities of guanidine carbonate and 31 D. M. Salunke and M. Vijayan, Int. J. Peptide Protein Res., 1981, 4-nitrotoluene-2-sulfonic acid dihydrate as very fine colourless 18, 348. to light tan aggregates of needles. An opaque solid was also 32 Y. Yokomori and D. J. Hodgson, Int. J. Peptide Protein Res., 1988, isolated on the sides of the crystallization vessels, suggesting 31, 289. 33 V. A. Russell, PhD Thesis, University of Minnesota, 1995. that the solid may be a desolvated solvate form. The IR 34 A. Gavezzotti, in Structure correlation ed. H.-B. Burgi and J. D. spectrum of the opaque solid matched that of the colourless Dunitz, VCH, New York, 1994, vol. 2, ch. 12. The van der Waals needles. DSC mp 249 °C; n/cm-1 3469 (m), 3369 (s), 3340 (s, volumes have also been reported as 13.67 and 16.8 (for nitro sh), 3267 (m), 3193 (s), 1673 (m), 1585 (m), 1519 (s, nN-O asym), attached to carbon atom) cm3 mol-1 for methyl and nitro groups, 1465 (m), 1380 (m), 1355 (s, nN-O sym), 1308 (w), 1268 (w), 1229 respectively: A. Bondi, J. Phys. Chem., 1964, 68, 441. (s), 1198 (m), 1150 (m), 1079 (s), 1028 (s), 922 (w), 915 (w), 895 (w), 833 (w), 801 (w), 741 (m), 720 (m), 704 (m), 617 (s); d 8.51 Paper 7/00023E; Received 2nd January, 1997 J. Mater. Chem., 1997, 7(7), 1123–1133 1133

