摘要:

FEATURE ARTICLE Chromatography on chiral stationary phases Stig Allenmarka and Volker Schurigb aDepartment of Organic Chemistry, Go� teborg University, S-41296 Go�teborg, Sweden bInstitut fu� r Organische Chemie, Universita�t Tu� bingen, Auf der Morgenstelle 18, D-72076 T u� bingen, Germany An overview is given of the development of the variety of chiral stationary phase materials that are available today for the direct separation of enantiomers by diVerent chromatographic methods.Applications of these range from trace analytical determinations of enantiomeric composition in biological materials to large scale preparative isolation of enantiopure compounds of industrial importance. These chromatographic techniques are also of interest for studies of mechanism and dynamics, like enantiomerization processes due to chiral inversion taking place in thermally labile chiral molecular structures.when in 1966 acetylated microcrystalline cellulose was used with ethanol as the mobile phase to achieve separation of Introduction enantiomers.4 It was shown later5 that heterogeneous acetylation with almost complete preservation of the microcrystallin- The separation of enantiomers by chromatography can be performed in two modes: indirect method (a), in which oV- ity of the cellulose to give a triacetate of each glucose unit could be performed easily, giving a widely useful chiral sorbent.column conversion of enantiomers into diastereomeric derivatives occurs, by chemical reaction with an enantiomerically It was also found that the microcrystallinity was essential for the enantioselective properties of the material and that it was pure resolving agent and subsequent chromatographic separation of the diastereomers on a conventional achiral stationary lost on dissolution and reprecipitation. It also became clear that this material, at least to some extent, operated by means phase; and direct method (b), in which chromatographic separation of the enantiomers takes place on a chiral stationary of inclusion of the solute into molecular cavities, and therefore the term ‘inclusion chromatography’ was often used to charac- phase (CSP) containing a resolving agent in high but not necessarily complete enantiomeric purity.terize the complex separation process. While method (a) involves the formation of diastereomers before separation, method (b) relies on the diVerent diastereo- Chiral stationary phases (CSPs) in gas meric molecular association between the chiral, non-racemic chromatography stationary phase (selector) and the chiral analyte (selectand). Since diastereomers usually possess diVerent physical proper- The situation prevailing at the outset of enantioselective gas ties, an unintended discrimination may arise during detection chromatography in the late sixties is vividly described by when using method (a).Also fractionation may occur as the Gil-Av, the early pioneer in the field:6 result of incomplete recovery, decomposition and losses during work-up, isolation and sample handling. Furthermore, racemization and kinetic resolution must be absent in the formation of diastereomers by the indirect method (a).Thus, the more refined direct method (b) is preferred in modern enantiomer separation and analysis whenever possible. Ever since the days of Louis Pasteur, separation of racemates into enantiomers has been a challenge. Still a rather unpredictable matter, the situation has been drastically improved, however, owing to the development of chiral phase systems in chromatography, enabling direct enantiomer separation in a flow system. The crucial issue for the success of these techniques is the chiral material used as the stationary phase in the system.Some early experiments with liquid chromatography on columns packed with lactose1,2 demonstrated that such separations could be realized; however, no systematic research was made until Gil-Av and co-workers started to use amides and oligopeptides as chiral stationary phases (CSPs) in gas chromatography for the separation of racemic amino acid derivatives.3 The chiral discrimination between the enantiomers in this case was caused by a slightly more favourable multiple hydrogen bonding interaction with the stationary phase for the more retained enantiomer.As illustrated by Fig. 1, the CSP and the solute were complementary to each other, forming transient diastereomeric complexes possessing a small diVerence in stability which caused the resolution. Fig. 1 Selector–selectand hydrogen-bonding interactions in amidebased CSPs A similar breakthrough took place in liquid chromatography J. Mater.Chem., 1997, 7(10), 1955–1963 1955‘‘When we started this work in 1964, this topic was in a ‘state of frustration’. Nobody believed it could be done. In fact, people were convinced that there could not possibly be a large enough diVerence in the interaction between the D- and Lsolute with an asymmetric solvent. This was the feeling people had, even those known as unorthodox thinkers.This view had also some experimental basis, because a number of communications had been published, in which it was claimed that such resolutions could be eVected, but nobody was able to reproduce these results, and some of them were shown to be definitely wrong.’’ Paradoxically, the situation is almost reversed 30 years later. According to the Chirbase databank most classes of volatile chiral compounds are amenable to enantiomer separation by gas chromatography on various chiral stationary phases (CSPs).7 High eYciency, sensitivity and speed of separation are important advantages of enantiomer separation by high resolution capillary gas chromatography.Established ancillary techniques such as multi-dimensional gas chromatography (in series-coupled column operation), use of interfacing and coupling methods (gas chromatography–mass spectrometry, GC–MS) can also be adapted readily for chiral separations.Sensitivity can be extended to the picogram level by GC–MS, Scheme 1 Hydrogen-bonding-type CSPs8 electron capture detection (ECD) or element-specific detection. Employing GC–MS selected ion monitoring (SIM) trace amounts of enantiomers present in complex matrices can be detected easily.The direct method (b) is especially useful for chiral analysis when no sample derivatization is required, e.g., by head space analysis whereby volatile enantiomers are directly analysed from the vapour phase of the sample matrix. Owing to the enormous separation power of capillary gas chromatography, contaminants and impurities are usually separated from the analytes and the simultaneous analysis of multi-component mixtures of enantiomers (e.g., the proteinogenic amino acids, Fig. 2) is possible in one analytical run. A prerequisite for the use of method (b) is volatility, thermal stability and resolvability of the chiral analyte. Unless specific derivatization techniques are utilized to increase volatility, capillary supercritical fluid chromatography (SFC) and capillary electrochromatography (CEC) are important complementary methods applicable to nonvolatile analytes.A new Fig. 2 Enantiomer separation of proteinogenic a-amino acids as Ndevelopment is the use of micro-packed liquid chromatography (O,S)-pentafluoropropanoyl isopropyl esters ( histidine as Nim-ethoxy- (m-LC) or open-tubular liquid chromatography (OTLC).carbonyl derivative) on Chirasil-Val 4 (20 m×0.27 mm id glass capillary, 85–185 °C, 0.35 bar H2). All D enantiomers are eluted before the Enantiomer separation by gas chromatography is routinely L enantiomers.11 performed with three types of chiral stationary phases (CSPs):8,9 enantiomer separation on chiral amino acid derivatives via hydrogen bonding;6,10–13 enantiomer separation on and (2-carboxypropyl)methylsiloxane of appropriate vischiral metal coordination compounds via complexation;14 and cosity.10 The resulting polymeric CSP Chirasil-Val 4 exhibits enantiomer separation on cyclodextrin derivatives via (inter excellent gas chromatographic properties for the enantiomer alia) inclusion.15–17 separation of a number of classes of chis over the temperature range 0–250 °C.12,16 The simultaneous enantiomer separation of all proteinogenic amino acids in less than Chiral stationary phases based on hydrogen bonding 25 min is illustrated in Fig. 2.11 Enantiomer separation by As mentioned beforehand, the first successful separation of hydrogen bonding CSPs generally requires derivatization of racemic N-trifluoroacetyl amino acid esters on a glass capillary the analyte in order to increase volatility and/or to introduce column coated with non-volatile N-trifluoroacetyl-L-isoleucine suitable functions for additional hydrogen bonding association.lauryl ester 1 (Scheme 1) was achieved in 1966 by Gil-Av et al.3a Subsequent studies revealed that in the dipeptide phase Chiral stationary phases based on coordination 2 (Scheme 1) the C-terminal amino acid was not essential for chiral recognition; however, the second amide function was As a first example of enantiomer discrimination by complexation gas chromatography, the chiral metal coordination important for additional hydrogen bonding.This observation led to the development of the diamide 3 derived from valine compound dicarbonylrhodium(I) 3-trifluoroacetyl-(1R)-camphorate 5 (Scheme 2) was used for the enantiomer separation which represents the most eYcient selector possessing a single chiral centre.18 Previously, the chiral selectors were used as of the chiral alkene 3-methylcyclopentene.14 This method was later extended to oxygen-, nitrogen- and sulfur-containing non-volatile neat liquids which were coated on the inner capillary surface. Subsequently, Frank et al.synthesized analytes using manganese(II), cobalt(II ) and nickel(II) bis[(3- heptafluorobutanoyl)-(1R)-camphorate] as stationary phases Chirasil-type stationary phases by chemically linking the selector to a polysiloxane. Thus, the diamide 3 was coupled via the dissolved in squalane or dimethylpolysiloxane.8,14 In Fig. 3 the enantiomer separation of eight stereoisomeric amino function to a statistical copolymer of dimethylsiloxane 1956 J. Mater. Chem., 1997, 7(10), 1955–1963employed in a packed column for the enantiomer separation of a- and b-pinene and cis- and trans-pinane.20 Subsequently, it was recognized that alkylated cyclodextrins (CDs) can be used in capillary columns for analytical enantiomer separation.Thus, undiluted permethylated b-cyclodextrin 8 (Scheme 3) was employed above the melting point.21 Since per-n-pentylated and 3-acyl-di-2,6-n-pentylated cyclodextrins are liquids even at room temperature, the derivatives 9–14 (trade name Lipodex) have been used in the undiluted form for the separation of enantiomers of many classes of compounds on deactivated Pyrex glass and fused silica capillary columns by Ko� nig.16 The more polar CD derivatives containing hydroxypropyl, free hydroxy groups or trifluoroacetyl groups, respectively, 15–19, developed by Armstrong et al., were coated on fused silica capillary columns.22 In another approach aimed at combining the enantioselectivity of the selectors with the excellent gas chromatographic properties of polysiloxanes, alkylated cyclodextrins like 8 were dissolved in moderately polar silicones such as OV-1701.15 Thus, the selectors could be employed for gas chromatographic enantiomer separation irrespective of their melting point or possible phase transitions.The separation of enantiomers of saturated cyclic hydrocarbons, devoid of any functionality, is demonstrated in Fig. 4. The presence of three hydroxy groups which can be regiose- Scheme 2 Complexation-type CSPs8 lectively alkylated and acylated oVers an enormous number of possible a-, b- and c-cyclodextrin derivatives. The readily available permethyl-b-cyclodextrin 8 proved to be the most versatile chiral selector, although for selected applications other derivatives such as octakis(3-O-butanoyl-2,6-di-O-npentyl)- c-cyclodextrin (Lipodex E) 14 are very useful.16 In analogy to Chirasil-Val, permethylated b-cyclodextrin has been chemically linked via a mono-6-octamethylene spacer to Fig. 3 Rapid simultaneous separation of eight stereoisomers (enantiomers and diastereoisomers) of 2-methyl-3-(1¾-methylpropyl)oxirane on 0.125 M nickel(II ) bis[3-heptafluorobutanoyl-(1R)-camphorate] 6 hexakis(2,3,6-tri-O-pentyl )-a-cyclodextrin Lipodex A Q in SE 30 (20 m×0.25 mm id glass capillary, 90 °C, 1 bar N2).Peak hexakis(3-O-acetyl-2,6-di-O-pentyl )-a-cyclodextrin Lipodex B N assignment:8 trans: 1: 2S,3S,1¾S; 2: 2R,3R,1¾R; 3: 2S,3S,1¾R; 4: 2R,3R,1¾S. heptakis(2,3,6-tri-O-pentyl )-b-cyclodextrin Lipodex C N 9–14 heptakis(3-O-acetyl-2,6-di-O-pentyl )-b-cyclodextrin Lipodex D R cis: 5: 2S,3R,1¾R; 6: 2R,3S,1¾S; 7: 2S,3R,1¾S; 8: 2R,3S,1¾R. octakis( 2,3,6-tri-O-pentyl )-c-cyclodextrin NN octakis(3-O-butanoyl-2,6-di-O-pentyl )-c-cyclodextrin Lipodex E S hexakis[-O-{(S)-2-hydroxypropyl}-per-O-methyl]-a-cyclodextrin PMHP-a-CD Q aliphatic oxiranes by complexation gas chromatography is heptakis[-O-{(S)-2-hydroxypropyl}-per-O-methyl]-b-cyclodextrin PMHP-b-CD N15–19 illustrated.hexakis(2,6-di-O-pentyl )-a-cyclodextrin dipentyl-a-CDR heptakis(2,6-di-O-pentyl )-b-cyclodextrin dipentyl-b-CDN A limitation of the use of coordination-type CSPs 5 and 6 heptakis(3-O-trifluoroacetyl-2,6-di-O-pentyl )-b-cyclodextrin DPTFA-b-CDS is the low temperature range of operation (25–120 °C). The thermostability has been increased by the preparation of immobilized polymeric CSPs (Chirasil-Metal) 7 (Scheme 2).19 Chirasil-Metal stationary phases can also be applied with supercritical carbon dioxide as the mobile phase by capillary supercritical fluid chromatography (SFC).19 The use of low temperatures in SFC improves the enantioselectivity at the expense of a loss of eYciency.Chiral stationary phases based on inclusion As a first example of enantiomer discrimination by inclusion Scheme 3 Inclusion-type CSPs8 gas chromatography, native a-cyclodextrin in formamide was J.Mater. Chem., 1997, 7(10), 1955–1963 1957Fig. 4 Enantiomer separation of cis- and trans-1-ethyl-2-methylcyclohexane and cis- and trans-1-methyl-2-n-propylcyclohexane on 0.07 M permethyl-b-cyclodextrin 8 in OV-1701 (30 m×0.25 mm id fused silica capillary, 50 °C, 1.5 bar H2)8 a dimethylpolysiloxane backbone to obtain Chirasil-Dex 20 (Scheme 3).19 The chemically bonded chiral polymers combine the selectivity of the CSP with the eYciency of polysiloxanes, thus aVording high resolution capillary columns with an extended range of operating temperatures (0–220 °C).Even separations at temperatures as low as -20 °C are feasible.In addition, fused silica capillary columns coated with Chirasil- Dex 20 possess advantages such as the presence of a non-polar polysiloxane matrix resulting in low elution temperatures for polar analytes, a high degree of inertness allowing analysis of polar compounds without prior derivatization and long-term Scheme 4 Synthesis of Chirasil-man-18C6-C2524 stability.Chirasil-Dex stationary phases can also be thermally immobilized on the inner surface of fused silica capillary columns. Immobilization of Chirasil-Dex 20 is the prerequisite spent in the column. Consequently, the purpose of a chiral sorbent is to exhibit a diVerent aYnity towards the two for use in chiral supercritical fluid chromatography (SFC),19 open-tubular electro-chromatography (OTCEC) and open- enantiomers of a racemate applied to the column and thereby cause a diVerence in their migration rates large enough to tubular liquid chromatography (OTLC), or a unified approach involving all four methods GC, SFC, LC and CEC and cause their complete separation.The sorbent is most often composed of a solid support with a large surface area, usually employing a single column.23 A new Chirasil-type stationary phase 21 is based on a chiral silica, upon which the chiral stationary phase (CSP) has been immobilized.In this case the CSP constitutes only a surface crown ether derived from mannitol (Scheme 4).24 layer exposed to the mobile phase. Another category of sorbents, lacking the support phase, also exists. Here, the CSP ishiral phase systems in liquid chromatography used in the form of small beads, which are totally porous.This is an alternative for polymeric CSPs, giving a high column Separation of enantiomers by chiral liquid chromatography (CLC) provides a wide variety of possibilities in experimental capacity at the expense of eYciency. The general design and morphology of chiral sorbents are given in Fig. 5. design and today there is a large number of stationary phases which can be combined with diVerent mobile phase systems In liquid chromatography it is customary to distinguish between normal phase and reversed phase modes of separation, to achieve resolution of a racemate.25 Furthermore, improvements in selective detection techniques, e.g., use of chiroptical26 depending upon whether the mobile phase is less or more polar than the stationary phase.Generally speaking, inter- and mass spectrometric27 detection, add to the usefulness of CLC which spans from trace analytical to truly preparative- actions based on hydrogen bonding, charge transfer or Coulombic forces are favoured in normal phase systems, scale resolutions. In the following we will review the achievements made in whereas hydrophobic interactions dominate in reversed phase systems.However, intermediate cases, which are diYcult to the field of chiral sorbents useful for the direct separation of enantiomers in the liquid chromatographic mode. classify, also exist. Table 1 gives a general overview of the various categories and types of chiral sorbents available, Generally, a column for CLC is densely packed with small solid particles constituting the sorbent, i.e., the column bed.A ordered roughly in increasing complexity. It should be emphasized that even if the type of interactions compound migrating through the column will be partitioned between the sorbent and the mobile phase used for elution causing retention is often fairly well known, the mechanism behind the chiral discrimination observed is a far more complex and its relative aYnity to the sorbent will determine its time 1958 J.Mater. Chem., 1997, 7(10), 1955–1963Table 1 Main types of chiral sorbents in liquid chromatography sorbent main reference selector-based, brush-type chiral sorbents: ligand exchange 28 crown ether inclusion 29 cyclodextrin interaction 30 charge transfer complexation 31 other types: antibiotics 32 synthetic receptors 33 sorbents based on synthetic and natural polymers: polyacrylamides and polymethacrylates 34,35 polysaccharides: 36 esters carbamates proteins 37 the first type in Fig. 5, i.e., the selector is immobilized via a two to three carbon spacer to the silica surface. Columns packed with these sorbents are now commercially available (such as ChirobioticTM, Astec Corp.) and have been used for the resolution of large number of diVerent racemates.Another complex, basket-shaped selector, although not naturally occurring, but synthesized in the laboratory [23, Fig. 6(b)], was found to possess almost receptor-like properties towards certain members a series of amides of N-acyl amino acids.33 Note that this selector has C3 symmetry and is based on (S)-tyrosine as the chiral building block.Interestingly, when the N-Boc methylamide derivatives were resolved on the column, the (S)-enantiomers were consistently most retained, whereas for the N-3,5-dinitrobenzoyl (DNB) hexylamides the opposite elution order was found. This was interpreted in terms of the operation of two opposing mechanisms, viz.inclusion (i.e., binding to the interior of the basket) in the former case and p-stacking in the latter (interactions at the outside of the basket between the p-donating aromatic rings of the selector and the p-accepting DNB groups in the selectand). For some of the racemates the a values were exceptionally high, with a=43 in the case of N-Boc-Thr-NHMe, corresponding to a Gibbs free energy diVerence in binding aYnity (DDG) of 2.2 kcal mol-1.Normal phase conditions, employing dichloromethane with 0.5–1%of methanol as retention modifier, were used for these separations. Selectors based on aromatic p–p interactions have undergone a dramatic development over the last few years and have recently been designed [24, Fig. 6(c)] to make use of an additional face-to-edge p–p interaction.38 As seen in Fig. 6(c), a selector of this type is designed to have a cleft, consisting of p-acidic and p-basic aromatic parts directed almost perpendicular to each other, to which one enantiomer is preferentially Fig. 5 (a) Morphology of diVerent kinds of chiral sorbents; (b) scanning bound. The enantioselective binding has been shown by NMR electron microscopic appearance of beads of derivatized cellulose studies to be due to a simultaneous face-to-face and face-to- (From ref. 36, reproduced with permission from VCH Verlagsgesellschaft mbH, Weinheim) edge p–p interaction.39 These phases (commercially available as Whelk-OTM, Regis Co.) typically operate under normal phase conditions with hexane containing a retention modifier like propan-2-ol.problem and in most cases it is incompletely understood. Below some of the more recent advances in the development A series of new CSPs based on N,N¾-diallyl-L-tartardiamide (DATD) as the chiral building block was recently introduced.40 of chiral sorbents of diVerent categories are summarized. Naturally occurring macrocyclic peptides (antibiotics pro- These sorbents contain O,O¾-diacyl-DATD selector units [25, Fig. 6(d)] anchored in a network polymeric structure which is duced by fungi) like vancomycin and analogues have been recently exploited as CSPs in reversed as well as normal phase covalently bound to the silica matrix [corresponding to the second type in Fig. 5(a)]. The columns based on this sorbent systems.32 These chiral selectors are of a relatively complex structure [22, Fig. 6(a)] and the mechanism behind their chiral operate in a normal phase mode with mobile phases based on hexane with alcohols or ethers as hydrogen bonding modifiers. discrimination ability is virtually unknown, although it has been suggested that they act by a combination of hydrogen The nature of the acyl substituents was found to greatly influence the chiral recognition behaviour of these CSPs which bonding, p–p complexation, dipole stacking, inclusion and steric interaction.32 Vancomycin contains 18 stereogenic are supposed to cause retention and enantiomer discrimination via a combination of hydrogen bonding and p–p interactions.centres, whereas the others used, rifamycin B, thiostrepton and teichoplanin, contain 9, 17 and 23, respectively.They represent Two of the CSPs investigated, 25a and 25b, are now commer- J. Mater. Chem., 1997, 7(10), 1955–1963 1959Fig. 6 Some diVerent types of chiral selectors used in liquid chromatography. (a) Vancomycin 22 (used in Chirobiotic-V@), (b) C3-symmetric receptor 23 of Gasparrini et al.,33 (c) selector 24 based on p–p interaction [used in (S,S)-Whelk-01@], (d) chiral diaroyl-DATD unit 25 used in network polymers (Kromasil-CHI@ DMB and TBB, respectively).In the 3D representation of 23, the tyrosyl side chains have been omitted for clarity. 1960 J. Mater. Chem., 1997, 7(10), 1955–1963cially available as KromasilA CHI-DMB and CHI-TBB, changing its net charge, conformation and hydrophobicity as a function of the pH, ionic strength, organic solvent modifier respectively.Synthetic polymers as CSPs date back to the early 1970s or other additive used. Invariably, immobilization of the protein makes use of the terminal primary amino groups in and the pioneering work of Blaschke and Donow,34 who created intrinsically chiral polymer particles via suspension the lysine side chains which are easily reacted with the functionalized silica, either directly or via a bifunctional reagent.copolymerization of optically active acrylamides and acrylates with a suitable crosslinker like ethylenediacrylate or methylene- The most widely used, and commercially available, sorbents thus far are based on serum albumins (BSA, HSA), a1-acid bisacrylamide.Some years later, a diVerent type of chiral polymer was prepared for the first time.35 This was obtained glycoprotein (AGP) and ovomucoid (OVM). Interestingly, a recent study of the last of these has shown that the chiral by anionic polymerization of triphenylmethyl methacrylate in the presence of a chiral initiator, leading to polymers which recognition is not caused by ovomucoid but by another protein from egg white, tentatively named ovoglycoprotein, present as were chiral by virtue of their single-handed helicity.An outline of the reactions used and structures obtained is given in a contaminant.45 Considerable progress has been made in the understanding of the mechanism of chiral discrimination by Fig. 7(a). Synthetic polymers based on a diVerent concept, viz.poly- proteins via a combination of 2D NMR spectroscopy and molecular modelling.46 mers containing chiral cavities obtained via ‘molecular imprinting’ techniques41–43 by the use of chiral templates, constitute another interesting class of materials intended for Current trends in analytical and preparative direct liquid chromatographic resolution purposes. These chiral phases are prepared by first allowing polymerizable molecules, applications bearing suitable functional groups, to arrange in solution The real advantage of the direct separation of enantiomers in around the template molecules.Then polymerization is a flow system appears when the racemate has a structure initiated to give a solid polymer from which, after grinding which precludes the application of classical resolution tech- and sieving to obtain suitable fine particles, the template is niques.For example, it has been possible to resolve hydro- removed by hydrolysis or by a simple washing procedure. carbon racemates like cis,trans-cycloocta-1,3-diene (a When used in an enantioselective chromatographic column representative of a compound with planar chirality) on micro- the cavities left in the material will cause longer retention of crystalline cellulose triacetate (MCTA).47 In liquid chromatog- the enantiomer that has been used as the template.The raphy the possibility of resolution on a truly preparative scale drawbacks resulting from the usually rather low resolution also exists. Here there are progressive developments taking eYciency obtained have been partly overcome by a technique place with respect to both large-scale production of eYcient involving deposition of the polymer on silica.CSPs and continuously operating chromatographic systems CSPs based on biopolymers of the polysaccharide type have like the recently introduced simulated moving bed (SMB) been extensively studied with respect to their chiral recognition technique.48 Basically, SMB technology means that the solute mechanism, mainly by NMR spectroscopy.44 Since the spin– is applied continuously to the bed and the two separated lattice relaxation time, T1, is sensitive to molecular motion, compounds are collected from separate outlets of the system. this can be used to probe the selector–selectand dynamics.The bed size can be increased to very large dimensions, Further, NOESY spectra give information regarding intermolenabling large throughputs per time unit.Chiral sorbents ecular distances. From such data, in combination with results which can be prepared easily in large quantities, like MCTA, from molecular modelling, a detailed chiral discrimination have already been used successfully on a very large scale.49 As rationale for the cellulose tris(5-fluoro-2-methylphenylcarbaan example, in 1992 a 500 mm×450 mm id column packed mate)–1,1¾-bi-2-naphthol system is now available.44 with 32 kg of MCTA was used for the partial separation of The structurally most complex CSPs, used entirely in the the enantiomers of 1.2 kg of a drug substance in a single run.reversed phase mode, are proteins. They are unique in so far The separation took ca. 13 h and required a flow rate of as regulation of retention via the mobile phase composition methanol of 300 ml min-1. This clearly shows that preparative can be made in many diVerent ways. The protein immobilized separations of enantiomers with chiral phase systems using to the silica surface will change its binding characteristics by SMB will be of great importance for the further development of industrial chirotechnology for a variety of applications.The temperature eVect in chiral separations A prerequisite for chiral separation by chromatography is a fast and reversible diastereomeric association equilibrium between selectand and selector. Thus, enantioselectivity is not governed by kinetics, but is determined exclusively by thermodynamics.Enantioselectivity is defined by the Gibbs free energy diVerence -DDG which is related to the chiral separation factor a (i.e., the ratio of the net retention times t¾=tR-tM, of the second and first eluted enantiomer, respectively) by eqn. (1) (this relationship is only valid in the absence of nonenantioselective interactions arising, for example, from achiral matrices used as solvents like polysiloxanes) -DDG=RT lna=RT ln(t¾2/t¾1) (1) The temperature dependence of -DDG is determined by the Gibbs–Helmholtz eqn. (2) (KR and KS refer to the association constants of the diastereomers formed between the selector and the enantiomers R or S of the selectand) Fig. 7 (a) Synthetic polymers and (b) biopolymers used as chiral stationary phase materials -DDG=-DDH+T DDS=RT ln(KR/KS) (2) J.Mater. Chem., 1997, 7(10), 1955–1963 1961which may be rewritten as the van’t HoV plot ane which rapidly interconverts from one chiral chair conformation to another representing the non-superimposible -DDG/T=-DDH/T+DDS=Rln(KR/KS) (3) mirror image. If the rate of enantiomerization is comparable to that of the As expected for a 151 association process, -DDH and DDS compensate each other in determining -DDG.Therefore, at chromatographic timescale of enantiomer separation, a characteristic interconversion profile is observed.14 It resembles a the isoenantioselective temperature Tiso enantiomers cannot be separated. plateau formation between the unchanged peaks for the enantiomers. By ‘dynamic’ chromatography, activation parameters Tiso=DDH/DDS for -DDG=0 (4) of enantiomerization (DG‡) can be obtained by comparing experimental and calculated elution profiles.Two examples Below Tiso, enantiomer separation is enthalpy controlled and the (R)-enantiomer is eluted after the (S)-enantiomer, while featuring enantiomerization are shown in Fig. 9. above Tiso, enantiomer separation is entropy controlled and the (S)-enantiomer is eluted after the (R)-enantiomer (reversal Summary and outlook of the elution order, peak inversion).An example is depicted in Fig. 8. The development of chiral stationary phase materials for enantioselective chromatography has been very rapid and In most cases, enantioselectivity is enthalpy controlled. Therefore, temperatures as low as possible should be employed today virtually any racemate can be at least partially resolved.The methodologies are rapidly implemented in various pro- to increase the chiral separation factor a. Low temperatures should also be used to avoid enantiomerization when the cesses for diVerent applications, not least within the fine chemicals and pharmaceutical industries where there is often chiral selectand is prone to configurational change.a need for pure enantiomers. This has created a new field of research, viz. chirotechnology. Enantiomerization Along with this development there is a need for a better understanding of the phenomenon of chiral recognition at the Owing to peak coalescence, only an unresolved peak will be observed in enantioselective chromatography when a racemic molecular level.Although this is a complex and diYcult problem, progress is being made owing to the continuous selectand undergoes rapid enantiomerization in the presence of a CSP. Thus, in contrast to cis-1-ethyl-2-methylcyclohexane improvement of NMR techniques for studies of solution structures of model systems44,50 as well as of methods for molecular (Fig. 4), only one peak is expected for cis-1,2-dimethylcyclohexmodelling of selector–selectand interactions in docking situations.51 The refinement of the chromatographic resolution techniques is perhaps best illustrated by the separations of race- Fig. 8 A, Temperature-dependent reversal of enantioselectivity for the enantiomers of isopropyloxirane by complexation gas chromatography Fig. 9 Interconversion profiles due to inversion of configuration on nickel(II) bis[3-heptafluorobutanoyl-8-(ethylidene)-(1R)-camphorate], derivative of 6. (a) 110 °C, (b) 70°C, (c) 55°C. B, Linear Van’t (enantiomerization) of 1-chloro-2,2-dimethylaziridine and 1,6-dioxaspiro[ 4.4]nonane by complexation gas chromatography on nickel(II) HoV plot and determination of the isoenantioselective temperature Tiso (89 °C).8 bis(3-heptafluorobutanoyl-(1R)-camphorate) 68,14 1962 J.Mater. Chem., 1997, 7(10), 1955–196321 Z. Juvancz, G. Alexander and J. Szejtli, J. High Resolut. mates based on chirality solely due to isotopic substitution, as Chromatogr. Chromatogr. Commun., 1987, 10, 105. demonstrated recently.52,53 Furthermore, the strive for miniatu- 22 D.W. Armstrong, W. Y. Li, C. D. Chang and J. Pitha, Anal. Chem., rization of analytical separations, e.g., by the use of electro- 1990, 62, 914. driven methods, like in capillary column techniques, or of ‘chip 23 V. Schurig, M. Jung, S. Mayer, M. Fluck, S. Negura and technology’, will lead to new possibilities for the study of H. Jakubetz, J. Chromatogr. A, 1995, 694, 119. 24 X.Zhou, C. Wu, H. Yan and Y. Chen, J. High Resolut. extremely small amounts of chiral materials. Chromatogr., 1996, 19, 643. 25 S. Allenmark, Chromatographic Enantioseparation, Horwood/ Wiley, Chichester/New York, 2nd edn., 1991. Figures from ref. 8 and ref. 24 have been reproduced with 26 A. Mannschreck, Chirality, 1992, 4, 163. permission from Georg Thieme Verlag, Stuttgart, and Hu�thig 27 J.Hermansson, I. Hermansson and J. Nordin, J. Chromatogr., Verlag, Heidelberg, respectively. 1993, 631, 79. 28 V. Davankov, in Chiral Separations by HPL C, ed. A.M. Krstulovic, Ellis Horwood, Chichester, 1989, p. 446. 29 L. R. Sousa, G. D. Y. Sogah, D. H. HoVman and D. J. Cram, J. Am. References Chem. Soc., 1978, 100, 4569. 30 D. W. Armstrong, T. J. Ward, R. D. Armstrong and T.E. Beesley, 1 G. M. Henderson and H. G. Rule, Nature (L ondon), 1938, 141, 917. Science, 1986, 232, 1132. 2 V. Prelog and P. Wieland, Helv. Chim. Acta, 1944, 27, 112. 31 W. H. Pirkle and T. C. Pochapsky, Chem. Rev., 1989, 89, 347. 3 (a) E. Gil-Av, B. Feibush and R. Charles-Sigler, T etrahedron L ett., 32 D. W. Armstrong, Y. Tang, S. Chen, Y. Chou, C. Bagwill and J.- 1966, 1009; (b) E.Gil-Av and B. Feibush, T etrahedron L ett., 1967, R. Chen, Anal. Chem., 1994, 66, 1473. 3345. 33 F. Gasparrini, D. Misiti, C. Villani, A. Borchardt, M. T. Burger 4 (a) A. Lu� ttringhaus and K. C. Peters, Angew. Chem., 1966, 78, 603; and W. C. Still, J. Org. Chem., 1995, 60, 4314. (b) A. Lu� ttringhaus, U. Hess and H. J. Rosenbaum, Z. Naturforsch 34 G. Blaschke and F.Donow, Chem. Ber., 1975, 108, 1188. B, 1967, 22, 1296. 35 Y. Okamoto, K. Suzuki, K. Ohta, K. Hatada and H. Yuki, J. Am. 5 (a) G. Hesse and R. Hagel, Chromatographia, 1973, 6, 277; 1973, 9, Chem. Soc., 1979, 101, 4763. 62; (b) G. Hesse and R. Hagel, L iebigs Ann. Chem., 1976, 966. 36 J. Dingenen, in A Practical Approach to Chiral Separations by 6 E. Gil-Av, J.Mol. Evol., 1975, 6, 131.L iquid Chromatography, ed. G. Subramanian, VCH, Weinheim, 7 B. Koppenhoefer, A. Nothdurft, J. Pierrot-Sanders, P. Piras, 1994, p. 115. C. Popescu, C. Roussel, M. Stiebler and U. Trettin, Chirality, 1993, 37 S. Allenmark, in A Practical Approach to Chiral Separations by 5, 213. L iquid Chromatography, ed. G. Subramanian, VCH, Weinheim, 8 V. Schurig, in Determination of Enantiomeric Purity by Direct 1994, p. 183. Methods, Methods of Organic Chemistry, Volume E21a, 38 C. J.Welch, J. Chromatogr. A, 1994, 666, 3. Stereoselective Synthesis, ed. G. Helmchen, R.W. HoVmann, 39 W. H. Pirkle and C. J.Welch, J. Chromatogr. A, 1994, 683, 347. J. Mulzer and E. Schaumann, Houben Weyl, George Thieme 40 (a) S. Allenmark, S. Andersson, P. Mo� ller and D. Sanchez, Verlag, Stuttgart and New York, ch. 3.1.5, pp. 168–192. Chirality, 1995, 7, 248; (b) S. Andersson, S. Allenmark, P. Mo� ller, 9 P. Schreier, A. Bernreuther and M. HuVer, Analysis of Chiral B. Persson and D. Sanchez, J. Chromatogr. A, 1996, 41, 23. Organic Molecules, Walter de Gruyter, Berlin, 1995. 41 M. Kempe and K. Mosbach, J. Chromatogr. A, 1995, 694, 3. 10 H. Frank, G. J. Nicholson and E. Bayer, J. Chromatogr. Sci., 1977, 42 G. WulV, Angew. Chem., Int. Ed. Engl., 1995, 34, 1812. 15, 174. 43 B. Sellergren and K. J. Shea, J. Chromatogr., 1993, 635, 31. 11 E. Bayer, Z. Naturforsch B., 1983, 38, 1281. 44 E. Yashima, C. Yamamoto and Y. Okamoto, J. Am. Chem. Soc., 12 V. Schurig, Angew. Chem., Int. Ed. Engl., 1984, 23, 747. 1996, 118, 4036. 13 W. A. Ko� nig, T he Practice of Enantiomer Separation by Capillary 45 J. Haginaka, C. Seyama and N. Kanasugi, Anal. Chem., 1995, Gas Chromatography, Hu� thig, Heidelberg, 1987. 67, 2539. 14 V. Schurig and W. Bu� rkle, J. Am. Chem. Soc., 1982, 104, 7573. 46 T. C. Pinkerton, W. J. Howe, E. L. Ulrich, J. P. Comiskey, 15 V. Schurig and H.-P. Nowotny, Angew. Chem., Int. Ed. Engl., 1990, J. Haginaka, T. Murashima, W. F. Walkenhorst, W. M. Westleer 29, 939. and J. L. Markley, Anal. Chem., 1995, 67, 2354. 16 W. A. Ko� nig, Enantioselective Gas Chromatography with Modified 47 R. Isaksson, J. Roschester, J. Sandsto�m and L.-G. Widstrand, Cyclodextrins, Hu� thig, Heidelberg, 1992. J. Am. Chem. Soc., 1985, 107, 4074. 17 J. Snopek, E. Smolkova�-Keulemansova�, T. Cserha� ti, K. Gahm and 48 M. Negawa and F. Shoji, J. Chromatogr., 1992, 590, 113. A. Stalcup, in Comprehensive Supramolecular Chemistry, ed. 49 E. Francotte and A. Junker-Buchheit, J. Chromatogr., 1992, 576, 1. J. Szejtli and T. Osa, Pergamon Press, Oxford, 1996, ch. 18, 50 W. H. Pirkle and C. J.Welch, J. Chromatogr. A, 1994, 683, 347. pp. 515–571. 51 K. B. Lipkowitz, J. Chromatogr. A, 1994, 666, 493. 18 B. Feibush, J. Chem. Soc., Chem. Commun., 1971, 544. 52 W. H. Pirkle and K. Z. Gan, T etrahedron: Asymmetry, 1997, 8, 811. 53 K. Kimata, K. Hosoya, T. Araki and N. Tanaka, Anal. Chem., 19 V. Schurig, D. Schmalzing and M. Schleimer, Angew. Chem., Int. 1997, 67, 2610. Ed. Engl., 1991, 30, 987. 20 T. Koscielski, D. Sybilska and J. Jurczak, J. Chromatogr., 1986, 364, 299. Paper 7/02403G; Received 8th April, 1997 J. Mater. Chem., 1997,

ISSN:0959-9428    

DOI:10.1039/a702403g

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年代:1997

数据来源: RSC

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MATERIALS CHEMISTRY COMMUNICATION First example of a conducting polymer synthesised in supercritical fluids Francesca M. Kerton, Gerard A. Lawless and Steven P. Armes* School of Chemistry, Physics and Environmental Science, University of Sussex, Falmer, Brighton, UK BN1 9QJ bar for CO2 and 46.9 bar for CHF3; see Table 1) and the rate of thermal decarboxylation of the pyrrole-2-carboxylic acid Polypyrrole is synthesised via thermal decarboxylation of a precursor monomer, pyrrole-2-carboxylic acid, using ferric was rapid.After 2–3 h the polymerisation was terminated by cooling the vessel to room temperature and slowly venting the salts in both supercritical carbon dioxide and supercritical fluoroform; pressed pellet conductivities were as high as gaseous CO2 or CHF3.Preliminary NMR studies indicated that both the pyrrole- 2×10-2 S cm-1 and scanning electron microscopy studies revealed an unusual non-spherical morphology. 2-carboxylic acid precursor monomer and the FeCl3 oxidant have relatively limited solubility in scCO2. However, both pyrrole monomer and the Fe(CF3SO3)3 oxidant were soluble in this medium, as judged by visual inspection of these reagents in sapphire NMR tubes (in the case of pyrrole, a good quality There is increasing interest in carrying out chemical reactions proton NMR spectrum was readily obtained in scCO2).The and extractions in environmentally benign solvents such as FeCl3 oxidant proved to be more soluble in scCHF3. These supercritical carbon dioxide (scCO2). This approach has several observations are consistent with the higher level of residues potential advantages, including low solvent cost and toxicity, found in polypyrroles prepared with FeCl3 oxidant in scCO2 ease of solvent removal, potential for recycling and variation (see Table 1).of reaction rates by relatively small changes in pressure.1 Prior to our experiments we were sceptical about the Recently DeSimone and co-workers published a series of eYciency of thermal decarboxylation ( loss of CO2) of the papers describing the synthesis of polystyrene, poly(methyl pyrrole-2-carboxylic acid precursor in the presence of scCO2.methacrylate), poly(acrylic acid) and perfluorinated polymers However, reasonable yields (50–60% based on monomer) of via dispersion, precipitation and solution polymerisation in polypyrrole were obtained within 2 to 3 h with FeCl3 (see scCO2.2–5 Table 1).A near-quantitative yield of polypyrrole was obtained Polypyrrole is a relatively air-stable organic conducting using the more soluble Fe(CF3SO3)3 oxidant. FT–IR spectra polymer which is easily prepared as an insoluble black powder of the scCO2-synthesised polypyrroles were very similar to by chemical oxidative polymerisation in water, ethyl acetate, that of conventional polypyrrole9 prepared in water at room acetonitrile, methanol or diethyl ether.6 In this work we temperature. Bulk powder densities were in the range describe the synthesis of polypyrrole in both scCO2 and 1.55–1.65 g cm-3 as measured by helium pycnometry, which supercritical fluoroform (scCHF3).The precursor monomer route7 described by workers at DSM Research was selected.This approach allowed good control over the onset of the polymerisation since the in situ generation of pyrrole monomer by decarboxylation only occurs at elevated temperature (see Scheme 1). As far as we are aware, this is the first example of the synthesis of a conducting polymer in supercritical fluids.Polymerisations were conducted either in a 300 ml Paar pressure vessel fitted with a mechanical stirrer or else a 10 mm sapphire NMR tube as described earlier.8 The reaction vessel was charged with the desired amounts of precursor monomer (0.3 to 1.0 g) and oxidant (either FeCl3 or Fe(CF3SO3)3; the initial molar oxidant/monomer ratio was fixed at 2.33 in each case) prior to pressurising with CO2 or CHF3.The vessel was heated to 80–110 °C; under these conditions a supercritical Scheme 1 continuous phase was achieved (i.e. the pressure exceeded 72 Table 1 EVect of synthesis parameters on the yield, conductivity, residues content and doping level of polypyrroles prepared in supercritical media entry oxidant solvent T /°C pressure/bar yielda (%) doping levelb conductivityc /S cm-1 residuesd (%) 1 FeCl3 toluene 110 1 92 0.16 9×10-4 1.5 2 FeCl3 scCO2 110 72 54 0.25 3×10-3 13.6 3 FeCl3 scCO2 80 150 63 0.24 5×10-2 7.6 4 Fe(CF3SO3)3 scCO2 80 150 87 0.21 2×10-2 0.8 5 FeCl3 scCHF3 90 48 57 0.20 1×10-3 0.0 aCalculated based on pyrrole monomer, assuming complete thermal decarboxylation of the pyrrole-2-carboxylic acid precursor.bCalculated from elemental microanalyses (Cl/N or S/N ratios).cAs measured on pressed pellets at room temperature using the four-point probe method. dAs determined from thermogravimetric analyses (scan rate: 20 °C per min in air). J. Mater. Chem., 1997, 7(10), 1965–1966 1965are in reasonable agreement with literature values.9 Pressed pellet conductivities (four-point probe technique) were as high as 5×10-2 S cm-1 for the polypyrrole prepared using the Fe(CF3SO3)3 oxidant.This value is two to three orders of magnitude lower than those of polypyrrole prepared in conventional solvents6,9 and probably reflects some over-oxidation of the conjugated chains. Indeed, a weak carbonyl feature at ca. 1700 cm-1 was observed in the IR spectra of these materials which is normally absent in the IR spectrum of polypyrrole synthesised using FeCl3 in conventional solvents.9 This is understandable given the elevated temperature required for this precursor route.Indeed, an even lower conductivity (9×10-4 S cm-1) was obtained for a ‘control’ polypyrrole synthesised in refluxing toluene (see entry 1 in Table 1). The Cl/N (or S/N) ratios calculated from elemental microanalyses indicate doping levels of 0.20 to 0.25, which are slightly lower than the normally accepted doping range of 0.25–0.33 for polypyrrole.6 Thermogravimetric analyses indicated significant levels of incombustible residues (7.6 to 13.6% at ca. 600 °C) in polypyrroles synthesised using FeCl3 in scCO2, even after extensive washing of the precipitated polymer with methanol and water.This is most likely due to polypyrrole deposition onto insoluble FeCl3 particulates. On the other hand, negligible residues (<1%) were obtained for polypyrroles prepared using Fe(CF3SO3)3 in scCO2 and FeCl3 in scCHF3, which suggests that these more soluble oxidants are easily removed during clean-up. Polypyrroles prepared via conventional precipitation polymerisation in aqueous or non-aqueous media invariably exhibit globular morphologies comprising pseudo-spherical features of sub-micrometre dimensions.9 A typical scanning electron micrograph is shown in Fig. 1(a). In contrast, scanning electron microscopy studies of the scCO2-synthesised polypyrroles (entries 3–5 in Table 1) revealed an unusual fibrillar morphology [see Fig. 1(b)]. The material is made up exclusively of thin fibres or strands, several micrometres in length and Fig. 1 Scanning electron micrographs of polypyrroles prepared using approximately 100 to 200 nm in diameter. Similar morpho- FeCl3 in (a) water at 25 °C and (b) scCO2 at 80 °C (entry 3 in Table 1) logies were reported by DeSimone’s group for poly(acrylic acid) prepared in scCO2.3 The relatively low viscosity of the supercritical fluid may be a significant factor in producing this polypyrrole morphology. However, BET measurements con- References firmed that the specific surface areas of these fibrillar polypyrroles were comparable to those reported by Maeda and Armes 1 M.A. McHugh and V. J. Krukonis, Supercritical Fluid Extraction: for conventional globular polypyrrole.10 Principle and Practice, Butterworths, Stoneham, MA, USA, 2nd In summary, polypyrrole has been synthesised in supercriti- edn., 1994. 2 D. A. Canelas, D. E. Betts and J. M. DeSimone, Macromolecules, cal fluids for the first time. Yields were moderate to high, 1996, 29, 2818. depending on the reaction conditions. The spectroscopic and 3 J. M. DeSimone, E. E. Maury, Y. Z. Menceloglu, J. B. McClain, physicochemical properties of these materials were generally T.J. Romack and J. R. Combes, Science, 1994, 265, 356. very similar to those of polypyrrole prepared in conventional 4 T. J. Romack, E. E. Maury and J. M. DeSimone, Macromolecules, solvents. However, pressed pellet conductivities were somewhat 1995, 28, 912. lower and unusual non-spherical morphologies were obtained. 5 J. M. DeSimone, Z. Guan and C. S. Elsbernd, Science, 1992, 257, 945. Clearly further investigations of the synthesis–structure–prop- 6 R. E. Myers, J. Electron. Mater., 1986, 2, 61. erty relationships of these materials are warranted. 7 H. van Dijk, O. Aagaard and R. Schellekens, Synth. Met., 1993, 55–57, 1085. F. M. K. thanks the EPSRC for a DPhil studentship. S. P. A. 8 I. T. Horvath and E. C. Ponce, Rev. Sci. Instrum., 1991, 62, 1104. thanks the EPSRC for capital equipment grants for the BET 9 S. Maeda and S. P. Armes, J.Mater. Chem., 1994, 4, 935. and helium pycnometer instruments (GR/K01841). The DRA 10 S. Maeda and S. P. Armes, Synth.Met., 1995, 73, 151. and DSM Research are thanked for partially funding the Communication 7/04479H; Received 25th June, 1997 purchase of the FT–IR spectrometer. 1966 J. Mater. Chem., 1997, 7(10), 1965–1966

ISSN:0959-9428    

DOI:10.1039/a704479h

出版商:RSC    

年代:1997

数据来源: RSC

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MATERIALS CHEMISTRY COMMUNICATION New p-conjugated polymers containing tetrathiafulvalene as the monomeric unit Takakazu Yamamoto* and Takahisa Shimizu Research L aboratory of Resources Utilization, T okyo Institute of T echnology, 4259 Nagatsuta,Midori-ku, Yokohama 226, Japan Four kinds of poly(arylene)- and poly(aryleneethynylene)-type polymers containing TTF units in the p-conjugated main chain, which are susceptible to chemical and electrochemical oxidation, have been prepared by organometallic polycondensation.Tetrathiafulvalene (TTF) and its analogues have been the The monomer 1 was prepared by lithiation of commercially subject of many recent papers.1 Molecular design of polymers available (Aldrich) diphenyl-TTF followed by treatment with containing TTF units has also been carried out,1a,2 however, perfluorohexyl iodide (62%).The precursor [trans-diphenylexamples of polymers containing TTF units in the p-conjugated bis(trimethylsilylethynyl)-TTF] of 2 was prepared via reaction main chain have been limited. Here we report the preparation of 1 with trimethylsilylacetylene in the presence of Pd(PPh3)4 of new p-conjugated polymers containing TTF units in the pand CuI in THF–NEt3 (151), and 2 was obtained by hydrolysis conjugated main chain and the redox behavior of the polymers.of the precursor with aqueous KOH. The trans structures of The Ni-3 [eqn. (1)] and Pd-promoted3a,4 [eqn. (2)] polycon- 1 and the precursor of 2 were confirmed by X-ray crystallogdensations and copolymerizations of monomers 1 and 2 with raphy; details of the preparation of 1 and 2 will be reported 3 give p-conjugated polymers 4–7 in high yields.elsewhere. The monomer 3 was prepared as previously reported.3,4 IR spectra of the polymers are consistent with their assigned structures and peaks due to the C–I bond and terminal acetylenic unit of the monomers are not observed. The nickel-promoted polycondensation proceeds smoothly using a mixture of bis(cycloocta-1,5-diene)nickel(0) [Ni(cod)2] and 2,2¾-bipyridyl (bpy) in DMF at 50 °C to yield 4 and 5 in S S Ph I S S I Ph S S Ph S S Ph H H 1 2 100 and 85% yields, respectively; the polymerization times were 48 and 24 h for 4 and 5, respectively.The p–p* absorption peak of 4 in THF appears at 430 nm, which is at a longer wavelength by 40 nm compared with the absorption peak of monomer 1.Elemental analysis data for 4 are consistent with S H13C6 I I 3 the assigned structure. The copolymer 5, obtained from a 151 mixture of 1 and 3, exhibits a p–p* absorption peak at 410 nm in THF. The GPC trace for 5 gives Mn (number average molecular weight) and Mw (weight average molecular weight) of 4700 and 5800 (eluent, DMF; polystyrene standard), respectively, and 5 has an [g] value of 0.17 dl g-1 in THF at 30 °C.The 1H NMR spectrum of 5 in CDCl3 is consistent with a 151 I Ar I Ar n (1) (2) I Ar I H Ar¢ H Ar Ar¢ n Ni(cod)2 Pd(PPh3)4 Cul, NEt3 + composition (m5n) of the copolymer: dH 0.86 (CH3, s, 3H), 1.0–1.8 (CH2, m, 8H), 2.53 and 2.78 (0.7350.27) (a-CH2, 2H) (the splitting of the a-CH2 peak is considered to be due to the presence of both head-to-tail and head-to-head sequences), 6.8–7.6 (11H, aromatic H).The palladium-promoted polycondensation also proceeds smoothly in a mixture of NEt3 and THF at 60 °C to yield 6 and 7 in 93 and 100% yield, respectively, after 24 h. The S S Ph S S Ph n S S Ph S S Ph m S H13C6 n 4 (lmax = 430 nm) hompolymer from 1 [eqn.(1)] 5 copolymer from 1 and 3 [eqn.(1)] homopolymer 6 is partly (about 60 wt%) soluble in THF, and the THF soluble part gives rise to an absorption peak at 502 nm, which is located at a longer wavelength by about 110 and 60 nm from the peak positions of monomers 1 and 2, respectively.The larger bathochromic shift of 6 from the peak position of the monomer, compared with the bathochromic shift observed with 4, is ascribed to eVective expansion of the p-conjugation system in 6 due to the presence of the spacing COC group, which will ease the steric repulsion between the S S Ph S S Ph n 6 (lmax = 502 nm) polycondensation between 1 and 2 [eqn.(2)] J.Mater. Chem., 1997, 7(10), 1967–1968 1967Polymers 6 and 7 have electrical conductivities of 2.1×10-7 and 8.5×10-8 S cm-1, respectively, as measured on compressed powders.Oxidation with iodine raises the electrical conductivities of 4.7×10-3 and 2.7×10-4 S cm-1, respectively, at 300 K. The electrical conductivity (s) of 4.7×10-3 S cm-1 for the iodine adduct of 6 (I/monomer unit=1.67) at 300 K varies with changes of temperature following eqn. (3) s=3×10-3 exp (-95T -0.25) (3) over a temperature range of 100–300 K.In contrast to polymers 6 and 7, poly(aryleneethynylene) type polymers do not usually undergo chemical oxidation with iodine.4 Polymer 6 seems to be partly oxidized in air and gives an EPR signal at g=2.0081 with a peak-to-peak line width DHpp of 1.44 mT and a spin concentration of 3.2×1018 g-1 in air. Oxidation with iodine leads to an increase in the spin concentration and causes broadening of the EPR signal.For example, the iodine adduct of 6 (I/monomer unit=0.85) gives rise to a Fig. 1 Cyclic voltammograms of cast films of (a) polymer 6 and symmetrical and very broad EPR signal at g=2.014 with a (b) polymer 7 (in an MeCN solution of 0.10 M [NEt4]BF4 at DHpp value of 14.2 mT and a spin concentration of 200 mV s-1). Both the polymer films exhibit essentially the same 9.6×1018 g-1.The marked broadening of the signal suggests colour changes, shown in this figure. coupling of the radical species with many H nuclei due to moving of the radical species over a wide range. Polymer 5 TTF monomeric units. The THF soluble part of 6 shows Mn also shows a very broad EPR signal upon oxidation with and Mw of 2320 and 4700, respectively, as determined by GPC iodine.As described above, 6 is crystalline, however, the iodine (eluent, THF; polystyrene standard). The THF insoluble part doped sample of 6 is not crystalline as proved by powder Xof 6 exhibits an IR spectrum identical to that of the THF ray crystallography, and the adduct may contain cis-TTF units soluble part and is considered to have a higher molecular in addition to the trans-TTF units since trans–cis isomerization weight.The powder X-ray diVraction chart of 6 shows clear of a TTF derivative via a cationic species of the TTF derivative peaks at 2h (Cu-Ka)=23.95, 27.09 and 31.90 °, supporting the has been reported.6 suggested formation of a crystalline polymer with a regular As described above, new p-conjugated polymers containing repeating unit.the highly electron-donating TTF unit can be prepared via The copolymer 7 obtained from a 151 mixture of 2 and 3 is organometallic polycondensation. Among the polymers, the also partly (about 60–80 wt%) soluble in THF, and the THF poly(aryleneethynylene) type polymers 6 and 7 have wellsoluble part gives an absorption peak at 495 nm in THF. The expanded p-conjugation systems due to COC spacer groups, THF soluble part shows an [g] value of 0.39 dl g-1 in THF and they are active for both chemical and electrochemical at 30 °C, and its light scattering analysis in 1,1,2,2-tetrachloro- oxidation even when containing strongly electron-withdrawing ethane using an He-Ne laser at 632.8 nm gives an Mw value COC groups. of 610 000.The 1H NMR data for 7 are consistent with a 151 We are grateful to Professor K.Kubota of Gunmma University composition between the monomeric units: dH (CD2Cl2) 0.82 (CH3, 3H), 1.08 (CH2, 6H), 1.50 (b-CH2 2H), 2.48 (a-CH2, for the light scattering analysis of 7. 2H), 6.82 (thiophene ring-H, 1H), 7.14 (m- and p-H of Ph, 6H), 7.67 (o-H of Ph, 4H). References All of the polymers are electrochemically active and their cast films give two-step oxidation peaks† at about 0.4 and 1 E.g.(a) G. Schukat, A. M. Richter and E. Fangha�nel, Sulfur Rep., 0.7 V vs. Ag/Ag+, respectively, which are characteristic of TTF 1987, 7, 155; (b) T. Otsubo, Y. Aso and K. Takimiya, Adv. Mater., 1996, 8, 203; (c) N. L. Narvor, N. Robertson, T. Weyland, J. D. and its analogues,1,2,5 although the oxidation pes locate Killurn, A.E. Underhill, M. Webster, N. Svenstrup and J. Becher, at fairly high potentials compared with those of other Chem. Commun., 1996, 1363. TTF derivatives. Poly(aryleneethynylene) type polymers 2 (a) L. van Hink, G. Schukat and E. Fangha�nel, J. Prakt. Chem., 1979, MArMCOCMn usually have electron-accepting properties due 321, 299; (b) M. R. Bryce, A. C. Chissel, J.Gopal, to the electron-withdrawing COC group,4a and the high P. Kathirgamanathan and D. Parker, Synth. Met., 1991, 39, 397; (c) C. Thobie-Gautier, A. Gorgues, M. Jubault and J. Roncali, oxidation potential reflects the electron-withdrawing eVect of Macromolecules, 1993, 26, 4094; (d) M. Fourmigue, I. Johannsen, the COC group. Poly(aryleneethynylene) type polymers are K.Boubekeur, C. Nalson and P. Batail, J. Am. Chem. Soc., 1993, ordinarily inert to electrochemical oxidation, even when they 115, 3752; (e) S. Frenzel, S. Arndt, R. M. Gregorious and K. Mu� llen, contain an electron-donating thiophene ring as the Ar unit.4 J. Mater. Chem., 1995, 5, 1529; ( f ) A. Charlton, A. E. Underhill, However, due to the strong electron-donating ability of the G. Williams, M.Kalaji, P. J. Marphy, D. E. Hibbs, M. B. Hursthous and K. M. A. Malik, Chem. Commun., 1996, 2423. TTF unit, polymers 6 and 7 are electrochemically active in the 3 (a) T. Yamamoto, Prog. Polym. Sci., 1992, 17, 1153; (b) T. Yamamoto, oxidation region, as depicted in Fig. 1. This is the first example K. Sugiyama, T. Kushida, T. Inoue and T. Kanbara, J. Am. Chem. of a poly(aryleneethynylene) type polymer which is active for Soc., 1996, 118, 3930.electrochemical oxidation. The oxidation is also accompanied 4 (a) T. Yamamoto, W. Yamada, M. Takagi, K. Kizu, T. Maruyama, by a colour change, as depicted in Fig. 1. Neutral 6 and 7 are N. Ooba, S. Tomaru, T. Kurihara, T. Kaino and K. Kubota, Macromolecules, 1994, 27, 6620; (b) K. Sanechika, T. Yamamoto and purple, and the colour of the films changes to green during A.Yamamoto, Bull. Chem. Soc. Jpn., 1984, 57, 752; Polym. Prepr. the oxidation. The polymers 6 and 7 are also active in the Jpn., 1981, 30, 160. reduction region, similar to other poly(aryleneethynylene) type 5 (a) V. Khodorkovsky, A. Edzifna and O. Neiland, J. Mol. Electron., polymers, and give several redox cycles in the reduction region. 1989, 5, 33; (b) Y. N. Kreitsberga, A. S. Edzhinya, R. B. Kampare and O. Y. Neiland, J. Org. Chem. USSR (Engl. T ransl.), 1989, 25, 1312. 6 A. Souizi, A. Robert, P. Batail and L. Ouahab, J. Org. Chem., 1987, † Epa (1), Epc (1), Epa (2) and Epc (2) vs. Ag/Ag+ (in V) are: 4: 0.27, 52, 1610. 0.07 [E0 (1)=(0.27+0.07)/2=0.17], 0.58 and 0.43 [E0 (2)=0.51]; 5: 0.29, 0.15 [E0 (1)=0.22], 0.62 and 0.46 [E0 (2)=0.54] in an MeCN solution of 0.10 M [NEt4]BF4; for 6 and 7, see Fig. 1. Communication 7/04753C; Received 4th July, 1997 1968 J. Mater. Chem., 1997, 7(10), 1967–

ISSN:0959-9428    

DOI:10.1039/a704753c

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Control of average size and size distribution in as-grown nanoparticle polymer composites of MSe (M=Cd or Zn) Stephen W. Haggata,a David J. Cole-Hamilton*a and John R. Fryerb aSchool of Chemistry, University of St. Andrews, St. Andrews, Fife, Scotland, UK KY16 9ST bDepartment of Chemistry, Glasgow University, Glasgow, Scotland, UK G12 8QQ The preparation of CdSe and ZnSe semiconductor nanoparticles in the quantum size regime (1–5 nm) within a polymer matrix is described. A carefully controlled reaction temperature and suitable choice of solvent is found to have a dramatic eVect on the size of the particles produced when a soluble pyridyl polymer adduct (a polymer that contains nitrogen and dimethylcadmium or dimethylzinc) is reacted with H2Se in solution.An expected change in colour (absorbance) of the material from black through to yellow is observed for cadmium selenide and dark yellow to light yellow for zinc selenide as the particles decrease in size.TEMs reveal that cadmium selenide particles are evenly distributed in the polymer film with a size distribution wider than analogous sulfide preparations recently reported. The CdSe particles exhibit size quantization and hence a blue shift of the band edge position is observed in the absorbance and photoacoustic spectra as the reaction temperature is decreased or the polymer solubility is improved.ZnSe nanoparticle growth has been found diYcult to control although a change in reaction temperature has an eVect on the particle size similar to that for CdSe. Semiconductor nanosized particles (or QDs) are small molecu- matrix in the past has usually involved the passing of a chalcogenide gas over the surface of a polymer blend or lar clusters in the size range of ca.ten to several hundred a° ngstro�ms in diameter and have attracted considerable co-polymer film which contains either organometallic blocks32,33a,b which have a complex synthesis, or coordinated research interest during the last several years because of their unique quantum eVects observed only at nanosized dimensions metal salts.34 The microphase separation domains in the block copolymer are able to control the particle dispersity and size and which lead to obvious diVerences from bulk macrocrystallites in terms of their electronic, optical and catalytic proper- distribution.However, these methods are carried out in a heterogeneous phase and consequently very low loading of ties.1–16 These materials have possible future applications as photocatalysts in photoreactions,4,13 in electro-luminescent semiconductor particles and varying metal/chalcogenide stoichiometry is observed if the morphology of the polymer film devices (electro-optics)8,9,10 for e.g.the development of flat panel luminescent displays; photoconductive and photovoltaic is poorly controlled. An alternative approach is to mix preprepared particles with suitable polymers and then use the com- devices11,12 in e.g. photocopiers and laser printers; all-optical (non-linear optical)14 devices in e.g. optical switches; and posite to form films.8,9,11,35 Methods in which the particle film composite is formed ‘in situ’ by using a soluble polymer/metal magneto-optics15 in e.g.erasable optical data storage. However, there are important criteria that have to be met before such complex adduct and reacting it with a suitable source of the group 16 element have been less explored, but is the basis for devices can be realised.The first is the preparation of a high concentration of monodisperse particles. The size and shape the successful production of nanoparticulates from aqueous solutions containing polyphosphates.36 In these systems, the of the particles has an influence on the wavelength of their absorbance and light emission which can be ‘tuned’ by altering polymer not only acts as the encapsulant for the particles, but is also capable of controlling the particle size by providing the particle size.Therefore, narrow size distributions are essential if pure colours are to be obtained in luminescence. A slow release of group 12 precursor and by binding to surface metal atoms to terminate the particle growth. Recently,10 the second, vital point is the control of the surface (grain boundary).A substantial percentage of crystallite atoms are surface copolymerisation of styrene and zinc methacrylate to form a soluble zinc containing microgel and subsequent reaction with atoms and these largely account for the chemical and physical properties of the particle. Therefore, in order to prevent higher H2S has produced ZnS nanocrystals in a polymer matrix. How luminescent devices can be made from such composite wavelength emission, surface states must be eradicated and this can be achieved by chemically binding the particles to a materials has also been described.10 Our previous work has described the functionalization of suitable material of higher band gap.4,5 Polymers are able to passivate the material and prevent particle agglomeration polybutadiene with various Lewis-base groups by homogeneous catalysis37–39 to form soluble polymer ligands.Further, whilst maintaining a good spatial distribution of particles. In some cases polymers are able to assist charge transfer in e.g. we have reported that the addition of group 12 metal alkyls forms soluble metal alkyl/polymeric adducts40 and we have photoconductive and electroluminescent devices.Here, it is essential to encapsulate the crystallites in a conducting polymer then observed their reaction with hydrogen sulfide gas41,42 to form semiconductor colloids. Polymer/semiconductor com- in order for charge extraction to proceed.11,12 There are many synthetic methods reported which target surface control and posite films can then be obtained after removing the solvent under vacuum or by filtration which has proved a simple, monodispersity and these include syntheses in colloidal suspensions, 17,18 solutions of single-molecule precursors,19,20 sol– flexible and alternative route to the preparation of nanoparticulate II/VI materials with narrow size dispersion.gels,21 zeolites,22 LB films,23 micelles7,24 and polymer films.8–11 Of these methods, the formation of semiconductor nanosized Particular advantages of this technique include the low reaction temperatures involved, the simplicity of the system, and especi- particles in a polymer medium has received intense interest owing to the ready processibility of the polymer films and ally the ability to obtain polymer nanoparticle composites in which the particle size distribution is narrow without sub- possible future application in device structures.8–11,25–31 The synthesis of nanosized semiconductor particles in a polymer sequent treatment or size fractionation.The average particle J. Mater. Chem., 1997, 7(10), 1969–1975 1969size can be controlled over a wide range by controlling Synthesis of nano-sized CdSe and ZnSe semiconductor particles the reaction parameters, especially temperature.The The preparation procedure for zinc/polymer composites folpolymer/ metal alkyl adducts readily dissociate on heating,43 lows that for cadmium which is now described. A 2pySiPB so they may also be useful for purifying metal alkyls through polymer–dimethyl cadmium solution (3%g cm-3 of polymer) the adduct purification process44 although simpler systems stirred in a flask (250 ml) under nitrogen was gradually exposed based on monomeric Lewis bases are currently preferred to a hydrogen selenide atmosphere until a solid precipitated.because of their lower cost. The transparent yellow polymer adduct solution quickly changed to either a yellow, orange, red, crimson, brown or black coloured suspension of cadmium selenide in polymer Experimental (yellow for zinc selenide), the colour depending on the reaction temperature or choice of solvent.As soon as the precipitate Experiments were carried out under dry oxygen-free argon was observed, the H2Se gas flow was interrupted and the purified by passing through a series of columns consisting of suspension was allowed to settle and then separated by fil- Cr2+ on silica and dry molecular sieveGreaseless joints and tration isolating the coloured polymer composite. taps were employed and manipulations were carried out using standard Schlenk-line and catheter tubing techniques. All the Results solvents were carefully dried by distillation from sodium diphenylketyl. 2-Methylpyridine was purchased from Aldrich Reactions in toluene of polypyridine bound Me2M with H2Se and was distilled prior to use.Butyllithium (1.6 M in hexane) produce composites consisting of nanoparticles embedded and dimethylzinc (2.0 mol dm-3 in toluene) were purchased within the polymer matrix (Fig. 1). A series of diVerent from Aldrich and used as received. Polybutadiene (83% pencoloured CdSe/polymer samples prepared by the method above dant, 17% trans-1,4, Mn=3000) was a commercial product under various synthesis conditions are illustrated in Fig. 2. The (Nippon Soda Company) and was used after pumping for 2 h. size quantization eVect can be seen most dramatically in Me2Cd was prepared by the standard literature method.45 samples Cd1 [-78 °C, Fig. 2(a)], Cd2 [r.t., Fig. 2(b)] and Cd3 Powder X-ray diVraction (PXRD) patterns were recorded [60 °C, Fig. 2(c)] where a darkening in colour indicates an on a Sto� e STADI/P diVractometer using Cu-Ka radiation.increase in particle size and a decrease in band gap energy. Data were collected in transmission mode with a sample The PXRD pattern of Cd3, Fig. 3(c), shows the sharpest mounted in vaseline on a rotating disc and compared with the and most intense peak in all CdSe patterns at the 2h value of standard pattern obtained from the JCPDS database or the 25.5° (002), broader hkl reflections at 42.0° (110) and 50.0° PXRD pattern of a wurtzite sample.(112) as well as very broad (103) reflection at 46.0°. These Transmission electron micrographs (TEMs) were obtained reflections are indicative of the cadmoselite (hexagonal) phase, using a Phillips EM 301 microscope at 80 keV. All samples Fig. 3(e), and not the sphalerite (cubic) phase of CdSe [see the were embedded in an epoxy resin and the sections were then JCPDS pattern of cubic CdSe, Fig. 3(f )]. The hexagonal phase cut on a microtome with a diamond knife. The dried specimen is also confirmed by the lattice spacings in the HRTEM images.sections were put onto a copper grid which had a carbon The broad peak of the polymer is observed at 2h=17°. support film present. Another layer of carbon was then evaporated onto the sample in order to prevent specimen charging. High resolution TEMs (HRTEMs) were obtained by using an ABT 002B microscope at 200 keV. The samples were either prepared as previously described or suspended in acetone and then ultrasonically dispersed and lifted oV onto a graphite grid.Both absorbance and photoacoustic spectroscopy (PAS) were used to measure the band edge of the material. Photoacoustic spectra were obtained using an OAS 400 photoacoustic spectrophotometer as described previously.42 Absorbance spectra were obtained using a Perkin Elmer Lambda 14P UV–VIS spectrophotometer. All samples were scanned from the UV region to the near IR (300–800 nm). The knees of the band edge for both photoacoustic and absorption spectra were recorded and taken as the closest estimation of the band edge value.Of course, the particle size distribution of the material will directly aVect the slope of the band edge and therefore calculated band gap values are indicative of the smallest sized particles only.For commercial CdSe (cadmoselite), the value of the band edge measured in this way is 1.74 eV ( lit. value, 1.74 eV46). Calculations of the band gap when the band edge value is taken from the tail of the spectrum are more inaccurate as the band edge is smeared out by lattice vibrations and falls oV exponentially in accordance with Urbach’s rule.47,48 Synthesis of a polymer adduct with dimethylcadmium or dimethylzinc The polymeric polybutadiene Lewis base containing 2-methylpyridyl (2pySiPB) groups and the subsequent dimethylcadmium or dimethylzinc polymer adduct were synthesized as previously reported.42 In all cases the M/N (M=Zn, Cd) mole Fig. 1 Formation of the 2pySiPB adduct and the reaction of the polymer adduct with hydrogen selenide ratio of metal alkyl to coordinating nitrogen was 0.5. 1970 J. Mater. Chem., 1997, 7(10), 1969–1975Fig. 3 PXRD patterns of CdSe prepared in toluene in the presence of 2pySiPB at (a) -78 °C, run Cd1; (b) 25 °C, run Cd2; (c) 60 °C, run Cd3. Pattern (d) corresponds to a CdSe sample prepared in petrol in the presence of 2pySiPB, run Cd4. Patterns (e) and (f ) correspond to commercial hexagonal (cadmoselite) CdSe and JCPDS pattern for cubic CdSe (sphalerite) respectively.Impurities present within vaseline are denoted as 1. diameter, measurements are therefore calculated from TEMs. The type of solvent chosen for the reaction also has a profound eVect on the crystallite growth. When light petroleum (bp 40–60 °C) is used (see Table 1, Cd4), the PXRD pattern, Fig. 2 CdSe samples showing quantum confinement. Samples (a) to Fig. 3(d), is sharper corresponding to an increase in particle (c) were prepared in toluene in the presence of 2pySiPB at (a) -78 °C, run Cd1; (b) 25 °C, run Cd2; and (c) 60 °C, run Cd3. Sample (d) r.t., diameter. run Cd4, is prepared in light petroleum in the presence of 2pySiPB. The PXRD patterns of ZnSe/polymer composites and the Sample (e) is commercial (bulk) cadmium selenide.JCPDS pattern of hexagonal ZnSe are shown in Fig. 4(a)–(c) where Zn1 [Fig. 4(a)] and Zn2 [Fig. 4(b)] are runs prepared in toluene at -78 °C and r.t. respectively. Again, an increase Estimates of the particle size based on width measurements of the single reflection observed at 42.0° can be made but are not in reaction temperature corresponds with a sharpening of the hkl lines indicating an increase in particle size. The h0l reflection included in this paper because they do not produce accurate values for particles in which distortions and defects occur.(103) at 49.5° is clearly observed for Zn1 and is more intense than other high-angle hkl. At higher temperature, sample Zn2, The reaction temperatures and solvents were varied for a series of experiments in order to correlate the reaction con- all h0l intensities are suppressed.The TEM of run Cd2, Fig. 5, shows an even distribution of ditions with the sizes of the CdSe particles formed and the results are summarized in Table 1. particles within the polymer matrix. A size distribution graph of a TEM obtained by measuring individual particles on a A change in reaction temperature (see Table 1, Cd1–Cd3) has a large eVect on the average particle size as found for TEM image of sample Cd2 (run at r.t.) is illustrated in Fig. 6. TEM studies of samples prepared from runs in toluene and analogous runs for CdS and ZnS.42 PXRD patterns obtained from these samples where the temperature has been varied electron diVraction of all CdSe samples prepared for TEM confirms the crystallite to have lattice spacings indicative of from-78 to 60 °C, Fig. 3(a)–(c), show significant hkl dependent sharpening of lines at higher temperature, i.e. an increase in the Cadmoselite (hexagonal) phase. After two days in an atmosphere of air and after ca. 1 week under nitrogen, the particle size.PXRD studies indicate that the particles exhibit lattice and turbostratic distortions49,50 (see later) and therefore colour of the material obtained from sample Cd1 slowly darkened from a yellow to a light brown colour. However, the calculations of the coherence length of the crystallite using a modification of Scherrer’s formula18 give very poor estimations PXRD patterns taken of the same sample immediately after preparation and then a week later were identical and gave no of crystallite size.For a more reliable indication of crystallite Table 1 Dependence of CdSe particle size on various synthesis conditions av. TEM polymer band particle size run no. Cd/Py ratio solvent T /°C conc. (%) gap/eV (range)/nm bulk CdSe 1.74a Cd1. 0.5 toluene -78 3.0 2.72a 2.3 (1–4.5) Cd2. 0.5 toluene R.T. 3.0 2.17a 2.9 (1.0&nda;5) Cd3. 0.5 toluene 60 3.0 2.15a 3.6 (1.5–6) Cd4. 0.5 light petroleum 25 3.0 2.06b 3.1 (1.5–5.5) Band gap calculated from band edge observed by photoacoustica or absorbanceb spectroscopy. J. Mater. Chem., 1997, 7(10), 1969–1975 1971Fig. 4 PXRD patterns of ZnSe prepared in toluene in the presence of 2pySiPB at (a) -78 °C, run Zn1; (b) 25 °C, run Zn2.Pattern (c) corresponds to the JCPDS pattern for hexagonal ZnSe. Impurities present within vaseline are denoted as 1. Fig. 6 Size distribution graph of CdSe particles, Cd2, prepared in the presence of 2pySiPB in toluene (2.63 eV). However, all the band edges observed for samples of this material are very shallow owing to very wide size distributions of particles and lattice vibrations.This observation suggests that the polymer has little control over ZnSe particle growth and termination, reasons for which are discussed shortly. Many of the composites were investigated for their photoluminescence behaviour. Although blue luminescence was observed for the polymer itself, there was no observable Fig. 5 Low resolution TEM of CdSe particles prepared in run Cd2 luminescence from the nanoparticles.This is probably because low-lying acceptor orbitals on the pyridine quench the emission. sharpening of hkl intensities which would be expected if the particle size increased forcing a change in colour (absorbance). This phenomenon is also observed for ZnSe samples and Discussion possible explanations will be discussed later.The band gap of commercial cadmoselite CdSe, as measured All the PXRD patterns for CdSe samples are consistent with their being of the hexagonal phase. The PXRD pattern of the from photoacoustic spectroscopy is 1.74 eV ( lit. value, 1.70 eV46) and diVers markedly from those of the CdSe/ sample run at high temperature [run Cd3, Fig. 3(c)] suggests that there are turbostratic distortions49,50 present within the polymer composites.Photoacoustic spectra [Fig. 7(a)–(c)] obtained from samples Cd1 (-78 °C), Cd2 (r.t.) and Cd3 crystallites. A perfect crystal has sharp hkl reflections whose structure factors describe the symmetry and contents of the (60 °C) respectively, show a gradual red shift in the band edge indicating an increase in particle size and a decrease in band unit cell.If the layer planes are rotated relative to each other the 00l reflections remain unchanged; the hk0 intensities will gap from 2.72 to 2.15 eV. The UV–VIS absorbance spectrum of the sample prepared in light petroleum [Cd4, Fig. 7(d)] change and the peaks will become broadened because the distortions aVect the unit cell eVectively reducing the crystal gives a calculated band gap value of 2.06 eV and clearly shows a band edge slope indicative of a large size distribution of size.Most aVected will be the hkl reflections, similarly reduced in intensity and broadened. These strong characteristics of particles and exciton relaxation caused by lattice vibrations. In fact, the slopes of the band edges obtained from all samples rotational distortion can be seen in all observed PXRD patterns as the (002) plane is always intense; in most cases the h0l are shallower than e.g.cadmium sulfide samples42 and could be interpreted as a collective result of (i) lattice vibrations planes, (101) at 27°, (102) at 35°, (103) at 46°, and the hkl plane, (112) at 49.5°, are severely broadened. Distortions (phonons); (ii) the faster rate at which cadmium selenide particles are formed compared to the sulfides, thus giving a involving random shifts of lattice planes along the direction of the specified hkl plane of the crystallite give changes in the larger and wider particle size distribution due to slower termination of CdSe particle growth (see later); (iii ) absorbance peak profile exhibiting asymmetry through the peak centre.This eVect is observed in the PXRD pattern of run Cd3, transitions to lower energy bands or trapped ‘surface’ states between the valence and conduction bands which are the result Fig. 3(c), where the (110) peak at 42° has a slightly diVerent rate of intensity decay on the high angle side (r.h.s.) of the peak. of disrupted crystallite/polymer Cd–N interactions present at the grain boundary formed after particle growth; or (iv) traps In preparations where toluene is the reaction solvent there appear to be fewer stacking faults within these crystallites as between the valence and conduction band which pertain to point defects within the bulk of the nanocrystallite. compared to previous CdS or ZnS42 and CdSe samples18 and this is confirmed by the presence of the (103) reflection in Cd3 The band gap for commercial ZnSe as calculated from the photoacoustic measurement is 2.58 eV ( lit. value, 2.58 eV46).[Fig. 3(c)], however, in all cases the (102) reflection is severely broadened and unobserved. While the strong (002) reflection The photoacoustic measurement of the band edge position for the polymer/composite sample Zn2, Fig. 7(e) is 470 nm, could indicate near perfect registry in this particular lattice 1972 J. Mater. Chem., 1997, 7(10), 1969–1975Fig. 8 Propagation and termination of ME (M=Cd, Zn; E=S, Se) particle growth Fig. 4(b)] suggests that there are significant defects within the crystallites, e.g. stacking faults.18 The (002) plane at 27.5° is again the most intense peak in both Zn1 and Zn2 samples.Fig. 7 Photoacoustic spectra of CdSe samples prepared in toluene in the presence of 2pySiPB at (a) -78 °C (run Cd1), (b) r.t. (run Cd2) The ZnSe sample obtained at low temperature [Zn1, Fig. 4(a)] and (c) 60 °C (run Cd3); absorbance spectrum of a CdSe sample shows very broad peaks corresponding to the hexagonal phase prepared in petrol in 2pySiPB at (d) r.t. (run Cd4); photoacoustic of ZnSe at 29.0° (101) and 49.5° (103).The collection of hkl spectrum of a ZnSe sample prepared in toluene in 2pySiPB at (e) r.t. intensities around the (002) region observed in Zn2 is narrower (run Zn1) than expected which could suggest that the number of stacking faults within the crystallite is comparable with the number of planes in perfect hexagonal registry, hence a substantial per- plane of the crystallite, another explanation could be a non-uniformity in the crystallite dimensions suggesting pre- centage of the crystallite co-exists in the form of the cubic phase of ZnSe along with the hexagonal phase.This form of ferred orientation along the direction of this plane. Preparations in light petroleum gave a dark brown colouration polytypism ensures that the suspected (002) reflection therefore more closely resembles the cubic (111) plane of ZnSe and of the reaction mixture indicating the formation of large sized particles.As similarly found in crystallites formed in analogous reinforces the explanation of why the (103) intensity is severely suppressed. reactions for CdS runs, the h0l reflections are noticeably sharper and the (002) reflection broader than found in other The region of quantum confinement for CdSe is very narrow and a slight increase or decrease in particle size would clearly patterns.TEM studies confirm that the particles are agglomerated into clusters with little apparent polymer content but are lead to a change in colour of the material. However, from our sample observations it has been found that although the colour nevertheless comparable in their size distribution (1.5–4.5 nm) to particles observed in sample Cd3.The cadmium selenide/ of the particles (both CdSe and ZnSe) gradually change over a long period of time, the hkl dependent line broadening of polymer adduct is insoluble in non-polar light petroleum and thus cannot disperse the growing nanoparticulates and hence the PXRD patterns remains unchanged indicating no change in particle dimensions. Therefore, we can consider two possibil- allows them to cluster.The suppression of the (103) plane in the PXRD pattern of ities for these observations and firstly, we postulate that on exposure of these samples to moisture a gradual deposition of the ZnSe sample obtained at room temperature [Zn2, J.Mater. Chem., 1997, 7(10), 1969–1975 1973red selenium metal on the surface of the particle occurs as Allen and Dr Douglas F. Foster for useful discussions; and the EPSRC for funding (S.H.). oxygen is a harder base than selenium and O2 thus displaces it forming a colourless, high band gap metal oxide (ZnO, CdO). The second possibility could be the presence of residual hydrogen selenide gas enveloped in the polymer which, on air References exposure, decomposes to elemental selenium in a similar 1 M.G. Bawendi, M. L. Steigerwald and L. E. Brus, Annu. Rev. Phys. manner to selenium alkyls. Chem., 1990, 41, 477. On the basis of the average particle size and the size 2 A. Henglein, Chem. Rev., 1989, 89, 1861. dispersion under a given set of conditions, the eVectiveness 3 Y.Wang and N. Herron, J. Phys. Chem., 1991, 95, 525. 4 H. Weller, Adv.Mater., 1993, 5, 88. of the polymer pyridyl group for controlling the growth 5 H. Weller, Angew. Chem., Int. Ed. Engl., 1993, 32, 41. of nanoparticles appears to decrease in the order 6 J. H. Fendler and F. C. Meldrum, Adv. Mater., 1995, 7, 607. ZnS>CdS>CdSe>ZnSe.This is the same order as we have 7 A. R. Kortan, R. Hull, R. L. Opila, M. G. Bawendi, M. L. observed in the production of nanoparticles from gas phase Stiegerwald, P. J. Carroll and L. E. Brus, J. Am. Chem. Soc., 1990, reactions of Me2M and H2E in the presence of pyridine,51,52 112, 1327. although in those cases, elemental analysis of the particles 8 B. O. Dabbousi, M.G. Bawendi, O. Onitsuka and M. F. Rubner, Appl. Phys. L ett., 1995, 66, 1316. obtained gives further information since, for ZnS and CdS 9 V. L. Colvin, M. C. Schlamp and A. P. Alivisatos, Nature (L ondon), there is approximately one mole of pyridine per surface atom, 1994, 370, 354. whilst for CdSe much less pyridine is present and for ZnSe 10 Y. Yang, J. Huang, S. Liu and J. Shen, J.Mater.Chem., 1997, 7, 131. there is hardly any pyridine present in the deposits. 11 Ying Wang and N. Herron, J. L umin., 1996, 70, 48. We have rationalised these observations in terms of a particle 12 G. Hodes, I. D. J. Howell and L. M. Peter, J. Electrochem. Soc., growth model in which surface metal atoms are bound to one 1992, 139, 3136. 13 K. Kalyanasundaram, in Energy Resources by Photochemistry and methyl group.Since the particle growth is carried out under Catalysis, ed. M. Gra�tzel, Academic Press, London, 1983, p. 217. H2E-rich conditions, these surface metal atoms bind H2E. It 14 E. Wolf, Progress in Optics XXIX, Elsevier Science Publishers, is the relative rates of elimination of methane (to give surface North Holland, 1991, p. 321.HE atoms and propagate growth), and displacement of H2E 15 S. H. Risbud, T he Encyclopedia of Advanced Materials, Cambridge by pyridine (to terminate growth), that determine the eYciency University Press, Cambridge, 1994, p. 2115–2121. with which particle growth is controlled by pyridine. For H2Se, 16 E. Corcoran, Sci. Am., 1990, 263, 74. 17 N. Herron, Y. Wang and H. Eckert, J.Am. Chem. Soc., 1990, which is much more acidic than H2S, loss of methane dominates 112, 1322. and this is more pronounced for ZnSe because the surface 18 C. B. Murray, D. J. Norris and M. G. Bawendi, J. Am. Chem. Soc., ZnMC bonds are more polar (d- on C) than those for Cd. 1993, 115, 8706. Hence control of growth is poor for the selenides, especially 19 J. G. Brennan, T. Siegrist, P.J. Carroll, M. Stuczynski, L. E. Brus ZnSe. For the less acidic H2S, the rate of elimination of and M. L. Steigerwald, J. Am. Chem. Soc., 1989, 111, 4141. methane is low, so that the surface bound H2S adduct has 20 T. Trindade and P. O’Brien, Chem. Mater., 1997, 9, 523. 21 B. Breitscheidel, J. Zieder and U. Schubert, Chem. Mater., 1991, suYcient lifetime for displacement by pyridine, thus terminat- 3, 559.ing growth. The harder nature of Zn than of Cd and of 22 Y. Wang and N. Herron, J. Phys. Chem., 1987, 91, 257. pyridine than of H2S will ensure that the displacement of H2S 23 X. K. Zhao, L. McCormick and J. H. Fendler, Chem. Mater., 1991, by pyridine will be more favored for ZnS than for CdS. We 3, 922. believe that similar arguments qualitatively explain the growth 24 J.N. Robinson and D. J. Cole-Hamilton, Chem. Soc. Rev., 1991, process and its control for the polypyridine–nanoparticle com- 20, 49. 25 Y. Wang, A. Suna, M. Mahler and R. Kasowski, J. Phys. Chem., posites described here. The relevant reaction schemes are 1987, 87, 7315. shown in Fig. 8. 26 M. E. Wozniak, A. Sen and A. L. Rheingold, Chem. Mater., 1992, 4, 753. 27 J. P. Kuczynski, B. H. Milosavljevic and J. K. Thomas, J. Am. Conclusions Chem. Soc., 1986, 108, 2513. 28 Y. Wang and W. Mahler, Opt. Commun., 1987, 61, 233. CdSe semiconductor nanoparticulates in the size range 1–5 nm 29 E. Hilinski, P. Lucas and Y. Wang, J. Chem. Phys., 1988, 89, 3435. with a relatively narrow size distribution can be prepared by 30 Y. Wang, A. Suna, J. McHugh, E.Hilinski, P. Lucas and R. D. reacting a polymer adduct solution with H2Se. When the Johnson, J. Chem. Phys., 1990, 92, 6927. reaction temperature is controlled and toluene is the reaction 31 S. Yanagida, T. Enokida, A. Shihdo, T. Shiragami, T. Ogata, T. Fukumi, T. Sakaguchi, M. Mori and T. Sakata, Chem. L ett., solvent, the size of the growing crystallite and hence band edge 1990, 1773.position can be controlled relatively easily. However, when the 32 V. Sankaran, J. Yue, R. E. Cohen, R. R. Schrock and R. J. Silbey, solvent is light petroleum, generally wide size distributions of Chem. Mater., 1995, 7, 1185. clustered CdSe nanoparticulates are observed, because of the 33 (a) M. MoYtt and A. Eisenberg, Chem. Mater., 1995, 7, 1178; (b) insolubility of the polymer.CdSe nanoparticulates appear to M. MoYtt, L. McMahon, V. Pessel and A. Eisenberg, Chem. be cadmoselite (hexagonal) in phase and in some cases appear Mater., 1995, 7, 1185. 34 Y. Yuan, J. H. Fendler and I. Cabasso, Chem. Mater., 1992, 4, 312. to exhibit partial registry and distortions of the crystallite 35 A. Chevreau, B. Phillips, B. G. Higgins and S. Risbud, J. Mater. lattice.Although the quantum confinement regime observed Chem., 1996, 6, 1643. in CdSe is narrow the diVerent colours exhibited by the 36 L. Spanhel, M. Haase, H. Weller and A. Henglein, J. Am. Chem. material in this region as the particles decrease in size are Soc., 1987, 109, 5649. clearly observed indicating that the particle size distribution is 37 A. Iraqi and D. J. Cole-Hamilton, J.Mater. Chem., 1992, 2, 183 relatively small. The particle size of ZnSe semiconductor and references therein. 38 P. Narayanan, B. Kaye and D. J. Cole-Hamilton, J. Mater. Chem., nanoparticulates is diYcult to control mainly because of the 1993, 3, 19. strong Brønsted acidity of the hydrogen selenide and its rapid 39 A. Iraqi, S. Seth, C. A. Vincent, D. J. Cole-Hamilton, M. D. reaction with surface methyl (ZnMMe) groups which carry Watkinson, I. M. Graham and D. JeVrey, J. Mater. Chem., 1992, more negative charge than cadmium methyl groups. 2, 1057. 40 X. Li, C. M. Lindall, D. F. Foster and D. J. Cole-Hamilton, J.Mater. Chem., 1994, 4, 657. We thank John Mackie at the Bute, St. Andrews, for sample 41 X. Li, J. R. Fryer and D. J. Cole-Hamilton, J. Chem. Soc., Chem. preparation for low resolution TEM. We are also grateful to Commun., 1994, 1715. the Nippon Soda Company for generous gifts of polybuta- 42 S. W. Haggata, X. Li, J. R. Fryer and D. J. Cole-Hamilton, J.Mater. Chem., 1996, 6, 1771. dienes; to Dr Barry Kaye for photography; Professor J. W. 1974 J. Mater. Chem., 1997, 7(10), 1969–197543 X. Li, D. F. Foster and D. J. Cole-Hamilton, Polym. Adv. T echnol., 49 Y. G. Andreev and T. Lundstro� m, J. Appl. Crystallogr., 1995, 28, 534. 50 Y. G. Andreev and T. Lundstro�m, J. Appl. Crystallogr., 1994, 27, 1994, 5, 541. 44 D. J. Cole-Hamilton, Chem. Br., 1990, 26, 852. 767. 51 N. L. Pickett, D. F. Foster, J. R. Fryer and D. J. Cole-Hamilton, 45 D. F. Foster and D. J. Cole-Hamilton, Inorg. Synth., 1997, 31, 29. 46 CRC Handbook of Chemistry and Physics, ed. D. R. Lide, CRC J.Mater. Chem., 1996, 6, 507. 52 N. L. Pickett, D. F. Foster, F. Riddell, J. R. Fryer and D. J. Cole- Press Inc., Boston, MA, 1992, vol. 12, p. 75. I. Pankove, Optical Processes in Semiconductors, Dover Hamilton, J.Mater. Chem., 1997, 7, 1855. Publications, New York, 1975, p.43. 48 F. Urbach, Phys. Rev., 1953, 92, 1324. Paper 7/01943B; Received 19th March, 1997 J. Mater. Chem., 1997, 7(10), 1969–1975 1975