ISSN:0959-9428    

DOI:10.1039/a700023e

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Engineering of peptide b-sheet nanotapes† Amalia Aggeli, Mark Bell, Neville Boden,* Je N. Keen, Tom C. B. McLeish, Irina Nyrkova, Sheena E. Radford and Alexander Semenov Centre for Self OrganisingMolecular Systems, School of Chemistry, T he University of L eeds, L eeds, UK L S2 9JT A set of principles are outlined for the design of short oligopeptides which will self-assemble in appropriate solvents into long, semi-flexible, polymeric b-sheet nanotapes.Their validity is demonstrated by experimental studies of an 11-residue peptide (DN1) which forms nanotapes in water, and a 24-residue peptide (K24) which forms nanotapes in non-aqueous solvents such as methanol. Circular dichroism (CD) spectroscopy studies of the self-assembly behaviour in very dilute solutions (mM) reveal a simple transition from a random coil-to-b-sheet conformation in the case of DN1, but a more complex situation for K24.Association of DN1 is very weak up to a concentration of 40 mM at which there is a sudden increase in the fraction of peptide in the b-sheet structure, indicative of an apparent ‘critical tape concentration’. This is shown to arise from a two-step self-assembly process: the first step being a transition from a random coil to an extended b-strand conformation, and the second the addition of this b-strand to a growing b-sheet.Both peptides are shown to gel their solvents at concentrations above 2×10-3 volume fraction: these gels are stable up to the boiling point of the solvents. Rheology measurements on gels of the 24-residue peptide in 2-chloroethanol reveal that the tapes form an entangled network with a mesh size of 10–100 nm for peptide volume fractions 0.03–0.003; the persistence length of the tape is 13 nm or greater, indicative of a moderately rigid polymer; the tapes are about a single molecule in thickness.The mechanical properties of the gels in many respects are comparable to those of natural biopolymers such as gelatin, actin, amylose and agarose.Self-assembly of complementary molecular components the networks in the gels formed at higher concentrations is obtained by rheological measurements. through hydrogen bonding is emerging as a novel synthetic route to linear polymeric structures. Polymer tapes have been synthesised1–3 from nucleoside-like components (triaminopyri- Peptide Design midines/triazine barbiturates) and shown to form gels in certain organic solvents.4 These are essentially self-assembling ladder The objective is to design a peptide of minimal complexity, polymers.5 Biaxial polymers are of special interest, since they which will self-assemble into elongated, antiparallel b-sheet are expected to have quite dierent physics from classical, tapes in a particular solvent. This requires a large negative uniaxial linear polymers.6 Whilst the polymer will bend freely free energy change for the transformation of a monomeric in one plane, it will be relatively rigid in the perpendicular helical/random coil peptide from solution to the end of a direction.However, at long enough length scales, out of plane growing tape (Scheme 1).fluctuations are expected to lead to disc-like objects and, at It is believed that a peptide must have a minimum of six even longer length scales, to three-dimensional coils. residues to form stable b-sheet structures.8,9 The relative Consequently, solutions of tape-like polymers are predicted to stability of a b-sheet as compared to helix or random coil is form mesophases at high enough concentrations.Self-assemb- believed to stem, not from the dierences in hydrogen bonding ling tape-like polymers can be expected, therefore, to exhibit energies, but rather from the forces between the side-chains of far more complex behaviour. neighbouring amino acids and the solvation energies of these We have been exploring an alternative generic route to tape- side-chains.These interactions must be sucient to fully like polymers. This exploits the propensity of peptide chains compensate for the loss of translational and conformational to self-assemble via intermolecular or intramolecular hydrogen entropies of the peptide molecule as it is ‘frozen’ into the bonding into b-sheet structures. Such structures exist widely relatively rigid b-sheet organisation.It is also essential to as short b-sheets or barrels in proteins, and also as extended incorporate an element of molecular recognition into the side- b-sheets in silk.7 chain interactions. This is to ensure that the peptides arrange Here, we demonstrate that oligopeptides of minimal com- themselves into the requisite antiparallel tape-like structures plexity, can be designed to self-assemble in solution to form in preference to an interdigitated two-dimensional b-sheet.It long, semi-flexible, polymeric b-sheet tapes, a single molecule is also essential that the medium acts as a good solvent for in thickness. At volume fraction concentrations 0.001–0.005, the polymer tape. the tapes become entangled to form gels with viscoelastic These considerations lead us to the following working properties in some ways analogous to, but in other ways criteria for the rational design of peptides to produce b-sheet distinct from, those observed for gels of classical synthetic tapes in solution: (i ) cross-strand attractive forces (hydro- polymers. The shape and dimensions of the polymers are phobic, electrostatic, hydrogen-bonding) between side-chains, established by transmission electron microscopy, the confor- (ii) lateral recognition between adjacent b-strands to constrain mation of the peptides and their self-assembly (secondary and their self-assembly to one dimension, and avoid heterogeneous tertiary structures) are monitored in dilute solutions by circular aggregated b-sheet structures, and (iii ) strong adhesion of dichroism spectropolarimetry, and in semi-dilute solutions by solvent to the surface of the tapes to control solubility.FTIR spectroscopy, whilst information about the properties of The work presented in this paper focuses on the behaviour of two distinctly dierent peptides K24 and DN1, which have been designed to self-assemble into b-sheet tapes in, respectively, moderately polar (non-aqueous) and highly polar (aque- † A preliminary account of this work has been published in Nature, 1997, 386, 259.ous) media. J. Mater. Chem., 1997, 7(7), 1135–1145 1135Scheme 1 Schematic representation of the self-assembly of six-residue peptide molecules to form a growing intermolecular antiparallel b-sheet tape. Hydrogen bonding between backbones of adjacent peptide b-strands is indicated.The design of the 11-residue peptide DN1, CH3CO-Gln- Gln-Arg-Phe-Gln-Trp-Gln-Phe-Glu-Gln-Gln-NH2 [Fig. 1(b)], is not based on any native protein. Rather it has been rationally designed to form b-sheet polymer tapes in water. The (-CH2-)2 moieties of the six glutamine residues are expected to provide attractive intermolecular hydrophobic interactions between side-chains.The residues Phe4, Trp6 and Phe8 are also hydrophobic, but they are also expected to provide intermolecular recognition by p–p interactions.12,13 Arg3 and Glu9 provide an additional degree of recognition via their strong coulombic attraction,12 and favour an antiparallel alignment of the strands. Gln, Arg and Glu side-chains make one surface of the b-sheet more hydrophilic than the other.Chemically blocked termini were used to avoid edge-to-edge coulombic attractions between tapes. (a) (b) Fig. 1 Space-filling models of (a) K24 and (b) DN1 peptides in an extended b-strand conformation. Colour code: white=hydrogen, red= Materials and Experimental Methods oxygen, blue=nitrogen, yellow=sulfur, black=carbon.Peptide synthesis Standard automated solid phase methods were employed for The primary structure of the 24-residue peptide K24, NH2- Lys-Leu-Glu-Ala-Leu-Tyr-Val-Leu-Gly-Phe-Phe-Gly-Phe-Phe- the synthesis of the peptides. The synthesis of K24 was carried out as described in ref. 11 for K27. Solid-phase synthesis of Thr-Leu-Gly-Ile-Met-Leu-Ser-Tyr-Ile-Arg-COOH, [Fig. 1(a)] is related to the single transmembrane domain of the IsK peptide DN1 was also performed using Fmoc-chemistry (Fmoc: fluoren-9-ylmethoxycarbonyl). The peptide was protein.10 Its longer 27-residue version, K27, is known to readily form b-sheet structures in lipid bilayers,11 suggesting assembled on PEG–PS (polyethylene glycol–polystyrene) resin, incorporating a linker to generate the C-terminal amide upon these peptides would be good candidates for formation of bsheet tapes in amphiphilic solvents such as methanol and 2- cleavage of the peptide from the resin.Fmoc-amino acids were C-terminally activated using Hbtu [2-(1H-benzotriazol-l- chloroethanol. Furthermore, the amphiphilicity along the K24 molecule (polar side-chains predominantly near the peptide yl)-1,1,3,3-tetramethyluronium hexafluorophosphate] with DIPEA (diisopropylethylamine). The resin-bound peptide was termini, and apolar side-chains predominantly in the middle of the peptide chain) is expected to favour the alignment of N-terminally acetylated by reaction with 0.3 M acetic anhydride–0.03 M pyridine in DMF (dimethylformamide) the peptide b-strands with respect to each other, which will lead to the one-dimensional propagation of the b-sheet.Such (10 min at room temp.). Cleavage of the peptide from the resin and deprotection of amino acid side-chains were achieved by an intermolecular arrangement would thus allow the establishment of several kinds of favourable contacts along adjacent incubation for 1 h in TFA (trifluoroacetic acid), containing 1% (w/v) phenol, 2% (v/v) water, 4% (v/v) ethane-1,2-dithiol peptide b-strands: for example, intermolecular interactions between polar side-chains (such as Lys1, Glu3 and Arg24) and 2% (v/v) anisole.The peptide was precipitated with diethyl ether, centrifuged and washed a further 4 times with diethyl near the termini, between non-polar side-chains (such as Leu5, Val7, Leu8, Leu16, Ile18 and Leu20) in the central region of ether.The diethyl ether was evaporated and the peptide dissolved in water for purification by reversed-phase HPLC, the peptide chain, as well as specific interactions such as p–p interactions among the four aromatic phenylalanine rings at which was carried out using water–acetonitrile gradient in the presence of 0.1% TFA. Mass spectrometry has shown that the positions 10, 11, 13 and 14. 1136 J. Mater. Chem., 1997, 7(7), 1135–1145molecular masses are as expected (K24: m/z 2800 and DN1: m/z 1593). K24 was shown by 19F NMR and FTIR to contain ca. 4 mols of residual TFA per 1 mol peptide. Less TFA (ca. 2 mol TFA/mol peptide) is present in DN1, on the basis of FTIR. Peptides were stored in their lyophilised state. The gelation properties and self-assembly behaviour of 0.007 v/v K24 solutions as monitored by FTIR are independent of the method used to prepare the samples, i.e.starting from either a predominantly helical or a mixture of helical–b-sheet conformations in the original lyophilised sample. Experimental techniques FTIR spectroscopy. Spectra were averages of four scans, recorded with a resolution of 4 cm-1, at 20°C in an FTIR liquid cell equipped with CaF2 crystals and a 50 mm Teflon spacer, using a Perkin-Elmer 1760X FTIR spectrometer.After subtraction of the solvent spectrum, the component peaks of the peptide amide I band were obtained by second derivative analysis and peak-fitting of the absorption spectra, using a home-written program, consisting of iterative adjustment of the relative heights, widths and ratios of Lorenzian–Gaussian functions of the lineshapes of the individual components, to obtain the best fit between the spectrum calculated by the program and the experimental one.CD spectroscopy. Measurements were obtained with a Jasco J-715 spectropolarimeter using 1 mm and 1 cm quartz cuvettes at 20°C. Spectra were recorded with a step resolution of 1 nm, a scan speed of 50 nm min-1, a sensitivity of 50 millidegrees and a response time of 1 s. Each spectrum was the average of 4–10 scans.The peptide concentration in solution was determined by amino acid analysis and ninhydrin assay. Transmission electron microscopy (TEM). Fig. 2(a) was obtained using a 0.001 v/v (500 mM) K24 gel in methanol after dilution to 25 mM peptide concentration.The copper EM grids were of mesh size 300 (corresponding to grids with 300 bars per inch) and were coated with carbon films. The carbon films were glow-discharged in order to build static charges. A droplet of the peptide solution was deposited on a clean surface. Each grid was then deposited on top of the sample droplet for 1 min, so that the peptide network was adsorbed onto the surface of the carbon film.Excess sample was drained o the grid. The grid was then introduced on top of a droplet of uranyl acetate (UA) negative staining solution in water (4 g UA per 100 ml water) for 20 s and excess of the staining solution was drained o the grid. Fig. 2(c) was obtained with a starting solution of 0.001 v/v (500 mM) K24 in deionised water.The opalescent solution was centrifuged to remove the largest particles. A droplet of the supernatant was used to prepare a grid in the same way as above. The specimens were allowed to air-dry. Fig. 2(a) was obtained with a Phillips CM10 TEM at 100 kV accelerating voltage, set at 105 000±5% magnification, whilst Fig. 2(b) and (c) were obtained with a Jeol 100S TEM operating at 100 kV and set at 50000±5% magnification.The pictures were enlarged during printing to attain the total sample magni- fication which appears in the individual figure legends. Care was taken to obtain a photographic record of the specimens as Fig. 2 TEM micrographs of b-sheet structures of K24 stained with quickly as possible after sample insertion, in order to minimise uranyl acetate and adsorbed on carbon-coated, glow-discharged grids: artefacts which can be caused by long exposure of the sample (a) network of a 0.001 v/v (500 mM) K24 gel in methanol, following to high vacuum and the intense electron beam.twentyfold dilution; (b) network of a 0.001 v/v K24 gel in methanol, without prior dilution; (c) insoluble peptide in water. Total magnifi- cation: ×262 000.Rheology. A Rheometrics Dynamic Analyser II, with 25 mm diameter parallel plate geometry, was employed. The thickness of the material between the plates was between 0.5 and 0.8 mm. ethanol. Its high boiling point compared to the other gelfavouring solvents minimises the solvent evaporation rate and Gels were prepared 3 d prior to the measurements. The following precautions were taken to minimise the problem of solvent changes of peptide concentration. Secondly, the oven, which surrounds the sample, was kept closed during data collection.evaporation from the periphery of the sample between the two parallel plates. First, most gels were prepared in 2-chloro- Thirdly, two containers full of solvent were kept near the J. Mater. Chem., 1997, 7(7), 1135–1145 1137sample, inside the oven, to saturate the atmosphere with Characterisation of secondary structure solvent vapour.Furthermore, the air flow system (which is Infra-red spectroscopy. FTIR spectroscopy was used to probe used to control sample temperature) was turned o, and the the conformation of the peptide chains as well as their supra- experiments were carried out at ambient temperature.molecular organisation. Peak-fitting analysis of the amide I Temperature fluctuations of the sample were closely monitored. band of all K24 gel samples studied (which correspond to gels produced in more than 20 dierent solvents), reveals a single major component centred at 1624.5±0.5 cm-1 [Fig. 3(d) and (e), red spectra] with a half-height bandwidth Dn� equal to Results 17–19 cm-1.An amide II band centred at ca. 1530 cm-1 is General observations also observed (not shown). These features are characteristic of a stable homogeneous intermolecular b-sheet structure.14 When K24 is added to solvents such as ethanol, methanol or It was estimated from the integrated intensities of the 2-chloroethanol, at a peptide volume fraction of ca. 0.002 v/v corresponding IR components that ca. 90% of the peptide (ca. 3 mg ml-1), the dry peptide particles initially swell, and adopts b-she structure, such as those obtained in methanol, eventually coalesce to form a transparent, homogeneous gel. propanol–D2O (90510 v/v) and 2-chloroethanol, and the This procedure takes place over several hours depending on remainder adopts a mixture of random coil and helical confor- peptide concentration, solvent and temperature.The gels are mations, as inferred from the weak components at ca. 1645 stable for months at room temperature. They are optically and 1655 cm-1. isotropic, but become optically birefringent when sheared. Theoretical as well as experimental evidence has shown that These properties are indicative of the presence of a network of the ratio of the integrated intensities of the weak and strong long polymer molecules in solution.IR components at ca. 1696 and 1625 cm-1 respectively, is The apparent viscosity of the K24 gel sample is dependent equal to 0.09–0.19, if the b-sheet consists of 100% antiparallel on the peptide concentration. For example, a 0.0011 v/v K24 strands.14 Peak-fitting analysis of spectra obtained with K24 solution in methanol (1.5 mg ml-1) is fluid, whilst a 0.0074 v/v gels revealed that this ratio is between 0.075 and 0.11, which peptide solution (10 mg ml-1) is a solid-like transparent gel.is indicative of predominantly antiparallel b-sheets. The anti- Gels with peptide concentration below 0.02 v/v peptide in parallel arrangement of peptide strands allows the establish- methanol are transparent, whilst in more concentrated solu- ment of stronger interpeptidic hydrogen bonds, leading to the tions, particles of insoluble peptide aggregates are dispersed in formation of more stable b-sheets than the alternative parallel the gel.In another experiment, a 0.007 v/v K24 rigid gel in arrangement.7 Other factors, such as specific complementary methanol was diluted tenfold: after gentle shaking for a few interactions between oppositely charged groups on the N- and seconds to ensure homogeneity, a fluid solution was obtained. C-termini of adjacent peptides, could further contribute to the This reversibility of gelation is a characteristic signature of an stabilisation of the antiparallel b-sheet.entangled polymer network. Furthermore, K24 gels in 2- Polarised IR spectra of K24 gels spread on a CaF2 plate chloroethanol were found to be stable when incubated for 2 h showed that the CNO stretching vibration of the peptide in a thermostatted water bath at various temperatures from backbone at 1625 cm-1 is 13% more intense in the direction 40 to 90°C (which is close to the boiling point of the solvent).parallel to the direction of shear as compared to the direction Similarly, the gels of DN1 in water were found to be stable up perpendicular to the shear. Assuming that the long axes of the to the boiling point of water. polymers are partially orientated on the direction of shear, this result suggests that the long axis of the peptide b-strand is perpendicular to the long axis of the b-sheet polymer.Transmission electron microscopy studies A typical micrograph, obtained with a 0.001 v/v K24 gel in methanol after twentyfold dilution, is shown in Fig. 2(a). A network consisting of long polymers randomly distributed on the grid can be seen. Use of a higher peptide concentration results in higher density of polymers on the grid [Fig. 2(b)].In some areas, the polymers form complex entanglements or bundles. In a few cases, they are seen to bend and twist around each other. In areas where their density is lower, individual polymers with straight or wavy edges can be observed. Regions where individual polymers can clearly be observed were chosen to measure polymer dimensions. More than twenty measurements were made from micrographs of several K24 gel samples.The width was measured to be between 6.6 and 8.1 nm. Overlap of polymers makes it dicult to identify their ends, and to obtain an estimate of their length. Pieces of polymer between points where they meet/cross each other were found to be as long as 120 nm. K24 does not form a gel in water. Rather, it precipitates out of solution and forms fine, white, solid particles.Such particles were observed with TEM, in order to compare them with the structures in the gel. The micrograph in Fig. 2(c) is typical of such a sample. Individual polymers, scattered on the grid are observed. Their width has values between 7.6 and 8.5 nm, and Fig. 3 FTIR amide I bands of 0.004 v/v (2 mM) K24 solutions in: their length is between 38 and 76 nm, i.e.they are shorter than (a) HFIP, (b) 753 HFIP–methanol, (c) 151 HFIP–methanol, (d) meth- the ones in gel forming solvents. Moreover, they do not appear anol, (e) 83517 propanol–D2O, ( f) 154 propanol–D2O and (g) propa- to interact with each other to form bundles or networks as in nol. Colour code: red=gel, green=fluid solution, black=insoluble the case of the gel structure.Rather, a large number of polymers peptide. The band at ca. 1675 cm-1 is due to residual TFA, and it is particularly intense in polar solvents. aggregate to form numerous thick amorphous structures. 