ISSN:0959-9428    

DOI:10.1039/a701943b

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Monodisperse liquid crystalline peptides Peter A. G. Cormack, Barry D. Moore*† and David C. Sherrington* Department of Pure and Applied Chemistry, University of Strathclyde, T homas Graham Building, 295 Cathedral Street, Glasgow, UK G1 1XL Solid phase peptide synthesis (SPPS) has been used as a synthetic tool to prepare monodisperse mesogen–oligopeptide conjugates with novel molecular architectures. Building upon previous success in this area, we have extended our methodologies to include a mesogenically substituted L-glutamic acid residue, and incorporated this derivative, as well as the L-lysine derivative previously reported, into a variety of new structures.Some novel liquid crystalline materials have been discovered. Both a-helical and b-sheet secondary structures have been explored as alternative scaVolds for the pendant mesogenic groups. The helical systems show most promise in terms of their ease of synthesis and handling, but at the relatively lower levels of mesogenic substitution studied so far, compared to the previously reported homo-oligopeptide materials, mesophases are not observed.The b-sheet materials prepared are rather insoluble and too high melting to have any practical value as thermotropic materials without further manipulation of their structures.For a number of years, one of the major goals in polymer primary sequences, utilising both the L-lysine derivative science has been to prepare high polymers with narrow polydis- reported previously, as well as a mesogenically substituted persities and pre-determined molecular mass.This synthetic amino acid based on L-glutamic acid. Amino acid sequences control would allow tailoring of physical properties and lead which support either a-helical or b-sheet secondary structures to a widening of polymers’ potential usefulness, and has now are also explored as alternative scaVolds for the mesogenic been achieved successfully for a number of polymer classes, groups.using a variety of diVerent methodologies. This synthetic aim is no diVerent in the field of liquid crystalline polymers (LCPs), where it has been shown that the degree of polymerisation can exert a significant influence upon the mesophase stability, as Results and Discussion well as the nature of the mesophase itself.1 It is also known Synthetic concepts and approaches that polydisperse LCPs can display multi-component mesophases. 2 As a result of these eVects, many of the methods for At the beginning of this current study, we identified two controlled polymerisation which are now well known in the possible routes into structurally well-defined, side-chain liquid literature have been applied to the synthesis of LCPs, with crystalline peptides, which were directly analogous in concept some success.These include cationic,1,3 anionic,4,5 group- to those utilised in the preparation of side-chain functionalised transfer6,7 and ring-opening metathesis8,9 polymerisation polymers. Thus one could modify, by attachment of a mesogen, studies. a pre-formed peptide which has been prepared by solid phase As outlined in a preliminary communication,10 we became means.Alternatively, an amino acid derivative appropriately interested in applying the synthetic control oVered by solid functionalised with a mesogen could be utilised in the solid phase methods in the preparation of side-chain liquid crystal- phase synthesis. Although the former is certainly attractive in line oligopeptides and polypeptides.This approach is very several respects, it was the latter approach which we adopted attractive in terms of preparing truly monodisperse materials, because it oVered potentially the greatest degree of synthetic but also, by the very nature of the technique, it oVers unpre- control. cedented control over the primary and secondary structures of In the design and synthesis of functionalised amino acids the peptide backbones, promising entry into structurally inter- suitable for our purposes, there were a number of aspects esting and unusual materials.Although to date solid phase which were considered, to give our systems synthetic and methods have been used extensively for the routine synthesis structural versatility, whilst maximising the likelihood of mesoof peptides, and have become central to the preparation of phase formation.These included not only their ease of synthesis libraries of compounds by combinational methods,11–13 it had and their stability, but also their potential to facilitate the never before been reported, to our knowledge, for the premedi- formation of ordered structure over and above that of the tated synthesis of structurally well-defined liquid crystalline liquid crystal phase.We concentrated upon the a-amino acids materials prior to our preliminary communication.10 because of their ready commercial availability in an optically In the latter, we described the successful evolution of a solid pure form, their well established chemistry and their interesting phase approach to the synthesis of L-lysine oligomers substi- secondary structure forming abilities.14 Bearing in mind the tuted in the side-chain with a mesogenic group. We detailed implications of the ‘spacer concept’15 for side-chain liquid the preparation of the dimer, the trimer and the tetramer of a crystalline polymers, as well as the structures of certain polydismesogenically substituted L-lysine derivative, and showed that perse liquid crystalline polypeptides already reported in the these did indeed display thermotropic behaviour, as antici- literature, we felt that the incorporation of a flexible spacing pated.In this full paper we give further details, and will show unit in all our designs was necessary to promote liquid how we have extended our methodologies to include other crystallinity.Mesogenic groups based on 4-cyano-4¾-hydroxybiphenyl were employed throughout, because these have been extensively studied in both low molecular mass materials and * Correspondence should be addressed to: Professor D. C. in polymers, and are reasonably stable under a variety of Sherrington. †E-mail: b.d.moore@strath.ac.uk conditions. J. Mater. Chem., 1997, 7(10), 1977–1983 1977Scheme 2 Reagents and conditions: i, Cl(CH2)6OH,KOH; ii, Ac2O; iii, toluene, heat; iv, PhNHNH2, Bu3N; v, Fmoc-OSu, Na2CO3 The eYciency with which the modified amino acid coupled to the growing peptide chain was particularly notable, and yielded materials which did not require further purification. This high purity was borne out by the sharp thermal transitions which were observed during heating and cooling cycles.Hot-stage polarised optical microscopy revealed thermotropic phases for all three materials which are visually of indistinct texture, but which we believe to be the nematic phase, since the samples Scheme 1 Reagents and conditions: i, MeOH, H2SO4; ii, K2CO3, displayed shear birefringence as well as showing no fan texture NCC6H4C6H4OH; iii, KOH; iv, SOCl2; v, CuLys2, NaOH; vi, EDTA 2Na; vii, Fmoc-Cl, Na2CO3 typical of some smectic phases.The peptides 16–18 are considered as ‘uncapped’ since the N-terminus is present as the free amine. We realised that Synthesis of amino acid derivatives perhaps an opportunity existed for modifying the mesophases The L-lysine derivative 8 was prepared by the route shown in and/or the transition temperatures through careful control of Scheme 1. 6-(4-Cyanobiphenyl-4¾-yloxy)hexanoic acid 4 was the capping group at the N-terminus, and so proceeded to prepared from 6-bromohexanoic acid 1 according to a litera- prepare the N-acetyl derivatives Ac-[Lys(M)]n-NH2, n=2 ture procedure.16 In the key step of the synthesis, the acid (19), 3 (20) and 4 (21) of the three peptides.To our surprise, chloride of 4 was coupled selectively to the e-amino group in acetylating the N-terminus completely destroyed the mesogenic the L-lysine copper complex using interfacial reaction con- properties of the peptides. We believe that this may be due to ditions, in a fashion similar to that previously reported by a hydrogen bonding interaction between the amide group at Gue�niVey for the reaction of lg-chain acid chlorides with the N-terminus and the amide group in the side-chain, but we the L-lysine copper complex.17 Thereafter, decomplexation are currently carrying out further capping studies to probe followed by fluorenylmethoxycarbonyl (Fmoc) protection gave this interesting eVect further. 8 in an overall yield of 14%, in eight steps.In a similar fashion to the L-lysine system we have also The L-glutamic acid derivative 15 was prepared via the route begun to construct the L-glutamic acid series of homo-oligopepshown in Scheme 2. 6-(4-Cyanobiphenyl-4¾-yloxy)hexanol 1018 tides. In the first instance we have prepared the dimer, with and N-phthaloyl-L-glutamic anhydride 1219 were prepared the N-terminus either acetylated or present as the free amine according to literature procedures, and coupled under con- X-[Glu(M)2-NH2, X=H (22) orAc (23).Again, the ‘as cleaved’ ditions similar to those employed by Feijen20 for a related materials were of apparently high purity, and this time both system, to give 13. Phthaloyl deprotection21 followed by Fmoc the acetylated and the uncapped peptide displayed a mesoprotection gave 15 in an overall yield of 10% in five steps.phase, which was readily identifiable as the nematic phase in both cases. We were unable to crystallise 22, and indeed this Solid phase synthesis of oligopeptides oily compound was observed to be birefringent at room temperature. Further extension of this series to include the Homo-oligopeptides.As detailed elsewhere,10 the dimer, higher analogues will be required to establish structure–prop- trimer and the tetramer H-[Lys(M)]n-NH2, n=2 (16), 3 (17) erty relationships, and also to ascertain the eVect of developing and 4 (18)‡ of the L-lysine derivative were successfully prepared. secondary structure upon the liquid crystalline behaviour as the main-chains become longer.‡Throughout, M=6-(4-cyanobiphenyl-4¾-yloxy)hexanoyl for lysine The fact that the acetylated derivative 23 was liquid crystal- derivatives and 6-(4-Cyanobiphenyl-4¾-yloxy)hexyl for glutamic acid derivatives. line in this case can be considered as lending some supporting 1978 J. Mater. Chem., 1997, 7(10), 1977–1983evidence to the reasoning behind why the acetylated lysine derivatives 19–21 were not liquid crystalline, since 23 has an ester functionality in the side-chain, and not an amide.Targetted a-helical peptides. In developing the concept of monodisperse liquid crystalline peptides with additional secondary structure imposed by the peptide backbone, we have become interested in the preparation of peptides where the main-chain is in the a-helical conformation, since this structural feature is common to many natural peptides and proteins, and is present in virtually all polydisperse polypeptides which display mesomorphic behaviour.22 To succeed in this goal, one must have an understanding of the structural parameters which dictate secondary structure formation. These include the primary sequence of the peptide,23 the length of the peptide chain,24,25 the presence or otherwise of substituents at the Nand C-termini26,27 and other factors such as the ability to form salt bridges or disulfide bridges which can stabilise secondary structures.Our first a-helical peptide target was the dodecapeptide 24, Ac-AlaAlaLys(M)AibAlaLys(M)AlaAibAlaLys(M)AlaAla- NH2, which contains seven alanine (Ala) residues, two aaminoisobutyric acid residues (Aib) and three mesogenically substituted L-lysine residues [Lys(M)]. We considered the level of mesogenic substitution to be rather lower (25% of amino acid residues are mesogenic) than would be considered ideal for the formation of liquid crystalline phases, but nonethe- Fig. 1 A molecular model of the dodecapeptide 24, showing the three less 24 provided an ideal initial target to assess the feasibility mesogenic groups pendant on one side of the a-helix.The main axis of the a-helix is normal to the plane of the page. of the approach. The amino acids in the sequence were chosen on the basis of their a-helix forming propensity; alanine is well known for its helix forming abilities,23 as is the non-natural amino acid would lie on one side of the helix, with the remaining three on a-aminoisobutyric acid.26,28 The conformational parameters of the opposite side.This, we felt, was the most favourable the non-natural Lys(M) residue were not known, but it was arrangement of side-chains at this level of mesogenic substi- anticipated that it would either be a weak helix former or tution, in terms of potential mesophase formation.This relatively neutral conformationally, and hence quite readily sit arrangement is most easily visualised in the molecular model in a helical conformation in the presence of other strong helix (Fig. 2). formers. To our advantage, not only do Aib residues show After purification by reverse phase HPLC and precipitation strong helical conformation preferences, but they also inhibit from the helicogenic solvent acetonitrile,22 the conformation the formation of b-sheets.The C-terminus and the N-terminus of 25 was investigated using infrared spectroscopy. Bands were of 24 were capped as the acid amide and the acetyl derivative observed at 1668 and 1543 cm-1.30 The former is outside the respectively, to enhance the solubility of the peptide in organic range for the amide I band of a helix and instead suggests 25 solvents and also to enhance the likelihood of helix formais folded into a series of beta turns or loops.Thermal studies tion.27,29 Crucially, the positions of the Lys(M) residues within indicated that the material did not exhibit any thermotropic the sequence were carefully chosen so that all three of the phases, with melting from the crystalline to the isotropic liquid mesogenic groups would lie on approximately the same side being observed at 75 °C, and crystallisation directly from the of the a-helix (Fig. 1).We believed that this arrangement of isotropic melt occurring upon cooling. This could be because side-chains oVered the best prospect for mesophase formation the level of mesogenic substitution is simply not at this level of mesogenic substitution.high enough to sustain a mesophase or because the poorly After purification by reverse phase HPLC and precipitation defined peptide conformation is unfavourable for ordering. from methanol, which in our hands has often proved to be Accordingly, we are currently working towards more robust helicogenic,22 24 was indeed shown to adopt an a-helical helical systems where the mesogenic content is still higher.conformation, as confirmed by the absorption bands present at 1659 (amide I) and 1541 cm-1 (amide II) in the solid state FTIR spectrum.30 Upon heating, it melted directly into the Targetted b-sheet peptides. To date, all of the polydisperse liquid crystalline polypeptides reported in the literature have isotropic liquid state at 196 °C, and crystallised directly from the melt upon cooling.That is, no thermotropic behaviour been based upon a-helical main-chains. We were very interested, therefore, in exploiting the opportunities lent to us by was observed, as perhaps anticipated. Nonetheless we at least demonstrated that this was a feasible synthetic approach our methodologies to prepare materials with diVerent secondary structures. Accordingly, a series of six peptides (26–31) towards these well-defined helical structures.As a second a-helical peptide target, we selected the tetra- based upon a simple repeat unit X-[ValLys(M)]n-NH2; X=H or Ac; n=1, 2 or 3 was prepared, designed to adopt an decapeptide 25, Ac-AlaAlaGlu(M)AibGlu(M)AlaGlu(M)Glu- (M)AiBGlu(M)AlaGlu(M)AlaAla-NH2, which again contains extended b-sheet conformation.Valine (Val ) has strong b-sheet preferences. Every second residue in the sequence was meso- predominately Ala and Aib residues. The level of mesogenic substitution is significantly higher (ca. 43% of residues are genically substituted. The solid phase synthesis of these peptides proceeded reason- mesogenic) than for 24 to favour mesophase formation, and because of this we elected to utilise the L-glutamic acid ably well, but the presence of deletion peptides was apparent in the mass spectra of the crude samples of the higher (n>2) derivative because the ability of L-glutamic esters themselves to support an a-helix is well known.22 The mesogenic residues analogues.The low solubility of these materials in a range of solvents meant that satisfactory purification was not possible, within the primary sequence were arranged such that three J. Mater. Chem., 1997, 7(10), 1977–1983 1979Table 1 Summary of the peptides prepared, showing their composition and their thermal behaviour no. of amino mesogenic peptide acids in chain content (%) thermal behaviour homo-oligopeptides-L-lysine series 16a 2 100 K 142 °C LC 173 °C I 17a 3 100 K 163 °C LC 190 °C I 18a 4 100 K 204 °C I 175 °C LC 19b 2 100 mp 116 °C 20b 3 100 mp 120 °C 21b 4 100 mp 118 °C homo-oligopeptides-L-glutamic acid series 22a 2 100 N 71.6 °C I 23b 2 100 K 109 °C I 105 °C N 60 °C K a-helical peptides 24 12 25 mp 196 °C 25 14 ~43 mp 75 °C b-sheet peptides 26–31 4, 6 or 8 50 mp~300–350 °C (decomp.) aUncapped, bCapped.much longer peptides, containing a number of mesogenic units, by this approach (up to 14 residues in length), and control their secondary structures through appropriate design of the amino acid sequence. Although we have not yet been able to prepare longer peptides which are thermotropic, either because the level of mesogenic substitution was too low or the materials were too high melting, we are nonetheless currently pursuing a number of approaches which will undoubtedly make this possible.Fig. 2 A molecular model of the tetradecapeptide 25, showing three mesogenic groups pendant on one side of the a-helix, and the Experimental remaining three pendant on the opposing side. The main axis of the a-helix is normal to the plane of the page. Materials All materials used in this study were commercial samples, and and as a result the thermal measurements were carried out on were used as supplied unless otherwise stated.For all the solid the ‘as cleaved’ materials. phase syntheses, the amino acids and coupling reagents FTIR spectroscopy showed that an extended chain (b-sheet) employed were of peptide synthesis grade, and were supconformation had indeed been adopted in every case, as shown plied by Novabiochem. N,N,-Dimethylformamide (DMF) by the amide I and amide II absorption bands present (Rathburn, peptide synthesis grade), piperidine (Rathburn, at ca. 1635 and ca. 1545 cm-1 respectively in each sample.30 peptide synthesis grade), N,N-diisopropylethylamine (Aldrich, However, thermal studies revealed that all these materials 99%) and trifluoroacetic acid (TFA) (Rathburn, gas phase either melted or decomposed at high temperatures sequencer grade) were also used as supplied.The resin support (typically>300 °C) without entering into a liquid crystal phase. used throughout (PR500, 0.36 mmol g-1 nominal loading) was This was deemed too high to be of any practical value, so we supplied by Novabiochem. All chromatographic solvents were are currently pursuing structural modifications aimed at lowerdistilled prior to use, and other solvents purified, where appro- ing the melt temperatures and allowing fluid phases to be priate, using standard procedures.accessed at more reasonable temperatures. In principle, these modifications, which involve disrupting the hydrogen bonding, should also make these materials more soluble and hence Instrumentation easier to purify and handle. 1HNMR spectra were recorded on a 250 MHz Bruker WM- 250 spectrometer. FTIR spectra were recorded either on a Summary and Prospects UNICAM Matteson 1000 spectrometer or on a Nicolet Impact 400D spectrometer. Elemental microanalysis was carried out In conclusion, we have demonstrated that the solid phase by the Microanalytical Laboratory at the University of approach to the synthesis of monodisperse liquid crystalline Strathclyde.Melting points were recorded on a Gallenkamp oligopeptides is indeed very powerful, and has allowed us to melting point apparatus, and are uncorrected. HPLC was prepare a number of interesting mesogen–peptide conjugates performed on a C8 column (25×4.6 mm, packed with 5 mm of unique structure; the materials prepared to date are summar- Spherisorb ODS2, supplied by Anachem) using Gilson 306 ised in Table 1.In the first instance we prepared the dimer, the pumps and a Gilson 117 UV detector. ES–MS spectra were trimer and the tetramer of a mesogenically substituted L-lysine recorded on a Fisons VG Platform spectrometer.DSC was derivative, and found all of these to be liquid crystalline when carried out on a Perkin-Elmer DSC 7 diVerential scanning the N-terminus was uncapped. We are currently extending this calorimeter. Molecular modelling was carried out using series to include the higher analogues, whilst at the same time MACROMODEL, Version 4.5, on a Silicon Graphics Indigo developing an alternative series based upon an L-glutamic acid derivative.We have also shown that we can readily prepare work-station. 1980 J. Mater. Chem., 1997, 7(10), 1977–1983Solid phase synthesis of peptides 6-(4-Cyanobiphenyl-4¾-yloxy)hexanoyl chloride 5. The acid 4 (3.00 g, 9.70 mmol) was stirred with thionyl chloride (10 cm3, Oligopeptides were synthesised on a Novasyn Crystal solid 137 mmol) for 3 h at room temp.in a flask fitted with a phase peptide synthesiser using Novasyn PR500 resin and calcium chloride guard tube, giving a clear yellow solution. PyBOP coupling chemistry. Typically, each residue was double This solution was then gently heated for 90 min to expel any coupled using a two-fold excess of amino acid.Cleavage of the remaining gases from solution. The excess thionyl chloride was peptide from the resin was with 10% TFA in CH2Cl2 for then removed under reduced pressure, and the oily residue co- 90 min. evaporated several times with dry diethyl ether. The acid chloride was then used in the next step without further Liquid crystal characterisation of peptides purification; nmax/cm-1 (selected band) 1805.Hot stage polarised optical microscopy was performed either Copper complex of e-[6-(4-cyanobiphenyl-4¾-yloxy)hexanam- on an Olympus CH-2 microscope fitted with a Mettler FP-5 ido]-L-lysine 6. To a boiling solution of L-lysine monohydro- hot-stage, JVC TK-1085E colour video attachment and a Sony chloride (3.54 g, 19.4 mmol) in water (60 cm3), was added basic UP-3000P colour video printer, or on an Olympus Vanox copper(II) carbonate (9.43 g, 42.7 mmol) over a period of microscope fitted with a Linkam TH600 hot-stage and a 10 min; the addition caused eVervescence and the supernatant Linkam PR600 thermal controller.Microscope slides were solution turned bright blue. The mixture was boiled for a pretreated with a homogeneous aligning agent.further 10 min, cooled to room temp. and the excess copper(II) carbonate filtered oV. A solution of sodium hydroxide (0.78 g, Syntheses 19.4 mmol) in water (5 cm3) was then added to the bright blue L-Lysine derivative 8. Methyl 6-bromohexanoate 2. 6- filtrate and the solution cooled to 0 °C on an ice-bath. A Bromohexanoic acid (22.00 g, 113 mmol), concentrated sulfuric solution of 5 (9.7 mmol) in dry dichloromethane (DCM) acid (3 cm3) and methanol (50 cm3) were refluxed for 3 h and, (30 cm3) was added to the rapidly stirred solution over a after cooling, excess methanol was removed under reduced period of 2.5 h, the temperature being maintained at 0 °C, and pressure.Portions of deionised water (100 cm3) and chloroform the mixture then stirred for a further 4.5 h at the same (100 cm3) were added to the colourless liquid residue, and the temperature.The lilac coloured precipitate which formed was organic layer separated. The aqueous layer was further filtered oV, washed with ethanol, water and diethyl ether, and extracted with portions of chloroform (3×50 cm3). The organic then air-dried (4.55 g, 100% technical yield).Further purifi- layers were combined, washed with 5% aqueous sodium cation was not carried out; nmax/cm-1 3430, 3306, 2936, 2865, hydrogen carbonate (100 cm3), then water (50 cm3), dried over 2225, 1620, 1604, 1541, 1496, 1390, 1291, 1253, 1181 and 822. magnesium sulfate and the chloroform removed under reduced pressure. The crude liquid residue was distilled under reduced e-[6-(4-Cyanobiphenyl-4¾-yloxy)hexanamido]-L-lysine 7.The pressure and 2 collected as a colourless liquid (17.16 g, copper complex 6 (94.43 g, 4.73 mmol) was stirred with 10% 73%), bp 40 °C at 0.01 mbar. (Found: C, 40.6; H, 6.6; Br, 37.7. aqueous ethylenediaminetetraacetic acid (EDTA) disodium salt C7H13BrO2 requires C, 40.2; H, 6.3; Br, 38.2%); nmax/cm-1 (200 cm3) for 20 h at room temp.The precipitate which formed 2947, 2865, 1740, 1436, 1362, 1255, 1202, 1173, 1122 and 1000; was filtered oV and again stirred with 10% aqueous EDTA dH (CDCl3) 1.40–1.52 (m, 2H), 1.57–1.76 (m, 2H), 1.78–1.98 disodium salt (200 cm3) for a further 20 h at room temp. After (m, 2H), 2.33 (t, 2H), 3.38 (t, 2H) and 3.66 (s, 3H). filtering, the solid was washed well with water and dried in vacuo at 50 °C for 64 h (3.70 g, 89% technical yield), mp 285 °C.Methyl 6-(4-cyanobiphenyl-4¾-yloxy)hexanoate 3. The ester 2 Further purification was not carried out; nmax/cm-1 3310, (8.80 g, 42.1 mmol), 4-cyano-4¾-hydroxybiphenyl (8.21 g, 3071, 2938, 2866, 2229, 1638, 1606, 1582, 1543, 1521, 1494, 42.1 mmol) and anhydrous potassium carbonate (5.82 g, 1407, 1289, 1253, 1181 and 824. 42.1 mmol) were stirred in DMF (20 cm3) for 24 h in a flask fitted with a calcium chloride guard tube. The viscous yellow e-[6-(4-Cyanobiphenyl-4¾-yloxy)hexanamido]-N-fluorenylslurry which formed was poured into water (200 cm3) and the methoxycarbonyl-L-lysine 8. Compound 7 (3.00 g, 6.86 mmol) resultant white precipitate filtered oV and washed with water.was suspended in a mixture of 10% aqueous sodium carbonate The crude product was dried over phosphorus pentoxide for (60 cm3) and dioxane (40 cm3), and the mixture stirred for 1 h 48 h, and then recrystallised from 80% aqueous ethanol at room temp. A solution of fluorenyl chloroformate (1.95 g, (175 cm3) giving 3 as a white powder (9.90 g, 73%), mp 7.54 mmol) in dioxane (20 cm3) was then added to the stirred 88–89 °C ( lit.,16 80–81 °C).(Found: C, 74.4; H, 6.8; N, 4.3. mixture; precipitation of a white solid was observed approxi- C20H21NO3 requires C, 74.3; H, 6.6; N, 4.3%); nmax/cm-1 2947, mately 5 min after the addition. After stirring for a further 2869, 2222, 1722, 1600, 1497, 1473, 1294, 1272, 1254, 1181, 24 h, the mixture was poured into water (75 cm3) giving a 1031 and 829; dH (CDCl3) 1.46–1.92 (m, 6H) 2.37 (t, 2H), 3.68 turbid solution.Acidification to pH 3 with 20% aqueous (s, 3H), 4.02 (t, 2H), 6.98 (m, 2H) and 7.50–7.73 (m, 6H). hydrochloric acid yielded a sticky white material which was extracted into chloroform (3×150 cm3). The organic layers were combined, washed with water (150 cm3) and dried over 6-(4-Cyanobiphenyl-4¾-yloxy)hexanoic acid 4.The ester 3 9.00 g, 27.8 mmol) and potassium hydroxide (16.00 g, magnesium sulfate. The solvent was removed under reduced pressure and the crude product recrystallised from chloroform– 285 mmol) were stirred in ethanol (150 cm3) for 3 h at room temp. in a flask fitted with a calcium chloride guard tube. The light petroleum, giving 8 as a white powder (2.05 g, 45%), mp 154–156 °C.(Found: C, 72.4; H, 6.4; N, 6.1. C40H41N3O6 resultant yellow slurry was poured into iced-water (450 cm3), the mixture allowed to warm to room temp. and then neutral- requires C, 72.8; H, 6.3; N, 6.4%); nmax/cm-1 3327, 3065, 2942, 2862, 2224, 1751, 1694 (sh. at 1721), 1602, 1544, 1494, 1451, ised with concentrated sulfuric acid. The white precipitate was filtered oV, washed with water and dried over calcium chloride. 1253, 1181, 821, 760 and 741; dH (DMSO) 1.06–1.84 (m, 12H), 2.07 (t, 2H), 3.02 (m, 2H), 3.84–4.06 (m, 3H), 4.18–4.38 (m, Recrystallisation from ethanol (400 cm3) gave 4 as a white powder (5.73 g, 67%), K 163.7 °C I 157.1 °C N 134.3 °C (from 3H) and 6.93–8.35 (m, 18H); M 659.8; found, m/z 682.5 (M+Na+) and 660.7 (M+H+).DSC) ( lit.,16 mp 165 °C) (Found: C, 73.6; H, 6.2; N, 4.1. C19H19NO3 requires C, 73.8; H, 6.2; N, 4.5%); nmax/cm-1 2942, 2869, 2229, 1703, 1601, 1445, 1251, 1183, 1076, and 820; dH L-Glutamic acid derivative 15. 6-(4-Cyanobiphenyl-4¾-yloxy) hexanol 10. 4-Cyano-4¾-hydroxybiphenyl (6.43 g, 32.9 ([2H8]THF) 1.30–1.88 (m, 6H), 2.25 (t, 2H), 4.04 (t, 2H) and 6.98–7.88 (m, 8H). mmol), potassium hydroxide (1.85 g, 32.9 mmol) and a few J.Mater. Chem., 1997, 7(10), 1977–1983 1981crystals of potassium iodide were dissolved in an ethanol– mixture was observed 15 min after the addition. The free- flowing white coloured mass which formed was poured into water mixture (451, 165 cm3) at room temp. 6-Chlorohexanol (4.87 cm3, 36.6 mmol) was then added, and the mixture refluxed water (500 cm3), and the resulting turbid solution–suspension acidified with glacial acetic acid.The white precipitate which for 24 h. After cooling, the ethanol was removed under reduced pressure, and the resultant yellow slurry filtered oV on a glass formed was filtered oV and washed with water and diethyl ether. Purification by flash column chromatography on silica sinter.The product was carefully washed with water, dilute aqueous sodium hydroxide, and then again with water. gel using ethyl acetate as the eluent gave 15 as a sticky, yellow solid (1.72 g, 69%). (Found: C, 72.9; H, 6.3; N, 4.1, C39H38N2O7 Recrystallisation from methanol (50 cm3) gave 10 as a white powder (4.10 g, 42%), K 92.3 °C N 113.0 °C I (from DSC) requires C, 72.4; H, 5.9 N, 4.3%); nmax/cm-1 3351, 3064, 3039, 2935, 2862, 2225, 1726, 1602, 1522, 1493, 1251, 1178, 1051, 822, ( lit.,18 K 93.5 °C N 110.9 °C I).(Found: C, 76.7; H, 6.7; N, 4.7. C19H21NO2 requires C, 77.2; H, 7.2; N, 4.7%); nmax/cm-1 3289, 760 and 741; dH (CDCl3) 1.32–2.70 (m, 12H), 3.73–4.58 (m, 8H), 6.82–7.83 (m, 17H); M 646.8; m/z 647.9 (M+H+) and 2943, 2868, 2226, 1602, 1494, 1473, 1291, 1252, 1183, 1072, 1012 and 829; dH (CDCl3) 1.23–1.96 (m, 9H), 3.68 (t, 2H), 4.02, 311.2 (M-C21H22NO3+H+).(t, 2H), 6.98 (d, 2H), 7.42–7.83 (m, 6H). Resin washing protocol N-Phthaloyl-L-glutamic anhydride 12. A mixture of NUpon completion of each peptide synthesis, and prior to phthaloyl-L-glutamic acid (5.07 g, 18.3 mmol) and acetic anhycleavage of the peptide from the resin, the resin–peptide dride (10 cm3, 106 mmol) was refluxed for 10 min under a dry conjugate was washed according to the following protocol to nitrogen atmosphere.Upon cooling to room temp., a white remove any residual traces of DMF as well as any other precipitate was seen for form. Dry diethyl ether (50 cm3) was soluble impurities. added to the mixture, the solids filtered oV, washed with a The resin–peptide conjugate was removed from the Novasyn further volume of dry diethyl ether, and then air-dried.Crystal reactor column, placed on a glass sinter and then Recrystallisation from dry ethyl acetate (75 cm3) furnished 12 washed sequentially with DMF, tert-pentyl alcohol, acetic acid, as colourless crystals, with a second crop being collected after tert-pentyl alcohol, DCM and finally diethyl ether.It was then cooling the filtrate overnight in the refrigerator (3.26 g, 69%), dried in vacuo for 8 h. mp 196–197 °C (decomp.) ( lit.,19 195–196 °C). (Found: C, 60.0; H. 3.5; N, 5.4. C13H9NO5 requires C, 60.2; H, 3.5; N, 5.4%); nmax/cm-1 1813, 1772, 1715, 1470, 1389, 1226, 1074, 1030, 974 Acetylation of resin-bound peptides and 723; dH (DMSO) 2.53–2.70 (m, 2H), 2.90–3.20 (m, 2H), Where appropriate, peptides were acetylated at the N-terminus 5.43–5.50 (m, 1H), 7.88–7.97 (m, 4H).as follows. The dry resin-peptide conjugate (20 mg) was placed in a round-bottomed flask and distilled DCM (4 cm3) added, 5-[6-(4-Cyanobiphenyl-4¾-yloxy)hexyl ] hydrogen N-phthalfollowed by a few crystals of dimethylaminopyridine (DMAP) oyl-L-glutamate 13.Compounds 10 (3.56 g, 12.0 mmol) and 12 and distilled acetic anhydride (1 cm3). The mixture was gently (3.12 g, 12.0 mmol) were refluxed in dry toluene (300 cm3) shaken for 4 h, the resin filtered oV and washed well with fresh under a dry nitrogen atmosphere for 24 h. After cooling, the DCM. The resin was then dried in vacuo for several hours. toluene was removed under reduced pressure and the oily residue purified by flash column chromatography on silica gel, Cleavage of peptides from the resin support using ethyl acetate as the eluent, giving 13 as a sticky yellow oil (5.34 g, 80%).(Found: C, 69.6; H, 5.4; N, 4.8. C32H30N2O7 The dry resin–peptide conjugate (20 mg) was placed in a round requires C, 69.3; H, 5.5; N, 5.1%); nmax/cm-1 2939, 2863, 2225, bottomed flask and 10% TFA in distilled DCM (10 cm3) 1776, 1720, 1603, 1494, 1468, 1391, 1251, 1178, 1114, 823 and added; the resin was observed to turn bright red shortly 720; dH (CDCl3) 1.32–1.90 (m, 8H), 2.34–2.73 (m, 4H) 3.92–4.22 thereafter.The flask contents were swirled every 10 min or so, (m, 4H), 4.87–5.05 (m, 1H), 6.98 (d, 2H) and 7.47–7.96 (m, 10H). and after 90 min the resin was filtered oV on a glass sinter and washed with 10% TFA in DCM (4×5 cm3) followed by DCM 5-[6-(4-Cyanobiphenyl-4¾-yloxy)hexyl ] hydrogen L-glutamate (4×5 cm3).The filtrate was concentrated under reduced press- 14. Compound 13 (5.34 g, 9.63 mmol) was dissolved in 96% ure, and the residue then co-evaporated several times with aqueous ethanol (10 cm3). Tributylamine (2.29 cm3, acetonitrile (HPLC grade).Addition of diethyl ether (glass 9.63 mmol) and freshly distilled phenylhydrazine (2.84 cm3, distilled grade) to the resultant oily residue yielded a mobile, 28.9 mmol) were then added and the mixture refluxed for 2 h; white precipitate which was collected by centrifugation and a solid was observed to precipitate from solution approximately then dried in vacuo for 8 h. 15 min after reflux was attained. Methyl ethyl ketone (20 cm3) was added, the mixture refluxed for a further 15 min, and then Homo-oligopeptides cooled to room temp. Glacial acetic acid (0.83 cm3, 14.4 mmol) was added, the mixture stirred at room temp. for approximately Satisfactory acylation and deprotection traces were obtained 10 min, and the white solid filtered oV.This was washed for all residues: carefully with methyl ethyl ketone and then dried in vacuo for 7 h at 40 °C giving 14 as a white powder (1.67 g, 41% technical H-[Lys(M)]2-NH2 16. K 142 °C LC 173 °C I; M 856.2, m/z yield) mp 159.7 °C (from DSC). Further purification was not 857.2 (M+H+). carried out. (Found: C, 67.1; H, 7.0; N, 6.7. C24H28N2O5 requires C, 67.9 H, 6.7; N, 6.6%); nmax/cm-1 3445, 2938, 2864, H-[Lys(M)]3-NH2 17.K 163 °C LC 190 °C I; M 1275.8, m/z 2228, 1730, 1604, 1580, 1496, 1414, 1329, 1254, 1181 and 823. 1298.2(M+Na+), 1276.3 (M+H+), 650.0 (M+H++Na+) and 639.0 (M+2Na+). 5-[6-(4-Cyanobiphenyl-4¾-yloxy)hexyl ] hydrogen N-fluorenylmethoxycarbonyl- L-glutamate 15. Compound 14 (1.63 g, H-[Lys(M)]4-NH2 18. K 204 °C I 175 °C LC; M 1695.3, m/z 3.84 mmol) was suspended in a mixture of 10% aqueous 870.4 (M+2Na+), 867.5 (M+H++K+), 859.4 sodium carbonate (50 cm3) and dioxane (35 cm3); partial dis- (M+H++Na+) and 848.5 (M+2H+).solution was observed. To the rapidly stirred suspension was added a solution of fluorenylmethoxycarbonyl-N-hydroxysuccinimide (1.30 g, 3.84 mmol) in dioxane (15 cm3), and the Ac-[Lys(M)]2-NH2 19.mp 116 °C; M 898.2, m/z 921.0 (M+Na+) and 899.0 (M+H+). mixture stirred at room temp. for 23 h; precipitation from the 1982 J. Mater. Chem., 1997, 7(10), 1977–1983Ac-[Lys(M)]3NH2 20. mp 120 °C; M 1317.8, m/z 1318.6 Ac-[ValLys(M)]3-NH2 30. mp 330 °C (decomp.); nmax/cm-1 (selected bands) 1633 and 1547. (M+H+), 670.9 (M+H++Na+) and 659.9 (M+2H+). Ac-[ValLys(M)]4-NH2 31.mp 340 °C (decomp.); nmax/cm-1 Ac-[Lys(M)]4-NH2 21. mp 118 °C; M 1737.4, m/z 1738.6 (selected bands) 1634 and 1548. (M+H+), 880.5 (M+H++Na+) and 869.7 (M+2H+). We gratefully acknowledge financial support from both the H-[Glu(M)]2-NH2 22. N 71.6 °C I; M 830.1, m/z 830.9 EPSRC and Merck Ltd. We would also like to thank Professor (M+H+), 495.6 (M-C21H22NO3+H+) and 477.7 George W.Gray, Dr David Coates and Ian Bonny for their (M-C21H22NO3-NH2-). useful discussions, and Novabiochem for assistance with the peptide syntheses. Ac-[Glu(M)]2-NH2 23. K 109 °C I 105 °C N 60°C; M 872.1, m/z 894.7 (M+Na+), 872.6 (M+H+), 559.1 (M-C21H22NO3+H+) and 537.0 (M-C21H22NO3+H+). References 1 V. Percec, M. Lee and C. Ackerman, Polymer, 1992, 33(4), 703. a-Helical peptides 2 H.Finkelmann, Angew. Chem., Int. Ed. Engl., 1987, 26, 816. 3 V. Percec and D. Tomazos, Adv.Mater., 1992, 4(9), 548. Satisfactory acylation and deprotection traces were obtained 4 N. Koide, T. Kumada and K. Iimura, Abstr., 14th Symp. L iquid for all residues. Crystals, Sendai, Sept. 29th, 1988, 164. 5 R. Bohnert, H. Finkelmann and P. Lutz, Makromol. Chem. Rapid Ac-AlaAlaLys(M)AibAlaLys(M)AlaAibAlaLys(M)AlaAla- Commun., 1993, 14(2), 139. 6 W. Kreuder, O. W. Webster and H. Ringsdorf, Makromol. Chem. NH2 24. mp 196 °C; nmax/cm-1 (selected bands) 1659 and 1541; Rapid Commun., 1986, 7, 5. M 1958.7, m/z M+H++K+), 1004.6 (M+H++Na+), 993.6 7 M.HeVt and J. Springer, Makromol. Chem. Rapid Commun., 1990, (M+2H+) and 985.2 (M-NH-2 +H+). 11(8), 397. 8 Z. Komiya, C. Pugh and R.R. Schrock, Macromolecules, 1986, Ac-AlaAlaGlu(M)AibGlu(M)AlaGlu(M)Glu(M)AibGlu(M)- 25, 6586. AlaGlu(M)AlaAla-NH2 25. mp 75°C; nmax/cm-1 (selected 9 C. Pugh, Macromol. Symp., 1994, 77, 325. 10 P. A. G. Cormack, D. C. Sherrington and B. D. Moore, Chem. bands) 1668 and 1543; M 3095.0, m/z 796.7 (M+4Na+), 701.8 Commun., 1996, 353. (M+2H++2Na+-C21H22NO3) and 575.6 (M+5Na+- 11 M.A. Gallop, R. W. Barrett, D. J. Dower, S. P. A. Fodor and C21H22NO3). E. M. Gordon, J.Med. Chem., 1994, 37, 1233. 12 R. M. Baum, Chem. Eng. News, 1994, 20. b-Sheet peptides 13 R. M. J. Liskamp, Angew. Chem., Int. Ed. Engl., 1994, 33, 633. 14 M. Bodanszky, Peptide Chemistry: A Practical T extbook, Springer The low solubility of these materials in a range of solvents Verlag, Berlin, 1988.meant that satisfactory purification was often not possible, and 15 Side Chain L iquid Crystal Polymers, ed. C. B. McArdle, Blackie, Glasgow, New York, 1989. as a result the thermal measurements were carried out on the 16 J. L. West, US Patent 5,093,471. ‘as cleaved’ materials. For some peptides it was not possible 17 H. Gue�niVey, R. Garnier and C. Pinazzi, Makromol. Chem., 1977, to acquire ES–MS data; in these cases the identity of the 178, 1277. peptides were inferred from the satisfactory acylation and 18 V. Percec and M. Lee,Macromolecules, 1991, 24(5), 1017. deprotection traces which were obtained during the solid phase 19 F. E. King and D. A. A. King, J. Chem. Soc., 1949, 3315. synthesis. 20 J. Feijen, W. M. Sederel, K. de Groot, A. C. de Visser and A. Bantjes, Makromol. Chem., 1974, 175, 3193. 21 I. Schumann and R. A. Boissonnas, Helv. Chim. Acta, 1952, 35, H-[ValLys(M)]2-NH2 26. mp 350 °C (decomp.); nmax/cm-1 2235. (selected bands) 1634 and 1550; M 1054.5, m/z 1077.3 22 W. H. Daly, D. S. Poche� and I. I. Negulescu, Prog. Polym. Sci., (M+Na+), 1055.3 (M+H+), 547.4 (M+H++K+) and 528.3 1994, 19(1), 79. (M+2H+). 23 P. Y. Chou and G. D. Fasman, Annu. Rev. Biochem., 1978, 47, 251. 24 M. Narita, Y. Tomotake, S. Isokawa, T. Matsuzawa and T. Miyauchi, Macromolecules, 1984, 17, 1903. H-[ValLys(M)]3-NH2 27. mp 275 °C (decomp.); nmax/cm-1 25 M. Mutter, Angew. Chem., Int. Ed. Engl., 1985, 24, 639. (selected bands) 1633 and 1546;M1573.2, m/z 1573.4 (M+H+), 26 C. Toniolo, G.M. Bonora, E. Benedetti, A. Bavoso, B. D. Blasio, 1154.3 [M-Lys(M)+H+], 1055.2 [M-Lys(M)-Val+H+], V. Pavone and C. Pedone, Biopolymers, 1983, 22, 1335. 806.8 (M+H++K+), 798.7 (M+H++Na+) and 787.7 27 A. Bierzynski, P. S. Kim and R. L. Baldwin, Proc. Natl. Acad. Sci. (M+2H+). USA, 1982, 79, 2470. 28 I. L. Karle and P. Balaram, Biochemistry, 1990, 29, 6747. 29 K. R. Shoemaker, P. S. Kim, O. N. Brems,Marqusee, E. J. York, H-[ValLys(M)]4-NH2 28. mp 340 °C (decomp.); nmax/cm-1 I. M. Chaiken, J. M. Stewart and R. L. Baldwin, Proc. Natl. Acad. (selected bands) 1634 and 1549. Sci. USA, 1985, 82, 2349. 30 S. Krimm and J. Bandekar, Adv. Protein Chem., 1986, 38, 181. Ac-[ValLys(M)]2-NH2 29. mp 330 °C (decomp.); nmax/cm-1 (selected bands) 1633 and 1550;M1096.5, m/z 1096.3 (M+H+). Paper 7/01084B; Received 17th February, 1997 J. Mater. Chem., 1997, 7(10), 1977–1983 1983

ISSN:0959-9428    

DOI:10.1039/a701084b

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Lyotropic liquid crystalline polyesters: synthesis of polyesters and copolyesters based on poly(sulfo-p-phenylene nitroterephthalate) David T.B. Hannah and David C. Sherrington* Department of Pure and Applied Chemistry, T homas Graham Building, University of Strathclyde, 295 Cathedral Street, Glasgow, UK G1 1XL The syntheses of the polyesters poly(sulfo-p-phenylene methoxyterephthalate) (4), poly(sulfo-p-phenylene 2-methoxy-5- nitroterephthalate) (5) and poly(sulfo-p-phenylene 2-bromo-5-nitroterephthalate) (6), and the random copolyesters poly([sulfo-pphenylene nitroterephthalate]x-co-[sulfo-p-phenylene terephthalate]) (7a–c), poly([sulfo-p-phenylene nitroterephthalate]y-co- [sulfo-p-phenylene bromoterephthalate]) (8a–c) and poly([sulfo-p-phenylene nitroterephthalate]z-co-[sulfo-p-phenylene nitroisophthalate]) (9a–f ) are reported.It was found that, of the three new homopolyesters prepared, 5 showed lyotropic behaviour in 151 DMSO–H2O, DMSO and DMF (DMSO=dimethyl sulfoxide; DMF=dimethylformamide). The copolymers were prepared in various monomer compositions in an eVort to establish the eVect of either (a) diluting the mesogenic character of a known liquid crystalline polyester by reducing the nitro ratio of the substituents, or (b) disrupting the main-chain linearity of the ester back-bone by the introduction of meta-arranged isophthalate units.It was found that the original polyester could incorporate up to 50% bromo- or un-substituted terephthalate units and retain the ability to form lyotropic solutions. In the case of the accommodation of nitroisophthalate units, only 9% could be tolerated.Poly(sulfo-p-phenylene nitroterephthalamide) (10) has also been synthesised and shown to form a lyotropic liquid crystalline phase in DMSO and aqueous DMSO. The development and commercialisation of liquid crystalline units, and also copolyesters which incorporate either bromo- (LC) polymers has recently acquired much interest in the or un-substituted terephthaloyl units into the main chain, or chemical community.1 This is in no small part due to the which contain some degree of non-linearity by the introduction success of liquid crystalline products such as Kevlar, a lyotropic of nitroisophthaloyl units.In this way, the eVect of diluting LC polyamide produced by Du Pont, and the series of the mesogenic character of the original polyester has been thermotropic LC polyesters marketed by Hoescht Celanese as monitored.Vectra. The ability of such materials to form liquid crystalline phases during processing imparts superior tensile strength and high moduli in the final products. The fabrication of these, Results and Discussion and other related, materials is restricted, though, by the conditions required for their processing.For example, Kevlar Monomer syntheses fibre is normally spun from a 98% sulfuric acid solution, whereas mouldable LC thermotropic polyesters such as Vectra 2-Nitroterephthaloyl chloride, 2-bromoterephthaloyl chloride, are typically processed at elevated temperatures. The economic 2-methoxyterephthaloyl chloride, 2-bromo-5-nitroterephthand ecological ramifications for industry are obvious.As a aloyl chloride and 2-methoxy-5-nitroterephthaloyl chloride result, an important research aim remains the synthesis of were prepared by reaction of their respective diacids with polymer materials which have the ability to form lyotropic LC thionyl chloride using DMF as a catalyst. Each was purified solutions in common organic solvents or even water, or by distillation and fully characterised.The diacids of the latter thermotropic mesophases accessible at moderate temperatures. pair were synthesised as described previously.10,11 It was found Some research groups have tackled the case of lyotropic that the diacid chlorides could also be prepared under the polyamides by the introduction of solubilising groups onto the mild phase-transfer conditions proposed by Burdett5 although polyamide main-chain.This work has been reviewed by us no significant yield enhancement was found. and others.2 In the case of lyotropic polyesters, we believe that success may be gained with the use of substituents which not only disrupt the main-chain regularity of the polymer, but which also oVer the possibility of interactions with an appropriate solvent.Recently, we achieved some initial success in this regard with the synthesis of poly(sulfo-p-phenylene nitroterephthalate) 1.3 This polyester was shown to be the first example of a wholly aromatic polyester which forms liquid crystalline solutions in aqueous organic solvents. This polyester forms lyotropic solutions in mixtures of dimethyl sulfoxide (DMSO) or dimethylformamide (DMF) with water, and in pure water and DMF.The critical concentration for mesophase formation lies between 18 and 50% and increases with the water content of the solvent. Somewhat surprisingly, the analogous bromo- and un-substituted polyesters (2 and 3 respectively) showed no lyotropic character. As part of a wider research project4 to synthesise other examples of polyesters in this class we have studied the phase behaviour of several polyesters prepared from alternative diacid J.Mater. Chem., 1997, 7(10), 1985–1991 1985Polymer syntheses and physical characterisation stretch of the methoxy group is present near 1030 cm-1. In all polymers a characteristic feature is a broad absorption around The polymers were synthesised using a standard interfacial 3000 cm-1 which can be attributed to the OMH stretch of procedure which was previously optimised for homopolyester any acidic or phenolic end-groups present or residual bound synthesis.3 The restricted solubility and ionic nature of the diol water molecules.prevented the use of either single-phase solution or melt procedures. 1H NMR spectra. All of the polyesters have very similar 1H Three novel polyesters were successfully prepared from the NMR spectra when observed in deuterated DMSO. Briefly, potassium salt of hydroquinonesulfonic acid with 2-methoxy-, two main features can be noted. Firstly, a complex set of 2-methoxy-5-nitro- and 2-bromo-5-nitro-terephthaloyl chlorsignals is seen in the aromatic range, ca. 7–9 ppm, which is in ides at room temperature (polyesters 4, 5 and 6) in yields of turn separated into two broad sets of signals. These can be 30, 78 and 63% respectively. In each case the reaction times attributed to the aromatic rings of the diol and diacid units in were comparatively short (ca. 1 h) and precipitates were the repeat unit. Further assignment of these peaks is diYcult aVorded during the polycondensation.In the case of the especially when we must take into account the random regio- copolyesters (7a–c, 8a–c and 9a–f ), the mixture of diacid regularity of the substituents with respect to each other. chloride monomers was added simultaneously in a single Secondly, one (or sometimes two) peak(s) are seen at around chloroform solution.In this way it was hoped that the copoly- 10–11 ppm. These are due to the acidic protons in the end- mer structures were consistent with the monomer compositions groups. Quantitative analysis of these was not attempted. and of an entirely random nature. Otherwise, their synthesis and isolation was identical to the homopolyester syntheses. The polyesters were characterised by infra-red spectroscopy, Solution viscometry.In our eVorts to ascertain the molecular mass characteristics of these polyesters we considered several 1H NMR spectroscopy, solution viscometry, diVerential scanning calorimetry, and elemental microanalysis. These results options. Previously,3 aqueous phase gel permeation chromatography (GPC) of 1 was conducted; however further GPC are summarised in Table 1.analysis of the other polyesters was severely limited by poor solubility. Since solubility tests show that, in general, the FTIR spectra. Absorptions typical of the functional groups expected were revealed: the strong CNO stretch of the ester polyesters are most soluble in 151 DMSO–H2O, capillary viscosity measurements were carried out in this solvent with carbonyl near 1740 cm-1; SO3- near 1235, 1070 and 630 cm-1 (SNO stretch); and the strong vibrations of the nitro function 0.1% LiCl added in an attempt to counteract the polyelectrolyte eVect due to the ionic nature of the polymer chains.The near 1540 (anti) and 1350 cm-1 (symmetric). The CMBr stretch will be below 400 cm-1 and is hence not observed. The CMO eZux time required for the solvent (t0) and the polymer 1986 J.Mater. Chem., 1997, 7(10), 1985–1991Table 1 Physical characterisation of polyesters found (%) calculated (%) polyester yield (%) C H N S Br Cl C H N S Br nmax/cm-1 ginh/dl-1a 4 30 41.9 2.9 — 9.2 — 0.5 46.4 2.3 — 8.2 — 2700–3500br (OMH), 1740s (CNO), 1200br, 1082 and 637 (SMO), 1030 (CMO) b 5 78 40.4 2.6 3.0 7.3 — 2.7 41.6 1.9 3.2 7.4 — 2700–3500br (OMH), 1760s (CNO), 1532 and 1351 (NMO), 1200br,1067 and b 643 (SMO), 1020 (CMO) 6 63 35.3 1.7 2.4 7.4 12.7 1.25 34.9 1.05 2.9 6.65 16.6 2700–3500br (OMH), 1760s (CNO), 1537 and 1362 (NMO), 1200br, 1036 and 633 (SMO) b 7a 67 42.7 2.1 0.3 8.8 — — 45.5 1.8 1.0 8.7 — — 0.20 7b 69 39.5 2.6 1.3 7.4 — — 44.2 1.7 1.8 8.4 — — 0.24 7c 69 40.6 2.4 2.2 7.3 — — 35.2 1.6 2.7 8.2 — 2700–3600br (OMH), 1755s (CNO), 0.15 1548 and 1357 (NMO), 1243, 1088 and 633 (SMO) 8a 46 37.4 2.4 0.5 6.9 13.0 — 39.2 1.4 0.8 7.5 14.0 — 0.20 8b 54 38.1 2.2 1.5 7.2 7.3 — 40.0 1.4 1.7 7.6 9.5 — 0.20 8c 54 40.1 2.1 2.5 7.5 3.1 — 40.8 1.5 2.6 7.8 4.8 3000–3600br (OMH), 1760s (CNO), 0.31 1548 and 1362 (NMO), 1228, 1098 and 638 (SMO) 9a 92 40.35 2.7 2.9 7.4 —- 4.1 41.7 1.5 3.5 7.95 — 2700–3500 (OMH), 1741s (CNO), 0.06 1540 and 1350 (NMO), 1236, 1070 and 628 (SMO) 9b 80 39.8 2.4 2.85 8.15 — 1.5 41.7 1.5 3.5 7.95 — 2700–3500 (OMH), 1740s (CNO), 0.11 1540 and 1352 (NMO), 1236, 1070 and 630 (SMO) 9c 73 40.4 2.4 2.8 7.8 — 0.9 41.7 1.5 3.5 7.95 — — 0.15 9d 86 40.6 2.55 3.0 7.8 — 1.3 41.7 1.5 3.5 7.95 — — b 9e 64 40.3 2.1 2.8 7.8 — 0.7 41.7 1.5 3.5 7.95 — — b 9f 79 41.1 2.3 2.9 7.8 — 0.8 41.7 1.5 3.5 7.95 — — b aMeasured at 0.2 g dl-1 polymer in 151 DMSO–H2O with 0.1% LiCl.bNot recorded. J. Mater. Chem., 1997, 7(10), 1985–1991 1987solution (t) were measured thrice and taken as an average. The crystalline texture observed in most cases was a marbled inherent viscosity was calculated as [ln (t/t0)]/c where the nematic type, similar to that seen previously for polyester 1 concentration (c) was 0.2 g dl-1. The viscosity measurements [see Fig. 4(a), ref. 3]. The corresponding nitro-bromo polyester tabulated indicate a relatively low molecular mass range for 6 shows no birefringence and has relatively poor solubility, these polyesters. As a comparison, for polyester 1 of ginh= solubilising in 151 DMSO–H2O, pure DMSO and pure DMF 0.18 dl g-1, a M9 w of 4300 was obtained from statistical light at concentrations no higher than ca. 11, 21 and 27 mass% scattering measurements.3 respectively. Similarly, the methoxy polyester 4 is highly insoluble and consequently shows no lyotropicity in the test sol- DiVerential scanning calorimetry. These measurements were vents. The importance of the nitro function for inducing performed on the polyesters over the range 25–400 °C under lyotropicity is clearly demonstrated, again, when polyesters 4 a nitrogen atmosphere with a heating rate of 20 °C min-1.All and 5 are compared. those materials containing a nitro function exhibited a large The degree and nature of the interaction between the exothermic peak with an onset tremperature of around 300 °C.macromolecule chains themselves and that between the chains This was also reported for 1.3 Polyester 4 showed only a small and solvent molecules can provide a major influence on the exotherm at ca. 270 °C and further decomposition thereafter. phase behaviour of these polymers. It is for this reason that In no case was a thermotropic transition observed. the polar nature of the nitro function appears to favour polymer–solvent interactions, thus solubilising the whole poly- Elemental microanalysis.The microanalytical data on these mer chain suYciently to allow a lyotropic mesophase. materials provide some useful information. In general terms it Interestingly, in thermotropic systems, steric factors have been can be seen from the chlorine content that even rigorous shown to play a more significant role than polar considerations washing does not in some cases remove all traces of the phase- in mesophase formation.6 transfer catalyst and NaCl formed in the reaction.The required The first two series of copolymers (7a–c and 8a–c) prepared analytical content has been calculated ignoring the end-groups were those where the introduction of diacid units, previously present.Nevertheless, the probable low molecular mass of the found to be unable to induce lyotropicity, was investigated in products will mean that these groups will contribute to any order to ascertain their eVect on the overall liquid crystallinity anomalies in the elemental data. Furthermore, for the copoly- of the copolymer. In both the cases where either the unesters, we are given an indication of the copolymer composition substituted or bromo-substituted diacid was used the eVect in comparison to that expected by the feed ratios of the was essentially the same, i.e.the incorporation of these units comonomers. It is assumed that the various diacid units will retards the mesophase formation of the copolymer.These be of comparative reactivity and hence that the comonomers results are summarised in Table 3. are most probably randomly distributed along the macromole- In both cases where the nitro content is low (ca. 25%) the cular chains in amounts proportional to their feed ratios. In copolymers are insoluble in the test solvent. This could be general, the microanalytical data would tend to support this.predicted from the previous results3 where it was found that The only significant exception is with polyester 7a where the the unsubstituted polymer was not soluble in 151 DMSO–H2O N content is low. An explanation for this is not immediately at concentrations as low as 7 mass% and the bromo polymer apparent. was insoluble at ca. 10 mass%. Essentially, these particular copolyesters retain the properties of the major constituent Solution phase characterisation of polyesters structural units.When the nitro content of the polymerisation feed was increased to ca. 50% and above it can be seen that For homopolymers 4–6 the phase behaviour is summarised in the copolyesters behave in the same way as the nitro homopoly- Table 2. Firstly, it can be seen that only polyester 5 which is ester and lyotropic solutions were obtained at 40 mass% in disubstituted on the diacid aromatic ring with a nitro and 151 DMSO–H2O for monomer feed ratios where x or y equal methoxy function shows any birefringence when viewed under 1 and 3.Liquid crystalline textures were as before, but copoly- cross-polarised light. This is apparent under shear in 151 ester 7c exhibited regions of Schlieren and marbled textures. DMSO–H2O only above ca. 62 mass% polymer. In pure DMF From this result it can be assumed that there exists a critical and DMSO, alignment is seen without shear at or above concentrations of ca. 68 and 42 mass% respectively. The liquid comonomer composition for the random copolyester of this Table 2 Solution properties of homopolyesters polyester solvent concentrationa observationb 4 H2O <10 insoluble 151 DMSO–H2O 21 insoluble 6 reaches isotropicity DMSO 21 insoluble 8 reaches isotropicity DMF 13 reaches isotropicity 5 H2O <10 insoluble 151 DMSO–H2O 48 isotropic gel 55 isotropic solution 62 nematic under shear DMSO 68 untextured birefringence DMF 42 untextured birefringence 6 H2O <10 insoluble 151 DMSO–H2O 33 insoluble 11 reaches isotropicity DMSO 56 insoluble 21 reaches isotropicity DMF 27 reaches isotropicity aExpressed as mass% polymer in solution to nearest integer.bUnder cross-polarised light. 1988 J. Mater. Chem., 1997, 7(10), 1985–1991Table 3 Solution properties of copolyesters with bromo- and un-substituted diacid units nitro diacid bromo diacid unsubstituted observation in 151 copolyester x y in feed (%) in feed (%) diacid in feed (%) DMSO–H2Oa 7a 0.33 — 25 — 75 insoluble 7b 1 — 50 — 50 nematic solution 7c 3 — 75 — 25 nematic solution 8a — 0.33 25 75 — insoluble 8b — 1 50 50 — nematic solution 8c — 3 75 25 — nematic solution aUnder cross-polarised light.type whereby, firstly, the copolyester will be soluble in in 151 mer structures, we can see that a more extended polymer chain will allow the nitro groups more freedom to interact with the DMSO–H2O (and other related solvent systems) and, secondly, where it will form a lyotropic solution.At present it is clear solvent molecules. A more compacted, meta-arranged chain may well find its ability to interact with solvent molecules that these two critical values (for they may be diVerent) lie in the region 25–50%.Further studies are required to provide a restricted. This solubility trend has an expected eVect on the ability of better approximation of these values. Secondly, a series of copolyesters (9a–f ) was prepared which the resulting copolymers to form lyotropic mesophases in 151 DMSO–H2O. The fully isophthalic polyester (9a) was insoluble incorporated a varying amount of the 5-nitroisophthaloyl unit.The use of meta-arranged substituents in the backbone of in 151 DMSO–H2O at a concentration as low as 11 mass%. The 151 copolymer 9b was soluble up to ca. 42 mass% but liquid crystalline polymers has been wide in the case of thermotropic polymers. Indeed, Cai and Samulski7 have formed only an isotropic solution.At isophthalic contents of ca. 22 and 33% (9c and 9d respectively) solubility could be reported thermotropic polyesters which contain up to 85% isophthalic acid units. However, these types of units have not achieved up to ca. 60 mass% and untextured birefringence (poorly aligned) was observed under cross-polarisation. At been considered as modifying groups in the synthesis of lyotropic polymers.It was our belief that the introduction of 12.5% isophthalic content 9e birefringence was seen at 53 mass%, but a fully aligned, lyotropic nematic sample was only the 5-nitroisophthloyl unit into our existing polymer chain would have a marked eVect on the solubility of the overall observed with a low isophthalic content of ca. 9% (9f) at a critical concentration of 56 mass%.This high concentration copolymer and it was this eVect which we wished to investigate. Six new copolyesters were prepared with isophthalic contents value is indicative of the high degree of alignment required in solution for the lyotropic mesophase to be observed. As was (based on comonomer feed ratios) ranging from ca. 9 to 100%. The phase behaviour results are summarised in Table 4.mentioned earlier, for the fully para-arranged polyester (1) the critical concentration in in 151 DMSO–H2O is considerably In general terms, it was found that the incorporation of the 5-nitroisophthaloyl unit did indeed change the solubility of less at 33 mass%. Interestingly, therefore, this work has confirmed the impor- the overall copolymer, giving a copolymer which becomes less soluble in aqueous organic solvents as the isophthaloyl compo- tance of the nitro substituent in the diacid aromatic ring with regard to generating lyotropicity in aqueous organic solvents. nent is increased.In fact, the wholly isophthaloyl homopolymer is a highly insoluble material. This result was somewhat Dimethyl 2-fluoroterephthalate has recently become available and so fluoroterephthalic acid is now accessible via hydrolysis surprising in that one may expect the decrease in linearity of the main-chain to favour solubility, i.e.the tendency towards of this diester. It would be interesting in due course to see if the powerfully electron-withdrawing fluoro group has the same crystallisation of the polymer chain is decreased. An explanation may lie in consideration of the role that the nitro influence as the nitro group.function plays in solubilising the polymer chain. Through fluorescence measurements, we have shown that the chains in Poly(sulfo-p-phenylene nitroterephthalamide) 10 the original polyester 1 adopt a roughly parallel arrangement in the lyotropic state.8 There is close association of the sulfonate In the course of synthesising the above polyesters it occurred to us to examine the same substituent pattern in an analogous moieties, whereas the nitro groups appear to be free to interact with the solvent molecules.Therefore, it would seem that it is aromatic polyamide. This led us to synthesise polymer 10. The polyamide was synthesised by the solution reaction of the nitro groups which act as solubilising groups for the polymer chain as a whole. Hence, if we reconsider our copoly- 2,5-diaminobenzenesulfonic acid with 2-nitroterephthalic acid Table 4 Solution properties of copolyesters with nitroisophthaloyl units isophthalic concentration in copolyester z contenta 151 DMSO–H2Ob observationc 9a 0 100 11 insoluble 9b 1 50 69 insoluble 52 insoluble 42 isotropic solution 9c 2 33.3 49 faint birefringence under shear 58 untextured birefringence 9d 3.55 22 36 untextured birefringence 60 untextured birefringence 9e 7 12.5 47 nematic solution under shear 53 nematic solution 9f 10 9.1 56 nematic solution 62 nematic solution aExpressed as a percentage of isophthaloyl diacid units with respect to all diacid units.bExpressed as mass% polymer to nearest integer.cUnder cross-polarised light. J. Mater. Chem., 1997, 7(10), 1985–1991 1989in a N-methylpyrrolidone (NMP)–LiCl–pyridine solution with spectrometer. In IR data, str=stretch. Microanalytical data were obtained from the Microanalytical Service in the triphenylphosphite. A DSC trace of this polyamide was very similar to that of the polyester analogue, exhibiting an exother- Department of Pure and Applied Chemistry of the University of Strathclyde. Optical studies were performed on an Olympus mic peak at ca. 300 °C and an endothermic trough at 180 °C. The solution behaviour of this polyamide was investigated in Polarising Microscope. DiVerential scanning calorimery studies were carried out on a Du Pont Instruments 910 DSC.water, DMSO and a 151 mixture of DMSO and water. The results are summarised in Table 5. Capillary viscosity measurements were made in an Ubbelohde viscometer in an equilibrated water-bath at 30 °C. Statistical It can be seen from the above data that this polyamide does indeed exhibit lyotropic LC phases in appropriate solvents. light scattering measurements were performed at ICI Films, Wilton.Glassware was silanised by rinsing with a 3% silicone Although 10 is insoluble in water itself, it is very soluble in 151 DMSO–H2O and shows a lyotropic LC phase above ca. oil in methyl ethyl ketone solution, decanting the liquid and placing in a furnace at 400 °C for 4 h. 50 mass%. Similarly, in DMSO, a lyotropic LC phase is seen above a concentration of ca. 45 mass%.This polyamide was not found to be soluble in any other common organic solvents. Synthesis of monomers These results can be compared to the published data on the 2-Methoxy-5-nitroterephthalic acid10 and 2-bromo-5-nitro- analogous poly( p-phenylene nitroterephthalamide).9 This terephthalic acid.11 These were prepared as described in the polymer is insoluble in water at room temperature and disliterature.The acid chlorides were prepared by refluxing the solves only at ca. 100 °C over a period of time. Aqueous appropriate diacid compound in thionyl chloride and purified polymer solutions with concentrations of 0.4–0.7 mass% are by micro-distillation including 2-bromoterephthaloyl chloride reported to be birefringent when sheared. Such a low concenand 2-nitroterephthaloyl chloride which were prepared as pre- tration solution showing birefringence was questioned by the viously described.3 authors as being indicative of liquid crystallinity, and is certainly markedly diVerent from the concentrations of ca. 2-Methoxyterephthalic acid. A mixture of 2,5-dimethylanisole 45–50 mass% required for polyamide 10 to show anisotropic (3.0 g, 22.03 mmol), potassium permanganate (12.0 g, behaviour in aqueous and pure DMSO. 75.93 mmol) and distilled water (300 ml) was refluxed for 5 h. The mixture was cooled to room temp. and poured into stirred Conclusions cold ethanol (200 ml). This mixture was then filtered, washed thoroughly with water, reduced under vacuum, and acidified Three novel homopolyesters formed from the interfacial reacwith conc.hydrochloric acid. The resulting white precipitate tion of hydroquinonesulfonic acid and various diacid chlorides was collected by filtration, washed with water and dried (2.26 g, are reported. Of these, the polyester 5 bearing a nitro and 52.3%), mp 287–288 °C ( lit., 280 °C12 or 296–297 °C13) [Found: methoxy function on the diacid unit exhibits lyotropicity in C, 54.8; H, 4.0.C9H8O5 (196.16) requires C, 55.1; H, 4.1%]; 151 DMSO–H2O, DMSO and DMF. Two series of copolyumax/ cm-1 (KBr) 1703 (CNO str); dH ([2H6]DMSO) 3.87 (3H, esters are reported which contain increasing amounts of unsubs, OCH3), 7.55 (2H, br d, J5,6 8, 5,6-H), 7.68 (1H, d, J3,5 8, 3- stituted and bromo-substituted diacid units. It was found that H), 13.15 (2H, br s, 2× CO2H); dC ([2H6]DMSO) 55.82 the fully nitro polyester 1 could incorporate up to 50% of the (OCH3), 112.51 (3-C), 121.00 (5-C), 125.72 (1-C), 130.33 (6- non-mesogenic diacid and retain the ability to form lyotropic C), 134.52 (4-C), 157.51 (2-C), 166.60 and 167.00 (both CO2H).mesophases. A series of copolyesters containing increasing amounts of nitroisophthaloyl units is also reported. It was 2-Methoxyterephthaloyl chloride.14 (100%), mp 51–52 °C found that the overall composition of the copolymer could [Found : C, 46.2; H, 2.6; Cl, 30.6.C9H6Cl2O3 (233.05) requires contain no more than ca. 9% of the meta-arranged diacid and C, 46.4; H, 2.6; Cl, 30.4%]; umax/cm-1 (CNO str); dH (CDCl3) retain the ability to form fully orientated mesophases. 4.02 (3H, s, OCH3), 7.67 (1H, d, J3,5 2, 3-H), 7.81 (1H, dd, Furthermore, as the nitroisophthaloyl content increased the J5,3 2, J5,6 8, 5-H), 8.09 (1H, d, J6,5 8, 6-H); dC (CDCl3) 56.8 copolymer became more insoluble in aqueous organic solvents.(OCH3), 114.0 (3-C), 123.1 (1-C), 128.8 (5-C), 133.5 (6-C), Finally, poly(sulfo-p-phenylene nitroterephthalamide) 10 has 138.8 (4-C), 158.7 (2-C), 164.1 and 167.7 (both COCl).been prepared and shown to form a lyotropic LC phase in DMSO and aqueous DMSO. 2-Bromo-5-nitroterephthaloyl chloride. (95.0%), mp 53–55 °C, bp 135–140 °C at 0.02 mbar [Found: C, 29.55; H, Experimental 0.8; N, 4.0; Cl, 21.9; Br, 23.7. C8H2NO4BrCl2 (326.92) requires C, 29.4; H, 0.6; N, 4.3; Cl, 21.7; Br, 24.0%]; umax/cm-1 (CHCl3) Ethanol-free chloroform was prepared by elution through silica 3030s (CMH arom str), 1778br (C=O str), 1540 (NMO anti and subsequent distillation. Other solvents were used as str), 1350 (NMO symm str); dH (CDCl3) 7.99 (1H, s), 8.74 (1H, received.Solid chemicals were supplied by Aldrich Chemical s); dC (CDCl3) 128.15 (6-C), 128.27 (2-C), 133.97 (4-C), 135.97 Co. and used as recieved except for hydroquinonesulfonic acid (3-C), 138.57 (1-C), 143.11 (5-C), 163.64 and 164.06 (both potassium salt, which was recrystallised from distilled water.COCl). 1H NMR spectra were recorded at 250 MHz on a Bruker AMX-250 spectrometer. J Values are in Hz. Fourier transform 2-Methoxy-5-nitroterephthaloyl chloride. (76.1%), bp 175– infrared spectra were obtained on a Nicolet Impact 400D 180 °C at 0.03 mbar [Found: C, 38.8; H, 1.9; N, 5.15; Cl, 25.45. C9H5NO5Cl2 (278.05) requires C, 38.9; H, 1.8; N, 5.0; Cl, Table 5 Solution properties of poly(sulfo-p-phenylene nitroterephthal- 25.5%]; umax/cm-1 (CHCl3) 1786br (CNO str); dH (CDCl3) amide) (10) 4.13 (3H, s, OCH3), 7.10 (1H, s, 3-H), 8.89 (1H, s, 6-H); dC (CDCl3) 57.96 (OCH3), 110.69 (3-C), 130.86 (1-C), 136.05 (4- solvent concentrationa observationb C), 139.30 (5-C), 162.11 (2-C), 162.92 and 164.95 (both CO2H).H2O <10 insoluble DMSO 45 nematic solution 5-Nitroisophthaloyl chloride. (97.2%), bp 145–150 °C at 40 isotropic solution 0.06 mbar ( lit.,15 mp 67–68 °C) [Found: C, 39.3; H, 1.3; N, 5.1; 151 DMSO–H2O 50 nematic solution Cl, 28.6. C8H3NO4Cl2 (248.02) requires C, 38.7; H, 1.2; N, 5.65; Cl, 28.6%]; umax/cm-1 (CHCl3) 3095, 1761s (CNO str), 1632, aExpressed as mass% polymer to the nearest integer.bUnder crosspolarised light. 1363, 1260, 1157; dH (CDCl3) 9.12 (1H, t, J2,4 and J2,6 2, 2-H), 1990 J. Mater. Chem., 1997, 7(10), 1985–19919.22 (2H, d, J4,2 and J6,2 2, 4-H and 6-H); dC (CDCl3) 131.09 of ICI Films and Paints, respectively, and Eric Nield, formerly of ICI Paints, for their helpful comments.Thanks are also (4-C), 136.19 (2-C) 137.62 (1-C and 3-C), 149.06 (5-C), 165.83 (2×COCl). extended to ICI Paints for an SRF award for D.T.B.H. Synthesis of polyesters 4–9 References General procedure.3 Hydroquinonesulfonic acid (6.57 mmol) 1 (a) L iquid Crystallinity in Polymers, ed. A. Ciuferri, VCH, was dissolved in distilled water (5 ml) containing sodium Weinheim, Germany, 1997; (b) A.M. Donald and A. H. Windle, L iquid Crystalline Polymers, Cambridge University Press, hydroxide (0.53 g, 13.2 mmol) and stirred under nitrogen in a Cambridge, 1992; (c) C. B. McArdle, Side-Chain L iquid Crystal silanised flask. To this was added a thoroughly mixed chloro- Polymers, Blackie, Glasgow, 1987. form solution (30 ml ) of the relevant diacid chloride(s) (to a 2 (a) J.Lin and D. C. Sherrington, Adv. Polym. Sci., 1994, 111, 177; total of 6.57 mmol) in one portion. This was followed by the (b) M. Takayamgi and T. Katayose, J. Appl. Polym. Sci., 1984, 29, addition of a phase-transfer catalyst, benzyltriethylammonium 141; (c) M. B. Gielselman and J. R. Reynolds, Macromolecules, chloride (0.2 equiv), in distilled water (1 ml). Normally rapid 1990, 23, 3118; (d) J.Y. Yadhav, W. R. Krigbaum and J. Preston, Macromolecules, 1988, 21, 538; (e) E. J. Vandenberg, W. R. Dively, precipitation occurred concurrent with a change in solution L. J. Filar, S. R. Patel and H. G. Barth, Polym. Mater. Sci. Eng., colour from brown through a vibrant red to a strong yellow, 1987, 57, 139; ( f ) E. J. Vandenberg, W. R. Dively, L. J.Filar, before typically aVording a peach coloured product. The S. R. Patel and H. G. Barth, J. Polym. Sci. Polym. Chem., 1989, polymer product was precipitated in methanol, dried, washed 27, 1387. with a methanol water (351) mixture and re-dried under 3 J. Lin, D. C. Sherrington, R. W. Richards, E. Nield and W. A. vacuum. Analytical data is contained in Table 1. MacDonald,Macromolecules, 1992, 26, 7107. 4 This work is derived from the PhD Thesis of D. T. B. Hannah, University of Strathclyde, 1996. Synthesis of poly(sulfo-p-phenylene nitroterephthalamide) 10. 5 K. A. Burdett, Synthesis, 1991, 441. 2-Nitroterephthalic acid (1.58 g, 7.48 mmol), 2,5-diaminoben- 6 Q.-F. Zhou, R. W. Lenz and J.-I. Jin, in Polymeric L iquid Crystals, zenesulfonic acid (1.41 g, 7.48 mmol) and anhydrous lithium ed.A. Blumstein, Polym. Sci. T ech., Plenum Press, New York, 1985, p. 257. chloride (0.79 g, 18.64 mmol) were dissolved in a mixture of 7 F. R. Cai and E. T. Samulski,Macromolecules, 1994, 27, 35. N-methylpyrrolidone (15 ml ), pyridine (3.75 ml) and triphenyl 8 H. Takahashi, K. Horie, T. Yamashita, S. Machida, D. T. B. phosphite (3.94 ml). On addition of the latter, the solution Hannah and D. C. Sherrington, Macromol. Chem. Phys., 1996, developed a green colour. This solution was stirred by means 197, 2703. of a mechanical overhead stirrer at 120 °C for 4 h. The solution 9 E. J. Vandenberg, W. R. Dively, L. J. Filar, S. R. Patel and was then cooled to room temp. and poured into cold, rapidly- H. G. Barth, J. Polym. Sci. Polym. Chem., 1989, 27, 3745. 10 J.-I. Jin, Y.-H. Lee and H.-K. Shim, Macromolecules, 1993, 26, stirred methanol (200 ml). The resulting yellow solid was 1805. collected by filtration, washed thoroughly with hot methanol, 11 C. A. Panetta, Z. Fang and N. E. Heimer, J. Org. Chem., 1993, and dried to give a pale-green solid powder (10, 1.86 g, 71%) 58, 6146. [Found: C, 46.5; H, 3.3; N, 11.4; S, 6.85; Cl, 0.4. [C14H9N3O7S]n 12 F.Wessely and M. Grossa,Monatsch. Chem., 1966, 97, 570. (363.30)n requires C, 46.3; H, 2.5; N, 11.6; S, 8.8%]; umax/cm-1 13 N. K. Kochetkov, L. I. Kudryashov and A. N. Nesmeyanov, Bull. (KBr) 1800–3600br (OMH str), 1687br (CNO str), 1610br, Acad. Sci. USSR, 1955, 729. 14 I. Goodman, J. E. McIntyre and D. H. Aldred, ICI Patent 993,272 1527 (NNO anti str), 1500, 1398, 1357 (NMO symm str), 1315, 1965; Chem. Abstr., 1965, 63, 8521e. 1243 (SMO str), 1181, 1082 (SMO str), 1031, 896, 829, 762, 15 E.-S. M. E. Mensour, S. H. Kandid, M. E. M. Soliman and 864, 637 (S-O str). M. Ibrahim, Polym. Int., 1992, 29, 61. Gratitude is expressed to Bill MacDonald and Mannish Sarkar Paper 7/03180G; Received 8thMay, 1997 J. Mater. Chem., 1997, 7(10), 1985–1991 1991