1138 J. Mater. Chem., 1997, 7(7), 1135–1145Fig. 4 Peak-fitted FT-IR amide I¾ band of 0.003 v/v (5 mg ml-1 ) DN1 in D2O. The outer black lines are the experimental and fitted spectra, respectively. Component peaks are shown in dierent colours.Assignment of component peaks (ref. 35) from right to left: (i) 1604 cm-1=arginine side chain; (ii) 1618.6 cm-1 (maximum of amide I¾)=b-sheet; (iii) 1633.9 cm-1=glutamine side-chain; (iv) 1645.5 cm-1=residual H2O; (v) 1673.5 cm-1=residual TFA; (vi) 1683.3 cm-1=antiparallel b-sheet.The half-height width of the major b-sheet component at 1618.5 cm-1 is 19.5 cm-1, whilst that of the weak peak indicating antiparallel b-sheet is 10 cm-1. In the case of DN1, a self-supporting, thermostable gel (up to at least 90°C) is produced at peptide concentrations above 0.01 v/v (ca. 15mgml-1) in water. DN1 adopts a b-strand configuration in 0.003 v/v solutions or above in D2O, as revealed by its IR spectrum (Fig. 4). The major component of Fig. 5 (a) Far-UV circular dichroism spectra of K24 in methanol as a function of peptide concentration: [h] is the mean residue molar the amide I¾ band is centred at 1619 cm-1 and has half-height ellipticity; (b) plot of [h] at 216 nm as a function of peptide bandwidth of 20 cm-1. This main peak is shifted 5 cm-1 to concentration lower wavenumbers compared to that on the spectrum of K24, due to the exchange of peptide amide protons by deuterium in the D2O solvent. A weak peak at ca. 1683 cm-1 is indicative sity of the positive band at 195 nm is indicative of the presence of a significant amount of antiparallel b-strand arrangements of a strong negative band in the region of 197 nm, characteristic in the b-sheet.of a disordered conformation. The positive band at 195 nm The absence of amide I bands (typically at 1660–1680 cm-1) and the negative band at 216 nm, which increase in intensity corresponding to turns for both peptides indicates that they with peptide concentration, are indicative of a transition to a do not adopt a b-hairpin structure but rather a straight b- b-sheet conformation at higher concentrations.The change in strand configuration. The simplest arrangement of the peptide, the shape of the spectra at ca. 10mM, but which nonetheless therefore, is an antiparallel arrangement of b-strands aligned have a minimum at 216 nm, characteristic of a b-sheet struc- perpendicular to the tape long axis, so that they grow in one ture, is likely to arise from a change in the stacking of aromatic dimension to form tapes (Scheme 1).The polarised IR results side-chains responsible for the negative band at 203 nm. The support such an arrangement. Given that the average separa- plot of the mean residue molar ellipticity in Fig. 5(b) is tion between adjacent residues in a b-strand is 0.335 nm, the indicative of the progression of the transition with concen- length of a 24 residue b-strand is 7.7 nm.This length falls into tration, but due to complexities in the nature of the spectra it the range of measured values for the width of the polymers by is dicult to relate this quantity to the absolute fraction of TEM (6.6–8.1 nm). This observation supports further the peptide in the b-sheet state. presence of tape-like polymers, consisting of extended peptide strands with the strand axis being normal to the polymer’s DN1.Similarly, experiments have been carried out on a long axis, whilst the direction of hydrogen bonding is parallel series of solutions of DN1 in water with concentrations ranging to the polymer axis. In this way, K24 peptides could form from 10 mM (15.94 mg ml-1) to 450 mM (717.08 mg ml-1) pre- tape-like structures, whose width corresponds to the length of pared by dilution of a stock solution of known concentration.a K24 b-strand and thickness to that of a single b-sheet The stock solution was birefringent after storage for 2 d. (see below). Fig. 6(a) shows the far-UV CD spectra of DN1 in water as a function of peptide concentration. At peptide concentrations Self-assembly in dilute solutions up to 40 mM, the CD spectra exhibit a distinct minimum at 200 nm.This is at a higher wavelength than the value of K24. A series of solutions of K24 in methanol were prepared with concentrations ranging from 1–20 mM by taking known 197 nm typically found with random coil structures, and is believed to stem from the presence of tryptophan and phenyl- amounts of a 2.5 mM stock solution of peptide in HFIP, evaporating to dryness, and dissolution of the resulting peptide alanine side chains which absorb strongly in this region.30 At higher peptide concentrations, a positive band develops in the film in the required amount of methanol.The far-UV CD spectra are shown in Fig. 5(a) as a function of peptide concen- place of the negative band at 200 nm, and a new minimum develops at ca. 224 nm which is attributed to b-sheet structures. tration. At peptide concentrations up to 5.2 mM, the spectra exhibit a distinctive isodichroic point at 198 nm indicative of Again this peak is at a higher wavelength than the typical value for b-sheet proteins of 217 nm, presumably due to CD a two-state conformational transition.The variation in inten- J. Mater. Chem., 1997, 7(7), 1135–1145 1139where df and elink are defined above, nlink is the eective volume of a peptide–peptide bond (it characterises how freely a bstrand can move around its neighbour), and n0 is the intrinsic volume per strand. In order that tapes are stable at some concentration, elink<df<0. The minimisation of the free energy [eqn.(2)] under condition eqn. (1) gives the formulae (3) and (4) for the fractional composition, N1 nlink V =EL , E�eelink-df (3) Nm nlink V =ALm, A�eelink (m2), (4) with ln L being the Lagrange multiplier (0<L<1). The latter can be found using the condition eqn. (1): it is determined indirectly by the total concentration of peptides, eqn. (5). N nlink V =EL+AL2C 2-L (1-L )2D (5) Finally, the fraction of peptides in tapes (with m2), i.e.the b-sheet fraction, is given by eqn. (6), wtape� 1 N .2 m=2 mNm=1-G1+A E L (2-L ) (1-L )2 H-1 (6) with L being determined by the total concentration as in eqn. (5). Thus, one can use eqns. (5) and (6) to fit the wtape versus peptide concentration data in Fig. 6(b) using coecients A and E as fitting parameters.In doing this we assumed that the Fig. 6 (a) Far-UV circular dichroism spectra of DN1 in H2O as a measured [h] are a linear function of wtape. Note that the fitted function of peptide concentration; (b) plot of fraction of peptide in b- coecients crucially depend on the value chosen for the link sheet states (wtape) as a function of peptide concentration. The continu- volume nlink: A and E being proportional to nlink.The best fit ous line represents the fit of the experimental data to eqns. (5) and (6) which is represented by the continuous line in Fig. 6(b) was using A/nlink ca. 1.125 mM and E/nlink ca. 45 mM. achieved for: A/nlink ca. 1.125 mM; E/nlink ca. 45 mM. The estimations for the energies entering eqn. (3) and (4) signals from the aromatic side chains in the same region.The depend on the choice of nlink. For nlink=1 A° 3, we obtain A ca. isodichroic point at 210 nm is indicative of a simple two state 6.75×10-10 and E ca. 2.7×10-8, and hence: elink random coil<b-sheet equilibrium. The mean residue molar ca. -21.1; elink-df#-17.4; df#-3.7 (in units of kBT ), ellipticity at 224 nm has been taken as a measure of the whilst for nlink=0.5 A° 3, we obtain A#8.4×10-11 and fractional concentration of peptide in the b-sheet state.E#3.4×10-9, therefore: Fig. 6(b) shows a slow growth of the fraction of peptide in b-sheet structures up to a concentration of 40 mM, at which it elink ca. -23.2 elink-df ca. -19.5 df ca. -3.7 (in units increases suddenly. This behaviour is more complex than of kBT ). expected for a simple one-dimensional association of peptides.Thus, the values of elink and df are only weakly dependent on In fact, the sudden jump in the fraction of peptide in b-sheet the choice of nlink. In fact, df is necessarily insensitive to it. structures is indicative of a critical concentration in the aggregation process, reminiscent of the aggregation of surfactants into sphere-like micelles in aqueous solution.It arises here Properties of semi-dilute solutions from the fact that before a peptide monomer can add to a Gelation behaviour. The conformational state and gelation growing tape (see Scheme 1), it must first undergo a transition properties of both K24 and DN1 peptides have been explored from, in this case, a random coil to an extended b-strand in a variety of solvents.Our observations for K24 are rep- conformation. The change in free energy involved df (the resented in Fig. 7, as a plot of the solvent polarity, er, as a dierence between the free energies of a coil and a b-strand), function of its hydrogen-bonding ability, a.15 though significantly smaller than the free energy change, elink, Thermostable, self-supporting, mostly transparent gels are on breaking a peptide–peptide bond in a tape, suppresses found to be stable in solutions of K24 in moderately polar aggregation at low peptide concentrations.To demonstrate solvents, such as methanol, with er in the range 25–68, a<1.5, that this is the case and to obtain values for df and elink we and peptide concentrations equal to or higher than 0.002–0.004 have developed and applied an appropriate theoretical model.v/v (region of phase diagram with red dots). For example, We consider a solution at equilibrium containing tapes 100% propanol (er=20.1) produces an insoluble peptide pre- having a distribution of sizes such that there are Nm tapes of cipitate; mixtures of propanol–D2O, with volume fraction of aggregation number m.If the total number of peptide molecules D2O 0.1–0.7 and er 26–62, produce gels whose apparent in a given volume V of solution is N, then eqn. (1) holds. rigidity decreases in mixtures of high volume fraction of D2O; .2 i=1 iNi=N (1) and mixtures of propanol–D2O, with volume fraction of D2O 0.8–1 and er 71–80, produce again progressively more insoluble The total free energy of the solution is given by eqn.(2), peptide solutions in parallel to increased D2O content. Studies in methanol–water mixtures as well as in a wide range of other mixed solvents and pure solvents also gave similar results. F=N1(df)+ .2 m=2 Nm(m-1)Celink-lnAnlink n0 BD+.2 i=1 Ni lnANin0 Ve B In less polar solvents (15<er<25) than the gel-favouring ones, the peptide is not suciently soluble to form gels (black (2) 1140 J.Mater. Chem., 1997, 7(7), 1135–1145of peptide solution in HFIP; maximum and half-height bandwidth of principal component of amide I are 1655 cm-1 and 30 cm-1, respectively, maximum of amide II at 1545 cm-1], and by CD (minima of negative ellipticity at 208 and 222 nm). A dramatic decrease in the content of the b-sheet is eected by increase of the a-value (and hence of the hydrogen-bond donor strength) of the solvent from methanol to HFIP (Fig. 8).The peptide is also completely soluble in such solvents as benzyl alcohol, o-nitrotoluene, 1,2-dichlorobenzene and benzene (greenes with numbers 27, 26, 25 and 23, respectively, in Fig. 7). The solubilising eect of these solvents is comparable to that of solvents with high a-value.It seems that aromatic groups in the solvent can interact favourably with the six aromatic rings on the peptide and thus compete with peptide– peptide interactions, causing some destabilisation of the bsheet structure (for example only 60% of the peptide is in b-sheet structure in benzyl alcohol). Control of the self-assembly process of the b-sheet polymers, Fig. 7 A correlation plot between the macroscopic properties of 0.004 as well as of the interactions between polymers, provides ways v/v (2 mM) K24 solutions in various solvents, the relative permittivity of tuning the mechanical properties of the peptide gel. One er of the solvent and its ability to act as hydrogen bond donor, a. The way that the gel-to-fluid transition can be brought about is by plot was constructed from observations of the IR spectra and mechan- varying the hydrogen bond donor strength (a) of the solvent.ical properties of the samples. The dotted lines show the boundaries This can be achieved, for example, in the case of the K24 of the gel region. Each dot represents a solvent with the corresponding er and a values. Red=gel, green=transparent fluid solution, black= methanol system quite simply by adding HFIP.The CD insoluble peptide. The a and er values of mixed solvents were estimated spectra in Fig. 9 show the sequential changes in the confor- assuming linear relationship between the two component solvents. The mation brought about by 10% increments of HFIP: spectra number next to each dot refers to a particular solvent: (1) dimethylfor- (i) (10%) to spectra (x) (100%) HFIP. Fig. 9(a) shows the mamide (DMF), (2) 753 DMF–formamide, (3) 753 DMF–methanol, conversion from the b-sheet (characteristic minima at 216 nm) (4) 357 DMF–methanol, (5) 951 propanol–formamide, (6) 151 propa- to the a-helix (characteristic minima at 208 and 222 nm) nol–formamide, (7) 951 propanol–water, (8) 83517 propanol–water, (9) 451 propanol–water, (10) 753 propanol–water, (11) 352 propanol– conformation [curves (i) to (vii)].The isodichroic point at water, (12) 151 propanol–water, (13) 253 propanol–water, (14) 357 202 nm is confirmation of the simple two state nature of this propanol–water, (15) methanol, (16) 951 methanol–HFIP, (17) 451 transition. The accompanying change in secondary structure methanol–HFIP, (18) 753 methanol–HFIP, (19) ethanol, (20) 2- is illustrated in Fig. 10. Fig. 9(b) shows a second isodichroic chloroethanol, (21) glycerol, (22) ethylene glycol, (23) benzene, point associated with a subsequent helix-to-random coil trans- (24) toluene, (25) 1,2-dichlorobenzene, (26) ortho-nitrotoluene, ition [curves (viii), (ix) and (x)]. (27) benzyl alcohol, (28) 151 HFIP–CH2Cl2 , (29) 159 methanol–HFIP, (30) HFIP, (31) formamide, (32) 357 DMF–formamide, (33) water, DN1 shows similar behaviour to K24, apart from the fact (34) 159 propanol–water, (35) 154 propanol–water, (36) 154 methanol– that its gel region is shifted upwards towards regions of higher water, (37) 151 HFIP–water, (38) 151 TFE–water, (39) 151 polarity (higher er).For example, it produces a self-supporting, methanol–HFIP, (40)357 methanol–HFIP, (41) TFE, (42) acetonitrile, thermostable gel, up to at least 90°C, and at concentrations (43) acrylonitrile, (44) acetone, (45) THF, (46) diethyl ether, of 0.009 v/v (ca. 5 mg ml-1) or above in water, by self- (47) hexane, (48) cyclohexane, (49) cyclohexene, (50) dichloromethane, assembling into b-sheet polymers.In solvents with er equal to (51) chloroform, (52) 151 methanol–CH2Cl2, (53) propanol, (54) 151 TFE–CH2Cl2, (55) hexanol, (56) butanol. or less than ca. 33, or in the very polar solvent formamide (er=109), the peptide is precipitated from solution. Similarly to the behaviour of K24, in solvents with high a-value such as circles near the bottom of Fig. 7).The fraction of b-structure in these solvents is still as high as in the gel-favouring solvents, i.e. equal to ca. 0.7–0.9. For example, the peptide which is insoluble in pure propanol (er=20.1), has an IR amide I band in this solvent almost identical to the one obtained with its gels [Fig. 3(g), estimated percentage of peptide in b-structure: 90%]. A similar situation pertains in solvents with er>68 (black circles near the top of Fig. 7). For example, the amide I band in the FTIR spectrum of a peptide solution in 20% v/v propanol–80% v/v D2O is shown in Fig. 3( f ). The amide I¾ band is centred at 1620 cm-1 and the estimated percentage of peptide in b-sheet conformation is equal to 78%. Consequently, gels are obtained in such solvents which prevent stacking and precipitation of the b-sheets.The average polarity of a K24 molecule, which was estimated to correspond to er ca. 26, may be a major contributor in determining the optimal solvent polarity required for solvation of tapes and gelation. In solvents such as 1,1,1,3,3,3-hexafluoroisopropyl alcohol (HFIP) or solvent mixtures with a1.5, clear, low-viscosity solutions are obtained (green circles in Fig. 7), even at peptide Fig. 8 Plot of the fraction of K24 peptide in helical conformation in concentration higher than 0.02 v/v. These simple Newtonian ca. 0.002 v/v (1 mM) peptide solutions, as a function of the hydrogen fluid solutions contain peptide molecules in the monomeric bonding strength (a) of the solvent. The fraction of helical conformation state. This is manifested by the absence of b-sheet and the was estimated by calculating the ratios of the intensities of the FT-IR presence of mixtures of helical and random coil peptide peaks at 1655 cm-1 (characteristic of helices) and the sum of the intensities at 1655 and 1625 cm-1 (characteristic of b-sheet).conformations, as shown both by FTIR [Fig. 3(a): spectrum J. Mater. Chem., 1997, 7(7), 1135–1145 1141Fig. 11 Typical mechanical spectrum of a 0.02 v/v (9 mM) K24 gel in 2-chloroethanol at 24.8°C, obtained with small oscillatory shear in the linear viscoelastic region (c=1%) rheological measurements on solutions of K24 in methanol and 2-chloroethanol. A typical mechanical spectrum of a K24 gel in 2-chloroethanol (0.019 volume fraction) is shown in Fig. 11. This was measured using small-strain oscillatory shear experiments (strain: 1–10%).Gels with peptide volume fraction as low as 0.006 were also studied and found to behave in a similar way. The elastic modulus G¾ is an order of magnitude larger than the viscous modulus G indicative of an elastic rather than a viscous material. G¾ and G are seen to be very weakly dependent on frequency v of the oscillatory shear for frequen- Fig. 9 Circular dichroism spectra of a 27 mM solution of K24 in HFIP– cies 10-2–102 rad s-1, implying that the relaxation time t of methanol mixtures: (a) spectra (i) (10% HFIP) to (vii) (70% HFIP); the network is very long and not reflected in the measurements. (b) spectra (viii) (80% HFIP) to (x) (100% HFIP) By comparison, for networks of linear synthetic polymers, t is equal to the time trept required for a polymer strand to reptate out of its entanglement.The longer the