ISSN:0959-9428    

DOI:10.1039/a703180g

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

New chiral side chains for ferro- and antiferro-electric liquid crystals derived from the preen-gland wax of the domestic goose† Gerd Heppke,*a Detlef Lo�tzsch,a Michael Morrb and Ludger Ernstc aT echnische Universita� t Berlin, Sekr. ER11, Str. des 17. Juni 135, 10623 Berlin, Germany bGBF-Gesellschaft fu� r Biotechnologische Forschung mbH,Mascheroder Weg 1, 38124 Braunschweig, Germany cT echnische Universita� t Braunschweig, NMR-L aboratorium der Chemischen Institute, Hagenring 30, 38106 Braunschweig, Germany (2R,4R,6R,8R)-2,4,6,8-Tetramethyldecanoic acid and (2R,4R,6R,8R)-2,4,6,8-tetramethyldecanol, as well as the (2R,4R,6R,8R) and the (2S,4R,6R,8R) diastereomers of 4,6,8-trimethyldecan-2-ol, have been obtained from the preen-gland wax of the domestic goose.Starting from these alkanols and alkanoic acid, novel ferro- and antiferro-electric liquid crystals bearing four methyl branchings in the chiral side chain have been synthesized and their mesomorphic and electro-optical properties have been investigated. The results obtained are compared with the properties of the respective chiral (S)-2-methyldecanoic acid, (S)-decan-2-ol and (S)-2- methyldecanol derivatives.The compounds with four methyl branchings in the chiral side chain are found to exhibit lower melting points, broader SmC* phase ranges, higher values of spontaneous polarization and larger tilt angles in comparison to the respective compound with only one methyl branching. Many physical properties which are used in modern appli- optical tilt angles and switching times) of the novel ferro- and antiferro-electric liquid crystals are discussed.cations of liquid crystals depend entirely on the presence of chiral molecules, e.g. the helical structure of cholesteric and some smectic phases, the ferroelectricity of uniformly tilted smectic phases and the antiferroelectricity of alternating tilted Experimental smectic phases.1 Moreover, in certain systems high chirality Synthesis causes the induction of novel phases (Blue phases, Twist Grain Boundary phases, Q phases etc.)1,2 Several chiral phases possess Methyl (2R,4R,6R,8R)-2,4,6,8-tetramethyldecanoate was frustrated structures displaying competition between chiral obtained by transesterification and Spaltrohr distillation of the forces and the tendency of the molecules to pack in a space- preen gland-wax of the domestic goose.According to the filling arrangement. For a better understanding of the chiral reaction scheme shown in Scheme 1, the methyl ester was forces as well as for electro-optical applications the develop- transformed into the free acid 1f as well as into (2R,4R,6R,8R)- ment of new chiral liquid crystals plays an important role. 2,4,6,8-tetramethyldecanol 1b, which was partially further However, the design of novel structures is restricted by the transformed into the (2R,4R,6R,8R) and the (2S,4R,6R,8R) available chiral moieties, which can be obtained either by diastereomers 1d and 1e of 4,6,8-trimethyldecan-2-ol. All these enantioselective reactions or by using the natural chiral pool. tetramethylalkanols, as well as (2R,4R,6R,8R)-2,4,6,8-tetra- A novel natural source of chiral mono-, di-, tri- or tetra- methyldecanoic acid, were obtained with a diastereomeric methyl branched alkanoic acids is the preen-gland wax of excess of more than 99%.6,7 Compounds 1b, 1d, 1e and 1f, as poultry.3 For example, the wax of the domestic goose consists well as the commercially available compounds (purchased of about 90% octadecyl (2R,4R,6R,8R)-2,4,6,8-tetramethyl- from the Japan Energy Corporation) (S)-2-methyldecanol 1a, decanoate,4,5 so that after transesterification and Spaltrohr (S)-decan-2-ol 1c and (S)-2-methyldecanoic acid 1g, were then distillation large quantities of methyl (2R,4R,6R,8R)-2,4,6,8- used as chiral starting materials for the synthesis of three series tetramethyldecanoate are obtained,6 which can be transformed of liquid crystalline products.These series diVer by the linking into the free acid by standard methods. As recently shown, group between mesogenic core and chiral side chain. Within methyl (2R,4R,6R,8R)-2,4,6,8-tetramethyldecanoate can also each series, the number and the position of the chiral methyl be transformed into (2R,4R,6R,8R)-2,4,6,8-tetramethyldecanol branchings are varied.As outlined in Scheme 2, compounds and further into (4R,6R,8R)-4,6,8-trimethyldecan-2-ol, from 2a–d were synthesized by esterification of 4-benzyloxybenzoic which both the (2R,4R,6R,8R) and the (2S,4R,6R,8R) diastereo- acid with the chiral alkanols 1a–d, followed by hydrogenation mers can be isolated by column chromatography.7 These to remove the benzyloxy protecting group and finally esterifi- tetramethylalkanols, as well as (2R,4R,6R,8R)-2,4,6,8-tetra- cation of the obtained phenols with 4¾-octyloxybiphenyl-4- methyldecanoic acid, are promising chiral side chains for the carboxylic acid.The respective liquid crystalline ethers 3a–d design of novel liquid crystals. were obtained by a reaction between 4-hydroxyphenyl 4¾- Here we present the first ferro- and antiferro-electric liquid octyloxybiphenyl-4-carboxylate and the chiral alkanols 1a–c crystals having tetramethylalkyl groups in the chiral side chain.and 1e in the presence of diethylazodicarboxylate (DEAD) In order to study the influence of the additional optically and triphenylphosphine8 (see Scheme 3).Compound 4c active methyl branchings, the respective chiral 2-methylde- was synthesized by esterification of the acid chloride of 1g canoic acid, decan-2-ol and 2-methyldecanol derivatives have with 4-hydroxyphenyl 4¾-octyloxybiphenyl-4-carboxylate (see also been synthesized. Polymorphy, phase transition tempera- Scheme 3), whereas compound 4d could not been obtained tures and electro-optical properties (spontaneous polarization, optical pure in a similar way.Compound 4d was synthesized by esterification of 1f with 4-hydroxyphenyl 4¾- octyloxybiphenyl-4-carboxylate in the presence of dicyclo- † Presented in part at the 5th International Conference on Liquid Crystals, Cambridge, 1995. hexylcarbodiimide (DCC). All products were purified by J. Mater. Chem., 1997, 7(10), 1993–1999 1993Scheme 1 Reagents and conditions: i, LiAlH4; ii, PCC, CH2Cl2; iii, 2,2¾- bipyridyl–Cu complex, DABCO, ButOH, air; iv, methyloxazaborolidine, BH3–THF, THF; v, column chromatography [silica gel, Scheme 3 Reagents and conditions: i, MeOH, H2SO4; ii, K2CO3, CH2Cl2–Pri2O (951)]; vi, NaOH, dioxane C8H17Br, DMF; iii, KOH, EtOH; iv, HCl; v, SOCl2; vi, 4- BnOC6H4OH, pyridine; vii, H2, Pd; viii, ROH (1a–c,e), DEAD, PPh3, THF; ix, 1f, DCC; x, RCOCl (from 1g and SOCl2) were then transferred to the remaining compounds while considering the usual substituent eVects on chemical shifts11 and attempting maximum internal consistency.The purity of the products was checked by TLC and HPLC (pump: Knauer HPLC pump 64, column: Nucleosil 120-5 C18, solvent: methanol, detector: Severn Analytical SA 6503 at 303 nm) and their optical purity was characterized by measuring the optical rotation (Perkin-Elmer polarimeter 241).All compounds were found to be of high purity (HPLC purity above 99%). The synthesis of compound 2c has already been described;12 however no detailed information about the physical properties of this compound were reported.Synthesis of 2a–d A synthetic procedure for 2d is given as an example. Dicyclohexylcarbodiimide (DCC) (1.24 g, 6 mmol) was added Scheme 2 Reagents and conditions: i, MeOH, H2SO4; ii, K2CO3, C8H17Br, DMF; iii, KOH, EtOH; iv, HCl; v, SOCl2; vi, ROH (1a–d); at room temperature to a solution of 4¾-octyloxybiphenylvii, H2, Pd; viii, DCC, CH2Cl2 4-carboxylic acid (0.98 g, 3 mmol), (1R,3R,5R,7R)-1,3,5,7- tetramethylnonyl 4-hydroxybenzoate (0.95 g, 3 mmol) and 4-dimethylaminopyridine (DMAP) (0.06 g, 0.5 mmol) in dry chromatography, followed by recrystallization until the transdichloromethane (50 ml ), and the mixture was stirred for 24 h.ition temperatures remained constant. The structure of the After filtration the solvent was evaporated and the resulting products irmed by 1H and 13C NMR experiments product was purified by chromatography over silica gel using (Bruker ARX-400, AM-400 and DPX-300; 9.4 and 7.0 T, dichloromethane as the eluent (Rf 0.70), followed by re- respectively) and, in case of compounds 2d, 3d and 4d, additioncrystallization from ethanol until the transition temperature ally by IR (Perkin-Elmer PE 257) and mass (Varian MAT remained constant.Yield: 1.34 g (71%); n (CCl4)/cm-1 2955, 44F) spectroscopy. Signal assignments in the 13C NMR spectra 2925, 2871, 2854, 1735, 1715, 1604, 1504; m/z 628.5 (M+); 1H were achieved by DEPT-135 experiments,9 by two-dimensional and 13C NMR data are given in Tables 1 and 2; [a]20D -0.30 13C, 1H COSY10 and COLOC10 experiments for 2a, 3b and 4d and by comparison with literature data.6,7 These assignments (c 5, CHCl3) (2b, -2.36; 2c, +18.29; 2d, -18.68). 1994 J. Mater. Chem., 1997, 7(10), 1993–1999Table 1 1H NMR data for compounds 2a–d, 3a–d and 4c–d (400 or 300 MHz; CDCl3; Me4Si) chemical shift (multiplicity, coupling constant)a,b proton 2a 2b 2c 2d 3a 3b 3c 3d 4c 4d O1 4.01 4.01 4.01 4.01 4.01 4.01 4.01 4.01 4.01 4.01 (t,6.6) (t,6.6) (t,6.6) (t,6.6) (t,6.6) (t,6.6) (t,6.5) (t,6.6) (t,6.6) (t,6.6) O2 1.81 1.82 1.81 1.81 1.88 1.81 1.80 (qi,7.0) (qi,7.0) (qi,7.0) (qi,7.0) (qi,7.0) (qi,7.0) (qi,7.0) O8 0.89† 0.89† 0.89† 0.89 0.89† 0.89† 0.89 (t,6.9) (t,6.9) (t,6.9) (t,ca. 7) (t,6.9) (t,6.9) (t,6.9) B2 7.69 7.70 7.69 7.69 7.67 7.67 7.67 7.68 7.68 7.68 (8.4) (8.3) (8.4) (8.4) (8.3) (8.4) (8.3) (8.5) (8.4) (8.4) B3 8.23 8.23 8.23 8.22 8.22 8.22 8.21 8.22 8.22 8.22 B2¾ 7.59 7.59 7.59 7.59 7.59 7.58 7.58 7.58 7.59 7.58 (8.7) (8.7) (8.7) (8.7) (8.7) (8.7) (8.6) (8.8) (8.7) (8.7) B3¾ 7.00 7.00 7.00 7.00 7.00 6.99 6.99 7.00 7.00 7.00 P2 7.31 7.32 7.30 7.31 7.12 7.12 7.11 7.12 7.24‡ 7.24† (8.6) (8.6) (8.6) (8.7) (9.0) (9.0) (8.9) (9.0) (9.0) (9.0) P3 8.13 8.13 8.12 8.12 6.93 6.93 6.91 6.92 7.13‡ 7.13† D1 4.22 4.25 1.34 1.35 3.82 3.84 1.30 1.30 (dd,10.7,5.8) (dd,10.7,5.0) (d,6.3) (d,6.1) (dd,8.9,5.8) (dd,8.9,5.0) (d,ca. 6) (d,ca. 6) 4.12 4.09 3.72 3.70 (dd,10.7,6.7) (dd,10.7,6.8) (dd,8.9,6.8) (dd,8.9,6.8) D2 1.93 2.06 5.16 5.28 1.93 2.04 4.32 4.43 2.69 2.82 (oct,6.4) (oct,6.5) (sext,6.3) (m) (oct,6.4) (oct,6.5) (sext,6.0) (m) (sext,7.0) (dqd,9.8,6.9,4.9) D10 0.88† 0.88† 0.88† 0.85 0.88† 0.88† 0.85 (t,6.9) (t,6.9) (t,6.9) (t,7.3) (t,6.9) (t,6.9) (t,ca. 7) D2-Me 1.02 1.04 1.02 1.04 1.30 1.31 (d,6.7) (d,6.6) (d,6.7) (d,6.8) (d,6.9) (d,6.9) aCoupling constants (J) are given in Hz; qi=quintet, sext=sextet, oct=octet. Footnote symbols (†,‡) indicate interchangeable assignments. For the B and P protons, which are parts of AA¾XX¾ spin systems, the N values (JAX+JAX¾ ) are given in parentheses.bFurther signals: 2a: d 1.52–1.27 (m); 2b: d 1.71–0.81 (m); 2c: d 1.73–1.26 (m); 2d: d 1.90–0.80 (m); 3a: d 1.54–1.21 (m); 3b: d 1.68–0.82 (m); shifts assigned by 2D experiments: d 1.64 (m, D4), 1.60 (m, D6), 1.47 (m, D3), 1.46 (m, O3), 1.43 (m, D8), 1.38 (m, D9), 1.36, 1.32 (m, O4, O5), 1.30 (m, O7), 1.28 (m, O6), 1.22 (m, D5, D7), 1.05 (m, D9), 0.98 (m, D3), 0.90, 0.86 (m, D5, D7), 0.90 (d, 6.6, D4-Me), 0.85 (d, 6.6, D8-Me), 0.83 (d, 6.5, D6-Me); 3c: d 1.73–1.28 (m); 3d: d 1.87–0.83 (m); 4c: d 1.86–1.28 (m); 4d: d 1.96–0.83 (m); shifts assigned by 2D experiments: d 1.90 (ddd, 13.9, 9.7, 4.3, D3), 1.66 (m, D4), 1.63 (m, D6), 1.46 (m, O3)1.43 (m, D8), 1.36 (m, D9), 1.36, 1.32 (m, O4, O5), 1.30 (m, O7), 1.28 (m, O6), 1.23 (m, D5), 1.20 (m, D7), 1.16 (m, D3), 1.08 (m, D9), 0.97 (m, D5), 0.96 9d, 6.5, D4-Me), 0.90 (m, D7), 0.85 (d, ca. 7, D6-Me, D8-Me). Synthesis of 3a–d 13C NMR data are given in Tables 1 and 2; [a]20D+3.05 (c 5, CHCl3) (3b, -2.02; 3c, -2.11; 3d, -3.44). A synthetic procedure for 3d is given as an example. 4- Hydroxyphenyl 4¾-octyloxybiphenyl-4-carboxylate (1.26 g, Synthesis of 4c 3 mmol), triphenylphosphine (0.79 g, 3 mmol) and (2S,4R,6R,8R)-4,6,8-trimethyldecan-2-ol 1e (0.4 g, 2 mmol) 4-Hydroxyphenyl 4¾-octyloxybiphenyl-4-carboxylate (1.26 g, 3 mmol) was dissolved in dry pyridine (50 ml ) and (S)-2- were dissolved in dry tetrahydrofuran (50 ml ).While the temperature was kept at 0 °C, diethyl azodicarboxylate methyldecanoyl chloride (0.71 g, 3 mmol) was added while the temperature was kept at 0 °C.Stirring was continued for (DEAD) (0.52 g, 3 mmol) was added dropwise under a nitrogen atmosphere and stirring was continued for 36 h at room 16 h at room temperature. After hydrolysis in an excess of diluted HCl, the product was extracted into dichloromethane temperature. Afterwards the solvent was removed and the resulting product was purified by chromatography over silica (3×150 ml).The combined dichloromethane solutions were washed with water (200 ml) and dried (MgSO4). The solvent gel using dichloromethane as the eluent (Rf 0.90), followed by recrystallization from ethanol until the transition temperature was removed and the resulting product purified by chromatography over silica gel using dichloromethane as the eluent remained constant.Yield: 0.64 g (53%); n(CCl4)/cm-1 2954, 2926, 2870, 2854, 1733, 1606, 1504; m/z 600.5 (M+); 1H and (Rf 0.71), followed by recrystallization from ethanol until the J. Mater. Chem., 1997, 7(10), 1993–1999 1995Table 2 13C NMR data for compounds 2a–d, 3a–d and 4c–d (101 or 75 MHz; CDCl3) chemical shifta carbon 2a 2b 2c 2d 3a 3b 3c 3d 4c 4d O1 68.2 68.2 68.3 68.2 68.2 68.2 68.2 68.2 68.2 68.2 O2 29.3 29.3 29.3 29.3 29.3 29.3 29.3 29.3 29.3 29.3 O3 26.1 26.1 26.1 26.1 26.1 26.1 26.1 26.1 26.1 26.1 O4 29.3† 29.3† 29.3† 29.3† 29.3† 29.3† 29.3† 29.3† 29.3† 29.3† O5 29.4† 29.4† 29.4† 29.4† 29.4† 29.4† 29.4† 29.4† 29.4† 29.4† O6 31.9‡ 31.8 31.9 31.9 31.9‡ 31.9 31.9 31.9 31.9‡ 31.9 O7 22.7 22.7 22.7 22.7 22.7 22.7 22.7 22.7 22.7 22.7 O8 14.1 14.1 14.1 14.1 14.1 14.1 14.1 14.1 14.1 14.1 B1 146.3 146.3 146.3 146.3 145.9 145.9 145.9 145.9 146.1 146.1 B2 126.7 126.6 126.7 126.7 126.6 126.6 126.6 126.6 126.6 126.6 B3 130.8 130.8 130.8 130.8 130.7 130.7 130.7 130.7 130.7 130.8 B4 127.1 127.0 127.1 127.1 127.7 127.7 127.7 127.7 127.3 127.4 B4-CO 164.6 164.6 164.7 164.7 165.5 165.5 165.5 165.5 165.0 165.0 B1¾ 131.9 131.8 131.9 131.9 132.1 132.0 132.1 132.0 131.9 132.0 B2¾ 128.4 128.4 128.4 128.4 128.4 128.4 128.4 128.4 128.4 128.4 B3¾ 115.1 115.0 115.1 115.1 115.0§ 115.0 115.0 115.0 115.0 115.1 B4¾ 159.7 159.7 159.7 159.7 159.6 159.6 159.6 159.6 159.6 159.7 P1 154.7 154.7 154.6 154.6 144.4 144.3 144.3 144.3 148.3 148.4 P2 121.8 121.8 121.7 121.8 122.4 122.4 122.5 122.5 122.6§ 122.6‡ P3 131.2 131.2 131.2 131.1 115.2§ 115.1 116.6 116.5 122.5§ 122.5‡ P4 128.2 128.1 128.6 128.5 157.1 157.1 156.0 156.2 148.3 148.4 P4-CO 166.0 165.9 165.5 165.5 D1 70.1 69.8 20.1 20.6‡ 73.7 73.5 19.8 20.4‡ 175.3 175.3 D2 32.8 30.2 72.0 70.0 33.2 30.7 74.6 72.4 39.7 37.7 D3 33.5 41.4 36.1 43.1 33.6 41.3 36.6 44.1 33.8 41.3 D4 26.9 27.6‡ 25.5 26.7 27.0 27.7 25.6 26.6 27.3 28.5 D5 29.9† 44.7 29.5† 45.0 30.0† 44.7‡ 29.7† 44.9 29.6† 45.6 D6 29.6† 27.5‡ 29.5† 27.4 29.6† 27.5 29.6† 27.4 29.5† 27.4 D7 29.3† 45.7 29.5† 45.5 29.4† 45.6‡ 29.3† 45.7 29.3† 45.0 D8 31.8‡ 31.5 31.9 31.6 32.0‡ 31.6 31.9 31.6 31.8‡ 31.6 D9 22.7 28.8 22.7 29.1 22.7 28.9 22.7 29.0 22.7 29.1 D10 14.1 11.2 14.1 11.2 14.1 11.2 14.1 11.2 14.1 11.2 Me-D2 17.1 18.3 17.1 18.3 17.0 18.3 Me-D4 20.9 20.6‡ 21.1 20.7‡ 20.5 Me-D6 20.9 21.0‡ 21.0 20.7‡ 20.8 Me-D8 20.0 19.9 20.0 20.0 20.0 aFootnote symbols (†,‡,§) indicate interchangeable assignments.transition temperature remained constant. Yield: 1.18 g (67%); heptyloxycarbonyl)phenyl 4¾-octyloxybiphenyl-4-carboxylate (MHPOBC)13 have been used as reference compounds. 1H and 13C NMRdata are given in Tables 1 and 2; [a]20D+8.60 (c 5, CHCl3). Commercially available test cells (E.H.C.) with a layer spacing of 10 mm were used for the electro-optical investigations.The spontaneous polarization was measured by the triangular wave Synthesis of 4d method. Optical tilt angles were obtained by an extrapolation Dicyclohexylcarbodiimide (DCC) (0.70 g, 3.4 mmol) was of the switching angles to zero field. Switching times, defined added at room temperature to a solution of 4-hydroxyphenyl as the rise time from 10 to 90% transmission, were determined 4¾-octyloxybiphenyl-4-carboxylate (0.71 g, 1.7 mmol), by measuring the optical response to an applied electric field (2R,4R,6R,8R)-2,4,6,8-tetramethyldecanoic acid 1f (0.82 g, (rectangular wave) at a field strength of ±10 V mm-1 (com- 3.4 mmol) and 4-dimethylaminopyridine (DMAP) (0.27 g, pounds 2a–d, 3a–d and 4c) or ±20 V mm-1 (compound 4d). 2.2 mmol) in dry dichloromethane (50 ml ), and the mixture was stirred for 24 h. After filtration, the reaction mixture was washed twice with a solution of 5% citric acid in water (50 ml ) Results and once with water (50 ml ). The organic phase was dried (Na2SO4), the solvent removed and the resulting product Liquid crystalline properties purified by chromatography over silica gel using dichloro- Textural observations and miscibility studies have been carried methane as the eluent (Rf 0.81), followed by recrystallization out in order to determine the liquid crystalline properties of from ethanol until the transition temperature remained conthe compounds of series 2–4.On cooling from the isotropic stant. Yield: 0.42 g (39%); n(CCl4)/cm-1 2954, 2925, 2870, phase for all compounds the SmA phase appears showing 2854, 1756, 1732, 1604, 1504; m/z 628.5 (M+); 1H and 13C NMR characteristic textures (planar oriented region: focal-conic fan data are given in Tables 1 and 2; [a]20D-10.53 (c 5, CHCl3).texture, homeotropic oriented regions: no texture). On further cooling a phase transition into the SmC phase occurs: in the Equipment and methods planar oriented region the focal-conic fan texture of the SmA phase transforms into a broken focal-conic fan texture and in Phase transition temperatures were determined optically by observing the textural changes with a polarizing microscope.the homeotropic regions a schlieren texture appears. Below the SmC phase, most of the compounds show a direct transition Transition enthalpies were measured by diVerential scanning calorimetry (DSC) using a Perkin-Elmer DSC 7.Miscibility into a higher ordered smectic phase (denoted as SmIII phase in the following text, probably SmI) which exhibits either a studies were carried out by the contact method and, in one case, in addition, by choosing specific concentrations.broken focal-conic fan texture with thin round coloured bands or a schlieren texture. In the other two compounds, a SmCA For these investigations both enantiomers of 4-(1-methyl- 1996 J. Mater. Chem., 1997, 7(10), 1993–1999antiferroelectric phases. For both antiferroelectric phases (SmCA and SmIA) of compounds 2c and 4d, a ‘tristate’ switching has been observed, confirming the antiferroelectric nature of these phases.In order to allow a comparison to the electrooptical properties of the SmC phases of the other compounds, in the temperature range of the antiferroelectric SmCA phases polarization and optical tilt angle of the field induced ferroelectric states, as well as the rise time from 10 to 90% transmission for the direct switching between the two ferroelectric switching states, have been determined.Since no switching was observed for the SmCA phase of compound 4d at E±10 V mm-1, the switching times of this compound were measured at E±20 V mm-1. For the compounds of series 2 the results of the electrooptical investigations are shown in Figs. 2–4. Both tetramethyl derivatives (2b, 2d) are found to exhibit two to three times higher values of spontaneous polarization in comparison to their respective reference compound (2a, 2c) with only one chiral methyl branching.Additionally, an increase of the optical Fig. 1 Phase diagram between (R)-MHPOBC and 4d tilt angle by about a factor of 1.5 is observed; e.g. 50 K below phase occurs between the SmC and the higher ordered smectic phase SmIA. At the transition from the SmC to the SmCA phase the number of chirality lines in the broken focal-conic fan texture strongly decreases and the number of chevron defects strongly increases.The SmC–SmCA phase transition is also indicated by an inversion of the helical twist sense which can be observed in the schlieren texture. Moreover, in the supercooled region of three compounds a transition into a high ordered smectic phase SmIV can clearly be observed by the formation of a mosaic texture.The classification of the SmA, SmC, SmCA and SmIA phases was confirmed by miscibility studies. As an example, the phase diagram between the compounds 4d and (R)-MHPOBC, which has been investigated in more detail, is shown in Fig. 1. The Fig. 2 Temperature dependence of spontaneous polarization of compounds 2; (#) 2a, ($) 2b, (%) 2c and (&) 2d SmA, SmC, SmCA and SmIA phases of (R)-MHPOBC are uninterruptedly miscible, with the respective smectic modifications of 4d confirming thereby the phase sequence SmIA–SmCA–SmC–SmA for compound 4d.Polymorphy, phase transition temperatures and transition enthalpies of the liquid crystalline products are shown in Table 3 [NB: compounds with diVerent linking groups X are distinguished by diVerent numbers (2–4), whereas the letters (a–d) label the kind of the chiral side chain].As can be seen, the introduction of three additional methyl branchings leads to a decrease of the melting points of about 10 K for series 2 and 4 and 25 K for series 3, as well as to about 30 K lower clearing temperatures, resulting in a smaller liquid crystalline phase range for the tetramethyl derivatives.However, the SmC temperature range increases slightly in the case of series 2 and 4 and by more than a factor of three in case of series 3. The Fig. 3 Temperature dependence of the tilt angle of compounds 2; largest SmC phase ranges are observed in series 2, ranging (#) 2a, ($) 2b, (%) 2c and (&) 2d from 47 to 62 K.In the two compounds 2c and 4d, an alternating tilted SmCA phase occurs in a broad temperature range. With respect to the appearance of this SmCA phase the influence of the three additional methyl branchings is puzzling. Whereas in series 2 the SmCA phase of the 2-methyldecanol derivative 2c is replaced by a SmC phase in the respective tetramethyl derivative 2d, the opposite eVect is observed in series 4.Electro-optical properties All compounds of series 2–4 (see Table 3) show ferroelectric switching in the SmC and SmIII phases. To characterize the ferroelectric properties the temperature dependence of spontaneous polarization, optical tilt angle and switching time (t10–90 at E±10 V mm-1) of the SmC phases have been measured.In two of the compounds (2c, 4d) the ferroelectric Fig. 4 Temperature dependence of the switching time of compounds 2; (#) 2a, ($) 2b, (%) 2c and (&) 2d smectic modifications (SmC, SmIII) are almost replaced by J. Mater. Chem., 1997, 7(10), 1993–1999 1997Table 3 Polymorphy, phase transition temperatures and transition enthalpies of the liquid crystalline products compound X R transition temperatures/°C [enthalpies/kJ mol-1] 2a CO2 Cr 66.4 (SmIII 54.5) SmC 108.0 SmA 164.1 I [30.6] [1.02] [0.00] [6.24] 2b CO2 Cr 53.9 SmIII 54.0 SmC 115.7 SmA 134.3 I [22.9] [0.80] [0.00] [7.08] 2c CO2 Cr 62.8 (SmIA 62.5) SmCA 109.3 SmC 113.4 SmA 140.4 I [29.4] [1.50] [0.021] [0.00] [5.83] 2d CO2 Cr 53.5 (SmIII 38.0) SmC 94.8 SmA 102.1 I [19.9] [0.43] [0.50] [1.98] 3a O Cr 84.4 (SmIV 56.6) SmIII 101.6 SmC 107.9 SmA 164.4 I [31.2] [1.96] [2.96] [0.00] [7.04] 3b O Cr 55.1 (SmIV 44.7) SmIII 80.6 SmC 104.6 SmA 133.6 I [18.8] [1.84] [2.40] [0.00] [4.80] 3c O Cr 65.2 SmIII 81.3 SmC 94.5 SmA 144.2 I [23.5] [2.16] [0.00] [6.07] 3d O Cr 41.6 SmIII 49.5 SmC 90.4 SmA 116.3 I [22.7] [1.04] [0.00] [4.22] 4c O2C Cr 62.6 (SmIV 58.5) SmIII 91.7 SmC 136.1 SmA 157.5 I [26.1] [1.66] [2.72] [0.00] [5.48] 4d O2C Cr 56.3 SmIA 59.5 SmCA 108.5 SmC 109.4 SmA 119.2 I [23.2] [1.32] [0.12] [0.28] [2.68] Table 4 Spontaneous polarization, optical tilt angles and switching Table 5 Spontaneous polarization, optical tilt angles and switching times of the compounds of series 4 at 5, 10, 20, 30 and 40 K below times of the compounds of series 3 at 5, 10, 20, 30 and 40 K below the SmA–SmC transition temperature the SmA–SmC transition temperature compound T-Tc/K Ps/nC cm-2 h(°) t/ms compound T-Tc/K Ps/nC cm-2 h(°) t/ms 3a -5 1.8 9.1 22.3 4c -5 8.0 18.0 17.6 4c -10 10.7 21.5 19.1 3b -5 5.1 17.7 48.6 3b -10 6.8 20.4 70.0 4c -20 13.9 24.5 21.3 4c -30 16.2 25.9 22.9 3b -20 8.8 22.5 90.0 3c -5 24.4 11.1 9.6 4c -40 18.0 26.5 26.4 4d -5 25.6 27.1 12.4a 3c -10 32.6 13.5 9.9 3d -5 49.2 19.0 12.3 4d -10 29.6 29.6 14.4a 4d -20 35.7 32.2 20.2a 3d -10 63.6 23.0 13.8 3d -20 82.8 26.3 18.4 4d -30 41.0 33.5 32.9a 4d -40 44.3 33.8 62.1a 3d -30 93.3 27.5 25.5 3d -40 97.9 27.2 45.8 aMeasured at a field strength of ±20 V mm-1.the SmCA phase of compound 2c remains almost constant the SmA–SmC transition temperatures, optical tilt angles of about 30° for the tetramethyl derivatives are measured, com- close above the transition to the SmIA phase.For the electro-optical properties of the compounds of series pared to about 20° for the reference compounds. Thus the strong increase of the spontaneous polarization can not only 3, similar results are obtained (see Table 4). Again, the tetramethyl derivatives are found to exhibit nearly two times larger be attributed to an increase of the polarization–tilt angle coupling constant (as expected for the introduction of three optical tilt angles and two to three times higher values of spontaneous polarization in comparison to their respective additional chiral centres), but is also caused by the remarkable increase of the tilt angle.Although the spontaneous polariz- reference compounds.In series 4 only two compounds with the (first) chiral centre ation of the tetramethyl derivatives are much higher, their switching times are of the same order. In the vicinity of the in the a-position have been synthesized. The electro-optical properties of compounds 4c and 4d are summarized in Table 5. higher ordered smectic phase (probably SmI), an exponential increase of the switching times is observed for the SmC phases As in series 2 and 3, the introduction of the additional methyl branchings leads to a remarkable increase of the optical tilt of compounds 2a, 2b and 2d, whereas the switching time of 1998 J.Mater. Chem., 1997, 7(10), 1993–1999angle and of the spontaneous polarization. In comparison to References the respective compounds of series 2, whose molecular struc- 1 J.W. Goodby, A. J. Slaney, C. J. Booth, I. Nishiyama, J. D. Vuijk, tures diVer only by the direction of the ester group between P. Styring and K. J. Toyne, Mol. Cryst. L iq. Cryst., 1994, 243, 231. the mesogenic core and the chiral side chain, the spontaneous 2 A.-M. Levelut, D. Bennemann, G.Heppke and D. Lo� tzsch, Mol. polarization of compounds 4c and 4d is reduced by about a Cryst. L iq. Cryst., in the press. 3 J. Jacob, Fortschr. Chem. Org. Naturst., 1976, 34, 373. factor of five. 4 K. E. Murray, Aust. J. Chem., 1962, 15, 510. 5 G. Odham, Ark. Kemi, 1963, 21, 379. Conclusion 6 M. Morr, V. Wray, J. Fortkamp and R. D. Schmid, L iebigs Ann. Chem., 1992, 433. Starting from the natural source of the preen-gland wax of 7 M.Morr, C. Proppe and V.Wray, L iebigs Ann., 1995, 2001. poultry, novel ferro- and antiferro-electric liquid crystals bear- 8 O. Mitsonubu, Synthesis, 1981, 1. 9 D. M. Doddrell, D. T. Pegg and M. R. Bendall, J. Magn. Reson., ing tetramethylalkyl chains have been synthesized. In compari- 1982, 48, 323. son to the respective compounds with only one methyl 10 W. R. Croasmun and R. M. K. Carlson, T wo-Dimensional NMR branching, lower melting points and broader SmC phase Spectroscopy. Applications for Chemists and Biochemists, VCH, ranges are exhibited which favour the tetramethyl derivatives Weinheim, 2nd edn., 1994. for use in broad range SmC room temperature mixtures. 11 H.-O. Kalinowski, S. Berger and S. Braun, 13C-NMRAccording to the electro-optical investigations, the introduc- Spektroskopie, Thieme, Stuttgart 1984. tion of the additional methyl branchings leads to an increase 12 A. Fukuda, Y. Takanishi, T. Isozaki, K. Ishikawa and H. Takazoe, J.Mater. Chem., 1994, 4, 997. of the spontaneous polarization and the optical tilt angle. The 13 A. D. L. Chandani, T. Hagiwara, Y. Suzuki, Y. Ouchi, H. Takezoe use of new chiral tetramethylalkyl side chains seems to be and A. Fukuda, Jpn. J. Appl. Phys., 1988, 27, L729. promising, especially for the development of new materials for 14 J. Fu� nfschilling and M. Schadt, J. Appl. Phys., 1989, 66, 3877. device applications where switching angles of 45° are required 15 A. G. H. Verhulst and G. Cnossen, Ferroelectrics, 1996, 179, 141. (e.g. for deformed helix ferroelectric liquid crystal displays14,15 16 K. Nakamura, A. Takeuchi, N. Yamamoto, Y. Yamada, Y.-I. or for antiferroelectric liquid crystal displays).16 Suzuki and I. Kawamura, Ferroelectrics, 1996, 179, 131. The authors thank the Deutsche Forschungsgemeinschaft (Sfb 335) for financial support. Paper 7/04503D; Received 26th June, 1997 J. Mater. Chem., 1997, 7(10), 1993–1999