polymer, the larger the number of entanglements it participates in, and the longer the network relaxation time. For self-assembling polymers, the relaxation time of the network is equal to the geometric mean of two separate terms, namely the relaxation time trept due to overcoming entanglements by reptation of the polymers, and the relaxation time tbreak due to the probability of a selfassembled polymer to break.For example, in the case of wormlike surfactant micelles,22 G¾ and G cross at ca. 1 rad s-1, indicative of much shorter relaxation times compared to the peptide gel network. The long relaxation time of the peptide network is also consistent with the observation that a 0.019 volume fraction K24 gel was resistant to flow for several hours, following inversion of the sample tube. The mechanical spectrum in Fig. 11 supports the presence of stable peptide tapes which are either long and entangled or which form stable chemical crosslinks with each other. Calculations based on the rheological data to be presented below favour the presence of tape entanglements rather than chemical cross-links. Using rubber-like elasticity theory,16,17 we have been able to extract information about the mesh size of the peptide network, as well as the persistence length and the thickness of Fig. 10 Relationship between the mean residue molar ellipticity [h] at the tapes. The magnitude of G¾N0 (plateau elastic modulus) in 208 nm of the CD spectra of K24 solutions in methanol, as a function the linear viscoelastic region (Fig. 11) is related to j, the of the volume fraction of HFIP in these solutions. The more negative average distance between two nearest entanglements in space the value of the ellipticity, the higher the fraction of peptide in helical [Fig. 12(a)]: eqn. (7), conformation. Peptide concentration: ca. 0.00006 v/v (ca. 81 mg ml-1). G¾N0=gNntkBT (7) where gN is a numerical factor not far from unity, nt is the HFIP, the peptide DN1 is fully soluble, and the fraction of peptide in b-sheet is low. density of network tapes (mol of tapes per cm3), kB is the Boltzmann’s constant, and T the absolute temperature. j is in eect an upper limit to the mesh size of the network. As there Mechanical properties.To gain further insight into the structure and the dynamics of the gels, we have carried out is only one entanglement in volume j3, thus the density ne of 1142 J. Mater. Chem., 1997, 7(7), 1135–1145on the same tape is equal to or larger than j [Fig. 12(a)]. The polymer segment between two nearest crosslinks can be represented as a Kuhn chain consisting of a series of Ne segments each having a persistence length l.As a response to shear, the polymer segments between crosslinks straighten out. The strain at which the tapes are fully extended corresponds to cyield. Further increase in strain causes the tapes to break and the elastic stress to relax. When the applied strain is cyield, then the chain is fully uncoiled and the distance between the same entanglement points is L e=Nel.By definition, the shear strain Fig. 12 (a) Representation of a network tape, defined as the segment c is equal to the ratio of the length under stress and the length of the random walk tape between nearest crosslink points on the same at equilibrium, eqn. (11). tape. The crosses are individual entanglement points.For simplicity, no other network strands are shown. (b) Entanglement point of a network of polymer chains. Four network chains are involved with it, cyield# L e Re # Nel Ne1/2l #Ne1/2 (11) thus f=4. Ne was found to be 5.3±2.0 for a 0.019 volume fraction gel entanglements is given by eqn. (8) and approximately three times bigger for a 0.006 volume fraction peptide gel. From Fig. 12(a), we see that Rej. Thus, ne=1 j3 (8) since Re#Ne1/2l, we can calculate an upper limit to the persistence length of the tapes using eqn. (12). ne is related to nt by eqn. (9), l j Ne1/2 (12) nt=f2 ne= f 2j3 (9) This procedure yields l(12.7±3) nm, indicative of a moderwhere f is the number of tapes attributed to each entanglement ately rigid polymer, consistent with the intrinsic rigidity of b- [Fig. 12(b)]. sheet structures. Eqn. (7), (8) and (9) can be combined to yield eqn. (10). Since, the density of network tapes nt is related to their individual volumes Vt via nt=wp/Vt, where wp is the volume fraction of polymer in solution and Vt=L e wt=Nelwt, we can j=AgN fkT 2G3N0B1/3 (10) get from eqn. (7) an expression of the thickness of a network tape eqn. (13). Assuming that f is equal to or greater than 4, and using the magnitudes of G3N0 derived from the mechanical spectra, t= wpgNkT G3N0Nelw (13) we estimate the lower limit of j to be in the range (29–43)±13% nm, for gels with peptide volume fraction in the range of ca. 0.019–0.006, i.e. the mesh size decreases as the Using the following values for a 0.019 K24 volume fraction peptide volume fraction increases.gel: wp=0.019±0.001, gN=(1±0.2), G3N0=(328.8±111) Pa, The stress growth versus strain curve in Fig. 13 was obtained Ne=5.3±2.1, l(12.7±3) nm, T=297.8 K, and w= at a constant shear rate cÿ of 100% s-1, with a 0.019 K24 (7.35±0.70) nm, we deduce an upper limit of 0.7 nm for the volume fraction gel in 2-chloroethanol. The linear region thickness t of a tape. This value is consistent with tapes a implies that stress is proportional to strain up to a strain cyield single molecule thick (0.5<tb-sheet1.2 nm).This result is also of (230±45)%. Similar experiments carried out with a 0.006 in agreement with the notion of an entangled network of tapes. K24 volume fraction gel gave cyield ca. 415% independent of the shear rate cÿ, 50–100% s-1. These observations as well as Discussion the long network relaxation time evident in the mechanical spectra show that there is no significant network relaxation Structure and properties of tapes and networks going on during these measurements due to polymer The FTIR and CD results for K24 indicate that this peptide reptation or self-assembly.adopts an extended b-strand conformation in a pleated b-sheet At equilibrium, the distance Re between nearest crosslinks supramolecular aggregate.The electron micrographs reveal that the aggregates are elongated polymers rather than extended two-dimensional b-sheets. The analysis of the rheological measurements is also consistent with these observations and suggests that the aggregates are nanotapes a single molecule in thickness, i.e.thickness equal to 0.5–1 nm depending on the packing of the side-chains (Fig. 14). The estimated length of a 24 residue b-strand is 7.7 nm which falls into the range of the tape width (6.6–8.1 nm) measured by TEM. The rheological measurements have also provided an estimate of a lower limit of 13 nm for the persistence length of the tape, which is indicative of a moderately rigid polymer, consistent with the intrinsic rigidity of b-sheet structures.The tapes in the gels are of the order of microns in length. Whilst an extensive study of DN1 has yet to be made, the IR, CD and gelation studies suggest that it has similar structure and properties to K24. The initial concentration for gelation is of the order 0.002 volume fraction (1 mM K24). At lower concentrations, the Fig. 13 Stress–strain curve obtained with steady shearing of a 0.02 v/v polymer tapes must exist individually. The mesh size of the (9 mM) K24 gel in 2-chloroethanol at 24.8°C, at constant shear rate cÿ=100% s-1 entangled network in the gels decreases with increasing concen- J. Mater. Chem., 1997, 7(7), 1135–1145 1143gel than in the other biopolymer gels.Thus, our peptide gels are less brittle, and stronger than classical biopolymer gels. The thermal stability of biopolymer gels is shown in Table 1. The peptide gel is more stable at high temperature than most biopolymer gels, apart from alginate polysaccharide gel, which also possesses high thermal stability. This thermal stability of the peptide gels is indicative of a network of strong polymer chains.We note that other linear biological peptides, such as leucinerich (LRR) fragments of the drosophila Toll protein,23 a 28- Fig. 14 Schematic representation of an entangled network of K24 btapes in a gel, and a single self-assembled peptide nanotape. Each residue fragment of the b-amyloid protein24 and peptides vertical line of the tape represents the long axis of a peptide in a b- modelled on conserved domains of desmin,25 as well as syn- strand conformation (length of the b-strand=7.7 nm).The mesh thetic peptides incorporating non-natural chemical groups26–28 dimensions of the network correspond to gels with peptide volume have been reported to self-assemble into b-sheet polymers and fractions from 0.03 to 0.003. gel solvents. However, our aim has been to demonstrate the potential for production of b-sheet nanotapes by peptide design.tration: 10–100 nm for K24 gels with volume fraction It is also interesting that the peptide tapes show remarkable 0.03–0.003 (Fig. 14). similarity in structural terms, to protein fibrils formed in vitro The thermostability of the tapes is remarkable and presum- and in vivo from several dierent proteins includingthe polyglu- ably stems from the extensive cross-strand side chain–side tamine-containing proteins responsible for Huntington’s dis- chain interactions and the extensive network of intermolecular ease29 and the more complex structure of amyloid fibres.31 For backbone hydrogen bonds.This property, combined with the example, amyloid fibrils are rigid, non-branching structures, intrinsic chemical stability of the peptide bond, suggests that 10–15 nm in diameter, each consisting of two to five filaments these new polymers are quite robust.with cross b-structure, arranged in a twisted ribbon pattern. Thus, the peptide tapes may oer a simple model system for Comparison with other biopolymers studies of the mechanism of fibrillogenesis, which is of crucial The peptide gels (particularly the aqueous ones) in general importance in many physiological and pathological processes should be biodegradable and biocompatible. In this respect in biology.they can be compared with natural biopolymer gels18 such as gelatin, actin, amylose and agarose. The elastic (G¾) and dissi- Design and self-assembly behaviour pative (G) moduli of K24 gels are also quite similar with the moduli of most of these biopolymer gels (Table 1).Self-assembly mechanism. The results for the self-assembly behaviour of K24, depicted in Fig. 5(b), are of only qualitative The mechanical spectrum of the peptide gel in Fig. 11 is flat over the frequency range 10-1–102 rad s-1. Thus G¾ and G significance due to the complexity of the interpretation of the far-UV CD spectra [Fig. 5(a)]. However, the behaviour of are insensitive to the shear rate v, which indicates that the dominant viscoelastic relaxations of the network are at lower DN1 whose CD spectra [Fig. 6(a)] are interpreted in terms of a simple transition from a random coil to a b-sheet structure frequencies than we have measured (i.e. the relaxation time t of the network is long).This is indicative of either very long are quite similar. They are interesting in that the behaviour is more complex than a simple one-dimensional self-assembly and stable polymers which are highly entangled, or of strong, non-covalent chemical crosslinks between highly stable poly- process. This contrasts with the random coil to b-sheet transition of the Alzheimer’s b-amyloid fragment which can be mers.The latter mechanism is responsible for the shear rate insensitivity of G¾ and G of gelatin and agarose gels (Table 1). described by a simple non-cooperative one-dimensional aggregation model.32 From Fig. 6(b) we see that there is a simple b-Tape stacking would be an obvious crosslinking mechanism of the peptide polymers.However, the rheology measurements linear association of DN1 up to a concentration of 40 mM, above which there is a sudden increase in the fraction of show that the tapes are only one molecule thick, which implies that the best candidates for the crosslink points of the peptide peptide present in b-sheets, suggesting the requirement of a ‘critical tape concentration’. The behaviour is reminiscent of network are topological entanglements of the self-assembled tapes (Fig. 14). the aggregation of surfactants into spherical micelles in aqueous solution.We have shown that it has its origin in the requirement The stress response to strain for a 0.02 v/v peptide gel remains linear up to 230% strain (Fig. 13), which is much that before a peptide monomer can add to a growing tape it must first undergo a transition from, in this case, a random higher than for conventional biopolymer gels which typically break at strains of 50% (Table 1).The dierences in cyield coil to an extended b-strand conformation. The change in free energy df for this conversion, though significantly smaller than values indicate that the polymer segments between crosslinks are more flexible (i.e.contain more Kuhn segments) in the K24 the free energy change elink on breaking a peptide–peptide Table 1 Comparison of K24 with other aqueous polymer gels G¾ plateau/ temperature gel G¾/Pa G/Pa rad s-1 cyield/% stability/°C 2.5%w K24 in 2-chloroethanol 330 30 10-1–102 230 >90 2.2%w gelatin in water 40 — 10-2–102 <50 35 0.7%w pectin (polysaccharide) in water 100 ca. 2 10-3–10 <50 2%w amylose in water 700 200 1%w agarose (marine polysaccharide) in water 3500 300 10-2–102 %50 ca. 80 Alginate (marine polysaccharide) in water >100 Carageenan (marine polysaccharide) in water 20–50 2%w xanthane (microbial polysaccharide) in water %50 Worm-like micellar aqueous gel 1000 1–102 Entangled synthetic polymer network 100 1144 J. Mater. Chem., 1997, 7(7), 1135–1145bond in a b-sheet tape, is quite significant and destabilises References small aggregates. 1 J.-M. Lehn, M. Mascal, A. DeCian and J. Fischer, J. Chem. Soc., To observe the corresponding transition for K24 will necessi- Chem. Commun., 1990, 479. tate shifting the ‘critical tape concentration’ to higher concen- 2 J. A. Zerkowski, C. T. Seto, D. A. Wierda and G.M. Whitesides, trations so as to be detectable by CD spectroscopy. This could J. Am. Chem. Soc., 1990, 112, 9025. 3 J. A. Zerkowski and G. M. Whitesides, J. Am. Chem. Soc., 1994, be achieved by adding HFIP to the solution. This and other 116, 4298. experiments, aimed at controlling the pH and ionic strength 4 K. Hanabusa, T. Miki, Y. Tanaguchi, T. Koyama and H. Shirai, of the solution, are in progress.J. Chem. Soc., Chem. Commun., 1993, 1382. 5 G. J. Vroege and H. N. W. Lekkerkerker, Rep. Prog. Phys., 1992, 55, 1241. Peptide Design. The results from our studies of K24 and 6 I. A. Nyrkova, A. M. Semenov, J. F. Joanny and A. R. Khokhlov, DN1 suggest a working hypothesis for the design of peptides J. Phys. II, 1996, 6, 1411. which will assemble into b-sheet tapes in various solvents, and 7 T.E. Creighton, Proteins: Structures and molecular properties, we are currently involved in a programme of research to Freeman, New York, 1993, 2nd edn. rigorously test its validity. The results to date do however 8 M. T. Krejchi, E. D. T. Atkins, A. J. Waddon, M. J. Fournier, establish the viability of engineering nanotapes by peptide T.L. Mason and D. A. Tirell, Science, 1994, 265, 1427. 9 D. G. Osterman and E. T. Kaiser, J. Cell. Biochem., 1985, 29, 57. design. This opens up opportunities for producing materials 10 T. Takumi, H. Ohkubo and S. Nakanishi, Science, 1988, 242, 1042. with fascinating properties and applications. 11 A. Aggeli, N. Boden, Y. L. Cheng, J. B. C. Findlay, P. F. Knowles, The potential to vary the length of a peptide molecule (i.e.P. Kovatchev and P. J. H. Turnbull, Biochemistry, 1996, 35, 16 213. the tape width), the nature of its side-chains (natural and non- 12 K. C. Smith and L. Regan, Science, 1995, 270, 980. natural), and its environment, provides the means to control 13 G. D. Fasman, Prediction of Protein Structure and the Principles of the energetics and dynamics of the self-assembly process and Protein Conformation, Plenum Press, New York, 1989. 14 Yu. N. Chirgadze and N. A. Nevskaya, Biopolymers, 1976, 15, 607 consequently, the physical properties of the polymer solutions. and 627. There is also the unique opportunity to vary the structure on 15 M. J. Kamlet, J. L. M. Abboud, M. H. Abraham and R. W. Taft, opposing faces and edges of a tape.In this way tape–tape, J. Org. Chem., 1983, 48, 2877. tape–surface, and tape–ligand interactions can be controlled. 16 M. Doi and S. F. Edwards, T he T heory of Polymer Dynamics, These properties, together with the high temperature stability, Clarendon Press, Oxford, 1986. biocompatibility and biodegradability make these materials 17 J. D. Ferry, V iscoelastic Properties of Polymers, Wiley, New York, 1970.attractive for a wide range of applications. 18 A. H. Clark and S. B. Ross-Murphy, Adv. Polym. Sci., 1987, 83, 57. 19 H. McEvoy, S. B. Ross-Murphy and A. H. Clark, Polymer, 1985, Prospects for materials chemistry 26, 1483. 20 A. H. Clark, M. Watase, K. Nishinari and S. B. Ross-Murphy, It is interesting to compare the self-assembly of peptide nano- Macromolecules, 1989, 22, 346. tapes with other studies which have attempted to exploit the 21 S. B. Ross-Murphy and K. P. Shatwell, Biorheology, 1993, 30, 217. intrinsic self assembly behaviour of polypeptides as a route to 22 T. M. Clausen, P. K. Vinson, J. R. Minter, H. T. Davis, Y. Talmon and W. G. Miller, J. Phys. Chem., 1992, 96, 474. supramolecular materials. Tirell and co-workers8 have recently 23 D. A. Kirschner, H. Inouye, L. K. Duy, A. Sinclair, M. Lind and produced novel macromolecular solids by controlling the D. J. Selkoe, Proc. Natl. Acad. Sci. USA, 1987, 84, 6953. crystallisation of rationally designed polypeptides. In another 24 N. Geisler, T. Heimburg, J. Schuneman and K. Weber, J. Str. Biol., study, Ghadiri and co-workers33 have designed a ring-like 1993, 110, 205. peptide molecule, consisting of eight amino acid residues, 25 K. Hanabusa, Y. Naka, T. Koyama and H. Shirai, J. Chem. Soc., which self-assembles into long nanotubules. In another study, Chem. Commun., 1994, 2683. 26 H. T. Stock, N. J. Turner and R. McCague, J. Chem. Soc., Chem. Zhang and co-workers,34 have demonstrated that biologically Commun., 1995, 2063. derived linear oligopeptides can self-assemble into extensive b- 27 R. Vegners, I. Shestakova, I. Kalvinsh, R. M. Ezzell and sheets which condense into macroscopic membranes. Thus, a P. A. Janmey, J. Pept. Sci., 1995, 1, 371. peptide supramolecular materials chemistry appears to be 28 A. H. Clark and S. B. Ross-Murphy, Adv. Polym. Sci., 1987, 83, 57. emerging. 29 Yu. N. Chirgadze, B. V. Shestopalov and S. Yu. Venyaminov, Our work has as its focus the engineering of polymeric b- Biopolymers, 1973, 12, 1337. 30 S. Brahms and J. Brahms, J. Mol. Biol., 1980, 138, 149. sheet nanotapes. We believe it should be possible to design 31 C. Blake and L. Serpell, Structure, 1996, 4, 989. and synthesise peptides to generate an entire hierarchy of 32 E. Terzi, G. Ho�lzemann and J. Seelig, Biochemistry, 1994, 33, 1345. responsive self-assembled polymer architectures, including 33 M. R. Ghadiri, J. R. Granja, R. A. Milligan, D. E. McRee and homo- and hetero-tapes, dendrimers and cross-linked N. Kzazanovich, Nature, 1993, 366, 324. networks. 34 S. Zhang, T. Holmes, C. Lockshin and A. Rich, Proc. Natl. Acad. Sci. USA, 1993, 90, 3334. 35 W. K. Surewicz and H. H. Mantsch, Biochem. Biophys. Acta, 1988, This research was supported by the EPSRC, the Wellcome 952, 115. Trust and Schlumberger Cambridge Research. We also thank Dr P. McPhie for assistance with the electron microscopy and Mrs J. L. Johnson for assistance with peptide synthesis. Paper 7/01088E; Received 17th February, 1997 J. Mater. Chem., 1997, 7(7), 1135–1145 11

ISSN:0959-9428    

DOI:10.1039/a701088e

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Detecting a transition-metal ammine at tailored surfaces Sayeedha Iqbal,a Felix J. B. Kremer,b Jon A. Preece,*a Helmut Ringsdorf,b Martin Steinbeck,b J. Fraser Stoddart,b Jie Shenc and Nigel D. Tinkerd aSchool of Chemistry, University of Birmingham, Edgbaston, Birmingham, UK B15 2T T bInstitut fu� r Organische Chemie, Johannes Gutenberg-Universita�t, Becher Weg 18–20, 55099Mainz, Germany cDepartment of Applied Physics, De Montford University, T he Gateway, L eicester, UK L E1 9BH dBNFL , Springfields Works, Salwick, Preston, L ancashire, UK PR4 0XJ The fabrication of surfaces by forming Langmuir films, which incorporate amphiphiles containing hydrophilic 18-crown-6 (18C6) derivatives, at a gas/water interface is described.These Langmuir films can be transferred to a hydrophobised quartz crystal microbalance (QCM), using the Langmuir–Blodgett technique. The QCM response has been measured in aqueous solution as a function of the concentration of the transition metal complex [Co(NH3)6]Cl3 which was injected into a vial in which the filmcoated QCM had been immersed.By comparing various surfaces covered with hydrophilic polyether and hydroxy functions and hydrophobic methyl groups, and by varying the composition of the films so as to increase the separation between the 18C6 macrocycles, it has been demonstrated that surfaces can be tailored that will enhance the binding of the [Co(NH3)6]3+ trications.In 1959, Sauerbrey1 showed that, when a quartz crystal comes ammine complexes depend significantly on the nature of the solvent molecules. In the times of Werner, these non-covalent into contact with a gas, the change in frequency DF of the quartz crystal, sandwiched between two excitation electrodes, bonding interactions were not well understood at a molecular level. However today, with the advent of various spectroscopic of natural resonant frequency F0 (s-1), density r (2.648 g cm-3), and shear modulus m (2.947×1011 dyn cm-2 techniques and X-ray crystallography, intermolecular interactions are becoming more fully understood.Indeed, new areas for AT-cut quartz) is related to the adsorbed mass Dm by the relationship: of science are emerging from the study of molecular interactions: they include crystal engineering,27 host–guest28 and supramolecular chemistry.29 They are all disciplines which rely DF=-2Fo2 A(rm) Dm 1/2 (1) upon the natural concepts of self-assembly30 and self-organiswhere A is the exposed surface area (m2) of the quartz.This pioneering activity led to the development of the quartz crystal microbalance (QCM). The QCM has proved to be a highly versatile instrument for the determination of the amount of material deposited from the gas phase2 on to a solid surface.Applications include thickness monitors in metal evaporation and deposition processes, and the detection of gas-phase analytes, such as hydrocarbons, water vapour and other volatile compounds. A more demanding, but potentially more important, area in which the QCMis being employed currently, is in liquid media,3 where it has been used to measure interfacial processes at electrode surfaces.4 It has also been employed as an immunosensor5 at the nanogram level to monitor antibodies, 6 bacterial growth,7 cells,8 proteins,9 and microbes,10 as well as to detect surfactants,11 anaesthetics,12 antibiotics,13 bitter and odorous substances,14 DNA hybridisation,15 pH changes,16 enzyme reactions,17 liposomes,18 chiral recognition, 19 intercalation,20 and even cell growth.21 Many of these experiments involve the molecular recognition of a substrate from the subphase to a biological receptor which has been deposited on the QCM surface.There are, however, very few examples in the literature where the QCM has been utilised to detect recognition events involving totally synthetic systems.22 The research reported in this paper relates to detecting molecular recognition events within a wholly synthetic system.It is known from many crystal structures23 that transitionmetal (ammine) complexes (TMCs) hydrogen bond24 via the hydrogen atoms of their ammine ligands to the oxygen atoms of crown ethers, e.g. 18-crown-6 (Fig. 1). This type of molecular Fig. 1 Second-sphere coordination in the solid state: (a) illustrating recognition is an example of second-sphere coordination,25 a the first- and second-sphere ligands, and (b) illustrating the supramol- concept first discussed by Alfred Werner26 in 1912 when he ecular 151 polymer formed between [Cu(NH3 )4(H2O)]2+ and 18C6 (hydrogen atoms are omitted for clarity) noted that the optical properties of chiral transition-metal J.Mater.Chem., 1997, 7(7), 1147–1154 1147ation31 to construct arrays of molecules held together by non- Systems I and III form poorly expanded monolayers with extremely low collapse pressures of approximately 32 and 28 covalent bonding interactions.32 Here, we report the chemical modification of the 18-crown- mNm-1, respectively (Fig. 3). Presumably, the poor stability of these films is a result of (i) the larger area requirement of 6 (18C6) macrocycle in order to facilitate its incorporation33 into Langmuir films which can be deposited on to a QCM by the polyether moieties relative to the alkyl chains (especially in the case of the crown ether lipid), (ii) the electronic repulsion the vertical dipping method, such that the hydrophilic head groups are exposed to the aqueous environment.An aqueous between electron lone pairs on the oxygen atoms, and (iii) the high solvation of the polyether moieties by the water molecules. solution of [Co(NH3)6]Cl3 can then be injected into the aqueous environment in which the LB film-coated QCM is Conversely, the isotherm of the octadecanol 3 monolayer is very stable, collapsing at just below 60 mN m-1, forming a immersed, and the frequency response can be measured as a function of the concentration of [Co(NH3)6]Cl3 to yield solid analogous phase.Thus, a compromise is required between the poorly expanded films of 1 and 2 and the well condensed information about the kinetics and thermodynamics of the recognition process.34 film of 3. This compromise was achieved by cospreading the two ether compounds 1 and 2 with octadecanol 3.Fig. 3 shows the isotherms of the monolayers formed from systems II (1 Results and Discussion with 3) and IV (2 with 3) in which one molar equivalent of octadecanol 3 is cospread with the ether amphipiles 1 and 2, General remarks respectively. The isotherms of systems II and IV are less The compounds, which were used in the present research, are expanded, relative to systems I and III, and these two compo- listed in Fig. 2. Compound 1 is a chemically modified 18C6 nent films are stable until around 45 mN m-1. It should be derivative in which one of the methylene hydrogen atoms has noted that, at pressures between 30 and 45 mN m-1, the been replaced by an oxymethylene octadecanoate chain to isotherms have a short phase change from a liquid analogous make it amphiphilic in nature.Compound 2 is a linear phase to a close-packed phase. Isotherms of cospread mixtures polyether analogue of 1 in which the polyether is bonded with molar ratios of the ether compounds to octadecanol 3 of covalently to the aliphatic chain by an ester linkage. 152 and 154 (and 158 in the case 1) were recorded and showed Compounds 3 and 4 are simply the commercially available the general trend that, as the amount of octadecanol was octadecanol and thiooctadecanol, respectively.Compound 5 is increased, the film became less expanded. This point can be the kinetically inert transition-metal ammine complex appreciated from inspection of data recorded in Table 1 where [Co(NH3)6]Cl3. Compounds 1–4 were chosen for several the area per molecule at 25 mN m-1 (the pressure at which reasons.Firstly, 1 and 2 were utilised to establish if a macro- the films were transferred to theQCM for all systems) decreases cyclic eect was operating in addition to purely non-specific as the proportion of octadecanol 3 increases. binding. The hydroxy compound 3 was employed to establish if the transition-metal ammine complex had any anity for a QCM studies hydrophilic surface.Conversely, the thiowas utilised to establish if the [Co(NH3 )6]Cl3 5 had an anity for a hydro- The monolayers at the gas/water interface transferred with good transfer ratios (0.85–0.95) on to hydrophobised quartz phobic surface. Additionally, the hydroxy amphiphile 3 was employed to increase the separation of the polyether head- supports by the vertical dipping mode into the aqueous subphase, thus achieving X-type deposition.groups in cospread monolayers incorporating 1 and 2. This incorporation of 3 into monolayers formed from ether contain- The QCM was covered with a monolayer of the various systems I–V by the same vertical dipping technique, after the ing compounds 1 and 2 allows control over the intermolecular separation between the ether moieties in these monolayers, QCM surface was hydrophobised with a polymer solution, leading to an X-type deposition which is illustrated in Fig. 4. something which is crucial for the transition-metal ammine complex to become inserted into the monolayer. Thus, various The QCM is held on to a Teflon case by a vacuum and remains immersed in a vial which contains 3 ml of water and systems were investigated in which the molar ratio of the octadecanol to the ether amphiphiles was increased systemati- a stirrer bar.Several 50 ml aliquots of a 0.1 mM aqueous [Co(NH3)6]Cl3 were injected into the vial and the change in cally. These systems are illustrated in Fig. 2. frequency of the QCM was monitored as each injection was performed.Monolayer formation The QCM responses for the 152 ratios for systems II and A few points are noteworthy about the Langmuir film forming IV, together with system V, the one component octadecanol 3 ability of the single and mixed component systems. Firstly, system, are shown as a function of time in Fig. 5. It can be consider the single component systems, systems I, III and V.observed that initially the QCM frequency drops rapidly after each injection and then reaches a plateau at equilibrium between the [Co(NH3)6]3+ trications being adsorbed and not Fig. 2 Lipid compounds and transition-metal ammine complexes used in this research, showing the orientation of the molecules when adsorbed on to the QCM. The tabulated information describes the Fig. 3 Isotherms of systems I–VI constitutions of systems I–VI. 1148 J. Mater. Chem., 1997, 7(7), 1147–1154Table 1 Thermodynamic data for systems II, IV, V, VI adsorbed on to the QCM upon injections of 50 ml of 0.1 mM aqueous [Co(NH3)6]Cl3 solution into 3 ml of water in which the QCM was immersed relative system binding (ratio) Aa/nm2 n×10-14b DFc/Hz indexd Ka/dm3 mol-1 e Ka/dm3 mol-1 f II (151)A 0.318 1.62 1234 20 373 297 II (151)B 0.318 1.62 1308 22 354 363 II (152)A 0.289 1.78 4399 66 121 131 II (152)B 0.289 1.78 4452 67 128 141 II (154)A 0.253 2.04 3508 46 150 163 II (154)B 0.253 2.04 3796 49 131 143 II (158)A 0.201 2.56 1765 18 395 448 II (158)B 0.201 2.56 1900 20 319 339 IV (151)A 0.299 1.71 2422 22 262 283 IV (151)B 0.299 1.71 2259 35 253 278 IV (152)A 0.281 1.82 1568 23 343 383 IV (152)B 0.281 1.82 1960 19 250 282 IV (154)A 0.254 2.02 1778 26 361 328 IV (154)B 0.254 2.02 2063 27 272 289 VA 0.189 2.73 1735 17 339 375 VB 0.189 2.73 2213 22 224 244 VI —g 2.73 102 1 87 97 aArea per molecule when film is transferred to the QCM.bNumber of amphiphiles transferred to the QCM; calculated from the area per molecule at 25 mN m-1 for the Langmuir films of systems II and VI and 50 mN m-1 for system V multiplied by the area of the gold electrode on to which the films were transferred (area=0.513 cm2).cChange in frequency at infinite [Co(NH3 )6]Cl3; calculated from the Lineweaver Burke plot of the reciprocal of concentration of TMC against the reciprocal of the change in frequency.dCalculated from normalising the change in frequency at infinite [Co(NH3)6]Cl3 concentration. Normalisation achieved by accounting for the fact that dierent numbers of amphiphiles were transferred to the QCM for the various systems. eAssociation constant (k1/k-1). fAssociation constant calculated from the quotient of the gradient and the value of the intercept on the y-axis of the straight line obtained from the Lineweaver Burke plots (LWB).gChemisorbed from solution. Fig. 4 Diagrammatic representation of the QCM experimental set-up with a magnification of a cartoon representation of the recognition event on the QCM to detect complexation on a receptor-derivatised QCM surface in contact with a solution containing complementary guest molecules adsorbed on to the surface.The three curves demonstrate that, the self-assembled monolayer of thioocatedecanol 4. The first point to note is that system V, which presents a purely after each successive injection of the [Co(NH3)6]Cl3 aqueous solution, (i) the change in frequency is less, (ii) the crown ether hydrophobic surface to the aqueous environment, has a very small response to the TMC as subsequent injections are made.containing film, system II, has a considerably greater response than its linear counterpart, namely system IV, and (iii) the Indeed, the same response was recorded when pure water was injected. Thus, the small changes in frequency are not a result one-component octadecanol 3 system, which presents a purely hydrophilic surface to the aqueous environment, has a similar of the TMC having an anity for this hydrophobic surface, but merely a result of the physical changes experienced by the response to system IV.This data is more clearly presented in a concentration QCM as the water level rises which, in turn, exerts a slightly greater pressure on the QCM.35 Contrast this result with the dependent titration curve for the QCM response.Fig. 6 shows the QCM response for system IV, the octadecanol 3 and the hydrophilic surfaces of systems IV and V. In these cases, the QCM response is much larger than for system VI, or when amphiphilic polyether 2, in the molar ratios 151, 152 and 154, together with system V, pure octadecanol 3, and system VI, pure water is injected into the vial containing the QCM with J.Mater. Chem., 1997, 7(7), 1147–1154 1149Fig. 7 QCM concentration-dependent titration curves for the hydrophilic system II upon injection of 50 ml aliquots of 0.1 mM aqueous [Co(NH3)6]Cl3, illustrating the significantly dierent responses for the various ratios of 153 and the significantly greater response for the Fig. 5 Comparison of the time-dependent titration curves for system II 152 ratio system II, relative to the hydrophilic surfaces of system IV (152) and system IV (152) as 50 ml aliquots of a 0.1 mM [Co(NH3)6]Cl3 and V are injected into the vial containing the QCM DFt , DFeq, and DFmax are the changes in frequency after 10 s from the first injection, at equilibrium for the first injection and at infinite concentration of the TMC, i.e.when all the surface recognition sites are filled, obtained from the y-intercept of the Lineweaver Burke plot of the reciprocal of concentration against the reciprocal of change in frequency.The results from Table 2 are depicted graphically in Fig. 8. The ratio of the molar equivalents of octadecanol 3 against the ether amphiphiles 1 and 2 is plotted on the x-axis and the rate of complexation (rate on, k1) plotted on the left hand side y-axis and the rate of decomplexation (rate o, k-1) plotted on the right hand side y-axis.First of all, consider the rates of complexation and decomplexation for the linear polyether lipid in system IV. Here, it is evident that there is very little variation of these two parameters, illustrating once again the very non-specific nature of the complexation event involving the surfaces containing Fig. 6 QCM concentration-dependent titration curves for the hydro- the linear polyether and octadecanol 3. However, when one philic systems IV and V, and the hydrophobic system V, upon considers the crown ether lipid containing films of system II, injection of 50 ml aliquots of 0.1 mM aqueous [Co(NH3)6]Cl3, it is evident that, at the 152 to 154 ratio of crown ether lipid illustrating the non-specificity of all the hydrophilic surfaces to octadecanol 3, maxima result for both the rate on (k1) and rate o (k-1).However, it is slightly surprising that, on closer inspection of Fig. 8, the dierence between the complexation these hydrophilic surfaces. However, for all of these hydrophilic surfaces, the responses are very similar.This result indicates and decomplexation rate is at its smallest at the 152 ratio of system II. This result means that the binding of the TMC is that the complexation event on the surfaces formed from system IV and V is not a result of a complementary molecular apparently weakest for this ratio, even although it has been established that the QCM response is largest for system II recognition event, i.e., it is a result of non-specific non-covalent bonding interactions.