ISSN:0959-9428    

DOI:10.1039/a704503d

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Formation of smectic and columnar liquid crystalline phases by cyclotriveratrylene (CTV) and cyclotetraveratrylene (CTTV) derivatives incorporating calamitic structural units Ralph Lunkwitz,a Carsten Tschierske*a and Siegmar Dieleb aInstitute of Organic Chemistry,Martin-L uther-University Halle, D-06120 Halle/Saale, Kurt-Mothes-Straße 2, Germany E-mail: coqfx@mlucom.urz.uni-halle.de bInstitute of Physical Chemistry,Martin-L uther-University Halle, D-06099 Halle/Saale, Mu�hlpforte 1, Germany Novel liquid crystalline oligomesogens, which consist of five, six or eight calamitic 4-cyanobiphenyl, 2-alkyl-5-phenyl-1,3,4- thiadiazole or 5-octyl-2-phenylpyrimidine units covalently linked by spacers of diVerent length with a cyclotribenzylene central core (CTV derivatives, 2,3,7,8,12,13-hexa-substituted 10,15-dihydro-5H-tribenzo[a,d,g]cyclononenes) or a cyclotetrabenzylene central core (CTTV derivatives, 2,3,6,7,10,11,14,15-octa-substituted 5,10,15,20-tetrahydrotetrabenzo[a,d,g,j]cyclododecenes) have been synthesized. These compounds were investigated by polarizing microscopy, diVerential scanning calorimetry and some of them were also studied by X-ray diVraction. Many of the CTV derivatives show liquid crystalline properties.The cyanobiphenyl derivatives incorporating long spacer units have enantiotropic SA phases which can easily be supercooled. The thiadiazole derivatives and the pyrimidine derivatives have liquid crystalline phases only if the spacer units are rather short. Thiadiazole derivatives with an odd number of connecting atoms between the CTV core and the calamitic mesogens form SA phases, whereas those with even-numbered spacers display columnar mesophases.We propose that the columnar mesophases observed for these oligomesogens do not result from their ability to adopt a nearly disc-like shape. Instead, they represent ribbon phases which result from a steric frustration caused by the diVerent space filling of the central cone-like core and the rod-like mesogenic groups.A chevron-like (banana-like) average shape of the molecules with even-numbered spacers is assumed to be responsible for the formation of the ribbon phases. Also a CTTV derivative with eight appended phenylthiadiazole mesogens displays a columnar mesophase. A nematic phase was found for the corresponding cyanobiphenyl derivative, whereas the pyrimidine derivative is not liquid crystalline.Liquid crystalline materials have become increasingly import- from the CTV-unit by spacers with a medium length (type B) ant for many applications.1 Besides their well-established appli- form smectic A phases, whereas long spacers cause the loss of cation in electrooptical display devices, they are also interesting the mesogenic properties (type C).However, the materials candidates for information storage, nonlinear optics2 and reported so far have not only diVered in spacer length, but photoconductivity.3 also in the type of calamitic units incorporated and no system- It is well known that thermotropic liquid crystalline phases atic studies concerning the influence of the spacer length on may be formed by rod-like as well as by disc-shaped molecules.the mesomorphic properties have been carried out. Thereby the molecular shape determines the kind of mesophase observed. Rod-like molecules give rise to nematic and/or lamellar liquid crystalline phases whereas flat discotic, on average disc-shaped and also cone-shaped molecules can self organize to give discotic nematic phases and/or columnar mesophases.4 The investigation of the transition between layered (smectic) and columnar liquid crystalline phases is a topic of current interest.New materials and novel mesophases with interesting properties (e.g. biaxial nematic5 and cubic phases6) can be expected in this intermediate region. Initially, catenated molecules were designed to bridge the gap between these two diVerent types of mesophases.7 These molecules represent hybrid molecules with a long rod-like rigid core ending in two half-disc moieties and can exhibit nematic, lamellar and columnar phases.The covalent linkage of diVerently shaped molecules via spacer units is an alternative way to create new supramolecular structures.8 This was first realized by Ringsdorf et al., who connected two disc-like triphenylene units with a rigid core.9 We have recently reported the first cyclotriveratrylene (CTV) derivatives connected with five or six calamitic units.10,11 We found that, depending on the length of the spacers connecting the central cone-shaped CTV-unit to the rod-like rigid cores, either columnar or smectic phases can be formed.If the Compounds with short spacers are of special interest because calamitic unit was directly attached to the central unit via a they represent borderline cases between materials forming carboxy group, then broad columnar mesophases were found (type A). Compounds in which the calamitic units are separated lamellar and columnar phases. J. Mater. Chem., 1997, 7(10), 2001–2011 2001Three types of rod-like rigid cores have been chosen for our investigations: 4-cyanobiphenyls, 2-phenylpyrimidines and 2- phenylthiadiazoles. These rigid cores diVer in the direction of their dipole moments (along or perpendicular to the long molecular axis) and in the kind of mesophases formed by their simple alkyl derivatives (4-cyanobiphenyls: nematic, 2-phenylpyrimidines and 2-phenylthiadiazoles: smectic A and C).Scheme 1 Also, two diVerent types of disc-like central units have been used. The CTV unit is rigid but not strictly flat. Nevertheless, derivatives with a suYciently large number of aliphatic chains connected to this cone-shaped central unit exhibit columnar mesophases (bowlic liquid crystals).12,13 The possibility that these achiral compounds can form mesophases with ferroelectric properties is another interesting aspect which will not Fig. 1 Electrospray mass spectrum of compound 3e however be discussed here.12,14 Cyclotetraveratrylene (CTTV) derivatives in contrast are much more flexible and can be regarded as being disc-shaped molecules on the average.15 We octahydroxy-5,10,15,20-tetrahydrotetrabenzo[a,d,g,j]cyclodo- hoped that the type of supermolecular arrangements could be decene)13,18 respectively with appropriate carboxylic acids influenced by combining these diVerent calamitic and disc-like incorporating a 4¾-cyanobiphenyl, a 5-phenylthiadiazole or a units.Here we report two homologous series of CTV deriva- 5-phenylpyrimidine rigid core. As an example, the synthesis of tives in which six calamitic phenylthiadiazole or cyanobiphenyl the pyrimidine derivatives 5 and 9 is shown in Scheme 1.rigid cores are fixed via flexible spacers of diVerent length to Cyclotricatechylene and cyclotetracatechylene were obtained the macrocyclic cyclotribenzylene unit. Additionally, two pheaccording to standard procedures by cyclocondensation of nylpyrimidine derivatives have been prepared. Furthermore, veratrole with formaldehyde, followed by cleavage of the we have synthesized selected asymmetric cyclotribenzylene methyl ethers by means of boron tribromide.13,17,18 For the derivatives carrying only five rigid cores and cyclotertrabensynthesis of the asymmetric compounds a stepwise conden- zylene derivatives (CTTV derivatives), even with eight sation procedure was used to prepare 2-methyl-3,7,8,12,13- appended rigid cores.pentahydroxy-10,15-dihydro-5H-tribenzo [a,d,g]cyclononene17 which was esterified as described above. The structure of all Synthesis final compounds was confirmed from their 1H NMR spectra and combustion analysis. Furthermore electrospray–MS The synthesis of the symmetric CTV and CTTV derivatives was achieved by esterification10,11,16 of cyclotricatechylene investigations indicate the correct molecular mass. Peaks for only partially acylated products were not detected (see for (2,3,7,8,12,13-hexahydroxy-10,15-dihydro-5H-tribenzo [a,d,g] cyclononene)17 and cyclotetracatechylene (2,3,6,7,10,11,14,15- example Fig. 1). 2002 J. Mater. Chem., 1997, 7(10), 2001–2011Results and Discussion We have studied systematically the dependence of the mesomorphic properties on spacer length.The trantion temperatures and the corresponding enthalpy values of the cyclotribenzylene derivatives are summarized in Tables 1, 2 and 4. 4-Cyanobiphenyl derivatives In the series of 4-cyanobiphenyl derivatives 1 the melting points and the clearing temperatures decrease with increasing chain length (Table 1).Thereby the decrease in the melting temperatures is more pronounced than the decrease in the clearing temperature. Owing to the high melting points of the compounds with short spacers no mesophases could be Fig. 2 DSC heating and cooling traces of compound 1d (5 K min-1) detected for compounds 1a and 1c and only monotropic phases were found for 1b and 2a.The other cyanobiphenyl derivatives with long chains (compounds 1d and 1e) display enantiotropic liquid crystalline properties. cyanobiphenyl mesogens to the cyclotribenzylene unit gives All mesophases of the cyanobiphenyl derivatives show the rise to a significant mesophase stabilization and additionally same optical texture. On cooling, the appearance of a nonthe nematic phase of the monomeric 4-cyanobiphenyl derivaspecific birefringent texture and homeotropically aligned tives is replaced by smectic A phases in the CTV derivatives 1.regions were observed. On annealing these samples close to This means that the cyclotribenzylene linking unit stabilizes a the clearing temperature, the formation of a focal conic fan layered arrangement of the individual calamitic molecules.The texture could be observed. clearing temperatures and also the melting points decrease by The mesophase of compound 1b was investigated using a elongation of the spacers. Because the melting temperatures Guinier goniometer. The typical diVraction pattern of a layered are more strongly influenced than the clearing temperatures, structure without order within the layers was observed.The compounds with short spacers have monotropic mesophases layer thickness was calculated to be 3.03 nm. The length of the and those with long spacers are enantiotropic liquid crystals. molecule is estimated (CPK models) to be about 4.5 nm. From In the cases of compounds 1a and 1c it was not possible to this interdigitation of the cyanobiphenyl groups can be supercool the samples to suYciently low temperatures to allow deduced. From the textural observations and from the X-ray the observation of liquid crystalline phases.However, compattern we conclude that a smectic A phase exists. Because the pounds 1d and 1e with long spacer units form enantiotropic optical textures are identical for all investigated compounds 1, SA phases which can easily be supercooled to -30 °C without it is reasonable to assume that all mesomorphic cyanobiphenyl crystallization.Crystallization occurs only after prolonged derivatives 1 and 2 display SA phases. storage.† Sections of the DSC heating and cooling curves of Comparison of these compounds with related monomeric 4- compound 1d are shown in Fig. 2. cyanobiphenyl liquid crystals (e.g. C6H13OMC6H4MC6H4M Replacing one of the rigid cores with a methyl group CN: Cr 57 °C N 75.5 °C I)19 indicates that appending the (compounds 2) gives rise to a mesophase destabilization (cf.compound 1b with six rigid cores and compound 2b with only Table 1 Phase transition temperaturesa and transition enthalpies five rigid cores) and additionally, the melting points and the (lower lines) of the cyclotribenzylene derivatives 1 and 2 crystallization tendency are decreased by this desymmetrization of the molecules.11 A monotropic SA phase was observed by cooling the cyclotribenzylene derivative 2a which is a desymmetrized analogue of the non-liquid crystalline compound 1a.Compound 2a with only five rigid cores, but with the shortest spacer length, has the most stable mesophase of all cyanobiphenyl derivatives investigated.Thiadiazole derivatives The phase behaviour of the thiadiazole derivatives 3 depends transition temperatures, T /°C on the spacer length and diVers significantly from that of the R1 n comp. transition enthalpies, DH/kJ mol-1 cyanobiphenyl compounds 1 (Table 2). Again, the clearing temperatures decrease on elongation of R 3 1a Cr 220 I 75.8 the spacer units.However, the melting points of compounds 3 R 4 1b11 Cr 165 (SA 133) I remain nearly constant. Therefore, in this series of compounds 76.6 6.6 mesomorphic properties are only observed for the short chain R 5 1c Cr 159 I derivatives and are lost on increasing the spacer length. 83.9 The optical textures of compounds 3a, 3c, 3i and 4b which R 6 1d Cr1 65 Cr2 112 SA 123 I have three or five methylene groups in the spacers diVer from 18.8 7.0 8.2 R 10 1e Cr 95 SA 116 I those of the compounds 3b, 3g, 3h and 4a with four methylene 58.3 21.3 groups in the spacer units.The next homologue with six CH3 3 2a Cr 163 (SA 145) I methylene units (compound 3d) is only a crystalline solid. The 40.4 liquid phase of 3d can be supercooled to 145 °C and at this CH3 4 2b11 Cr 80 SA 118 I temperature no liquid crystalline phase is detected.CH3 10 2c Cr 78 SA 107 I 48.3 18.1 † Often, only part crystallization can be observed even after prolonged storage. This explains the low melting enthalpy values found for some aAbbreviations: Cr=crystalline, SA=smectic A-phase, I=isotropic phase. compounds.J. Mater. Chem., 1997, 7(10), 2001–2011 2003Table 2 Phase transition temperaturesa and transition enthalpies of methylene groups (i.e. with an even total number of con- (lower lines) of the cyclotribenzylene derivatives 3 and 4 incorporating necting atoms in each spacer) diVer significantly from the a 2-phenylthiadiazole unit compounds discussed above. They form highly viscous mesophases and display spherulitic flower textures which are typical of columnar mesophases (see Fig. 4). The mesophases of compounds 3a and 3c were investigated by X-ray diVraction using a Guinier film camera. Both compounds have a diVuse scattering in the wide angle region. Therefore a higher ordered smectic mesophase can be excluded. Additionally, besides a strong scattering there are several scatterings of low intensity in the small angle region. Table 3 displays the observed reflections of compound 3a, which have been evaluated on the basis of an oblique cell.The resulting transition temperatures, T /°C lattice parameters are a=5.6, b=2.26 nm and b=48°. R1 m n comp. transition enthalpies, DH/kJ mol-1 To propose a model for the arrangement of the molecules in this columnar phase, diVerent molecular conformations have R 7 3 3a Cr 152 Col 182 I 10.7 54.0 to be considered.The CPK models of three possible confor- R 7 4 3b11 Cr 117 SA 159 I mations of compound 3a are shown in Fig. 5. The star-shaped R 7 5 3c Cr 151 Col 155 I conformation [Fig. 5(a)] cannot explain the observed lattice 30.6 65.8 parameters. If the averaged molecules exist in a stretched R 7 6 3d Cr 156 I double forked conformation with three calamitic units placed 144.9 side by side [Fig. 5(b)], then their total length in the most R 7 7 3e Cr 158 I 135.6 R 7 10 3f11 Cr 142 I R 9 4 3g11 Cr 152 SA 160 I R 15 4 3h Cr1 102 Cr2 143 SA 155 I 87.1 0.9 R 9 5 3i Cr 80 Col 162 I CH3 9 4 4a11 Cr <20 Sx 126 SA 142 I CH3 9 5 4b Cr 62 Col 148 I aAbbreviations: Sx=unknown smectic phase, Col=columnar mesophase.Fig. 3 Model of the SA phase of compound 3b, 3g and 3h Smectic phases of thiadiazole derivatives. The above mentioned compounds with an even number of methylene groups (i.e. with an odd total number of connecting atoms in each spacer, because of the ether oxygen and the carboxy group) show small focal conic fan-textures, which can be homeotropically aligned on shearing the samples to give optically isotropic regions. These textural features are typical for SA phases.The X-ray patterns of compounds 3b and 4a have been discussed in a recent paper11 and confirm a smectic layer structure Fig. 4 Optical textures of the thiadiazole derivative 3a as obtained by without order in the layers. The layer period in the SA phase cooling from the isotropic melt (crossed polarizers): (a) transition I–Col of compound 3g is 5.3 nm.The molecules can be considered at 182 °C; (b) Col phase at 170 °C as oligomesogens consisting of rod-like rigid cores appended to the CTV central unit with an average double forked Table 3 Lattice parameter of the mesophase of compound 3a at conformation. Assuming this conformation, the molecular T=170 °C length in the most extended form (CPK models) is 6.3 nm.With respect to the calamitic phenylthiadiazole mesogens, no. Hexp hk these smectic phases can be regarded as bilayer structures 1 1.05 10 which are formed by the segregation of the central CTV units 2 2.07 11 from the terminal aliphatic chains (Fig. 3). 3 2.61 01 4 3.11 30 Columnar mesophases of thiadiazole derivatives. The thiadia- 5 3.69 32 zole derivatives 3a, 3c, 3i and 4b which have an odd number 2004 J.Mater. Chem., 1997, 7(10), 2001–2011Fig. 6 Structure of the CTV central unit (right-hand side) and possible packing model of the molecules of compound 3a in the ribbon phase ( left-hand side) sional arrangement of ribbons which are extended in the third dimension (ribbon-phase).According to this structure, the lattice parameter a should be equal to twice the length of the mesogenic units (2.4 nm) plus the diameter of the CTV unit (1.0 nm).‡ The observed lattice parameter a=5.6 nm is in excellent agreement with this molecular parameter. The angle b=48° of the oblique lattice is provided by the angle w=47±2° between the planes of the benzene rings of the CTV unit and the C3-axis of the CTV unit,20 which is orientated parallel to b.Assuming two molecules per unit cell and considering the tilt of the molecular moieties, then the value of the parameter b can also be well understood. Considering the mesogenic groups as structural units, which are linked via the CTV moieties, then the ribbons can be regarded as one-dimensionally extended fragments of a smectic bilayer.Such a ribbon arrangement, which partly destroys the natural segregation between the aromatic CTV units and the alkane chains must be favoured for geometric reasons. The diVerent space filling of the CTV unit and the spacers on the one hand and the calamitic mesogens with their appended alkyl chains on the other are likely responsible for the destabilization of the layer-like organization of the molecules and give rise to the formation of ribbons.This model of the columnar mesophase is related to other two-dimensional modulated phases.21–30 Modulated phases were also detected for terminally connected dimesogens.31–33§ In this case an alternation of the mesomorphic behaviour with the parity of the spacer length was found.When the total number of atoms in the spacer is even, the molecules adopt a rod-like shape and form smectic Fig. 5 Structure of compound 3a: (a) fully stretched (star-shaped) phases. When this number is odd, the dimesogens have a bent conformation; (b) double-forked structure showing aligned rigid cores average conformation. The formation of antiferroelectrically (side view and top view); (c) chevron-shaped conformation (side view) ordered and modulated phases is then favoured.An alternation of the mesomorphic properties with the parity of the spacer length is also found for the CTV derivatives extended conformation is ca. 5.7 nm, which agrees well with synthesized by us (see above).¶ In our case however, spacers the lattice parameter a=5.6 nm.However, in order to realize with an odd total number of connecting atoms can partly this conformation the cone-like shape of the CTV unit must compensate for the bent structure of the CTV unit and give be compensated for by a bend in the spacer units. In the third rise to a more rod-like total shape [see Fig. 5(b)] which favours conformation [Fig. 5(c)] a chevron-like molecular shape is smectic phases.The chevron-like molecular shape provided by proposed. It should be provided by the cone-like shape of the CTV unit and is transferred to the total shape of the molecules ‡ This roughly corresponds to the molecular length in the biforked conformation [Fig. 5(b)]. by the even-numbered spacers. Interestingly, the angle b=48° § In this respect some unusual features of nematic and smectic phases of the oblique lattice of the columnar phase of 3a corresponds of terminally connected oligomesogens, such as cyclotriphosphaz- exactly to the angle w=47±2° between the C3-axis and the enes,38 oligosiloxanes39 and cyclohexane 1,3,5-tricarboxylates40 are planes of the benzene rings of the CTV unit (see Fig. 6).20 under discussion. A possible model of the columnar phase based on a chevron- ¶ Because only three homologues are liquid crystalline, the question like average conformation of compound 3a is sketched in remains open as to whether this odd–even parity is also valid for the higher homologues.Fig. 6. This columnar phase can be regarded as a two-dimen- J. Mater. Chem., 1997, 7(10), 2001–2011 2005the even-numbered spacers favours the formation of ribbon phases.Thus the cone-like shape of the CTV linking unit provides an inverted odd–even parity in comparison to dimesogens. Pyrimidine derivatives Three 2-phenylpyrimidine derivatives have been synthesized. Compound 5a exhibits liquid crystalline properties in the temperature range between 143 and 177 °C, whereas the next homologue 5b is only a crystalline solid (Table 4).It seems that the dependence of the mesophase stability on the spacer length is analogous to that of the thiadiazole derivatives, i.e. decreasing mesophase stability is observed on elongation of the spacers. However, in the case of the pyrimidine derivatives the melting temperatures are significantly higher and therefore the mesogenic properties are lost in the tetramethylene derivative 5b.Even desymmetrization of the molecule by replacing one mesogenic unit by a methyl group (compound 6) cannot give rise to liquid crystalline properties. The mesophase behaviour of compound 5a is rather interesting because a phase transition occurs within the mesomorphic range at a temperature of 170 °C. The high temperature mesophase shows a spherulitic flower texture [Fig. 7(a)], which is similar to the texture observed for the columnar mesophases of the thiadiazole derivatives (see Fig. 4). However the X-ray pattern displays only a layer reflection with a periodicity of 2.9 nm and a diVuse halo in the wide angle region. It is remarkable that the observed periodicity corresponds only to half the total molecular length [L=5.9 nm, CPK model in the most extended conformation similar to Fig. 5(b)], which supports a smectic ‘monolayer’ structure. At the transition to the low temperature mesophase the texture becomes broken and nonspecific as shown in Fig. 7(b). Fig. 7 Optical textures (crossed polarizers) of the 2-phenylpyrimidine Here the X-ray studies suggest an oblique cell with a=5.82, derivative 5a as obtained by cooling from the isotropic melt: b=2.27 nm, b=47.4°.The diVuse halo in the wide angle region (a) transition I–M at 177 °C; (b) transition M–Col at 170 °C is maintained. The parameter a corresponds to the molecular length, whereas the parameters b and b are nearly identical to the corresponding parameters of the thiadiazole derivative 3a. can lead to an enlarged and non-uniform diameter of the Therefore, the same ribbon model as for compound 3a can be ribbons (more than one molecule is found in the diameter of discussed for the low temperature mesophase of the pyrimidine the ribbons).The well-defined oblique lattice is lost and only derivative 5a. a layer-like scattering corresponding to approximately half the A possible model for the high temperature phase is based molecular length remains.If this is valid, the M phase should on the assumption that increasing the temperature can lower also be a ribbon phase; only a layer reflection can be seen in the steric frustration between terminal chains and CTV units the X-ray diVraction pattern, however, suggesting a smectic and therefore give rise to a partial segregation of the CTV layer structure.units from the alkyl chains. The aggregation of the CTV units CTTV derivatives Table 4 Phase transition temperaturesa and transition enthalpies The mesomorphic properties of the synthesized CTTV deriva- (lower lines) of the pyrimidine derivatives 5 and 6 tives 7–9 are summarized in Table 5. In contrast to the CTV derivatives 1, which have SA phases, the cyanobiphenyl derivative 7 is a nematic liquid crystal as is obvious from the typical nematic schlieren texture.A spherulitic texture was observed for the thiadiazole derivative 8 on cooling from the isotropic melt (Fig. 8). From this texture we can conclude that an SA phase is absent. This mesophase could possibly be a columnar phase (ribbon phase). However, the high melting temperature and the onset of decomposition at temperatures above 200 °C does not allow the X-ray investigations of this mesophase and therefore no transition temperatures, T /°C confirmation of the phase structure was possible. R1 n comp.transition enthalpies, DH/kJ mol-1 The pyrimidine derivative 9 is a crystalline solid with no R 3 5a Cr 143 Col 170 M 177 I mesophase. 7.4 32.4 22.2 Interestingly, in the case of the CTTV derivatives, columnar R 4 5b Cr 168 I mesophases can be observed for compounds with an odd 84.6 number of connecting atoms in the spacers. Because the CTTV CH3 4 6 Cr1 126 Cr2 138 I unit adopts an averaged disc-like shape rather than a bowl- 20.6 70.3 like shape, this observation is in accordance with the proposed model. The odd–even parity is possibly inverted for the CTTV aAbbreviations: M=unknown liquid crystalline phase. 