(152). This apparently anomalous behaviour will be discussed later in the paper. The non-discriminatory nature of the molecular recognition event between the [Co(NH3)6]3+ trications and systems IV The thermodynamic data37 for the binding events of these systems are shown in Table 1.This data is summarised graphi- and V, illustrated in Fig. 6, contrasts extremely well with the amphiphilic crown ether containing films of system II (Fig. 7). cally in Fig. 9 and 10. Fig. 9(a) illustrates the maximum38 QCM responses obtained from the Lineweaver Burke plots, Initially, when only one equivalent of octadecanol 3 is present in the film with the amphiphilic crown ether, an intermediate derived from the graphs illustrated in Fig. 6 and 7. It is clearly evident that, for the linear polyether lipid systems (system IV), QCM response is obtained between that of the purely hydrophobic surface of system VI and the non-specific hydrophilic the response is essentially independent of the composition of the film. This result is in contrast with system II, the crown surfaces of system IV and V.However, when two molar equivalents of octadecanol 3 are introduced into the film, the ether lipid containing films, where the largest response by the QCM is with the 152 molar ratio film. Fig. 9(b) represents a response of the QCM is much greater than those of the nonspecific hydrophilic surfaces of systems IV and V. When four normalisation of the data in Fig. 9(a), to compensate for the slightly greater number of amphiphilic molecules which are equivalents of octadecanol 3 are introduced into the films of system II, the response of the QCM is then intermediate transferred to the QCM at 25 mN m-1 as the molar proportion of the octadecanol increases in the film. Thus, one could expect between the non-specific hydrophilic surfaces and the film formed from 1.0 molar equivalent of 1 and 2.0 molar equival- greater QCM responses for films with more molecules per unit area.However, upon inspection, this expected increase in ents of 3. Finally, when 8.0 molar equivalents of octadecanol 3 are incorporated into the film with the crown ether amphiph- response is clearly not observed. System IV surfaces still have no distinct feature, and the maximum of system II is still at ile, the QCM response is very similar to the non-specific hydrophilic surfaces of systems IV and V.the 152 ratio. Thus, the 152 ratio film of system II complexes to more [Co(NH3)6]3+ trications than any of the other films The kinetic data for these systems is shown in Table 2, where 1150 J.Mater. Chem., 1997, 7(7), 1147–1154Table 2 Kinetic dataa for systems II, IV and V adsorbed on to the QCM upon injections of 50 ml of 0.1 mM aqueous [Co(NH3)6]Cl3 solution into 3 ml of water in which the QCM was immersed system DFt=10sb/s-1 DFeqc/s-1 DFmaxd/s-1 k1 k-1 II(151)A 222 474 1234 14.51 0.039 II(151)B 200 487 1308 13.17 0.037 II(152)A 679 738 4399 25.50 0.211 II(152)B 682 785 4452 21.43 0.167 II(154)A 626 702 3508 26.76 0.178 II(154)B 598 684 3796 22.36 0.169 II(158)A 319 700 1765 14.53 0.037 II(158)B 403 659 1900 19.57 0.061 IV(151)A 315 672 2259 11.22 0.044 IV(151)B 457 733 2422 17.58 0.067 IV(152)A 296 572 1568 15.94 0.046 IV(152)B 287 577 1960 12.17 0.049 IV(154)A 370 670 1778 18.09 0.050 IV(154)B 290 636 2063 11.05 0.040 VA 277 632 1735 12.58 0.037 VB 284 603 2213 10.44 0.046 aY.Ebara and Y. Okahata, J. Am. Chem. Soc., 1994, 116, 1209. bChange in frequency 10 s after the first injection of the TMC. cEquilibrium change in frequency after first injection of TMC. dMaximum change in frequency at infinite concentration of TMC extrapolated from the Lineweaver Burke plot of reciprocal of change in frequency against reciprocal of concentration of TMC.Fig. 8 Plot of the rate of complexation (rate on, &) and rate of decomplexation (rate o, $) as the amount of octadecanol increases in systems II (—) and IV (A) Fig. 10 Plot of the binding constants obtained both kinetically ($) and thermodynamically (&) for (a) system II and (b) system V as a function of the amount of octadecanol in the monolayer studied in this paper, while all system IV (and system V) surfaces complex only approximately one-third of the number of trications complexed by system II (152 ratio) and are independent of surface composition.Fig. 10 depicts the variation of the binding constants, calculated both kinetically (k1 /k-1 ) and thermodynamically (Lineweaver Burke plots of data in Fig. 6 and 7) of the films towards the TMC as a function of the ratio of the ether lipids 1 and 2 and octadecanol 3.These graphs illustrate the extremely good agreement between the kinetically established binding constants and the thermodynamically established ones. Again, it is evident that system IV and V surfaces, containing the linear polyether lipid, complex to the [Co(NH3)6]3+ trications independent of the surface composition, such that they all have a Ka of approximately 300 dm3 mol-1.In contrast, it is evident Fig. 9 Plots of (a) the maximum change in frequency at infinite once again that system II, containing the amphiphilic crown concentration of [Co(NH3)6]Cl3 (obtained from Lineweaver Burke ether 1 and octadecanol 3, has a minimum at the 152 ratio. plot), and (b) the normalised change in frequency on taking into account the dierent number of amphiphiles transferred to the QCM However, this minimum is actually illustrating that this surface J.Mater. Chem., 1997, 7(7), 1147–1154 1151has the lowest binding constant towards the [Co(NH3)6]3+ has the greatest QCM response, as a result of the 151 (trication5crown ether) stoichiometry, rather than the 15several with a value for Ka of approximately 125 dm3 mol-1.This behaviour is apparently anomalous, since it has already been (trication5polyether) stoichiometry for the system IV films. Furthermore, the low Ka value for system II (152) is a result established that this molar ratio of system II equates with the largest QCM response, i.e. it complexes to the largest amount of the fine balance between the correct steric fit of the TMC in the surface cavities and the weak second-sphere hydrogen- of the [Co(NH3)6]Cl3, as illustrated graphically in Fig. 7 and 9. In order to explain this anomalous behaviour, the following bonding interactions enabling the [Co(NH3)6]3+ trications to slip in and out of the surface cavities easily, such that the rate two models are proposed.Firstly, consider system IV, the linear polyether 2 and the octadecanol 3 containing films. o is not so dierent, relative, to the rate on, leading to a low Ka value for system II. These systems have a relatively large binding constant relative to system II (152) but bind substantially less [Co(NH3)6]Cl3. This behaviour can be explained by inspection of the model Conclusions in Fig. 11 where we considerthat each [Co(NH3)6]3+ trication, when it reaches the hydrophilic surfaces is ‘captured’ by several The observation in the solid state of the so-called secondsphere coordination between transition-metal ammine com- polyether arms. The net eect is that, in order to break free from the surface, several polyether arms have to unravel plexes and 18C6 ligands, has prompted the chemical modifi- cation of 18C6 to make it amphiphilic in nature, such that it simultaneously from the [Co(NH3)6]3+ trications.Thus, the rate of decomplexation is slow relative to the rate of com- could be incorporated into Langmuir–Blodgett films on solid supports. By utilising a QCM as the solid support, it has plexation, resulting in a relatively large binding constant, Ka.Now consider system II (the 152 ratio variation). Here, the proved possible to detect this second-sphere coordination upon introduction of the transition-metal ammine complex largest QCM response is observed (Fig. 9) compared with all the systems studied, yet this system has the lowest binding [Co(NH3)6]Cl3 into a solution in which the film-coated QCM was immersed, and has enabled the kinetic and thermodynamic constant (Fig. 10). Consider the model in Fig. 12. Here, the crown ether moiety is spaced out at just the correct distance characterisation of these very weak molecular recognition events, something which was not possible by other techniques. to allow the [Co(NH3)6]3+ trication to slip easily into the surface cavity created by two neighbouring crown ethers.It turned out that the complexation was critically dependent on the composition of the film. The evaluation of the kinetic Additionally, the spacing is such that each crown ether binds to two [Co(NH3)6]3+ trications such that the overall stoichi- and thermodynamic data has enabled a model of the surface recognition event to be formulated. This model highlights that, ometry of the film is 151 with respect to the trication and 18C6 head groups.This model then establishes why this film although these very weak NMH,O hydrogen-bonding interactions are competing with the competitive aqueous environment, the recognition still occurs. By tailoring the film, the 18C6 moieties are preorganised36 in the film such that they create many surface recognition sites which are stereoelectronically compatible with the [Co(NH3)6]3+. However, the binding of these trications is weak, relative to the linear polyether containing surfaces which bind many fewer trications but in a significantly stronger manner.The recognition in natural systems atinterfaces by hydrogenbonding interactions is of utmost importance, since it is responsible for immuneresponses,37 amongst other biologically important signals.It follows that the study of simpler synthetic systems38 is of considerable value in shedding light on the more complex biological recognition events as well as for the development of new sensors.4 Additionally, we have demonstrated that the QCM oers Fig. 11 Model of the complexation event between the system IV yet another valuable tool to the research worker who is surfaces and the [Co(NH3)6]3+ trications, illustrating that several studying unnatural supramolecular systems, where the recog- polyether arms complex with the [Co(NH3)6 ]3+, leading to a small nition event is between relatively small molecules (Mr#300 u), decomplexation rate, relative to the system II (152) model (depicted in Fig. 12) in contrast with the majority of studies carried out to date which have been concerned with the detection of naturally occurring macromolecules5–21 (Mr>30000 u). Experimental Solvents were dried using literature methods where necessary or used directly as obtained from the suppliers. Thin layer chromatography (TLC) was performed on aluminium sheets coated with Merck 5554 Kieselgel 60F. Developed plates were scrutinized in an iodine chamber.Column chromatography was performed using Kieselgel 60 (0.040–0.063 mm mesh, Merck 9385). Melting points were determined with an Electrothermal 9200 melting point apparatus and are uncorrected. Microanalyses were performed by the University of Birmingham and the University of Sheeld Microanalytical Services.Low-resolution mass spectra were obtained from a Fig. 12 Model of the complexation event between the system II Kratos Profile mass spectrometer, operating at 4 kV and using surfaces and the [Co(NH3 )6]3+ trications, illustrating the 151 molar 70 eV for electron impact mass spectrometry (EIMS). Proton ratio between the crown ether head groups and the [Co(NH3)6]3+, nuclear magnetic resonance (NMR) spectra were recorded on resulting in a high [Co(NH3)6]3+ uptake coupled with the small binding constant, relative to system IV (Fig. 11) Bruker AC 300 (300 MHz) spectrometer, using the deuteriated 1152 J. Mater. Chem., 1997, 7(7), 1147–1154solvents as the lock. 13C NMR spectra were recorded on the were removed in vacuo and the residue was taken up in CH2Cl2 (50 ml) and washed with aqueous Na2CO3 (50 ml) and H2O Bruker AC 300 (75 MHz) spectrometer.The isotherm measurements were all recorded on a self- (2×50 ml). The organic layer was then dried (MgSO4) and the solvents were removed to aord a clear liquid which was made trough with a Wilhelmy pressure pick up system. The spreading solutions consisted of CHCl3 and the lipids 1–3, and purified by silica gel column chromatography (eluent: CH2Cl2–MeOH) to aord 7 as a clear oil.Yield 9.6 g (42%); mixtures thereof, in the concentration range 0.5–1.0 mg ml-1, of which between 25 and 50 ml were spread from a syringe on 1H NMR (300 MHz, CDCl3) d 3.70–3.50 (14H, m, OCH2CH2O), 3.45 (3H, s, OCH3), 2.81–2.78 (2H, t, to an aqueous subphase or an aqueous subphase with the [Co(NH3)6]Cl3 dissolved in it.Each isotherm was carried out CH2OCH3); 13C NMR (75 MHz, CDCl3 ) d 72.5, 71.8, 70.5, 70.5, 70.5, 70.2, 70.2, 61.6, 58.9. EIMS: C9H20O5 requires m/z over a 20 min period. The [Co(NH3)6]Cl3 was of analytical quality as defined by the commercial supplier and was used 208 [M+]. Found: m/z 209 [M+H]+. without further purification. [Co(NH3)6]Cl3 subphases were prepared freshly each day from Milli Q water (resistivity ca.Lipid ether 2. A solution of monomethoxytetraethyleneglycol 7 (2.0 g, 9.6 mmol) and NEt3 (1.2 g, 11.5 mmol) dissolved in 18 V cm-1). The quartz crystal microbalance consisted of a quartz crystal (9 MHz, AT-cut, d=8 mm) covered with gold dry toluene (25 ml) was added dropwise to a stirred solution of octadecanoyl chloride (3.5 g, 11.5 mmol) in dry toluene electrodes (area=0.53 cm2) obtained from Quartzkeramik GmbH.The quartz crystal was mounted on a Teflon dipstick (10 ml) maintaining the temperature below 10°C. The reaction mixture was then allowed to warm to room temperature and (Fig. 4). In order to ensure no short circuits between the two gold electrodes, a silicon sealing ring was placed between the H2O (20 ml) was added.The solution was then concentrated in vacuo to give a waxy o-white solid which was dissolved Teflon holder and the crystal. The QCM was held in place by a vacuum on the non-covered face. The QCM was hydro- CH2Cl2 (50 ml) and washed with H2O (50 ml×2). The organic layer was dried (MgSO4), filtered, and the filtrate concentrated phobised with a ‘silicon solution’ obtained from Serva.The quartz crystal was driven by an in-house oscillator (15 V, in vacuo and purified by silica gel column chromatography (eluent: EtOAc–Me2CO, 352) to yield the acyclic polyether 2 100 mA), the oscillation shape controlled by a Hameg (HM604) oscilloscope. The frequency change was recorded with an as a white waxy solid. Yield 4.4 g (96%); 1H NMR (300 MHz, CDCl3) d 4.24–4.19 (2H, m, CH2CO2), 3.73–3.52 (14H, m, Iwatsu universal counter (SC7201).Transfer of the Langmuir films of systems I–IV to the QCM was achieved on a computer- OCH2CH2O), 3.46 (3H, s, OCH3), 2.32–2.28 (2H, t, CH2OCH3), 1.65–1.55 (2H, m, CH2CH2CO2 ), 1.33–1.20 (28, controlled trough from KSV Instruments (KSV5000) utilising a vertical dipping method at 20°C and a film pressure of 25 s, CH2CH2), 0.89–0.84 (3H, s, CH2CH3); 13C NMR (75 MHz, CDCl3) d 173.9, 72.6, 71.9, 70.6, 70.3, 69.2, 63.4, 61.7, 59.0, 34.2, mN m-1 for systems I–IV and 50 mN m-1 for system V, with a dipping speed of 2 mm min-1. The films were held at 25 or 31.9, 29.5, 29.4, 29.1, 24.9, 22.7, 14.1.EIMS: C27H54O6 requires m/z 474 [M+]. Found: m/z 475 [M+H]+. Anal. Calc.: C, 50 mN m-1 for 20 min before deposition to ensure they were stable and were compressed with a barrier rate of 5 mm min-1. 68.31; H, 11.46. Found: C, 68.38; H, 11.61%. System VI was chemisorbed on to the QCM clean gold surface from a solution (0.1 mM) of thiooctadecanol 4 in CHCl3 . The Financial support by the Royal Society (J.A.P.) and by BNFL (S.I.) is gratefully acknowledged. gold surface was cleaned with MeOH and CHCl3 .Synthesis References 2-Oxymethyl-18-crown-6-octadecanoate 1. A solution of 2- 1 G. Z. Sauerbrey, Phys., 1959, 155, 206. hydroxymethyl-18-crown-6 5 (0.50 g, 1.70 mmol) and NEt3 2 Applications of Piezoelectric Quartz Crystal Microbalances, ed. C. Lu, Elsevier, New York, 1984, vol. 7; J. F. Alder and (0.260 g, 2.60 mmol) was dissolved in dry toluene (25 ml) and J.J. McCallum, Analyst, 1983, 108, 1169; K. Bodenho�fer, it was added dropwise to a stirred solution of octadecanoyl A. Hierlemann, G. Noetzel, U.Weimar and W. Go�pel, Anal. Chem., chloride (0.64 g, 2.10 mmol) in dry toluene (10 ml) whilst 1996, 68, 2210. maintaining the temperature at 10°C. The reaction mixture 3 (a) R. Schumacher, Angew. Chem., Int. Ed.Engl., 1990, 29, 329; was then allowed to warm to room temperature and H2O (b) D. M. Ward and D. A. Buttry, Science (Washington, DC), 1990, (20 ml) was added. The solution was then concentrated in 249, 1000; (c) S. Bruckenstein and M. Shay, Electrochim. Acta, 1985, 30, 1295; (d) R. Schumacher, G. Borges and K. K. Kanazawa, vacuo to give a yellow oil, which was dissolved in CH2Cl2 Surf.Sci., 1985, 163, L621; (e) O. Melroy, K. K. Kanazawa, (20 ml) and washed with H2O (20 ml×2). The organic layer J. G. Gorgom and D. Buttry, L angmuir, 1986, 2, 697. was dried (MgSO4), concentrated in vacuo and purified by 4 (a) A. J. Tu� do�s, P. J. Vandeberg and D. C. Johnson, Anal. Chem., silica gel column chromatography (eluent: EtOAc–Me2CO, 1995, 67, 552; (b) Y. Ebara, H.Ebato, K. Ariga and Y. Okhata, 352) to yield the amphiphilic crown ether 1 as a colourless oil. L angmuir, 1994, 10, 2267; (c) A. C. Hillier and D. M. Ward, Anal. 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Nivens, J. Q. Chambers, T. R. Anderson and D. C. White, leneglycol 6 (2 g, 0.11 mol) in dry THF was added dropwise Anal. Chem., 1993, 65, 65. to a suspension of NaH (60% dispersed in mineral oil) (20 mg, 8 J. Redepenning, T. K. Schlesinger, E. J. Mechalke, D. A. Puleo and R. Bizios, Anal. Chem., 1993, 65, 3378. 5.4 mmol) in dry THF (50 ml) under nitrogen at room tempera- 9 (a) M.Masson, K. Yun, T. Haruyama, E. Kobatake and ture. The reaction mixture was then heated under reflux and M. Aizawa, Anal. Chem., 1995, 67, 2212; (b) M. Muratsugu, stirred for a further 30 min. A solution of MeI (0.76 g, F. Ohta, Y. Miya, T. Hosokawa, S. Kurosawa, N. Kamo and 5.4 mmol) in THF (20 ml) was then added over a period of H. Ikeda, Anal. Chem., 1993, 65, 2933;(c) Y.Ebara and Y. Okahata, 20 min. The reaction mixture was heated under reflux for a L angmuir, 1993, 9, 574; (d) R. C. Ebersole, J. A. Miller, J. R. Moran further 2 h. The reaction mixture was then allowed to cool to and M. D. Ward, J. Am. 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ISSN:0959-9428    

DOI:10.1039/a700325k

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Recognition of aqueous flavin mononucleotide on the surface of binary monolayers of guanidinium and melamine amphiphiles Katsuhiko Ariga, Ayumi Kamino, Hiroshi Koyano and Toyoki Kunitake*† Supermolecules Project, JST (Former JRDC), Kurume Research Park, 2432 Aikawa, Kurume 839, Japan Recognition of aqueous flavin mononucleotide (FMN) on the surface of binary monolayers of guanidinium amphiphiles (monoalkyl derivative, C18Gua, or dialkyl derivative, 2C18Gua) and the melamine amphiphile (2C18mela-NN) has been investigated by p–A isotherms, FTIR spectroscopy, and XPS measurements.p–A Isotherms and FTIR spectra of C18Gua–2C18mela-NN (151) monolayers show that there is no direct hydrogen bonding and/or coulombic interactions between C18Gua and 2C18mela-NN on pure water and that the C18Gua component is dissolved into the subphase upon compression.In contrast, the presence of aqueous FMN prevented C18Gua molecules from dissolving into the subphase. Maximum condensation was observed at a 151 ratio in C18Gua–2C18mela-NN mixed monolayers on aqueous FMN. XPS analyses revealed that one FMN molecule was bound to one C18Gua–2C18mela-NN (151) unit and binding was saturated at 5×10-6 mol dm-3 FMN.Peak shifts observed in FTIR spectra indicated that the isoalloxiazine ring in FMN formed hydrogen bonds with 2C18mela-NN. These results support a model that the isoalloxazine and phosphate functions in FMN are bound via hydrogen bonding to melamine in 2C18mela-NN and guanidinium in C18Gua, respectively. Similar binding behaviour of FMN was observed for mixed monolayers of 2C18Gua–2C18mela-NN.Hydrogen bonding is highly directive unlike other secondary from AFM observation that a regular pattern of monolayer components was formed upon binding of FAD to a valence forces and plays an important role in molecular design of receptor–guest systems with high specificity.1–4 It has been C18Gua–2C18Oro mixed monolayer.23 It is expected that one can make various patterns through combinations of dierent believed that molecular recognition via hydrogen bonding is dicult in polar media such as water due to competition from amphiphiles and aqueous guests.However, diculties in preparing desirable recognition sys- the latter, and most eective designs have been carried out in non-aqueous media. Thermodynamic analyses by Williams tems are still sometimes encountered.One of the major di- culties is undesirable interactions among component et al.5 and Adrian and Wilcox6 suggested that a molecular design to induce entropic gain upon releasing bound water amphiphiles. For example, recognition of aqueous AMP by a mixed monolayer of C18Gua and 2C18Oro [Fig. 1(A)] is not would compensate an enthalpic disadvantage in polar media.This disadvantage may be avoided by creation of a local non- ecient,22 because ion pairing between guanidinium and orotate competes with their binding to the guest.24 Two-dimen- aqueous environment in water. Nowick et al.7 and Bonar- Law8 incorporated recognition sites in the hydrophobic core sional crystallization of an azobenzene-type monolayer of C10AzoAT sometimes disturbs the formation of a desirable of micelles, while Komiyama et al.9 immobilized hydrogen bonding sites in a water-insoluble polymer.recognition site.25 In order to develop mixed monolayer systems which can recognize various kinds of aqueous guests, we Unlike these approaches, we have studied the aqueous phase near a hydrophobic phase, i.e., interfaces with water.Our have to establish a strategy to avoid unfavourable interactions within monolayers. quantum chemical calculation based on a multidielectric model revealed that hydrogen bonding was enhanced at the air/water Here, we demonstrate recognition of flavin mononucleotide (FMN) by mixed monolayers of guanidinium amphiphiles interface, since electronic properties of molecules located in water close to the hydrophobic phase are aected by the low (monoalkyl derivative, C18Gua, and dialkyl derivative, 2C18Gua) and a melamine amphiphile (2C18mela-NN) dielectric medium, thus the molecules behave as if they are in a less polar medium.10 We have also demonstrated experimen- [Fig. 1(B), (C)]. The following aspects were considered to achieve eective recognition: (1) a guanidinium component tally that molecular recognition through hydrogen bonding is eective at the air/water interface. Nucleotides,11 nucleic acid was selected, as the strong interaction between guanidinium and phosphate11a would enhance the recognition eciency; bases,12 sugars,13 amino acids14 and peptides15 dissolved in the aqueous subphase are eectively bound by receptor mono- (2) the N,N-disubstituted 2C18mela-NN molecule has only one face of the three-point recognition site of the melamine ring. layers.Molecular recognition at the air/water interface has been recently reported by other groups as well.16–20 This new We thus expect a 151 recognition in contrast to network formation observed for N,N¾-disubstituted melamines;26 (3) ion finding is also applicable to microscopic interfaces formed by micelles and bilayers dispersed in bulk water.21 pairing can be avoided between guanidinium and melamine under normal conditions.These monolayer interfaces can be composed of a variety of amphiphile molecules, thus creating varied recognition sites. We already reported that a ternary monolayer of guanidinium Results and Discussion (C18Gua), diaminotriazine (C10AzoAT) and orotate (2C18Oro) recognizes flavin adenine dinucleotide (FAD) [Fig. 1(A)].22 In Monolayer behaviour of mixtures of guanidinium and melamine this system, guanidinium, diaminotriazine and orotate func- amphiphiles on water tions in monolayers are bound to phosphate, isoalloxazine and The behaviour of mixed monolayers of C18Gua and 2C18mela- adenosine units in FAD, respectively.It was also confirmed NN was examined on pure water. The monoalkyl guanidinium amphiphile C18Gua is relatively hydrophilic forming only an † Permanent address: Faculty of Engineering, Kyushu University, 1 Hakozaki, Higashiku, Fukuoka 812, Japan. unstable monolayer which is readily dissolved into water upon J. Mater. Chem., 1997, 7(7), 1155–1161 1155Fig. 1 Multisite binding of mixed monolayers with complementary aqueous guests: A, mixed monolayer of C10AzoAT, C18Gua and 2C18Oro on FAD; B, mixed monolayer of 2C18mela-NN and C18Gua on FMN; C, mixed monolayer of 2C18M and 2C18Gua monolayers on FMN compression.24 Therefore, p–A isotherms of the C18Gua mono- component in the mixed monolayer. The isotherm of the layer have a poor reproducibility on pure water and the 2C18mela-NN monolayer displays only a condensed phase molecular area is smaller than the cross-sectional area of the with a limiting area of 0.44 nm2, indicating formation of a well monoalkyl chain.packed monolayer. The isotherm of C18Gua–2C18mela-NN p–A Isotherms of 2C18mela-NN and C18Gua–2C18mela-NN (151) monolayer is similar in shape to that of the 2C18mela- (151) monolayers on pure water are shown in Fig. 2(A). Since NN single-component monolayer. The dierence in the mol- the molecular areas are normalized by the number of 2C18mela- ecular area between the two isotherms is only 0.05 nm2, and NN molecules, the dierence in molecular area between the is much smaller than the cross-section of one monoalkyl chain.two isotherms represents the area occupied by the C18Gua The area of a mixed monolayer compressed at 30 mN m-1 was observed to decrease with time. These results strongly suggest that C18Gua is dissolved in water upon compression in spite of the presence of the 2C18mela-NN component. The monolayers on pure water were transferred onto golddeposited glass plates and their FTIR spectra were measured in the reflection–absorption mode (RAIRS).Fig. 3 shows spectra of a cast film of C18Gua and LB films of 2C18mela-NN and C18Gua–2C18mela-NN (151). The cast film is a substitute for an LB film of the C18Gua component, since LB transfer of the C18Gua monolayer from pure water was dicult. In the spectrum of C18Gua [Fig. 3(A)], n(CNN) and d(NH) peaks of the guanidinium moiety are seen at 1680 and 1628 cm-1, respectively.27 The triazine n(CNN) peak at 1579 cm-1 and broad d(NH) peak at 1600–1700 cm-1 are observed in the spectrum of 2C18MLB film [Fig. 3(B)].28 The spectrum of the C18Gua–2C18mela-NN (151) LB film [Fig. 3(C)] is essentially identical to that of the single-component 2C18mela-NN LB film. The former spectrum does not contain any feature of the C18Gua component.These spectral characteristics clearly reveal dissolution of the C18Gua component into subphase during compression. These results also indicate the absence of specific (hydrogen bonding and/or coulombic) interaction between C18Gua and Fig. 2 p–A Isotherms of (1) 2C18mela-NN and (2) C18Gua–2C18mela- 2C18mela-NN. In the case of a mixed monolayer of C18Gua NN (151) monolayers at 20 °C: A, on pure water; B, on 1×10-5 mol and 2C18Oro, guanidinium and orotate functions form a stable dm-3 of aqueous FMN.Molecular area was calculated on the basis of the number of 2C18mela-NN molecules. 151 ion pair with specific peak shifts in the FTIR spectrum.24 1156 J. Mater. Chem., 1997, 7(7), 1155–1161Fig. 3 FTIR spectra of multilayer films (5 Y-type films) on goldcoated glass plates: A, C18Gua cast film; B, 2C18mela-NN LB film transferred from pure water; C, C18Gua–2C18mela-NN (151) LB film transferred from pure water Thus, guanidinium and melamine functions should act on the FMN guest without mutual interference. Fig. 4 (A) p–A Isotherm of mixed monolayers of C18Gua and 2C18mela-NN at 20 °C: a, C18Gua 100.0%; b, 79.9%; c, 59.9%; d, p–A Isotherms of mixed monolayers of C18Gua–2C18mela-NN 39.9%; e, 19.9%; f, 0.0%.(B) Plots of normalized deviation against on aqueous FMN the fraction of C18Gua at 5, 10, 15, 20, 25, 30, 35 and 40 mN m-1 We subsequently investigated the interaction of aqueous FMN (from the bottom). with the mixed monolayer. In remarkable contrast with the preceding results, a monolayer of C18Gua becomes stable on upon mixing.However, it is more likely that condensation of 1×10-5 mol dm-3 FMN and gives satisfactory reproducibility the mixed monolayer via FMN binding is most pronounced in its p–A isotherm. Apparently, binding of FMN to the for the equimolar monolayer. As the surface pressure increases, monolayer prevents the guanidinium component from disthe condensation eect is suppressed because of increased solution into subphase, as also observed with aqueous molecular packing at all the mixing ratios.However, binding FAD.24 Fig. 2(B) shows isotherms of 2C18mela-NN and of FMN to the monolayer proceeds even at high surface C18Gua–2C18mela-NN (151) monolayers on 1×10-5 mol pressures, as confirmed by XPS and FTIR results as dis- dm-3 FMN.Again, molecular areas are normalized by the cussed below. number of 2C18mela-NN molecules. The dierence in the molecular area between two isotherms is 0.20–0.25 nm2, in Binding analysis of aqueous FMN to mixed monolayers of reasonable agreement with the cross-sectional area of mono- C18Gua–2C18mela-NN by XPS measurements alkyl C18Gua. In order to investigate the stoichiometry of interacting Quantitative analysis of FMN binding was conducted by XPS components, p–A isotherms of the mixed monolayer were measurements.The amount of FMN bound to monolayers measured in varying mixing ratios on 1×10-5 mol dm-3 was determined from the elemental ratio of phosphorus and FMN. The data were normalised by the total number of nitrogen in XPS measurement of the transferred monolayer amphiphile molecules used and are shown in Fig. 4(A). (Table 1). The FMN/C18Gua ratio is close to unity for a Deviations of the observed molecular area from that of an C18Gua monolayer, indicating that one FMN molecule is ideal mixture were calculated according to the following equa- bound to each C18Gua molecule; the guanidinium group is tions,29 stoichiometrically bound to a phosphate unit.Although weak interactions to carbonyl and/or the lone pair on nitrogen in Aideal=xAa+(1-x)Ab (1) the isoalloxazine unit of FMN are also conceivable,11b,17b the Normalized deviation=(Aobs-Aideal)/Aideal (2) equimolar binding ratio observed strongly suggests that the binding occurs between guanidinium and phosphate and that where Aa, Ab, Aideal, Aobs, and x represent the molecular area of component a, molecular area of component b, mean molecu- the other possibilities are unlikely.The ratio of FMN bound to single-component 2C18mela- lar area for an ideal mixture, observed mean molecular area, and mole fraction of component a, respectively. Normalized NN monolayer is 0.44 under the same conditions. The binding constant reported for aqueous cyclic imide (thymine) and a deviations obtained with eqn.(2) are plotted in Fig. 4(B). At low surface pressures, the negative deviation (condensation diaminotriazine monolayer is only 2×102 dm3 mol-1,12a while the constant between aqueous phosphate(AMP) and a guanidi- eect) is maximized at an equimolar mixing ratio. This result might be considered to originate from an entropic contribution nium monolayer is 3×106 dm3 mol-1.11a The hydrogen bond- J.Mater. Chem., 1997, 7(7), 1155–1161 1157Table 1 Binding of FMN to monolayers as determined by XPSa amphiphile unit [FMN]/mmol dm-3 P (%) N (%) Rb C18Gua 0.01 1.52 9.92 1.07 2C18mela-NN 0.01 0.50 11.28 0.44 C18Gua–2C18mela-NN (151) 0.00 0.00 11.05 0.00 C18Gua–2C18mela-NN (151) 0.0001 0.55 11.55 0.62 C18Gua–2C18mela-NN (151) 0.005 0.90 11.04 1.06 C18Gua–2C18mela-NN (151) 0.01 0.93 10.96 1.10 C18Gua–2C18mela-NN (151) 0.10 0.94 10.93 1.12 2C18Gua–2C18mela-NN (151) 0.01 0.76 10.10 1.06 aLB films (9 or 10 layers) were used for measurement.bR=Bound FMN/amphiphile. ing interactions between a neutral receptor and guest, e.g., 2C18mela-NN–isoalloxazine (FMN), is less ecient. The XPS results reveal that the binding ratio of FMN to C18Gua–2C18mela-NN (151) is close to unity at FMN concentrations >5×10-6 mol dm-3, consistent with the binding motif of Fig. 1(B) where one FMN molecule simultaneously binds to one guanidinium and one melamine. Since the binding eciency is ca. 50% at 10-7 mol dm-3 FMN and is saturated at 5×10-6 mol dm-3 FMN, the binding constant is estimated to be in the range of 107 dm3 mol-1.FTIR examination of the receptor–guest interaction The mode of the receptor–guest interaction was studied by FT RAIRS spectroscopy of monolayer receptors transferred onto a gold-deposited plate. IR spectral changes caused by FMN binding were characterized separately for the two functional components of the receptor monolayer (Fig. 5 and 6).Fig. 5 shows IR spectral characteristics of C18Gua and FMN in the region 1200–1900 cm-1. According to the literature,30,31 the peaks observed for an FMN cast film [Fig. 5(C)] are assigned as follows: n(C4NO) at 1729, n(C2NO) at 1681; n(CNN) at 1579 and n(CNN) at 1550 cm-1. The spectrum of a C18Gua LB film transferred from 1×10-5 mmol dm-3 of aqueous FMN [Fig. 5(B)] is basically a superimposition of the two components, although some peak broadening by overlapping is seen in the 1600–1700 cm-1 region. The presence of FMN Fig. 6 FTIR spectra of multilayer films (5 Y-type films) on goldcoated glass plates: A, 2C18mela-NN LB film transferred from pure water;B, 2C18mela-NN LB film transferred from aqueous 1×10-5 mol dm-3 FMN; C, FMN cast film peaks indicates the binding of FMN to the C18Gua monolayer.The absence of significant shifts of CNO peaks implies that the guanidinium unit in the monolayer is not bound to the isoalloxiazine ring of FMN. Fig. 6 summarizes similar FTIR data for the 2C18mela-NN monolayer. Comparison of IR spectra of 2C18mela-NN LB films transferred from pure water [Fig. 6(A)] and from 1×10-5 mol dm-3 aqueous FMN [Fig. 6(B)] reveals that the latter spectrum shows new peaks at 1676, 1620 and 1551 cm-1 which can be assigned to n(C4NO), n(C2NO) and a n(CNN) stretch, respectively. The former two peaks show shifts to lower wavenumbers relative to the corresponding peaks of the FMN cast film. This suggests that 2C18mela-NN binds to isoalloxazine of FMN via hydrogen bonding. Similar spectral shifts were reported by Kyogoku et al.32 for hydrogen bonding between FMN and adenine derivatives.Fig. 7 shows spectra of a C18Gua–2C18mela-NN (151) LB film transferred from aqueous FMNat dierent concentrations. The n(CNO) peaks of isoalloxazine are shifted to 1676 and 1626 cm-1, indicating that FMN is bound to the mixed monolayer through hydrogen bonding. The peak at 1580 cm-1 is an overlapped peak of 2C18mela-NN and FMN, while that at 1550 cm-1 arises from bound FMN only.Therefore, the intensity of the latter peak is relatively weak at 1×10-7 mol Fig. 5 FTIR spectra of multilayer films (5 Y-type films) on gold- dm-3 FMN where the ratio of bound FMN to amphiphile coated glass plates: A, C18Gua cast film; B, C18Gua LB film transferred from aqueous 1×10-5 mol dm-3 FMN; C, FMN cast film is low. 1158 J. Mater. Chem., 1997, 7(7), 1155–1161Fig. 7 FTIR spectra of the C18Gua–2C18mela-NN (151) LB films (5 Y-type films) transferred from aqueous FMN: A, 1×10-7 mol dm-3; B, 1×10-5 mol dm-3; C, 1×10-4 mol dm-3 The IR data are again consistent with the binding motif of Fig. 1(B), i.e., one FMN molecule is bound to one guanidinium and one melamine with formation of a guanidinium–phosphate Fig. 8 (A) p–A Isotherms of mixed monolayers of 2C18Gua and pair and isoalloxazine–melamine hydrogen bonding. 2C18mela-NN at 20°C: a, 2C18Gua 0.0%; b, 19.9%; c, 40.3%; d, 60.3%; e, 79.9%; f, 100.0%. (B) Plots of normalized deviation against Binding of aqueous FMN with 2C18Gua–2C18mela-NN the fraction of C18Gua at 5 (#), 10 ($), 15 ('), 20 (+), 25 (1), 30 monolayers (#), and 35 ($) mN m-1.We conducted a similar investigation by using a dialkyl guanidinium amphiphile, 2C18Gua. p–A Isotherms of 2C18Gua–2C18mela-NN monolayers on 1×10-5 mol dm-3 aqueous FMN are shown in Fig. 8(A). Normalized deviations of molecular area calculated using eqn. (1) and (2) are plotted in Fig. 8(B) as a function of the fraction of 2C18Gua. p–A Isotherms show a condensed phase alone, and condensation eects are not significant at any mixing ratio.Collapse pressures are minimized at a monolayer composition close to equimolar mixing [curve (c) for 40.3% 2C18Gua and curve (d) for 60.3% of 2C18Gua]. Fig. 9 shows FTIR spectra of a 2C18Gua LB film transferred from pure water [Fig. 9(A)], a 2C18Gua LB film transferred from 1×10-5 mol dm-3 aqueous FMN [Fig. 9(B)], and 2C18Gua–2C18mela-NN (151) LB film transferred from 1×10-5 mol dm-3 aqueous FMN [Fig. 9(C)]. The latter two spectra show evidence of FMN binding, i.e., n(CNN) peaks of FMN at 1579 and 1549 cm-1 and peak broadening in the 1600–1700 cm-1 region due to overlapped n(CNO) peaks of FMN. Therefore, FMN is bound to both the 2C18Gua monolayer and the 2C18Gua–2C18mela-NN (151) monolayer.Binding of FMN is also confirmed by XPS (bottom row in Table 1). The observed elemental ratio indicates the presence of one FMN molecule per 2C18Gua–2C18mela-NN (151) unit and the spectroscopic data are in accord with the binding motif shown in Fig. 1(C). Conclusion Fig. 9 FTIR spectra of multilayer films (5 Y-type films) on goldcoated glass plates: A, 2C18Gua LB film transferred from pure water; Binding of aqueous FMN to guanidinium–melamine binary B, 2C18Gua LB film transferred from 1×10-5 mol dm-3 of aqueous monolayers has been investigated.The monolayer behaviour FMN; C, 2C18Gua–2C18mela-NN (151) LB film transferred from 1×10-5 mol dm-3 of aqueous FMN on pure water shows the absence of specific functional inter- J. Mater. Chem., 1997, 7(7), 1155–1161 1159actions between the two kinds of amphiphiles.Both amphi- subphase temperature was kept at 20±0.2°C. The surface pressures were measured by a Wilhelmy plate, which was philes are basic and cannot form a strongly interacting complex between themselves. FTIR data suggests the presence of hydro- calibrated with the transition pressure of an octadecanoic acid monolayer.gen bonding between the melamine amphiphile and the isoalloxazine unit of FMN. Quantitative analysis by XPS LB films were transferred onto gold-deposited glass plates for reflection–absorption FTIR spectroscopy. The substrate measurements reveals that one FMN molecule binds to one guanidinium molecule and one melamine molecule with a was prepared as follows. A slide glass (pre-cleaned, 176×26×1 mm, Iwaki Glass) was immersed in a detergent binding constant of ca. 107 dm3 mol-1. This eective binding originates owing to the absence of competitive amphiphile– solution overnight (Dsn 90, Bokusui Brown). The glass was washed with a large excess of ion-exchanged water to remove amphiphile interactions. Multisite binding in a mixed monolayer would produce a the detergent completely, and subjected to sonication in fresh ion-exchanged water several times.After the glass was dried regular molecular arrangement in a two-dimensional plane as shown in Fig. 1. In fact, AFM observations revealed that a in vacuo for over 1 h, thin layers of chromium and gold were consecutively formed by the vapour-deposition method mixed monolayer of C18Gua and 2C18Oro on aqueous FMN formed a regular two-dimensional molecular pattern.23 A large (1000 A° Au/50 A° Cr/slide glass) with a vapour-deposition apparatus VPC-260 (ULVAC Kyushu).variety of molecular patterns can be created by appropriate combinations of receptor and guest molecules. For example, LB transfer was carried out with a FSD-21 instrument (USI System, Fukuoka) by the vertical dipping method.Monolayers system A, B, and C of Fig. 1 would form dierent patterns. However, mutual interactions in monolayers sometimes disturb were transferred on Au-coated glass plates at 30 mN m-1 with dipping speeds of 20 mm min-1 (downstroke) and 5 mm min-1 guest binding. A mixed monolayer of C18Gua–2C18Oro (151) does not bind AMP eciently because of ion pairing between (upstroke).Transfer of 2C18Gua–2C18mela-NN (151) monolayer from aqueous FMN was conducted at 20 mN m-1 the two components and shows that mutual interaction between receptor components can seriously limit the pat- because of its low collapse pressure. Transfer ratios were almost unity and Y-type transfer was used. terning.24 In order to obtain designed molecular patterns, we have to avoid such interactions and the present system of guanidinium, melamine and FMN meets these conditions.Characterization of LB films Further development of suitable recognition systems will lead FTIR spectra (reflection–absorption mode) were measured to an increased variety of two-dimensional molecular patterns. with LB films (5 Y-type films) transferred onto gold-deposited glass plates with a Nicolet 710 FTIR spectrometer.Experimental X-Ray photoelectron spectra (XPS) were measured for the LB films (5 Y-type films) on Au/Cr/glass with a Perkin-Elmer Materials PHI 5300 ESCA instrument using an Mg-Ka X-ray source (300W). Repeated scans over the same surface region at a Flavine mononucleotide monosodium salt (FMN) was comtake- o angle of 45° gave reproducible spectra.The elemental mercially supplied (Wako Pure Chem.). The water used for composition was obtained by dividing the observed peak area the subphase was deionized and doubly distilled using a by the intrinsic sensitivity factor of each element. Nanopure II-4P and Glass Still D44 System (Barnstead). Spectroscopic grade benzene and ethanol (Wako Pure Chem.) were used as spreading solvents.Gold (99.999%) and chro- References mium (99.99%) used for the surface modification of solid substrates were purchased from Soekawa Chemicals. Synthetic 1 (a) J. Rebek Jr. and D. Nemeth, J. Am. Chem. Soc., 1986, 108, 5637; methods for 2C18Gua and C18Gua are described elsewhere.33 (b) J. Rebek Jr., Acc. Chem. Res., 1990, 23, 399. 2 (a) S-K. Chang, D. Van Engen, E.Fan and A. D. Hamilton, J. Am. The melamine amphiphile, 2C18mela-NN, was synthesized Chem. Soc., 1991, 113, 7640; (b) A. D. Hamilton and D. Van Engen, as follows. J. Am. Chem. Soc., 1987, 109, 5035. 3 (a) Y. Aoyama, Y. Tanaka, H. Toi and H. Ogoshi, J. Am. Chem. 2,4-Diamino-6-(dioctadecylamino) triazine (2C18mela-NN) Soc., 1988, 110, 634; (b) Y. Aoyama, Y. Tanaka and S.Sugahara, J. Am. Chem. Soc., 1989, 111, 5397. A mixture of 2,4-diamino-6-chlorotriazine (290 mg, 4 (a) J-M. Lehn, Pure Appl. Chem., 1994, 66, 1961; (b) K. C. Russell, 1.99 mmol), dioctadecylamine21 (1.04 mg, 1.99 mmol) and E. Leize, A. Van Dorsselaer and J-M. Lehn, Angew. Chem., Int. Ed. KHCO3 (200 mg, 1.99 mmol) in 1,4-dioxane (20 cm3) was Engl., 1995, 34, 209; (c) A. Marsh, E.G. Nolen, K. M. Gardinier and J-M. Lehn, T etrahedron L ett., 1994, 35, 397. refluxed for 6 h. Water (50 cm3) was added to the mixture and 5 (a) A. J. Doig and D. H. Williams, J. Am. Chem. Soc., 1992, 14, 338; the insoluble material was collected by filtration. The solid (b) D. H. Williams, J. P. L. Cox, A. J. Doig, M. Gardner, collected on the filter was washed with water and dried to give U.Gerhard, P. T. Kaye, A. R. Lal, I. A. Nicholls, C. J. Salter and a slightly yellow powder. This was chromatographed on SiO2 R. C. Mitchell, J. Am. Chem. Soc., 1991, 113, 7020. (200 g; CH2Cl2–MeOH, 1051). The product fractions were 6 (a) J. C. Adrian and C. S. Wilcox, J. Am. Chem. Soc., 1991, 113, 678; collected and concentrated to give a solid. This was recrys- (b) 1992, 114, 1398. 7 (a) J. S. Nowick and J. S. Chen, J. Am. Chem. Soc., 1992, 114, 1107; tallized from EtOH–MeOH to give 2C18mela-NN (458 mg, (b) J. S. Nowick, J. S. Chen and G. Noronha, J. Am. Chem. Soc., 36%) as a colourless solid. Mp, 49.7–55.8°C; TLC Rf 0.49 1993, 115, 7636; (c) J. S. Nowick, T. Cao and G. Noronha, J. Am. (CH2Cl2–methanol, 1051); 1H NMR (CDCl3, 300 MHz) d 0.88 Chem. Soc., 1994, 116, 3285.(t, 6H, J=6.6 Hz, 2 CH3), 1.2–1.4 (m, 60H, 30 CH2), 1.4–1.6 8 R. P. Bonar-Law, J. Am. Chem. Soc., 1995, 117, 12397. (m, 4H, 2 CH2CH2N), 3.44 (t, 4H, J=7.6 Hz, 2 CH2N), 5.23 9 H. Asanuma, S. Gotoh, T. Ban and M. Komiyama, Chem. L ett., (br s, 4H, 2 NH2). Anal. Calc. for C39H78N6·0.5H2O: C, 73.18; 1996, 681. 10 M. Sakurai, H. Tamagawa, T. Furuki, Y. Inoue, K. Ariga and H, 12.44; N, 13.13.Found: C, 73.28; H, 12.41; N, 12.83%. T. Kunitake, Chem. L ett., 1995, 1001. 11 (a) D. Y. Sasaki, K. Kurihara and T. Kunitake, J. Am. Chem. Soc., p–A Isotherm measurement and LB transfer 1991, 113, 9685; (b) 1992, 114, 10994; (c) D. Y. Sasaki, M. Yanagi, K. Kurihara and T. Kunitake, T hin Solid Films, 1992, 210/211, p–A isotherms were measured with a computer-controlled film 776.balance system FSD-50 (USI System, Fukuoka). A mixture of 12 (a) K. Kurihara, K. Ohto, Y. Honda and T. Kunitake, J. Am. Chem. benzene–ethanol (80520, v/v) was used as a spreading solvent. Soc., 1991, 113, 5077; (b) T. Kawahara, K. Kurihara and Compression was started about 10 min after spreading at a T. Kunitake, Chem. L ett., 1992, 1839. 13 (a) K. Kurihara, K.Ohto, Y. Tanaka, Y. Aoyama and T. Kunitake, rate of 0.2 mm s-1 (or 20 mm2 s-1 based on area). The 1160 J. Mater. Chem., 1997, 7(7), 1155–1161J. Am. Chem. Soc., 1991, 113, 444; (b) K. Kurihara, K. Ohto, K. Taguchi, A. Kamino, H. Koyano and T. Kunitake, L angmuir, 1997, 13, 519. Y. Tanaka, Y. Aoyama and T. Kunitake, T hin Solid Films, 1989, 179, 21. 24 A. Kamino, K.Taguchi, H. Koyano, K. Ariga and T. Kunitake, manuscript in preparation. 14 Y. Ikeura, K. Kurihara and T. Kunitake, J. Am. Chem. Soc., 1991, 113, 7342. 25 K. Taguchi, A. Kamino, H. Koyano, K. Ariga and T. Kunitake, manuscript in preparation. 15 (a) X. Cha, K. Ariga, M. Onda and T. Kunitake, J. Am. Chem. Soc., 1995, 117, 11833; (b) X. Cha, K. Ariga and T. Kunitake, J. Am. 26 H. Koyano, K.Yoshihara, K. Ariga, T. Kunitake, Y. Oishi, O. Kawano, M. Kuramori and K. Suehiro, Chem. Commun., 1996, Chem. Soc., 1996, 118, 73; (c) 9545. 16 H. Kitano and H. Ringsdorf, Bull. Chem. Soc. Jpn., 1985, 58, 2826. 1769. 27 L. J. Bellamy, T he Infrared Spectra of Complex Molecules, 17 (a) T. M. Bohanon, S. Dezinger, R. Fink, W. Paulus, H. Ringsdorf Chapman and Hall, London, 3rd edn., vol. 1, 1975. and M. Weck, Angew. Chem., Int. Ed. Engl., 1995, 34, 58; 28 M. Scoponi, E. Polo, F. Pradella, V. Bertolasi, V. Carassiti and (b) R. Ahuja, P-L. Caruso, D. Mo�bius, W. Paulus, H. Ringsdorf P. Goberti, J. Chem. Soc., Perkin T rans. 2, 1992, 1127. and G. Wildburg, Angew. Chem., Int. Ed. Engl., 1993, 32, 1033. 29 A. Kamino, K. Ariga, T. Kunitake, V. Birault, G. Pozzi, 18 X.-C. Chai, S.-G. Chen, Y.-L. Zhou, Y.-Y. Zhao, T.-J. Li and J.- Y. Nakatani and G. Ourisson, Colloids Surf., 1995, 103, 183. M. Lehn, Chin. J. Chem., 1995, 13, 385. 30 S. Suzuki, S. Isoda and M. Maeda, Jpn. J. Appl. Phys., 1989, 28, 19 M. Shimomura, O. Karthaus and K. Ijiro, Supramol. Sci., 1996, 1673. 3, 61. 31 V. I. Birss, A. S. Hinman, C. E. McGarvey and J. Segal, 20 Y. Ebara, K. Itakura and Y. Okahata, L angmuir, 1996, 12, 5165. Electrochim. Acta, 1994, 39, 2449. 21 M. Onda, K. Yoshihara, H. Koyano, K. Ariga and T. Kunitake, 32 Y. Kyogoku and B. S. Yu, Bull. Chem. Soc. Jpn., 1969, 42, 1387. J. Am. Chem. Soc., 1996, 118, 8524. 33 A. Kamino, H. Koyano, K. Ariga and T. Kunitake, Bull. Chem. 22 K. Taguchi, K. Ariga and T. Kunitake, Chem. L ett., 1995, 701. Soc. Jpn., 1996, 69, 3619. 23 (a) Y. Oishi, Y. Torii, M. Kuramori, K. Suehiro, K. Ariga, K. Taguchi, A. Kamino and T. Kunitake, Chem. L ett., 1996, 411; (b) Y. Oishi, Y. Torii, T. Kato, M. Kuramori, K. Suehiro, K. Ariga, Paper 7/00081B; Received 3rd January, 1997 J. Mater. Chem., 1997, 7(7), 1155–1161 11