2006 J. Mater. Chem., 1997, 7(10), 2001–2011Table 5 Phase transition temperaturesa and transition enthalpies mesogens are not the result of the ability to adopt a disc-like (lower lines) of the CTTV derivatives 7–9 shape. Rather, they represent ribbon phases which arise from the steric frustration caused by the diVerent space filling of the central cone-like core and the attached rod-like mesogens.A chevron-like average shape of the molecules with even-numbered spacers is assumed to facilitate this steric frustration and be responsible for the formation of the ribbon phases. Oddnumbered spacers can partly compensate for the bent structure of the CTV unit and give rise to SA phases. A nematic phase was only found in the case of a CTTV derivative with appended cyanobiphenyl mesogens.Thus, the covalent fixation of calamitic mesogens to central connecting units can not only stabilize smectic mesophases but also transition temperatures, T /°C provide the possibility to change the phase structure of cala- R comp. transition enthalpies, DH/kJ mol-1 mitic mesogens from smectic to columnar with only minor adjustments of the chemical structure. The ability of these 7 Cr1 174 Cr2 222 (N 220) I molecules to adopt a chevron-shaped (banana-shaped) 53.8 63.6 geometry is of special interest with respect to potential ferro- 8 Cr1 142 Cr2 204 M 222 I electric properties.34 Furthermore, the special kind of meso- 70.3 10.6 44.5 phases found in the so-called banana-shaped molecules34 could be related to the ribbon-phases described here. 9 Cr 241 I Experimental aAbbreviations: N=nematic phase. Methods Confirmation of the structures of intermediates and products was obtained by 1H and 13C NMR spectroscopy (Bruker WP 200 spectrometer and a Varian Unity 500; coupling constants J are given in Hz), infrared spectroscopy (Specord 71 IR) and mass spectrometry (Intectra GmbH, AMD 402, electron impact, 70 eV; electrospray–MS, VG Bio-Q Fisons Instruments).Microanalyses were performed using an Carlo- Erba 1102 or a Leco CHNS-932 elemental analyser. Transition temperatures were measured using a Mettler FP 82 HT hot stage and control unit in conjunction with a Nikon Optiphot 2 polarizing microscope and these were confirmed using diVerential scanning calorimetry (Perkin-Elmer DSC-7).XRay studies were performed using a Guinier goniometer (Fa. Huber). Materials Fig. 8 Optical photomicrograph of the mesophases of the CTTV 4-(5-Alkyl-1,3,4-thiadiazol-2-yl)phenols35 and 4-(5-octylpyrim- derivative 8 (crossed polarizers) as obtained by cooling from the isotropic melt at 222 °C idin-2-yl )phenol,36 3,7,8,12,13-hexahydroxy-10,15-dihydro-5Htribenzo[ a,d,g]cyclononene,17 3,7,8,12,13-pentahydroxy-2- methyl-10,15-dihydro-5H-tribenzo[a,d,g]cyclononene11 and derivatives.Unfortunately no liquid crystalline CTTV deriva- 2,3,6,7,10,11,14,15-octahydroxy-5,10,15,20-tetrahydrotetrabenzotives with even numbered spacer units have been obtained. [a,d,g,j]cyclododecene18 were synthesized according to literature procedures. 4-(4-Cyanophenyl)phenol and 11-bromo- Summary undecanoic acid were obtained from Merck. The methyl and In conclusion, we have prepared liquid crystalline oligomeso- ethyl v-bromoalkanoates (Aldrich) were used as obtained. gens which consists of five, six and even eight calamitic 4- Methyl 7-bromoheptanoate was synthesized from 6-bromocyanobiphenyl, 2-alkyl-5-phenyl-1,3,4-thiadiazole or 5-octyl-2- hexanoic acid via chain elongation with diazomethane.37 phenylpyrimidine units covalently linked by spacers of diVerent lengths with cyclotribenzylene or cyclotetrabenzylene central Alkyl v-[4-(4-cyanophenyl)phenoxy]alkanoates 10, alkyl v- [4-(5-alkyl-1,3,4-thiadiazol-2-yl)phenoxy]alkanoates 12 and cores. The cyanobiphenyl derivatives incorporating the CTV unit have broad SA phases which can easily be supercooled if ethyl v-[4-(5-octylpyrimidin-2-yl)-phenoxy]alkanoates 14. 4- (4-Cyanophenyl )phenol, 4-(5-alkyl-1,3,4-thiadiazol-2-yl )phenol the spacer units are long. The thiadiazole derivatives and the pyrimidine derivatives have liquid crystalline phases only if or 4-(5-octylpyrimidin-2-yl)phenol (20 mmol) was dissolved in dry butan-2-one (200 ml), the appropriate alkyl v- the spacer units are rather short.Phenylthiadiazole substituted CTV derivatives with an odd number of connecting atoms bromoalkanoate (40 mmol), K2CO3 (8.3 g, 60 mmol) and tetrabutylammonium iodide (50 mg) were added and the resulting form SA phases. With respect to the calamitic phenylthiadiazole mesogens, these smectic phases can be regarded as bilayer mixture was stirred at reflux temperature until the phenol could not be detected by TLC (1–10 h).After cooling, the structures which are formed by the segregation of the central CTV units from the terminal aliphatic chains. solvent was evaporated and the residue was dissolved in water (50 ml ) and dichloromethane (100 ml). The organic phase was Columnar mesophases were found for those compounds in which phenylthiadiazoles and phenylpyrimidines were washed with sodium hydrogen carbonate (50 ml ), hydrochloric acid (10%, 50 ml) and water (50 ml ) successively.Afterwards appended via even numbered spacers to a CTV unit and for phenylthiadiazole derivatives grafted to CTTV units. We pro- the solution was dried with Na2SO4 and the solvent was removed in vacuo.The 4-cyanobiphenyl derivatives 10 and the pose that the columnar mesophases observed for these oligo- J. Mater. Chem., 1997, 7(10), 2001–2011 2007Table 8 Phase transition temperatures of the v-[-4-(5-octylpyrimidin- thiadiazole derivatives 12 were crystallized twice from light 2-yl)phenoxy]alkanoic acids 15 and their ethyl esters 14 petroleum (bp 60–85 °C)–ethyl acetate.The pyrimidine derivatives 14 were crystallized from ethanol. The transition temperatures of the compounds 10, 12 and 14 are summarized in Tables 6–8. Analytical data of the compounds 10a, 12a and 14a are given as representative examples. Ethyl 4-[4-(4-cyanophenyl) phenoxy]butanoate 10a. Elemen- phase transition tal analysis (%): found (calc. for C19H19NO3): C, 73.46 (73.77); n R comp.temperatures T /°C R comp. mp T /°C H, 5.96 (6.19); N, 4.52 (4.53). dH (CDCl3) 1.26 (m, 6H, CH3), 3 C2H5 14a Cr 51 (SA 44) I H 15a 137 2.15 (m, 2H, CH2), 2.54 (t, 2H, J 7.2, CH2COO), 4.05 (t, 2H, 4 C2H5 14b Cr 41 N 43 I H 15b 117 J 6.1, OCH2CH2), 4.15 (q, 2H, OCH2CH3), 6.99 (d, 2H, J 8.8, Ar-H), 7.52 (d, 2H, J 8.8, Ar-H), 7.62–7.75 (m, 4H, Ar-H). Ethyl 4-[4-(5-heptyl-1,3,4-thiadiazol-2-yl )phenoxy]butanoate filtered oV, washed with water and crystallized twice from light 12a.Elemental analysis (%): found (calc. for C21H30N2O3S): petroleum (bp 60–85 °C)–ethyl acetate. The transition tem- C, 64.79 (64.59); H, 7.75 (7.74); N, 7.25 (7.17); S, 8.27 (8.21). peratures of the compounds 11, 13 and 15 are summarized in dH (CDCl3) 0.86 (m, 6H, CH3), 1.23–1.8 (m, 10H, CH2), 2.12 the Tables 6–8.Analytical data of the compounds 11a, 13a (m, 2H, CH2), 2.5 (t, 2H, J 7.3, CH2COO), 3.08 (t, 2H, J 7.7, and 15a are given as representative examples. ArCH2), 4.05 (t, 2H, J 6.1, OCH2CH2), 4.13 (q, 2H J 7.1, OCH2CH3), 6.93 (d, 2H, J 8.85, Ar-H), 7.83 (d, 2H, J 8.85, 4-[4-(4-Cyanophenyl)phenoxy]butanoic acid 11a. Elemental Ar-H).analysis (%): found (calc. for C17H15NO3): C, 72.30 (72.58); H, 5.61 (5.37); N, 4.74 (4.98). dH (CDCl3) 2.15 (m, 2H, CH2), Ethyl 4-[4-(5-octylpyrimidin-2-yl)phenoxy]butanoate 14a. dH 2.60 (t, 2H, J 7.2, CH2COO), 4.07 (t, 2H, J 5.9, OCH2CH2), (CDCl3) 0.86 (m, 6H, CH3), 1.15–2.11 (m, 14H, CH2), 2.52 (t, 6.97 (d, 2H, J 8.8, Ar-H), 7.50 (d, 2H, J 8.8, Ar-H), 7.61 (d, 2H, J 7.2, CH2COO), 2.58 (t, 2H, ArCH2, J 7.7), 4.07 (t, 2H, 2H, J 8.55, Ar-H), 7.67 (d, 2H, J 8.55, Ar-H).MS m/z (relative J 6.1, OCH2CH2), 4.14 (q, 2H, J 7.2, CH2CH3), 6.95 (d, 2H, J intensity, %): 281 (26, M+), 228 (15), 195 (100), 166 (18), 151 8.9, Ar-H), 8.32 (d, 2H, J 8.9, Ar-H), 8.55 (s, 2H, Ar-H). (5), 140 (12), 101 (11), 87 (8). v-[4-(4-Cyanophenyl)phenoxy]alkanoic acids 11, v-[4-(5- 4-[4-(5-Heptyl-1,3,4-thiadiazol-2-yl)phenoxy]butanoic acid alkyl-1,3,4-thiadiazol-2-yl)phenoxy]alkanoic acids 13, and v- 13a.Elemental analysis (%): found (calc. for C19H26N2O3S): [4-(5-octylpyrimidin-2-yl )phenoxy]alkanoic acids 15. The com- C, 63.01 (62.96) H, 7.27 (7.23); N, 7.72 (7.73); S, 8.91 (8.84). pounds 10, 12 or 14 (10 mmol) were dissolved in methanol dH (CDCl3): 0.86 (t, 3H, J 7, CH3), 1.23–1.8 (m, 10H, CH2), (150 ml) and heated to reflux.A solution of potassium hydrox- 2.12 (m, 2H, CH2), 2.59 (t, 2H, J 7.2, CH2COO), 3.09 (t, 2H, ide (1.7 g, 30 mmol) in water (20 ml ) was added under stirring J 7.6, ArCH2), 4.07 (t, 2H, J 6.1, OCH2CH2), 6.94 (d, 2H, J and the solution was refluxed for 2 h in the cases of 12 and 14 8.7, Ar-H), 7.84 (d, 2H, J 8.7, Ar-H).dC (CDCl3) 178, 169.8, and only for 5 min in the case of compound 10. After cooling, 168.2, 161, 129.4, 123.2, 115, 66.8, 31.6, 30.3, 30.2, 30.1, 28.9, the mixture was poured on crushed ice. The precipitate which 28.8, 24.3, 22.6, 14. MS m/z (relative intensity, %): 362 (84, formed on acidification with dilute hydrochloric acid was M+), 347 (8, M+-CH3), 333 (34), 319 (32), 305 (12), 291 (92), 278 (100), 247 (8), 233 (6), 219 (4), 205 (18), 192 (55), 137 (8).Table 6 Phase transition temperatures of the v-[4-(4-cyanophenyl)- phenoxy]alkanoic acids 11 and their alkyl esters 10 4-[4-(5-Octylpyrimidin-2-yl )phenoxy]butanoic acid 15a. dH (CDCl3) 0.85 (t, 3H, J 6.4, CH3), 1.2–1.7 (m, 14H, CH2), 2.52–2.67 (m, 4H, CH2), 4.08 (t, 2H, J 6.05, OCH2CH2), 6.95 (d, 2H, J 8.3, Ar-H), 8.31 (d, 2H, J 8.8, Ar-H), 8.56 (s, 2H, Ar- H).dC (CDCl3): 177.8, 162.4, 160.7, 156.9, 132.2, 130.4, 129.5, 114.5, 66.7, 31.8, 30.8, 30.4, 30.2, 29.3, 29.2, 29.1, 24.5, 22.6, phase transition 14.1. MS m/z (relative intensity, %): 370 (38, M+), 284 (100), n R comp. mp T /°C R comp. temperatures T /°C 241 (5), 227 (10), 213 (6), 199 (31), 185 (74), 158 (11), 119 (4). 3 C2H5 10a 76 H 11a Cr 200 (N 192) I 5 C2H5 10b 89–90 H 11b Cr 173 (N 170) I Esterification of the polyphenols. 2,3,7,8,12,13-Hexahydroxy- 6 CH3 10c 103 H 11c Cr 123 (N 118) I 10,15-dihydro-5H-tribenzo[a,d,g]cyclononene, 3,7,8,12,13- 10 CH3 10d 104 H 11d Cr 129 (N 122) I pentahydroxy-2-methyl-10,15-dihydro 5H-tribenzo[a,d,g]- Table 7 Phase transition temperatures of the v–4-(5-alkyl-1,3,4-thiadiazol-2-yl)phenoxy]alkanoic acids 13 and their alkyl esters 12 phase transition m n R comp.temperatures T /°C R comp. mp T /°C 7 3 C2H5 12a Cr 67 (SA 63) I H 13a 123 7 5 C2H5 12b Cr 64 I H 13b 113 7 6 CH3 12c Cr 58 I H 13c 99 7 7 CH3 12d Cr 59 I H 13d 107 15 4 C2H5 12e Cr 78 I H 13e 116 2008 J. Mater. Chem., 1997, 7(10), 2001–2011cyclononene or 2,3,6,7,10,11,14,15-octahydroxy-5,10,15,20- 3, 7, 8, 12, 13-Pentakis{11-[4-(4-cyanophenyl)phenoxy]undecanoyloxy}- 2-methyl-10,15-dihydro-5H-tribenzo[a,d,g]cyclo- octahydrotetrabenzo[a,d,g,j]cyclododecene (0.2 mmol) and N-cyclohexyl-N¾-[2-(N-methylmorpholinio)ethyl]carbodiimide nonene 2c.Yield: 52%, phase transitions (°C): Cr 78 SA 107 I. Elemental analysis (%): found (calc. for C142H155N5O15): toluene-p-sulfonate (1.5 equiv.per OH group) were dissolved in 50 ml of dry chloroform. After 5 min the appropriate substi- C, 78.55 (78.52); H, 6.99 (7.29); N, 3.09 (3.23). dH (CDCl3) 1.2–1.86 (m, 70H, CH2), 2.05 (s, 3H, CH3), 2.1–2.2 (m, 10H, tuted alkanoic acids 11, 13 or 15 (1.3 equiv. per OH group) was added followed by addition of 4-(dimethylamino)pyridine CH2), 2.6–2.8 (m, 10H, CH2COO), 3.67 (d, 3H, ArCH2Ar, He), 3.95–4.15 (m, 10H, OCH2CH2), 4.7–4.85 (m, 3H, ArCH2Ar, (40 mg).The resulting mixture was stirred at room temp. for 20 h. After the reaction was complete, water (50 ml ) was added Ha), 6.9–7.73 (m, 46H, Ar-H). to the mixture, and the phases were separated. The aqueous phase was extracted with CHCl3 (30 ml ). The combined 2, 3, 7, 8, 12, 13-Hexakis{4-[4-(5-heptyl-1,3,4-thiadiazol-2-yl ) phenoxy]butanoyloxy}-10,15-dihydro-5H-tribenzo[a,d,g]cyclo- organic phase was washed successively with saturated aqueous sodium hydrogen carbonate (30 ml ), brine (30 ml ), and water nonene 3a. Yield: 56%, phase transitions (°C): Cr 152 Col 182 I.Elemental analysis (%): found (calc. for C135H162N12O18S6): (30 ml ). Afterwards, the solution was dried (Na2SO4) and the solvent was removed in vacuo.The products were purified by C, 66.23 (66.64); H, 6.82 (6.71); N, 6.84 (6.91); S, 8.02 (7.91). dH (CDCl3) 0.86 (t, 18H, J 6.9, CH3), 1.24–1.8 (m, 60H, CH2), column chromatography using chloroform–methanol (2551) followed by crystallization from nitromethane. 2.12 (m, 12H, CH2), 2.66 (t, 12H, J 7, CH2COO), 3.06 (t, 12H, J 7.7, ArCH2), 3.69 (d, 3H, J 14.1, ArCH2Ar, He), 4.0 (t, 12H, J 6.1, OCH2CH2), 4.76 (d, 3H, J 13.7, ArCH2Ar, Ha), 6.89 (d, 2, 3, 7, 8, 12, 13-Hexakis{4-[4-(4-cyanophenyl)phenoxy]buta- 12H, J 9, Ar-H), 7.14 (s, 6H, Ar-H), 7.79 (d, 12H, J 9, Ar-H).noyloxy}-10,15-dihydro-5H-tribenzo[a,d,g]cyclononene 1a. Yield: dC (CDCl3) 170.3, 169.6, 167.9, 160.8, 140.7, 137.2, 129.4, 124.7, 73%, mp 220 °C.Elemental analysis (%): found (calc. for 123.4, 114.9, 66.6, 36.3, 31.6, 30.4, 30.2, 30.0, 29, 28.9, 24.4, 22.6, C123H96N6O18): C, 75.56 (75.91); H, 5.05 (4.97); N, 3.94 (4.32). 14. Electrospray–MS: found (calc. for C204H264N16O24S8): M+ dH (CDCl3) 1.32–2.12 (m, 12H, CH2), 2.66 (t, 12H, J 6.95, 2431.72±0.22 (2431.04). CH2COO), 3.69 (d, 3H, J 14.2, ArCH2Ar, He), 4.0 (t, 12H, J 5.9, OCH2CH2), 4.76 (d, 3H, J 13.55, ArCH2Ar, Ha), 6.91 (d, 2, 3, 7, 8, 12, 13-Hexakis{6-[4-(5-heptyl-1,3,4-thiadiazol-2-yl ) 12H, J 8.8, Ar-H), 7.14 (s, 6H, Ar-H), 7.45 (d, 12H, J 8.75, Arphenoxy] hexanoyloxy}-10,15-dihydro-5H-tribenzo[a,d,g]cyclo- H), 7.56 (d, 12H, J 8.4, Ar-H), 7.64 (d, 12H, J 8.4, Ar-H).nonene 3c. Yield: 59%, phase transitions (°C): Cr 151 Col 155 I.Elemental analysis (%): found (calc. for C147H186N12O18S6): 2, 3, 7, 8, 12, 13-Hexakis{6-[4-(4-cyanophenyl)phenoxy]hexa- C, 68.13 (67.87); H, 7.38 (7.21); N, 6.42 (6.46); S, 7.21 (7.39). noyloxy}-10,15-dihydro-5H-tribenzo[a,d,g]cyclononene 1c. dH (CDCl3) (t, 18H, J 6.9, CH3), 1.2–1.8 (m, 96H, CH2), 2.51 Yield: 85%, mp 159 °C. Elemental analysis (%): found (calc. (t, 12H, J 7.05, CH2COO), 3.08 (t, 12H, J 7.6, ArCH2), 3.67 for C135H120N6O18): C, 76.43 (76.69); H, 5.64 (5.72); N, 4.21 (d, 3H, J 14.2, ArCH2Ar, He), 3.96 (t, 12H, J 6.3, OCH2CH2), (3.97).dH (CDCl3) 1.53–1.86 (m, 36H, CH2), 2.57 (t, 12H, J 4.75 (d, 3H, J 13.6, ArCH2Ar, Ha), 6.9 (d, 12H, J 8.4, Ar-H), 6.6, CH2COO), 3.74 (d, 3H, J 13.4, ArCH2Ar, He), 4.01 (t, 7.13 (s, 6H, Ar-H), 7.81 (d, 12H, J 8.4, Ar-H).Electrospray–MS: 12H, J 5.9, OCH2CH2), 4.81 (d, 3H, J 13.45, ArCH2Ar, Ha), found (calc. for C147H186N12O18S6): M+ 2600.5±1.66 6.99 (d, 12H, J 8.6, Ar-H), 7.19 (s, 6H, Ar-H), 7.53 (d, 12H, J (2599.23). 8.55, Ar-H), 7.64 (d, 12H, J 8.35, Ar-H), 7.69 (d, 12H, J 8.2, Ar-H). 2,3,7,8,12,13-Hexakis{7-[4- (5-heptyl-1,3,4-thiadiazol-2-yl ) phenoxy]heptanoyloxy}-10,15-dihydro-5H-tribenzo[a,d,g]cyclo- 2, 3, 7, 8, 12, 13-Hexakis{7-[4-(4-cyanophenyl)phenoxy]hep- nonene 3d.Yield: 87%, mp 156 °C. Elemental analysis (%): tanoyloxy}-10,15-dihydro-5H-tribenzo[a,d,g]cyclononene 1d. found (calc. for C153H198N12O18S6): C, 68.63 (68.43); H, 7.48 Yield: 40%, phase transitions (°C): Cr1 65 Cr2 112 SA 123 I. (7.43); N, 6.32 (6.26). dH (CDCl3) 0.92 (t, 18H, CH3, J 6.8), Elemental analysis (%): found (calc.for C141H132N6O18): 1.2–1.8 (m, 108H), 2.56 (t, 12H, J 7.2, CH2COO), 3.13 (t, 12H, C, 76.93 (77.03); H, 6.14 (6.05); N, 3.73 (3.82). dH (CDCl3) J 7.5, ArCH2), 3.72 (d, 3H, J 14.3 ArCH2Ar, He), 4.01 (t, 12H, 1.3–1.84 (m, 48H, CH2), 2.49 (t, 12H, J 7.35, CH2COO), 3.67 J 6.4, OCH2CH2), 4.79 (d, 3H, J 13.8, ArCH2Ar, Ha), 6.95 (d, (d, 3H, J 14.1, ArCH2Ar, He), 3.95 (t, 12H, J 6.35, OCH2CH2), 12H, J 8.6, Ar-H), 7.18 (s, 6H, Ar-H), 7.87 (d, 12H, J 8.6, Ar- 4.73 (d, 3H, J 13.3, ArCH2Ar, Ha), 6.92 (d, 12H, J 8.9, Ar-H), H); Electrospray–MS: found (calc.for C153H198N12O18S6): M+ 7.12 (s, 6H, Ar-H), 7.47 (d, 12H, J 8.8, Ar-H), 7.6–7.7 (m, 24H, 2683.70±0.24 (2683.32). Ar-H). 2,3,7,8,12,13-Hexakis{8-[4- (5-heptyl-1,3,4-thiadiazol-2-yl ) 2, 3, 7, 8, 12, 13-Hexakis{11-[4-(4-cyanophenyl)phenoxy]unde- phenoxy]octanoyloxy}-10,15-dihydro-5H-tribenzo[a,d,g]cyclocanoyloxy}- 10,15-dihydro-5H-tribenzo[a,d,g]cyclononene 1e.nonene 3e. Yield: 54%, mp 158 °C. Elemental analysis (%): Yield: 45%, phase transitions (°C): Cr 95 SA 116 I. Elemental found (calc. for C159H210N12O18S6): C, 69.16 (68.95); H, 7.64 analysis (%): found (calc. for C165H180N6O18): C, 78.50 (78.17); (7.64); N, 5.96 (6.07).dH (CDCl3) 0.86 (t, 18H, J 6.95, CH3), H, 7.23 (7.16); N, 3.21 (3.31). dH (CDCl3) 1.2–1.84 (m, 96H, 1.23–1.82 (m, 120H, CH2), 2.48 (t, 12H, J 7.5, CH2COO), 3.07 CH2), 2.45 (t, 12H, J 7.4, CH2COO), 3.65 (d, 3H, J 14.1, (t, 12H, J 7.7, ArCH2), 3.66 (d, 3H, J 14.4, ArCH2Ar, He), 3.96 ArCH2Ar, He), 3.96 (t, 12H, J 6.51, OCH2CH2), 4.72 (d, 3H, (t, 12H, J 6.4, OCH2CH2), 4.73 (d, 3H, J 13.4, ArCH2Ar, Ha), J 14.3, ArCH2Ar, Ha), 6.94 (d, 12H, J 8.8, Ar-H), 7.1 (s, 6H, 6.9 (d, 12H, J 8.65, Ar-H), 7.12 (s, 6H, Ar-H), 7.81 (d, 12H, J Ar-H), 7.49 (d, 12H, J 8.8, Ar-H), 7.58 (d, 12H, J 8.8, Ar-H), 8.85, Ar-H).dC (CHCl3) 170.8, 169.7, 168.2, 161.3, 140.7, 137.0, 7.65 (d, 12H, J 8.67, Ar-H). 129.4, 124.6, 122.5, 120.0, 144.9, 68.0, 34.0, 31.6, 30.9, 30.1, 30.0, 29.1, 29.0, 28.9, 28.85, 25.8, 25.75, 22.6, 14.1. Electrospray–MS: found (calc. for C159H210N12O18S6): M+ 2768.67±1.07 3,7,8,12,13-Pentakis{4-[4-(4-cyanophenyl)phenoxy]butanoyloxy}- 2-methyl-10,15-dihydro-5H-tribenzo[a,d,g]cyclononene 2a. (2767.42). Yield: 55%, phase transitions (°C): Cr 163 (SA 145) I. Elemental analysis (%): found (calc.for C107H85N5O15): C, 76.55 (76.45); 2, 3, 7, 8, 12, 13-Hexakis{5-[4-(5-pentadecyl-1,3,4-thiadiazol- 2-yl )phenoxy]pentanoyloxy}-10,15-dihydro-5H-tribenzo[a,d,g] H, 5.08 (5.10); N, 4.09 (4.17). dH (CDCl3) 2.05 (s, 3H, CH3), 2.1–2.27 (m, 10H, CH2), 2.6–2.8 (m, 10H, CH2COO), 3.67 (d, cyclononene 3h. Yield: 53%, phase transitions (°C): Cr1 102 Cr2 135 SA 158 I.Elemental analysis (%): found (calc. for 3H, ArCH2Ar, He), 3.95–4.15 (m, 10H, OCH2CH2), 4.7–4.85 (m, 3H, ArCH2Ar, Ha), 6.9–7.73 (m, 46H, Ar-H). C189H270N12O18S6): C, 71.23 (71.15); H, 8.8 (8.53); N, 5.3 J. Mater. Chem., 1997, 7(10), 2001–2011 2009(5.27). dH (CDCl3) 0.86 (t, 18H, J 7, CH3), 1.2–1.86 (m, 180H, 2, 3, 6, 7,10, 11, 14,15-Octakis{6-[4- (4-cyanophenyl)phenoxy] hexanoyloxy}-5,10,15,20-tetrahydrotetrabenzo[a,d,g,j]cyclodo- CH2), 2.57 (t, 12H, J 5.45, CH2COO), 3.06 (t, 12H, J 7.85, ArCH2), 3.67 (d, 3H, J 13.6, ArCH2Ar, He), 3.98 (t, 12H, J decene 7.Yield: 55%, phase transitions (°C): Cr1 174 Cr2 222 (N 220) I. Elemental analysis (%): found (calc. for 5.65, OCH2CH2), 4.74 (d, 3H, J 13.7, ArCH2Ar, Ha), 6.89 (d, 12H, J 9, Ar-H), 7.14 (s, 6H, Ar-H), 7.80 (d, 12H, J 8.85, Ar- C180H160N8O24): C, 76.78 (76.67); H, 5.94 (5.72); N, 3.78 (3.98).dH (CDCl3) 1.43–1.81 (m, 48H, CH2), 2.57 (m, br, 16H, H). Electrospray–MS: found (calc. for C189H270N12O18S6): M+ 3189.17±0.87 (3187.89). CH2COO), 3.7 (m, br, 8H, ArCH2Ar), 3.96 (t, 16H, J 6.2, OCH2CH2), 6.6–6.9 (s, br, 8H), 6.93 (d, 16H, J 8.7, Ar-H), 7.46 (d, 16H, J 8.9, Ar-H), 7.57 (d, 16H, J 8.7, Ar-H), 7.63 (d, 2, 3, 7, 8, 12,13-Hexakis{6- [4- (5-nonyl-1, 3, 4- thiadiazol-2-yl ) 16H, J 8.5, Ar-H).phenoxy]hexanoyloxy}-10,15-dihydro-5H-tribenzo[a,d,g]cyclononene 3i. Yield: 41%, phase transitions (°C): Cr 80 Col 162 I. 2,3,6,7,10,11,14,15-Octakis{5-[4-(5-nonyl-1,3,4-thiadiazol-2- Elemental analysis (%): found (calc. for C159H210N12O18S6): yl )phenoxy]pentanoyloxy} - 5, 10, 15, 20 - tetrahydrotetrabenzo C, 69.08 (68.95); H, 7.80 (7.64); N, 5.85 (6.07).dH (CDCl3) 0.86 [a,d,g,j ]cyclododecene 8. Yield: 20%, phase transitions (°C): (t, 18H, J 6.9, CH3), 1.2–1.8 (m, 120H, CH2), 2.51 (t, 12H, J Cr1 142 Cr2 204 Col 222 I. Elemental analysis (%): found (calc. 7.05, CH2COO), 3.08 (t, 12H, J 6.5, Ar-CH2), 3.67 (d, br, 3H, for C204H264N16O24S8): C, 68.22 (68.43); H, 7.52 (7.43); N, 6.14 J 14.2, ArCH2Ar, He), 3.98 (t, 12H, J 6.25, OCH2CH2), 4.75 (6.26).dH (CDCl3) 0.86 (t, 24H, J 7, CH3), 1.2–1.9 (m, 160H, (d, br, 3H, J 13.6, ArCH2Ar, Ha), 6.92 (d, 12H, J 9, Ar-H), CH2), 2.6 (m, 16H, CH2COO), 3.08 (t, 16H, J 7.5 ArCH2), 3.7 7.16 (s, 6H, Ar-H), 7.83 (d, 12H, J 9, Ar-H). (m, br, 8H, ArCH2Ar), 3.99 (m, 16H, O-CH2-CH2), 6.6–6.9 (m, br, 8H, Ar-H), 6.89 (d, 16H, J 8.2, Ar-H), 7.81 (d, 16H, J 8.2, 3, 7, 8, 12, 13 - Pentakis{6 - [4 - (5-nonyl-1, 3, 4 - thiadiazol-2-yl ) Ar-H).Electrospray–MS: found (calc. for C204H264N16O24S8): phenoxy]hexanoyloxy} - 2-methyl - 10, 15-dihydro - 5H- tribenzo M+ 3579.91±0.86. (3580.91). [a,d,g]cyclononene 4b. Yield: 45% phase transitions (°C): Cr 62 Col 148 I. Elemental analysis (%): found (calc.for 2, 3, 6, 7, 10, 11, 14,15 - Octakis{5-[4- (5- octylpyrimidin -2 -yl )- C137H180N10O15S5): C, 69.55 (69.51); H, 7.87 (7.66); N, 5.77 phenoxy]pentanoyloxy} - 5, 10, 15, 20 - tetrahydrotetrabenzo (5.92). dH (CDCl3) 0.85 (t, 15H, J 6.4, CH3), 1.24–1.55 (m, [a,d,g,j ]cyclododecene 9. Yield: 10%, mp 241 °C. dH (CDCl3) 60H, CH2), 1.71–1.92 (m, 40H, CH2), 2.06 (s, 3H, ArCH3), 0.85 (t, 24H, CH3), 1.15–1.95 (m, 128H, CH2), 2.5–2.57 (m, 2.57–2.62 (m, 10H, CH2COO), 3.06 (t, 10H, J 7.6, ArCH2), 32H, ArCH2 and CH2COO), 3.7 (m, br, 8H, ArCH2Ar), 3.98 3.65 (d, br, 3H, J 13.9, ArCH2Ar, He), 3.94–4.05 (m, 10H, (t, 16H, OCH2CH2), 6.6–6.9 (m, br, 8H, Ar-H), 6.92 (d, 16H, CH2O), 4.75–4.85 (m, 3H, ArCH2Ar, Ha), 6.89–6.95 (m, 10H, J 8.7, Ar-H), 8.31 (d, 16H, J 8.9, Ar-H), 8.53 (s, 16H, Ar-H).Ar-H), 7.13–7.25 (m, 6H, Ar-H), 7.80–7.87 (m, 10H, Ar-H). This work was supported by the BMBF and the Fonds der 2,3,7,8,12,13-Hexakis{4-[4-(5-octylpyrimidin-2-yl )phenoxy] Chemischen Industrie. butanoyloxy} - 10,15 - dihydro - 5H- tribenzo[a,d,g]cyclononene 5a. Yield: 37%, phase transitions (°C): Cr 143 Col 170 M 177 References I. Elemental analysis (%): found (calc. for C153H186N12O18): C, 74.39 (74.06); H, 7.74 (7.56); N, 6.54 (6.77).dH (CDCl3) 0.86 1 B. Bahadur, L iquid Crystals Application and Uses,World Scientific, (t, 18H, J 6.55, CH3), 1.2–1.85 (m, 72H, CH2), 2.11 (m, 12H, Singapore, 1990, Vol. 1–3. 2 D.M.Walba, M. B. Ros, T. Sierra, J. A. Rego, N. A. Clark, R. Shao, CH2), 2.57 (t, 12H, J 7.7, ArCH2), 2.67 (m, 12H, CH2COO), M.D. Wand, R. T. Vohra, K. E. Arnett and S. P. Velsco, 3.67 (d, 3H, J 13.75, ArCH2Ar, He), 4.0 (t, 12H, J 6.1, Ferroelectrics, 1991, 121, 147; K. Schmitt, R.-P. Herr, M. Schadt, OCH2CH2), 4.75 (d, 3H, J 13.55, ArCH2Ar, Ha), 6.91 (d, 12H, J. Fu� nfschilling, R. Buchecker, X. H. Chen and C. Benecker, L iq. J 9, Ar-H), 7.12 (s, 6H, Ar-H) 8.30 (d, 12H, J 9, Ar-H), 8.52 Cryst., 1993, 14, 1735.(s, 12H, Ar-H). dC (CHCl3) 170.4, 160.8, 159.6, 156.9, 140.6, F. Closs, K. Siemensmeyer, Th. Frey and D. Funhof, L iq. Cryst., 137.1, 132.2, 129.6, 124.7, 120.5, 114.4, 66.4, 31.8, 31.2, 30.9, 1993, 14, 629. 4 S. Chandrasekhar, L iq. Cryst., 1993, 14, 3. 30.7, 30.1, 29.9, 29.3, 29.2, 24.4, 22.6, 14.1. Electrospray–MS: 5 S. Chandrasekhar, Mol. Cryst.L iq. Cryst., 1994, 243, 1. found (calc. for C153H186N12O18): M+ 2480.13±0.43 (2479.4). 6 K. Borisch, S. Diele, P. Go� ring, H. Mu� ller and C. Tschierske, L iq. Cryst., 1995, 18, 175. 7 H.-T. Nguyen, C. Destrade and J. Malthete, Adv. Mater., 1997, 2,3,7,8,12,13-Hexakis{5-[4-(5-octylpyrimidin-2-yl )phenoxy] 9, 375. pentanoyloxy} - 10, 15 - dihydro - 5H- tribenzo[a,d,g]cyclononene 8 I.D. Fletcher and G. R. Luckhurst, L iq. Cryst., 1995, 18, 175. 5b. Yield: 45%, mp 168 °C. Elemental analysis (%): found (calc. 9 W. Kreuder, H. Ringsdorf, O. Herrmann-Scho� nherr and for C159H198N12O18): C, 74.22 (74.44); H, 7.91 (7.78); N, 6.48 J. H. WendorV, Angew. Chem., 1987, 99, 1300. (6.55). dH (CDCl3) 0.86 (t, 18H, J 6.95, CH3), 1.15–1.95 (m, 10 H. Budig, S. Diele, P.Go� ring, R. Paschke, C. Sauer and 96H, CH2), 2.40 (t, 12H, J 7.1, CH2COO), 2.57 (m, 12H, C. Tschierske, J. Chem. Soc., Chem. Commun., 1994, 2359. 11 H. Budig, S. Diele, P. Go� ring, R. Paschke, C. Sauer and ArCH2), 3.66 (d, 3H, J 13.9, ArCH2Ar, He), 3.99 (t, 12H, J 5.5, C. Tschierske, J. Chem. Soc., Perkin T rans. 2, 1995, 767. OCH2CH2), 4.73 (d, 3H, J 13.7, ArCH2Ar, Ha), 6.93 (d, 12H, 12 J.Malthete and A. Collet, J. Am. Chem. Soc., 1987, 109, 7544. J 8.8, Ar-H), 7.13 (s, 6H, Ar-H), 8.33 (d, 12H, J 8.8, Ar-H), 13 H. Zimmermann, R. Poupko, Z. Luz and J. Billard, Z. Naturforsch. 8.55 (s, 12H, Ar-H). Electrospray–MS; found (calc. for T eil A, 1985, 40, 149. C159H198N12O18): M+ 2564.19±0.73 (2563.49). 14 L. Lei, Mol. Cryst. L iq. Cryst., 1987, 146, 41. 15 H.Zimmermann, R. Poupko, Z. Luz and J. Billard, Z. Naturforsch. T eil A, 1986, 41, 1137. 3,7,8,12,13-Pentakis{5-[4-(5-octylpyrimidin-2-yl )phenoxy] 16 C. Tschierske and H. Zaschke, J. Prakt. Chem., 1989, 331, 365. pentanoyloxy}-2-methyl-10,15-dihydro-5H-tribenzo[a,d,g]-cyclo- 17 A. S. Lindsey, J. Chem. Soc., 1965, 1685. nonene 6. Yield: 35%, phase transitions Cr1 125 Cr2 138 I. 18 V. Percec, C. G. Cho and C. Pugh, J.Mater. Chem., 1991, 1, 217. 19 G. W. Gray, K. J. Harrison and J. A. Nash, Pramana, 1975, 1, Elemental analysis (%): found (calc. for C137H170N10O15): 381. C, 75.19 (74.90); H, 8.1 (7.8); N, 6.09 (6.38). dH (CDCl3) 0.86 20 A. Collet, T etrahedron, 1987, 43, 5725; J. A. Hyatt, E. N. Duesler, (m, 15H, J 6.5, CH3), 2.08 (s, 3H, CH3), 1.15–2.12 (m, 60H, D. Y. Curtin and I. C. Paul, J. Org. Chem., 1980, 45, 5074; A. Collet, CH2), 2.5–2.7 (m, 20H, ArCH2 and CH2COO), 3.6–3.7 (m, 3H, J. Gabard, J. Jacques, M. Cesario, J. Guilhem and C. Pscard, ArCH2Ar, He), 3.95–4.1 (m, 10H, OCH2CH2), 4.62–4.80 (m, J. Chem. Soc., Perkin T rans., 1981, 1630. 3H, ArCH2Ar, Ha), 6.91–8.38 (m, br, 26H, Ar-H), 8.57 (s, 10H, 21 P. Barois, J. Pommier and J. Prost, in Solitons in L iquid Crystals, ed. L. Lam and J. Prost, Springer, Heidelberg, 1991, p. 191. Ar-H). 2010 J. Mater. Chem., 1997, 7(10), 2001–201122 B. J. Ostrovskii, L iq. Cryst., 1993, 14, 131. 32 F. Hardouin, M. F. Achard, J.-I. Jin and Y.-K. Yun, J. Phys. II, 1995, 5, 927. 23 F. Hardouin, A. M. Levelut, M. F. Achard and Sigaud, J. Chim. Phys., 1983, 80, 53. 33 V. Faye, F. Babeau, F. Placin, H. T. Nguyen, P. Barios, V. Laux and N. Isaert, L iq. Cryst., 1996, 21, 485. 24 F. Hardouin, A. M. Levelut and G. Sigaud, J. Phys. Fr., 1981, 42, 71. 34 T. Niori, T. Sekine, J. Watanabe, T. Furukawa and H. Takazoe, J.Mater. Chem., 1996, 6, 1231. 25 W. Weissflog, A. Wiegeleben, S. Diele and D. Demus, L iq. Cryst., 1984, 19, 983. 35 C. Tschierske, H. Zaschke, H. Kresse, A. Ma�dicke and D. Demus, Mol. Cryst. L iq. Cryst., 1990, 191, 223. 26 J.Watanabe, Y. Nakata and K. Simizu, J. Phys. II, 1994, 4, 581. 27 P. Davidson, P. Keller and A. M. Levelut, J. Phys., 1985, 46, 36 H. Zaschke and R. Stolle, Z. Chem., 1975, 15, 441. 37 Organicum, Deutscher Verlag der Wissenschaften, Berlin 1993, 939. 28 B. W. Endres, M. Ebert, J. H.WendorV, B. Reck and H. Ringsdorf, pp. 577 and 591. 38 A. M. Levelut and K. Moriya, L iq. Cryst., 1996, 20, 119. L iq. Cryst., 1990, 7, 217. 29 T. A. Lobko and B. I. Ostrovskii,Mol.Mater., 1992, 1, 99. 39 G. H. Mehl and J. W. Goodby, Chem. Ber., 1996, 129, 521. 40 P. Go� ring, G. Pelzl, S. Diele, P. Delavier, K. Siemensmeyer and 30 H. T. Nguyen, G. Sigaud, M. F. Achard, F. Hardouin, R. J. Twieg and K. Betterton, L iq. Cryst., 1991, 10, 389. K. H. Etzbach, L iq. Cryst., 1995, 19, 629. 31 R. W. Date, G. R. Luckhurst, M. Shuman and J. M. Seddon, J. Phys., 1995, 42, 587. Paper 7/02034A; Received 24thMarch, 1997 J. Mater. Chem., 1997, 7(10), 2001&n