ISSN:0959-9428    

DOI:10.1039/a700081b

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Supramolecular dimeric liquid crystals. The liquid crystalline behaviour of mixtures of a-(4-pyridyloxy)-v-[4-(4-butylphenylazo)phenoxy]alkanes and 4-octyloxybenzoic acid Marc J. Wallage and Corrie T. Imrie* Department of Chemistry, University of Aberdeen,Meston Walk, Old Aberdeen, UK AB24 3UE The thermal behaviour of binary mixtures of 1-(4-pyridyloxy)-5-[4-(4-butylphenylazo)phenoxy]pentane (Bu-azo-5-Pyr) and 1-(4- pyridyloxy)-6-[4-(butylphenylazo)phenoxy]hexane (Bu-azo-6-Pyr) with 4-octyloxybenzoic acid is reported.Both systems are miscible over the complete composition range; this miscibility is attributed to the formation of a hydrogen bond between the unlike species, a view confirmed by IR spectroscopy. Bu-azo-5-Pyr and Bu-azo-6-Pyr do not exhibit liquid crystallinity but enhanced liquid crystal behaviour is observed for the mixtures with the acid.In particular, smectic A behaviour is injected into the phase diagrams of both systems. The clearing temperatures of the mixtures containing Bu-azo-6-Pyr are considerably higher than those for the analogous mixture containing Bu-azo-5-Pyr. In addition, the entropy change associated with the clearing transition for the equimolar mixture containing Bu-azo-6-Pyr is significantly higher than that for the analogous mixture containing Bu-azo- 5-Pyr.These observations strongly suggest the formation of a supramolecular dimeric liquid crystal. For the Bu-azo-6-Pyr-based system the specific molecular interaction between the pyridyl and acid fragments has not only strongly enhanced the liquid crystalline behaviour but also increased the degree of molecular ordering.In recent years increasing research activity has focused on Experimental liquid crystalline systems consisting of supramolecular com- Bu-azo-5-Pyr, 1, and Bu-azo-6-Pyr, 2, were prepared using the plexes assembled via noncovalent interactions1–3 but this is by synthetic route shown in Scheme 1.no means a new idea. Indeed, some forty years ago Gray and Jones4 attributed the liquid crystalline behaviour of the 4- 4-Butyl-4¾-hydroxyazobenzene, 4 alkyl- and 4-alkoxy-benzoic acids to hydrogen bonded dimers rather than to discrete molecular units. The novelty of the Compound 4 was prepared as described in detail elsewhere.7 recent work, however, is that the two interacting species are not identical and a hetero-intermolecular bond is used to 1-Bromo-5-[4-(4-butylphenylazo)phenoxy]pentane, 5 assemble the liquid crystal unit.Furthermore, the two interacting molecules need not be, and are often not, liquid crystal- Compound 5 was prepared using the method described by line individually but the resulting supramolecular complex is Attard et al. elsewhere.8 Thus, a mixture containing 4 (4.5 g, mesogenic.This amplification of a specific molecular inter- 17.6 mmol), 1,5-dibromopentane (40.6 g, 176 mmol) and potaction into a macroscopically observable phenomenon, in this assium carbonate (20.68 g, 150 mmol) in acetone (100 ml) was case liquid crystallinity, is very much a central theme in refluxed with stirring overnight. The reaction mixture was supramolecular chemistry.5,6 allowed to cool, filtered and the acetone removed using a The most commonly used interaction in assembling liquid rotary evaporator.Light petroleum (bp 40–60°C) (300 ml) crystalline complexes is the hydrogen bond1–3 and particular interest has centred on mixtures of molecules containing pyridyl and carboxylic acid fragments.2 In these mixtures the pyridyl unit serves as the hydrogen bond acceptor and the carboxylic acid as the hydrogen bond donor.The focus of much of this research has been the role of the hetero-hydrogen bond in determining the transition temperatures and phase behaviour of the system. In comparison, complex formation has not been used in an attempt to manipulate the degree of molecular ordering within the mesophase.In order to investigate this possibility we have now characterised the thermal behaviour of mixtures of two a-(4-pyridyloxy)-v-[4-(4-butylphenylazo) phenoxy]alkanes, 1 and 2, with 4-octyloxybenzoic acid, 3. The acronyms used to refer to 1 and 2 are Bu-azo-5- Pyr and Bu-azo-6-Pyr, respectively. These particular structures were chosen because the analogous covalently bonded structures containing two mesogenic groups linked via a flexible spacer, the so-called dimeric liquid crystals, are known to exhibit mesophases in which the degree of molecular order is critically dependent on the length and parity of the alkyl spacer.Scheme 1 J. Mater. Chem., 1997, 7(7), 1163–1167 1163was added to the organic extracts and the solution cooled to Results and Discussion -20 °C for ca. 2–3 h. The resulting precipitate was collected, Bu-azo-5-Pyr–4-octyloxybenzoic acid mixtures washed with light petroleum and dried under vacuum. The product was recrystallised from ethanol. Yield: 3.93 g, 55%. dH Bu-azo-5-Pyr and 4-octyloxybenzoic acid were miscible over (CDCl3; J values in Hz throughout) 7.9, 7.8, 7.3, 7.0 (m, the complete composition range and this was presumably aromatic, 8H), 4.1 (t, ArOCH2, 2H, J 6.3), 3.4 (t, CH2Br, 2H, attributable to the formation of a hydrogen bond between the J 6.7), 2.7 (t, ArCH2 , 2H, J 7.8), 1.3–2.0 [m, CH3CH2CH2, unlike components in the mixture.This view was supported OCH2CH2)3CH2Br, 10H], 1.0 (t, CH3, 3H, J 7.3). by IR spectroscopy; specifically, the spectra of the complexes contain bands centred at ca. 2490 and 1900 cm-1, indicative 1-Bromo-6-[4-(4-butylphenylazo)phenoxy]hexane, 6 of strong hydrogen bonding.10–13 In addition, the carbonyl band has a shoulder at ca. 1690 cm-1, a characteristic value Compound 6 was prepared using the procedure described for for free carbonyl groups. Representative IR spectra of the 5. Yield: 5.54 g, 71%. dH (CDCl3) 7.9, 7.8, 7.3, 7.0 (m, aromatic, individual components and the equimolar complex are shown 8H), 4.0 (t, ArOCH2, 2H, J 6.4), 3.4 (m, CH2Br, 2H), 2.7 (t, as Fig. 1.Unfortunately, the complexity of the spectrum of the ArCH2, 2H, J 7.6), 1.3–2.0 [m, CH3CH2CH2, complex is such that it prevents even a semi-quantitative OCH2(CH2)4CH2Br, 12H], 0.9 (t, CH3, 3H, J 7.3). assessment of the equilibrium constant for the formation of the complex.Bu-azo-5-Pyr, 1 The dependence of the transition temperatures determined A mixture containing 5 (3.8 g, 9.5 mmol), 4-hydroxypyridine using dierential scanning calorimetry on the mole fraction of (0.92 g, 9.7 mmol) and caesium carbonate (15.6 g, 47.7 mmol) Bu-azo-5-Pyr in the mixture is shown in Fig. 2. Bu-azo-5-Pyr in N,N-dimethylformamide (DMF) (50 ml) was refluxed with melts directly into the isotropic phase at 89°C and can be stirring overnight.The reaction mixture was allowed to cool, supercooled to ca. 70°C without the observation of liquid poured into ice cold water (1 l) and stirred for ca. 30 min. The crystallinity. Similarly, the mixtures containing greater than mixture was extracted with chloroform; the organic layer was 0.5 mol fraction of Bu-azo-5-Pyr do not exhibit liquid crystal- washed with water, dried and the chloroform removed using line behaviour.In comparison, the remaining mixtures are a rotary evaporator. The crude product was passed through liquid crystalline; specifically nematic, smectic C and smectic A silica gel using acetone as the eluent. Yield: 0.6 g, 14.9%.Mp phases are observed. These phases were identified on the basis 85–86°C. dH (CDCl3 ) 8.4, 7.9, 7.3, 6.9 (m, aromatic, 12H), 4.0 of the observation of clear characteristic optical textures when (m, 2 ArOCH2, 4H), 2.7 (t, ArCH2, 2H, J 7.6), 1.2–2.0 [m, viewed through the polarised light microscope.14,15 Specifically, (CH2 )3, CH2CH2 , 10H], 1.0 (t, CH3, 3H, J 7.3). Bu-azo-6-Pyr, 2 Bu-azo-6-Pyr was prepared using the procedure described for Bu-azo-5-Pyr.Yield 0.53 g, 9.7%. Mp 118–120°C. dH (CDCl3) 8.4, 7.8, 7.3, 7.0, 6.8 (m, aromatic, 12H), 4.0 (m, ArOCH2, 4H), 2.7 (t, ArCH2, 2H, J 7.6), 1.3–1.9 [t, (CH2)4, CH2CH2, 12H], 0.9 (t, CH3 , 3H, J 7.3). 4-Octyloxybenzoic acid, 3 4-Octyloxybenzoic acid (Aldrich) was recrystallised from ethanol prior to use.The transition temperatures were in good agreement with those reported elsewhere:9 crystal–smectic C, 101 °C, smectic C–nematic, 108 °C, and nematic–isotropic, Fig. 1 IR spectra of (a) Bu-azo-5-Pyr, (b) 4-octyloxybenzoic acid and 147 °C. (c) the equimolar complex Preparation of complexes The mixtures were prepared by codissolving the components in pyridine and the solvent allowed to evaporate slowly.The complexes were dried under vacuum for at least 24 h prior to characterisation. Characterisation The proposed structures of all the compounds were verified using 1H NMR and IR spectroscopy. 1H NMR spectra were measured in CDCl3 on a Bruker AC-F 250 MHz NMR spectrometer. IR spectra were recorded using a Nicolet 205 FTIR spectrometer. Thermal characterisation The thermal behaviour of the materials was characterised by dierential scanning calorimetry using a Mettler-Toledo DSC Fig. 2 Dependence of the transition temperatures on the mole fraction 820 system equipped with an intracooler accessory and cali- of Bu-azo-5-Pyr 1 for mixtures of Bu-azo-5-Pyr and 4-octyloxybenzoic brated using an indium standard. The heating and cooling acid. The broken line represents the melting point; (#) rates in all cases were 10°C min-1.Phase identification was smectic A–isotropic transition; (1) smectic C–nematic transition, (%) performed by polarised light microscopy using an Olympus nematic–isotropic transition; (() smectic C–smectic A transition. C= BH-2 optical microscope equipped with a Linkam THMS 600 crystal; N=nematic; SC=smectic C; SA=smectic A; I=isotropic.Crystal–crystal transitions have been omitted for the sake of clarity. heating stage and TMS 91 control unit. 1164 J. Mater. Chem., 1997, 7(7), 1163–1167for the nematic phase a Schlieren texture was observed contain- and a nematic phase whereas the 0.7, 0.8 and 0.9 mixture exhibit solely nematic behaviour. The 0.95 mixture and ing both types of point singularity and which flashed when subjected to mechanical stress, while for the smectic C phase pure Bu-azo-6-Pyr do not exhibit liquid crystallinity.The virtual clearing temperature of Bu-azo-6-Pyr, estimated by a Schlieren texture was also observed but in which only one type of point singularity was evident. For the smectic A phase extrapolating the clearing temperature curve, is ca. 74°C. As with the Bu-azo-5-Pyr-based mixtures, the melting points a focal conical fan texture was observed in coexistence with regions of homeotropic alignment. of the Bu-azo-6-Pyr-based mixtures tend to be significantly higher than expected on the basis of a linear variation of the The mixtures containing 0.05 and 0.1 mol fraction of Buazo- 5-Pyr exhibit the same phase sequence as the pure acid, melting point on composition, see Fig. 3, and again this is indicative of a specific interaction between the unlike compo- namely smectic C–nematic–isotropic. The nematic–isotropic transition temperature falls on increasing the concentration of nents. The clearing temperatures for the Bu-azo-6-Pyr-based mixtures are also considerably higher than expected and Bu-azo-5-Pyr whereas the smectic C–nematic transition temperature appears to be insensitive to changes in composition.smectic A behaviour is injected into the phase diagram. For the 0.2 mol fraction Bu-azo-5-Pyr mixture, a smectic A phase is injected between the smectic C and nematic phase. Comparison of the systems The mixtures containing 0.3, 0.4 and 0.5 mol fraction of Bu- The estimated virtual transition temperature of Bu-azo-6-Pyr azo-5-Pyr exhibited exclusively smectic A behaviour.Thus, is ca. 22°C higher than that for Bu-azo-5-Pyr. This dierence increasing the concentration of Bu-azo-5-Pyr strongly pro- is in accord with the pronounced alternation observed for motes smectic A rather than nematic behaviour; we will return conventional low molar mass mesogens containing a bulky to this observation below.group attached via a flexible alkyl spacer to the mesogenic The melting points of the Bu-azo-5-Pyr–4-octyloxybenzoic core.16–19 Thus, these estimated values are both self-consistent acid mixtures are typically significantly higher than those of and reasonable. The miscibility observed for both systems the individual components, see Fig. 2. This behaviour strongly impliesthat the hydrogen bond between the unlike components suggests that the unlike components exhibit a specific inter- is more favourable than that present in the self-associated acid action. By comparison, the clearing temperature initially dimers. Furthermore, the strength of this hydrogen bond is decreases in essentially a linear fashion on increasing the sucient to counteract the unfavourable entropic term which concentration of Bu-azo-5-Pyr, see Fig. 2. The extrapolation tends to promote phase separation.9 of the linear segment of the clearing temperature curve allows The smectic A–isotropic transition temperature for the equi- the virtual clearing temperature of Bu-azo-5-Pyr to be esti- molar mixture containing Bu-azo-6-Pyr,135 °C, is considerably mated at ca. 52°C. The clearing temperatures for the 0.3, 0.4 higher than that observed for the Bu-azo-5-Pyr-based mixture, and 0.5 mol fraction of Bu-azo-5-Pyr mixtures deviate in a 109°C. This large dierence, similar to that observed for positive sense away from a linear dependence on composition. conventional dimeric liquid crystals,20,21 may be accounted for by considering the relative shapes of the complexes.Thus, for Bu-azo-6-Pyr–4-octyloxybenzoic acid mixtures the Bu-azo-6-Pyr–acid complex the mesogenic groups are coparallel if the spacer is in the all-trans conformation, see The dependence of the transition temperatures on the mole fraction of Bu-azo-6-Pyr in the mixtures is shown in Fig. 3. Fig. 4(a); this arrangement reinforces the intramolecular orientational correlations between the mesogenic cores and, hence, The phase assignments were performed using the arguments oered for the Bu-azo-5-Pyr-based mixtures.In addition, the enhances the clearing temperature. In contrast, for the Bu-azo- 5-Pyr–acid complex when the spacer is in the all-trans confor- IR spectra obtained for the mixtures were essentially identical to that shown in Fig. 1. The mixtures containing 0.05 and mation the mesogenic groups are constrained to lie at an angle with respect to each other, see Fig. 4(b), so reducing the 0.1 mol fraction of Bu-azo-6-Pyr exhibit smectic C and nematic phases and the associated transition temperatures are similar clearing temperature. All the mixtures containing Bu-azo-6- Pyr exhibit higher clearing temperatures than the correspond- to those of the pure acid.The 0.2 mol fraction of Bu-azo-6- Pyr mixture exhibits smectic C and A phases while the 0.3, 0.4 and 0.5 mol fraction mixtures exhibit exclusively smectic A behaviour. The 0.6 mol fraction mixture shows a smectic A Fig. 3 Dependence of the transition temperatures on the mole fraction of Bu-azo-6-Pyr 1 for mixtures of Bu-azo-6-Pyr and 4-octyloxybenzoic Fig. 4 Schematic representation of the eect on the shape of the acid. Crystal–crystal transitions have been omitted for the sake of clarity. (') Smectic A–nematic transition; all other symbols are as complex on introducing a single gauche defect into the spacer for (a) an even-membered and (b) an odd-membered spacer defined in Fig. 2. J. Mater. Chem., 1997, 7(7), 1163–1167 1165ing mixture containing Bu-azo-5-Pyr and this simply reflects the higher clearing temperature of the Bu-azo-6-Pyr–acid complex. The entropy change associated with the smectic A–isotropic transition, expressed as the dimensionless quantity DS/R, for the equimolar mixture containing Bu-azo-6-Pyr, 3.61, is considerably larger than that for the Bu-azo-5-Pyr-based equimolar mixture, 1.65; these values have been calculated assuming the formation of a 151 complex. This dramatic dierence in the entropy change associated with the clearing transition on adding just a single methylene unit is characteristic behaviour for conventional dimeric liquid crystals20,21 and may be accounted for by considering the inherent flexibility of the spacer.Indeed, the rationalisation of the dependence of the clearing temperatures on the parity of the spacer considered just the all-trans conformation of the spacer which is clearly unrealistic. If instead we consider the eects of introducing a single gauche defect into the spacer and allow it to move sequentially along the chain, then there are still conformations of an even-membered spacer in which the mesogenic units lie coparallel, see Fig. 4(a). In comparison, all the conformations of the odd-membered spacer constrain the mesogenic units to lie at an angle with respect to each other, see Fig. 4(b). The smectic A phase is an anisotropic environment which selects the more elongated conformations in which the mesogens are coparallel. Thus, at the smectic A–isotropic transition, there is a greater change in the conformational distribution of an evenmembered spacer than for an odd-membered spacer.Consequently, the conformational component of the overall smectic A–isotropic entropy change is higher for even-membered dimers, i.e. the equimolar Bu-azo-6-Pyr–acid mixture. We must remember, however, that are three main contributions Fig. 5 Probable local packing arrangements in the smectic A phase for mixtures containing (a) an equimolar ratio of the two components, to the overall entropy change, conformational, orientational (b) an excess of the acid and (c) an excess of 1. Shaded ellipses represent and translational, and it is the subtle interplay of these that azobenzene, open ellipses represent hydrogen bonded mesogenic units, determines the overall value.Indeed, calculations suggest that filled ellipses represent hydrogen bonded acid dimers and the pear- the conformational component may only be a relatively small shape represents free pyridyl units. component of the overall entropy change.21 Thus, further speculation on the molecular significance of the pronounced dierence in the smectic A–isotropic entropy change between these supramolecular complexes must now await further inves- chain interaction while entropically, the unfavourable inter- tigation.It is apparent, however, that the specific molecular action between a core and a chain which acts to order the interaction between the pyridyl and acid fragments in the Bu- chain drives phase separation. Thus, the equimolar complex azo-6-Pyr–acid equimolar mixture is manifested at the macro- would be expected to, and indeed does, exhibit smectic rather scopic level not only by the induction of smectic A behaviour than nematic behaviour. If we now increase the mole fraction into the phase diagram but also by the high degree of molecular of the acid component then hydrogen bonded acid dimers will ordering within the mesophase.It should be noted that if the be present. Initially, these may be accommodated within the two components are simply miscible but exhibit non-ideal smectic phase structure, see Fig. 5(b), but as their concentration behaviour then the clearing entropy would not be so critically increases the terminal octyl chains, which are too long to be dependent on spacer length.The non-ideality in the transition accommodated within the spacer domains, destabilise the temperatures of some liquid crystal mixtures is driven by an smectic phase and nematic behaviour is observed. Conversely, enthalpic term and the clearing entropy is essentially unaec- if the Bu-azo-Pyr component is in excess than the packing ted. Thus, the magnitude of the clearing entropies for the density in the smectic phase is reduced, see Fig. 5(c), and again systems discussed here are indicative of complex formation. nematic behaviour is favoured. The strong induction of smectic A behaviour in both phase diagrams, see Fig. 2 and 3, may be accounted for in terms of the relative tendencies of dimeric liquid crystals to form smectic Conclusion phases and the probable local structure within the smectic phase.It has been shown that if the lengths of the terminal In this study, we have seen that the phase behaviour and alkyl chains attached to a dimer exceed half the length of the transition temperatures of mixtures 1 and 2 with 3 are deter- flexible spacer, as, for example, in both equimolar complexes, mined by the formation of a hydrogen bond between the then a monolayer smectic phase results.21 The formation of unlike components; this is quite general behaviour.1–3 For the this phase may be understood in terms of a microphase first time, however, we have shown how the degree of molecular separation in which there exists three distinct regions: meso- order within the mesophase can also be controlled by hydrogen genic groups, terminal alkyl chains and spacers, see Fig. 5(a). bonding at the molecular level. In these dimeric structures the terminal chains are simply too long to be accommodated within the region comprising the spacers. The driving force resulting in this phase separation We are pleased to acknowledge support from the EPSRC, grant number GR/J32701 and from the University of Aberdeen may be considered as either energetic or entropic.Energetically, phase separation will occur if the mean of the core–core and for the award of a grant to purchase the Mettler-Toledo DSC 820. chain–chain interactions is more favourable than the core– 1166 J. Mater. Chem., 1997, 7(7), 1163–116713 S. E. Odinokov, A. A. Mashkovsky, V. P. Glazunov, References A. V.Iogansen and B. V. Rassadin, Spectrochim. Acta, 1976, 32A, 1 C. T. Imrie, T rends Polym. Sci., 1995, 3, 22. 1355. 2 T. Kato and J. M. J. Fre� chet,Macromol. Symp., 1995, 98, 311. 14 D. Demus and L. Richter, T extures of L iquid Crystals, Verlag 3 C. M. Paleos and D. Tsiourvas, Angew. Chem., Int. Ed. Engl., 1995, Chemie,Weinheim, 1978. 34, 1696. 15 G. W. Gray and J. W. Goodby, Smectic L iquid Crystals—T extures 4 G. W. Gray and B. J. Jones, J. Chem. Soc., 1953, 4179. and Structures, Leonard-Hill, Glasgow, 1984. 5 J.-M. Lehn, Angew. Chem., Int. Ed. Engl., 1990, 29, 1304. 16 G. W. Gray and K. J. Harrison, Mol. Cryst. L iq. Cryst., 1971, 6 J.-M. Lehn, Angew. Chem., Int. Ed. Engl., 1988, 27, 89. 13, 37. 7 A. A. Craig and C. T. Imrie, Polymer, 1997, in press. 17 G. W. Gray and K. J. Harrison, Symp. Faraday Soc., 1971, 5, 54. 8 G. S. Attard, C. T. Imrie and F. E. Karasz, Chem. Mater., 1992, 18 G. W. Gray, J. Phys. (Paris), 1975, 36, 337. 4, 1246. 19 D. Coates and G. W. Gray, J. Phys. (Paris), 1975, 36, 365. 9 K. I. Alder, D. Stewart and C. T. Imrie, J. Mater. Chem., 1995, 20 G. S. Attard, R. W. Date, C. T. Imrie, G. R. Luckhurst, 5, 2225. S. J. Roskilly, J. M. Seddon and L. Taylor, L iq. Cryst., 1994, 16, 10 T. Kato, T. Uryu, F. Kaneuchi, C. Jin and J. M. J. Fre� chet, L iq. 529. Cryst., 1993, 14, 1311. 21 R. W. Date, C. T. Imrie, G. R. Luckhurst and J. M. Seddon, L iq. 11 S. L. Johnson and K. A. Rumon, J. Phys. Chem., 1965, 69, 74. Cryst., 1992, 12, 203. 12 J. Y. Lee, P. C. Painter and M.M. Coleman, Macromolecules, 1988, Paper 7/00848A; Received 5th February, 1997 21, 954. J. Mater. Chem., 1997, 7(7), 1163–1167

ISSN:0959-9428    

DOI:10.1039/a700848a

出版商:RSC    

年代:1997

数据来源: RSC