ISSN:0959-9428    

DOI:10.1039/a702034a

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

The laser exposure requirements of liquid crystal polymer thin films for photomasking applications David Goldie,*a James Cairns,a Mark Verrallb and David Coatesb aDepartment of A.P.E.M.E., University of Dundee, Perth Road, Dundee, UK DD1 4HN bL iquid Crystal Research,Merck L td, Quay Road, Poole, UK BH15 1TD Thin solid films of a cyanobiphenyl based polymethacrylate liquid crystal polymer (LCP) coated onto quartz substrates have been thermally processed to give a UV light scattering texture for use as a photomask opaque layer.UV transparent line patterns in the scattering layer may subsequently be written by scanning a focused laser beam across the LCP surface. The exposed linewidths are found to be strongly dependent upon the thickness of the LCP film, the incident power of the laser, and the overall residence time of the focused beam waist at a given location on the film surface.Successful exposure of the LCP films demands that the absorbed laser energy and beam waist residence time should exceed minimum threshold magnitudes however, and it is demonstrated that the smallest feature size which may be defined in the scattering layer is eVectively limited by heat conduction from the LCP to the quartz substrate.Possible approaches towards improving the exposure requirements of the LCP are considered, and the compatibility of the proposed LCP photomask with current industrial direct-write laser systems is appraised. The unprecedented growth in consumer products containing in principle no longer required. Potential materials for onemicroelectronic components is largely founded upon the avail- step photomask production are liquid crystal polymers (LCPs).ability of cheap microchips which in many instances are The bulk optical properties of LCPs are sensitive to the relative designed to perform a particular self-dedicated function. order which exists between neighbouring domains and thus in Microchip manufacturers are constantly striving to increase the isotropic and non-isotropic phases of these materials the the density of individual components on a single chip and to relative strength of optical scattering is accordingly classed as achieve this it is necessary that the smallest feature size which weak or strong respectively. Transformation between the may be lithographically produced on a silicon wafer be continu- strongly and weakly scattering phases may be reversibly ously reduced.At present, feature sizes down to 0.3 mm may accomplished through the application of an aligning electric be routinely produced using standard ultraviolet (UV) photo- field,2 or by heating the material above a critical clearinglithography which involves projection printing through a point temperature and quench cooling through the glass photomask onto silicon wafers coated with photoresist.The transition temperature of the polymer.3 The electric field projection process normally uses optics to give an image alignment properties have previously been proposed for the reduction by a factor of 5 so that the smallest corresponding development of a photomask technology based upon liquid feature size recognisable on the photomask is about 1.5 mm.crystal cells4 in which the LCP is maintained in a liquid state Photomasks currently working down to a resolution of between plane-parallel aligning electrodes. By contrast, exploiabout 1 mm are conventionally produced using quartz plates tation of the clearing point temperature phenomenon for coated with chromium.The chromium layer is typically 0.1 mm photomask development involves depositing the LCP as a thick and is opaque at the standard UV wavelengths (desig- solid film onto a quartz substrate as depicted in Fig. 1(b). The nated G and I) used in projection lithography. The patterning LCP layer is then globally transformed into an optimised nonof these photomasks to produce UV transparent tracks isotropic (strong-scattering) state by suitable thermal proinvolves physical removal of the chromium however and this cessing.Non-scattering regions in the LCP film may then be is a multistage process.1 In particular, a detailed inspection of written by thermally inducing a local transition between the the photomask must always be made following the chromium non-isotropic and isotropic phases.Previous work by the GEC etch process and time-consuming repairs performed to rectify group using polyacrylate LCPs has demonstrated that clear identified defects in the chrome tracks. A schematic illustration of a chromium photomask is shown in Fig. 1(a). It should be noted that the finished product is often protected by a thin membrane layer (pellicle) before being shipped to the manufacturer. The pellicle prevents contamination of the photomask surface by dust particles and is an important consideration when it is realised that particulates lodged between the walls of the chromium tracks may be so strongly bonded as to be virtually immovable by high pressure water-jet cleaning.The most costly processing stages incurred in the production of conventional photomasks (inspection, repair and pellicle application) fundamentally arise because patterning of the photomask requires physical removal of the UV opaque chromium.A preferred method of photomask fabrication would clearly involve direct writing of UV transparent regions onto the photomask opaque. Such a one-step approach would produce a photomask structure depicted in Fig. 1(b) where in Fig. 1 Schematic illustration of the structure and operation of (a) a this case, due to the retention of a continuous smooth surface conventional chrome photomask and (b) an LCP photomask. Q= quartz substrate. on the photomask opaque layer, the provision of a pellicle is J. Mater. Chem., 1997, 7(10), 2013–2019 2013isotropic regions may be quenched into thin glassy films on 350 nm.The material should therefore be compatible with UV photolithography at the G (436 nm) and I (365 nm) exposure flexible substrates by laser addressing at room temperature.3,5 Negative contrast writing was produced on a non-isotropic wavelengths. To allow energy from the proposed HeNe laser writing wavelength of 633 nm to be eYciently coupled to scattering background with proposed applications for the polyacrylates as optical memory devices.LCP156, it was consequently necessary to add an anthroquinone based dye to the polymer. The spectral response of the The present photomask work is also concerned with the production of clear regions on a scattering background by anthroquinone dye ( hereafter referred to as AQD) was measured at a 2.5% concentration in an isotropic host liquid and scanning the focused output from a laser across thin LCP films.A cyanobiphenyl based polymethacrylate LCP coated found to exhibit a maximum absorption at approximately 638 nm with an extinction coeYcient of 44 l g cm-1. The added onto quartz substrates is used as the photomask opaque.The exposure requirements for the non-isotropic�isotropic writing concentration of AQD to LCP156 was nominally 2% by mass which maximised the laser absorption eYciency whilst ensuring process are experimentally determined, and the results used to evaluate whether LCPs provide a realistic commercial alterna- that the dye did not crystallise out of the LCP. tive to chromium for the fabrication of UV photomasks.Preparation of LCP solutions and spin-coating Experimental Solutions of LCP156 suitable for spin-coating were prepared using cyclopentanone (C5H8O) as a solvent. Appropriate solu- Materials tion viscosities were obtained by adding accurately weighed amounts of the LCP156+AQD to measured volumes of The liquid crystal polymer identified as promising for initial C5H8O where the ratio of LCP156+AQD to solvent was investigation was a side chain cyanobiphenyl polymethacrylate varied between 5 and 60% by mass.In order to ensure coded LCP156 whose chemical structure is shown in Fig. 2. complete dissolution of the polymer in the solvent, prepared LCP156 possesses an average molecular mass Mn=38 300 and solutions were generally allowed to stand for a period of 4–5 d is characterised by a glass transition temperature Tg of 66 °C.in sealed glass vials. This solution foation period was In the non-isotropic liquid crystal phase, LCP156 has a high particularly critical for more viscous concentrations where the birefringence with the cyanobiphenyl side chains aligning in mass ratio of polymer to solvent exceeded 30%.Immediately smectic A domains. The optically scattering liquid crystal prior to spin-coating, the LCP solutions were routinely filtered phase becomes an isotropic liquid at a clearing point temperausing glass syringes fitted with 0.2 mm PTFE filters. This final ture Tc of 101 °C. By thermally processing LCP156 from filtering process was performed to remove contaminating AQD temperatures above Tc to below Tg, the material becomes a particulates which were observed to form in the C5H8O as a glassy solid where the side chains are aligned in either the result of slow agglomeration of the dye molecules. As a result isotropic or non-isotropic states depending upon the rate of of filtering, the amount of AQD to LCP156 which remained cooling through the liquid crystal phase.Slow cooling allows in solution was somewhat reduced below the nominal 2% by the pendant cyanobiphenyl groups to align in smectic A mass concentration. domains, whereas quench cooling freezes-in the isotropic Thin films of LCP156+AQD were solvent cast from the liquid state. above solutions using a standard spin-coating unit operating The optical transmission spectrum of LCP156 reveals that at speeds between 500 and 5000 rpm.Simple two-stage spinning the cut-oV point for UV absorption in the polymer occurs at programs involving a pre-spreading step followed by a coating about 360 nm, with absorption becoming very strong below step were found to produce uniform films on two-inch square quartz substrates. A surface profilometer (model Dektak3 ST) was used to determine the thicknesses L c of the dried LCP layers and to evaluate the associated surface roughness of the deposited films.For the range of C5H8O solution viscosities prepared, it was found that films having thickness varying between 0.5 and 10.5 mm could be reproducibly coated where the corresponding surface roughness of the layers was typically less than 2% of L c.Thermal processing of LCP films The LCP films cast onto quartz substrates must be thermally processed in order to transform the layers into light scattering textures (i.e. to place the LCP into a non-isotropic state). The required thermal profile to achieve this is outlined in Table 1 and involves three basic stages; stage A: a heating step during which the specimen is raised above the clearing point temperature Tc; stage B: an annealing interval of duration tanneal during which time the sample is held at a temperature TA>Tc; stage C: a cooling period where the sample temperature is reduced to below the glass transition temperature Tg at a controlled cooling rate (dT /dt)c.To implement the above thermal recipe, an automated processing system was constructed which consisted of a custom-built hotplate linked to a HOTO TM55 temperature control unit. The hotplate was constructed using etched foil heaters to ensure a temperature accuracy of ±1 °C relative to the setpoint, and was fitted with under-plate nitrogen gas cooling at a working pressure of 30 psi to permit cooling rates approaching 5 K min-1 to be readily achieved.The controller Fig. 2 Chemical structures of the LCPs used in the present work unit was equipped with a serial interface to allow remote 2014 J. Mater. Chem., 1997, 7(10), 2013–2019Table 1 Thermal processing procdure used to transform LCP films across the LCP surface, a number of experiments were conducinto a non-isotropic state. TA annealing temperture; Tg glass transition ted in which the photomask was held stationary and exposed temperature to the incident beam for a finite time tp.A mechanical shutter was used to chop the continuous output from the laser and in stage thermal process temperature range t/min this manner light pulses down to tp~0.2 ms were produced. It A heat to TA room temp.�TA 20 is straightforward to show that under pulsed conditions, the B anneal TA tanneal exposed area of LCP film receives an amount of energy from C cool through Tg TA�(Tg–20) [TA–Tg+20]/(dT /dt)c the incident beam which would be obtained by continuous scanning at a speed given by eqn.(1) control via a computer, and software was developed to handle vs=do/tp (1) stages A–C in Table 1. where do is the diameter of the focused beam waist. For do= To determine the optimum cooling rate required to induce 6 mm, the eVective scan speed which can be experimentally maximum scattering for LCP156, a number of identically realised is therefore extended to 30 cm s-1 by pulsed exposure.prepared films were thermally processed into an optical scattering state keeping TA (=130 °C) and tanneal (=15 min) constant, but using diVerent values for (dT /dt)c.Optical Results and Discussion transmission experiments conducted on these samples revealed Measurement of written linewidths that the scattering power of LCP156 is relatively insensitive to the cooling rate but appears to be maximised in the vicinity Visual inspection of isotropic features written onto the LCP of (dT /dt)c=-1 K min-1. This was therefore adopted as the photomask was conducted using a microscope equipped with standard cooling rate for thermal processing of LCP156 into a CCD camera and a calibrated on-screen video measurement an optimised non-isotropic state. unit which allowed linewidths (2rl) and spot diameters (2rd) The integrity of the LCP layers following thermal processing to be directly measured.Examples of lines written onto a was evaluated by comparing the surface profiles of films in the 5.4 mm LCP156 film are shown for scanning speeds of isotropic and non-isotropic phases.For a single thermal process 1.2 cm s-1 in Fig. 3(a), and 3.0 cm s-1 in Fig. 3(b). Small dots stage, the surface roughness of the layers was observed to written onto the same film under pulsed conditions are also remain essentially unchanged from the as-cast magnitude, shown where the exposure times have been selected to give though repeated thermal cycling was found to progressively the same eVective scan speeds according to eqn.(1). It is increase the surface roughness by a factor of ca. 3 after the completion of 8–10 cycles. Thermal processing of the films into the non-isotropic phase was additionally found to reduce the average film thickness to about 90–95% of L c.Shrinkage of the films was only noticeable following the first thermal process cycle however and is therefore believed to be associated with a reduction in free volume of the LCP following internal reorientation of the polymer matrix at elevated temperatures. Overall, the changes observed for both the thickness and surface roughness of the processed films are relatively small, and these phenomena are not considered to represent a major drawback to the use of LCP156 as a photomask opaque material.Laser writing on LCP films The laser selected to perform writing tests on the LCP photomasks was a commercially available HeNe unit operating in a continuous wave mode at 633 nm with a nominal power output of 13 mW.The quoted output beam parameters were a beam waist of 800 mm, a beam divergence of 1 mrad and a Rayleigh distance of 80 cm. In order to successfully write clear isotropic regions on the photomask, the amount of energy absorbed from the beam must be suYcient to locally raise the temperature of the LCP above Tc. To achieve this a singlet lens having a focal distance of 0.7 cm was used to reduce the beam waist to 6 mm.By modifying the output beam with the focusing lens, the transformed beam divergence was estimated from simple gaussian optics to increase to 133 mrad and the corresponding focal depth to be reduced to 45 mm.6 The laser and auxiliary deflecting mirrors were consequently mounted along the (x-) axis of an optical guide, with the samples to be irradiated being attached in a horizontal plane to the arm of an x–y chart recorder under the focusing lens.The chart recorder was fitted with a manual y-adjustable platform, and modulation of the x-input of unit with a ramp drive allowed the photomasks to be scanned beneath the singlet lens at accurately controlled speeds vs between 0.01 and 3 cm s-1.Due to the relatively small depth of focus, the singlet lens holder was Fig. 3 Examples of isotropic features written onto a 5.4 mm thick connected to a micro-positioner arm to provide sensitive LCP156 photomask using a 13 mW HeNe laser: (a) line scan speed control over the lens to film distance in the z-direction. vs=1.2 cm s-1, pulsed exposure time tp=0.5 ms; (b) line scan speed vs=3.0 cm s-1, pulsed exposure time tp=0.2 ms In addition to continuous scanning of the focused beam J.Mater. Chem., 1997, 7(10), 2013–2019 2015evident that for equivalent exposure times the measured spot expression found to provide an acceptable regression fit was logarithmic in nature such that eqn. (2) holds. diameters closely agree with the corresponding linewidths (i.e. 2rd=2rl). The majority of work conducted on determining the (2rd)=a+b×loge (L c) (2) dependence of written isotropic feature dimensions upon laser exposure conditions was consequently carried out using pulsed The required values of the adjustable fitting parameters a and b were 7.2 and 11.9 mm, respectively. Whilst it is appreciated excitation due to the greater range of scan speeds accessible, and the potential to statistically determine average values for that such an expression can only provide an empirical fit to the data and has no physical basis [2rd does not saturate with 2rd from the measured diameters of a large number (ca. 20) of identically written dots. L c according to eqn. (2)] it may usefully be employed in the low L c region to determine L c*.Thus, setting 2rd=0 in eqn. (2), the extrapolated value of L c* using the a and b fitting param- Dependence of 2rd on exposure conditions eters is found to be 0.55±0.17 mm. For this threshold thickness, EVect of varying the LCP film thickness. A series of the fraction of incident light absorbed is only 20% and Pabs* spot writing experiments were conducted on samples of is correspondingly calculated to be 2.60±0.65 mW.LCP156+AQD in which the as-cast film thickness L c was varied between 1.65 and 10.0 mm. The incident laser power EVect of varying the incident HeNe irradiation power. An was held fixed at 13 mW in all cases and exposure times tp independent check of the Pabs* magnitude was made by ranged from 0.2 to 20 ms. The results of these tests are directly measuring the threshold HeNe power required to write summarised in Fig. 4 where the measured spot diameters 2rd an isotropic spot feature. Variation of the incident HeNe power are plotted as a function of L c. The following features of the between 0.5 and 13 mW was achieved by attenuating the data are noted: (i) As L c increases 2rd displays a tendency to output beam through the use of neutral density filters.Pulsed saturate. (ii ) Below a minimum threshold thickness it is imposswriting tests with 0.2<tp<5 ms were performed on LCP156 ible to write on the films. films 9.4 mm thick for which the amount of absorbed light Both of the above observations may be interpreted with energy was estimated to be 98%. A typical set of results is reference to the amount of energy absorbed from the HeNe shown in Fig. 5 which displays the measured spot diameters beam. For suYciently large L c, the amount of absorbed energy 2rd as a function of the incident HeNe power. The important is maximised and depends upon the amount and optical observations to be noted from these data are as follows. (i) For response of the incorporated dye. For AQD at an added mass a chosen tp, the spot diameters become larger as the incident concentration approaching 2%, it is estimated that about 98% power is increased.This presumably reflects the greater increase of the incident light is absorbed when L c=10 mm, and the in maximum local temperature induced for higher incident saturated spot size is dictated by the maximum temperature powers and parallels the increase in spot diameter found with rise locally induced by the corresponding absorbed energy.increasing L c in Fig. 4. (ii ) The rate of increase of 2rd with Conversely, when L c falls below some critical limit L c* the incident power is dependent upon tp, with a faster increase amount of absorbed energy is insuYcient to cause the local accompanying longer exposure times.Once again this is contemperature rise to exceed Tc and the LCP remains in a nonnected with the comparative maximum temperature rises isotropic scattering state. It should be noted that, irrespective expected locally. (iii ) Below a minimum threshold power of of how long one makes tp, no spot features may be written 2.6<3.9 mW, it is impossible to write spot features onto the when L c<L c*.This phenomenon is simply a manifestation of films irrespective of the length of illumination time tp. This the fact that for suYciently long times, the temperature at any corresponds to the fact that at these powers, Pabs>Pabs* is location within the film attains a local maximum value repnot being fulfilled. resentative of some global equilibrium state. The equilibrium From (iii ) above, the threshold power is therefore directly is dynamic in nature such that the rate of energy input Pabs is measured to lie between 2.6 and 3.9 mW taking into account exactly balanced by a rate of energy loss for any selected the small correction factor of 0.98 due to incomplete abvolume element within the LCP film.Consequently, to ensure sorption. Pabs* is accordingly estimated as Pabs*= that the temperature exceeds Tc at some point within the film, [(2.6+3.9)/2]×0.98=3.20±0.65 mW.This value is to be com- Pabs must exceed a threshold magnitude Pabs*, and since (for pared with the figure of 2.60±0.65 mW deduced from Fig. 4. a fixed incident HeNe power) Pabs3L c we require that L c>L c*. To estimate the critical thickness L c* from Fig. 4, regression Dependence of written spot size on illumination time tp. In curves shown as solid curves were fitted to the data points for order to successfully write onto LCP156 photomasks it is the various exposure times used. The simplest form of analytical necessary to ensure that not only does the absorbed power exceed Pabs*, but also that the exposure time tp is suYciently Fig. 4 Dependence of measured spot diameters 2rd upon film thickness Fig. 5 Dependence of measured spot diameters 2rd upon incident laser L c for LCP156+AQD films. The incident laser power was 13 mW and the exposure times tp were respectively (1) 0.2, (&) 1.0, (%) 5.0 power for LCP156+AQD films. The film thickness was 9.4 mm and the exposure times tp were (&) 5.0, (%) 1.0 and (×) 0.2 ms.and (2) 20.0 ms. The saturated data ($) corresponded to tp=10 s. 2016 J. Mater. Chem., 1997, 7(10), 2013–2019Fig. 7 Dependence of measured spot diameters 2rd upon illumination Fig. 6 Dependence of measured spot diameters 2rd upon illumination time tp for a 10.0 mm thick LCP156+AQD film. The incident HeNe time tp for LCP156+AQD films. The film thicknesses were (&) 9.4, powers were (&) 13.0, (%) 10.4 and (×) 6.5 mW.(%) 7.5 and (×) 4.5 mm. longer than some as yet undetermined onset time tp*. The LCP film and the failure to attain local temperatures greater requirement that tptp* is obvious from the data already than Tc. Alternatively, tp* may represent the minimum time presented in Fig. 4 and Fig. 5 where spot diameters 2rd become required for key molecular relaxation processes to occur in the progressively smaller as the exposure time is reduced.The transformation between the non-isotropic and isotropic phases question which arises is what is the functional dependence of of the LCP. 2rd upon tp and does this imply that written spot features will vanish before tp is reduced to zero? Estimation of the exposure requirement of LCP156+AQD Data illustrating an apparent logarithmic variation of 2rd under HeNe illumination with tp are presented in Fig. 6. These results were obtained on From the data analysed above we are now in a position to films of LCP156 having diVerent thicknesses which were estimate the minimum exposure requirements of LCP156. We subjected to an incident HeNe power of 13 mW.The fitted have seen that the threshold input power is Pabs*#2.9 mW, regression curves assume the form eqn. (3), and that this rate of energy input must be maintained for a (2rd)=a+b×loge (tp) (3) minimum exposure period of tp*=0.025 ms. The resulting energy threshold E* is accordingly given by eqn. (4), where a and b are fitting parameters. It was found that eqn. (3) provided an excellent fit to the data in the time regime where E*=Pabs*×tp* (4) tp10 ms and the regression parameters are summarised in so that E*=2.9 mW×2.5×10-5 s=73 nJ.This energy is Table 2. eVectively delivered over an area of the film surface bounded For longer tp, the spot diameters display a tendency to within the focused HeNe beam waist. Since the calculated saturate and eqn.(3) is not applicable. This is confirmed by beam waist is 6 mm, the corresponding exposure area is the columns in Table 2 comparing measured diameters at tp= 28.3 mm2 and the exposure sensitivity of LCP156 is therefore 10 s with extrapolated values from the fitted curves. It is 73 nJ/28.3 mm2=258 mJ cm-2. This estimate is consistent with recognised that the functional format of eqn. (3) has no the typical order of magnitude figure of >100 mJ cm2 quoted physical foundation and is purely empirical.The following for this class of LCP material.2 points are noted from Fig. 6. (i ) The rate of increase of 2rd with tp is dependent upon the film thickness L c. This of course Approaches towards reducing the exposure requirements of simply reflects the fact that Pabs is greater for thicker films.LCP photomasks (ii ) Extrapolation of the regression curves to 2rd=0 suggests that tp*~0.025 ms. The onset time appears to be independent The exposure sensitivity of LCP156 calculated above is in of L c and hence Pabs. practice an upper limit since it has been implicitly assumed To verify that tp* is indeed independent of Pabs, a second that all of the absorbed optical energy is converted into heat set of exposure time experiments were conducted on a 10.0 mm within the LCP alone.This situation is clearly unrealistic as film in which the incident HeNe power was attenuated directly. heat loss to the surroundings is anticipated to be significant. Results obtained for 13, 10.4 and 6.5 mW are shown in Fig. 7 Potential energy sinks include radiative and convective proand once again extrapolate in all cases to tp*~0.025 ms. cesses at the LCP surface, but the most important channel for The apparent independence of the threshold exposure time heat dissipation from the LCP layer is suspected to involve tp* upon Pabs is interesting as it implies that the use of direct heat conduction into the substrate.Possible approaches progressively higher incident powers will not permit successful to improve the exposure sensitivity of the LCP photomasks exposure to occur for correspondingly shorter times.Such a are now discussed. limitation may be connected with the flow of heat within the Substrate selection. An obvious approach to reduce heat Table 2 Summary of regression parameters used to fit the data of flow into the substrate is to employ a substrate material Fig. 6 using eqn. (3). tp* is the extrapolated onset exposure time below possessing a relatively poor thermal conductance. The quartz which spot features cannot be written. (i ) Extrapolated spot diameter substrates presently employed have a thermal conductance Kv at tp=10 s from regression parameters; (ii ) measured spot diameter at of approximately 10-3 W cm-1 K-1.7 In order to achieve a tp=10 s noticeable improvement in thermal insulation between the L c/mm a b tp*/ms 2rd(i)/mm 2rd(ii)/mm LCP and substrate, it is predicted from thermal simulation data8 that the thermal conductivity of the substrate [Kv(sub)] 4.5 20.2 5.3 0.023 69.0 39.9±1.6 will need to be an order of magnitude lower than the ther- 7.5 25.8 7.1 0.026 91.2 57.4±1.7 mal conductivity of the LCP which is estimated as 9.4 29.0 7.8 0.025 100.8 66.8±5.9 Kv(LCP156)~10-3 W cm-1 K-1.2 However, a survey of J.Mater. Chem., 1997, 7(10), 2013–2019 2017potential substrate glasses reveals that few possess Kv values LCP exposure requirements is to reduce the clearing point transition temperature Tc. This may be accomplished through significantly smaller than quartz.Irrespective of whether a suitable low Kv glass does exist, the substrate material must chemical modification of the underlying polymer structure. In order to function reliably in a photomask projection environ- primarily possess the necessary optical properties to function in a photomask environment.† For this latter reason, it is ment however, Tc is eVectively constrained to exceed a minimum value for the following reasons.(i ) Tg should always be diYcult to envisage that the microelectronics industry would consider replacing quartz as the photomask substrate and an higher than the maximum operating temperature of the LCP photomask which under projection printing conditions may alternative approach towards improving the exposure sensitivity of the LCP photomasks must be proposed.reach Tproj~40 °C. In order to form a scattering texture and maintain image integrity, Tc must exceed Tg by a margin of about 30 °C however so that a practical minimum limit for Tc Thermal buVering between the LCP layer and substrate. A possible solution to reduce conductive heat loss from the LCP is about 70 °C.(ii ) Reducing the clearing point temperature of the LCP is often accompanied by a decrease of the associated is to insert a thermally insulating buVer layer of thickness L b between the LCP layer and substrate. The buVer material glass transition temperature Tg. To ensure that the mechanical properties of the LCP film remain robust, Tg should at least selected must be optically inert at the standard G and I UV wavelengths employed in projection printing and for this exceed Tproj so that Tg (minimum)~40 °C.With the above criteria in mind, a new LCP (coded LCP95 reason should be kept as thin as possible (for example L b2 mm which would represent some 20–25% of a typical and shown in Fig. 2) having Tc=85, Tg=49 °C and Mn=5900 was evaluated as a photomask opaque.The clearing tempera- LCP thickness). The requirement that the buVer layer remains thin places a greater demand on the eYciency of thermal ture of LCP95 is 16 °C lower than that of LCP156 and should consequently show an enhanced response (i.e. larger spot insulation provided by the selected material and simulation studies suggest that for L b2 mm, Kv( lcp)/Kv(buVer) should diameters) under comparable exposure conditions (i.e.incident laser powers and exposure times). Data apparently consistent be greater than 10 in order to be eVective.8 Initial experimental work using thermal buVers has focused with these predictions are shown in Fig. 9 where a direct comparison is made between spot diameters for 3.3 mm thick upon an isotropic polymer (coded LCP96) whose chemical structure is illustrated in Fig. 2. This material has a molecular films of LCP95 and LCP156. Significantly, it is noted from the extrapolated regression curves that the threshold exposure mass of 23 000 and a glass transition temperature of 84 °C. The thermal conductivity of LCP96 is believed to be compar- time tp* is reduced by almost a factor of 2 for LCP95.The minimum incident power Pabs* to write onto LCP95 films was able to that of LCP156.2 Films of LCP96 ranging between 0.8 and 2.0 mm were deposited onto quartz substrates by spin- estimated to lie between 1.9 and 2.9 mW so that the overall exposure E* for the new polymer is calculated to be coating from cyclopentanone solutions. Following a suitable drying period, the buVer layers were then thermally processed E*~33±7 nJ.The corresponding sensitivity of LCP95 is therefore 117±25 mJ cm-2 which is a factor of 2–3 lower than using standard conditions to remove residual solvent and allow for possible film shrinkage. Identical layers of LCP156+AQD LCP156. Reduction of the clearing point transition temperature by comparatively modest amounts would thus appear to 10.4 mm thick were finally spin-coated onto these pre-coated substrates, with the entire structures being thermally processed result in significant improvements in LCP sensitivity. for a second time to place the LCP156 into a scattering state.The results of pulsed experiments using diVerent LCP96 Response of LCP materials to industrial laser writing systems thicknesses are compared in Fig. 8. The incorporation of the It is important to establish whether the current LCP materials LCP96 layer is found to have a negligible eVect upon the will respond positively to the writing conditions prevalent in measured spot diameters. EVective thermal buVering between direct-write-lasers (DWLs) commonly employed by industrial LCP156 and the substrate would have resulted in enhanced mask makers.With the results presented above, we are now lateral heat flow and larger values of 2rd for a given exposure in a position to predict whether exposure of the LCP phototime td. The thermal conductivity of LCP96 is consequently mask will be successful, based upon the nominal specification too high to provide eVective insulation for L b2 mm. of the laser writing equipment.The DWL systems of primary importance within the mask-making industry are the Core Reduction of LCP transition temperature through chemical 2564 system marketed by ETEC INC., and the HeCd and modification. The most direct approach towards reducing the Argon DWLs marketed by Heidelberg Instruments. The principal operating characteristics for each of these systems is † Quartz displays virtually no absorption at the UV wavelengths summarised in Table 3 where information has been drawn currently employed in projection printing.Fig. 9 Comparison of measured spot diameters 2rd for polymers Fig. 8 Comparison of measured spot diameters 2rd for 10.4 mm thick LCP156+AQD films deposited onto a thermal buVer layer (LCP96). LCP156 and LCP95 containing nominally equal amounts of AQD: (#) LCP156 and ($) LCP95.The film thickness was 3.3 mm in The thickness of the buVer layers were (%) 0, ($) 0.8, (×) 1.1 and (2) 2.0 mm. both cases. 2018 J. Mater. Chem., 1997, 7(10), 2013–2019Table 3 Operating conditions for some industrial DWLs. The laser used in the present work (HeNe) is included for reference beam scan exposure energy DWL wavelength power waist speed time density E system /nm /mW /mm 4P/pdo2 /cm s-1 t/s /mJ cm-2 HeNe 633 13 6.0 5e7 <30 >1×10-4 5520 Core 2564 364 3 1.0 4e8 1250 8×10-8 30 Heidelberg HeCd 442 10–20 0.8 3e9 4000 2×10-8 40–80 Heidelberg Ar 514 1000 0.8 2e11 4000 2×10-8 4000 primarily from the respective supplier’s documentation.9 The to be eVective using relatively thin (<2 mm) layers it is predicted that the insulating material should posses a thermal conduc- exposure conditions employed for the current HeNe setup have also been included for reference.tivity of less than a tenth of the overlying LCP. (iii ) The exposure sensitivity may be improved by chemically For LCP156 it has been established that for successful film exposure to occur the following three conditions must be modifying the LCP to reduce the clearing point temperature Tc.Pulsed exposure experiments performed upon LCP95 for simultaneously fulfilled. (i ) The absorbed energy density E must exceed the critical threshold E* for the photomask which Tc~85 °C indicate that E*~117 mJ cm-2. (iv) It has been established that in addition to the requirement material.For LCP156, the sensitivity E* was determined to be 258 mJ cm-2. (ii ) In addition to (i ), 4P/pdo2 for the selected that the sensitivity threshold E* be exceeded, successful exposure of LCP156 films will only occur if the absorbed laser addressing laser must exceed 4Pabs*/pdo2 where Pabs* is the critical threshold power absorbed by the film for a focused power and exposure times are greater than critical magnitudes Pabs* and tp* respectively.For LCP156 films on quartz sub- spot diameter do. For LCP156 Pabs*#3.2 mW and so 4P/pdo2 must exceed 1.1×107 mW cm-2. (iii ) In addition to (i ) and strates, Pabs*~1.1×107 mW cm-2 and tp*~2.5×10-5 s. Pabs* is believed to be governed by heat loss from the LCP and (ii ), the exposure time for the selected addressing laser must exceed tp* where tp* is the critical minimum exposure time.should therefore be reduced by thermal buVering. The physical origin of tp* is less certain but may also be connected with the For the present HeNe setup, it has been established that tp*= 2.5×10-5 s for LCP156. dynamics of heat flow; alternatively, tp* may be indicative of a fundamental minimum time required to perform molecular Assuming that appropriate dyes may be selected to ensure almost 100% energy-coupling at the various laser wavelengths, reorientation between the isotropic and non-isotropic states.It has been shown experimentally that tp* is reduced as Tc a survey of the system specifications collected in Table 3 reveals the following. The Core and Heidelberg HeCd systems would is lowered.(v) Currently available industrial DWLs are incapable of fail to provide LCP156 with suYcient energy E to induce an exposure. The absorbed power density in both cases fulfil exposing LCP156 photomasks. For all industrial systems, unsuccessful exposure is a direct consequence of the scanning criterion (ii ) however and the diYculty lies with too short an exposure period tp such that (iii ) is not satisfied. Both of these speeds being too high and the eVective exposure times being correspondingly too short.systems require the scanning rate to be reduced by a factor of 300–1000 in order to expose LCP156 photomasks. The Heidelberg Ar system satisfies conditions (i ) and (ii ) but tp is References still a factor of about 1000 too short to allow successful 1 S. P. Murarka and M. C. Peckerar, in ElectronicMaterials—Science exposure of LCP156. A considerable reduction in scanning and T echnology, Academic Press Inc., London, 1989. speed is therefore also required if this system is to provide a 2 Side Chain L iquid Crystal Polymers, ed. C. B. McArdle, Blackie, positive response. Glasgow and London, 1989. 3 C. Bowry and P. Bonnett, Optical Computing and Processing, 1991, 1, 13. Conclusions 4 Patent references (date of filing): (a) WO,A,90 10047 (1990); (b) GB,A,2 188 748 (1986); (c) GB,A,2 217 862 (1989); (d) US,A,4 The main points arising from the present work may be 013 466 (1975). summarised as follows. 5 C. Bowry, P. Bonnet and M. Clark, Proceedings Eurodisplay 90, (i ) Pulsed experiments conducted using a HeNe laser have Amsterdam, p. 158. allowed the exposure sensitivity E* of LCP156+AQD films 6 D. O’Shea, Elements of Modern Optical Design, John Wiley and to be estimated. For layers coated onto quartz substrates, Sons Ltd., Chichester, 1985. 7 Handbook of Chemistry and Physics, CRC, 49th edn., section E-8. E*~258 mJ cm-2. This figure represents an upper limit to 8 D. M. Goldie, simulation data, 1995 (unpublished). the underlying sensitivity of the LCP as no corrections have 9 (a) DWL technical specification, Heidelberg Instruments, been made for heat loss to the substrate or surrounding air. Mikrotechnik, Tullastrasse 2, 69126, Heidelberg; (b) Core 2564 (ii ) Attempts to reduce E* by incorporating a thermal buVer Technical Overview, ETEC Systems Inc., 9100 SW Gemini Drive, layer (LCP96) between the LCP and substrate have proved Beaverton, OR 970015, 1993. unsuccessful. The failure of LCP96 has been attributed to its comparatively high thermal conductivity. For thermal buVering Paper 7/02841E; Received 25th April, 1997 J. Mater. Chem., 1997, 7(10), 2013–2019 2019

ISSN:0959-9428    

DOI:10.1039/a702841e

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Hydrogen bonded lambda-shaped packing motif based on 4-nitrophenylhydrazones: a promising design tool for engineering acentric crystals Man Shing Wong,a Volker Gramlich,b Christian Bosshard*a and Peter Gu�ntera aNonlinear Optics L aboratory, Institute of Quantum Electronics, ET H-Ho�nggerberg, CH-8093, Zu�rich, Switzerland bL ab. fu� r Kristallographie, ET H-Zentrum, CH-8092, Zu�rich, Switzerland Two newly developed 4-nitrophenylhydrazone derivatives, 4-diethylaminobenzaldehyde 4-nitrophenylhydrazone, DEANPH, and 5-bromothiophene-2-carbaldehyde 4-nitrophenylhydrazone, BTNPH, which exhibit very strong second harmonic signals that are three orders of magnitude greater than that of the urea standard were investigated by single crystal X-ray analysis.The structural comparison with three other strongly optically nonlinear hydrazones revealed that the hydrogen bond-directed lambda-shaped supramolecular assemblies are the key and fundamental bases of all these non-centrosymmetric crystal packings.The high occurrence within this class of acentric crystals is likely due to this hydrogen bonded lambda-shaped packing motif. Since the chromophores are preferentially arranged in a lambda-shape, the acentric crystals are generally optimized for second-order nonlinear optical eVects such as second harmonic generation. The rational design and synthesis of novel crystalline materials used non-covalent interactions to assemble supramolecular structures.A great variety of one- and two-dimensional with desirable physical properties such as electrical, optical and magnetic properties, known as crystal engineering,1,2 has supramolecular structures such as wires, chains, tapes, ribbons, layers and rosettes, assembled by multiple hydrogen bonds or attracted considerable attention over the last decades.The specific solid state properties often depend on the relative hydrogen bond networks, have been designed and synthesized arrangement and orientation of the constituent functional to induce a desirable crystal packing.4,5,10,11 molecules, that is the crystal structures.Unfortunately, packing In the course of searching for useful and eYcient crystalline arrangements of three-dimensional crystals at the molecular materials based on the non-rod-shaped hydrazone skeleton, level are diYcult to predict and control because of the interplay we have found that donor-substituted (hetero)aromatic aldeof various thermodynamic and kinetic contributions during hyde 4-nitrophenylhydrazones (Fig. 1) show an unusually high the process of nucleation. In spite of such problems, important tendency to pack non-centrosymmetrically.12–14 Of the 28 4- progress has recently been made in improving the predictability nitrophenylhydrazone derivatives that have already been synof crystal packing by means of limiting the degrees of freedom thesized in our laboratory, 72% are SHG active in the Kurtz of the constitutent molecules during the assembly of the three- and Perry powder test which implies a non-centrosymmetric dimensional crystal structures.One of the widely exploited crystal packing.Furthermore, the majority of these acentric crystal engineering approaches is to design and synthesize crystals (43%) exhibit very strong SHG signals that are at reliable one- or two-dimensional predefined and self-organized least two orders of magnitudes greater than that of the urea aggregates, supramolecular assemblies, to limit the number of standard which implies a favourable chromophoric orientation possible packings and to impose a bias to the three-dimensional for an eYcient second harmonic generation.As a result, it crystal structures. Therefore, the identification and develop- would be valuable to probe and understand the packing motif ment of useful supramolecular synthons,3 robust aggregates4 of such an overwhelming bias in this class to turn this into a and packing motifs5 are important steps towards a successful useful and general design tool for crystal engineering.design of functional crystalline solids. We report here on the molecular properties, calculated by One of the important issues to be addressed in designing the AM1 semi-empirical method15 and the structural properties useful and eYcient crystalline materials for second-order non- of two newly developed hydrazones, 4-diethylaminobenzallinear optics is how to induce a favourable acentric arrange- dehyde 4-nitrophenylhydrazone, DEANPH, and 5-bromoment of the highly extended p-conjugated chromophores such thiophene-2-carbaldehyde 4-nitrophenylhydrazone, BTNPH, as donor–acceptor disubstituted stilbenes and tolanes in the which exhibit extremely strong SHG signals that are three solid state so that the intrinsic nonlinearities of the chromo- orders of magnitude greater than that of the urea standard.In phores are maximized in the bulk. For example, in second addition, the structural comparisons of these new hydrazones harmonic generation (SHG), the optimal angle between the with three of the most optically nonlinear, acentric charge transfer axis of a dipolar chromophore and the polar direction of a crystal for non-critical phase matching is ca. 55° for the most favourable point groups.6 Unfortunately, in addition to the diYculty in deducing the crystal packing from the molecular structure, achiral organic molecules show an extremely high preference for a centrosymmetric packing (90%) according to a statistical survey.7 However, there are possible strategies that have been used to enhance the probability of forming acentric crystals which include the incorporation of molecular chirality, the introduction of structural asymmetry, the use of coulombic interaction8 and the use of hydrogen bonding interaction.9 Hydrogen bonding, owing to its relatively strong bonding Fig. 1 Chemical structures of 4-nitrophenylhydrazone derivatives strength, directionality and flexibility, is one of the most often J. Mater. Chem., 1997, 7(10), 2021–2026 2021Table 1 Molecular and structural data of 4-nitrophenylhydrazone derivatives. The molecular conformation is derived from the X-ray crystal data and the dipole moment is calculated by the AM1 semi-empirical calculation.Torsional angle refers to the twist angle between the two (hetero)aromatic rings attached on the hydrazone backone. hp refers to the angles between the dipole moment of a molecule and the resultant polar direction of the corresponding crystal dipole moment/ space point distance/A° (angle/°) distance of torsional molecular conformation and dipole orientation 10-29 C m group group of [MO,HMNM] [MO,HM] angle/° hp/° DEANPH 3.57 Pca21 mm2 3.13 (162.9) 2.25 9.1 75 DANPH 3.41 Cc m 3.12 (147.4) 2.24 8.3 50 ACNPH 3.77 Pca21 mm2 3.17 (156.4) 2.32 11.8 73 BTNPH 2.50 Cc m 3.04 (159.8) 2.18 15.5 73 MTTNPH 2.54 Pca21 mm2 2.99 (162.9) 2.00 3.9 55 2022 J.Mater. Chem., 1997, 7(10), 2021–2026Table 2 Crystal data for DEANPH and BTNPH The melting points were determined by the diVerential scanning calorimetric method using a 5° min-1 heating rate.DEANPH BTNPH The ground state molecular properties including molecular geometries and dipole moments were calculated by the AM1 empirical formula C17H20N4O2 C11H8BrN3O2S semi-empiricial method15 in the MOPAC6 quantum chemical molecular mass 312.4 326.2 colour and habit red fragment red needle calculation package.crystal dimensions/mm3 0.2×0.3×0.4 0.1×0.15×0.35 Data collections for X-ray structure determination were crystal system orthorhombic monoclinic performed with a Picker-Stoe diVractometer (Cu-Ka, l= space group Pca21 Cc 1.54178 A ° ) at 293 K. The reflections were measured within point group mm2 m 3°<2h<100° with F>4.0s (F). The crystallographic data are a/A ° 30.18(2) 4.290(2) summarized in Table 2.Both structures were solved by direct b/A ° 7.320(4) 25.480(13) c/A ° 7.603(4) 11.832(6) methods and all heavy atoms were refined by full-matrix leastb( °) 98.12(4) squares procedures. All H atoms were located according to v/A° 3 1679.5(16) 1280.4(11) the riding model with fixed isotropic U. The bond lengths and Z 4 4 bond angles for both DEANPH are in Tables 3 density/g cm-3 1.235 1.692 and 4.† reflections collected 1075 685 independent reflections 955 664 observed reflections 833 655 DEANPH F(000) 664 648 dH (300 MHz; CDCl3; J values in Hz) 8.16 (d, J 9.27, 2H), 7.80 parameters refined 211 170 (s, 1H), 7.70 (s, 1H), 7.53 (d, J 8.94, 2H), 7.05 (d, J 9.21, 2H), R; Rw (%) 5.72; 7.82 5.10; 6.63 6.67 (d, J 9.00, 2H), 3.41 (q, J 7.08, 4H), 1.20 (t, J 7.08, 6H).Mp 190 °C. Table 3 Bond lengths and angles for DEANPH BTNPH bond length/A ° bonds angle/° dH (300 MHz, CDCl3) 8.18 (d, J 9.4, 2H), 7.93 (s, 1H), 7.84 (s, N(2)MN(3) 1.378(7) N(3)MN(2)MC(1) 112.6(5) 1H), 7.07 (d, J 9.2, 2H), 7.00 (d, J 3.87, 1H), 6.92 (d, J 3.87, N(2)MC(1) 1.277(9) C(7)MC(6)MC(5) 117.0(7) 1H).Mp 189°C. C(6)MC(7) 1.408(12) N(2)MN(3)MC(4) 118.5(5) C(6)MC(5) 1.394(10) N(3)MC(4)MC(5) 122.5(6) N(3)MC(4) 1.389(9) N(3)MC(4)MC(9) 116.8(6) Results and Discussion C(4)MC(5) 1.380(10) C(5)MC(4)MC(9) 120.7(6) C(4)MC(9) 1.405(9) C(6)MC(7)MC(8) 122.3(7) Unlike the stilbene derivatives, all the 4-nitrophenylhydrazone C(7)MC(8) 1.329(11) C(6)MC(7)MN(10) 119.2(7) derivatives adopt a non-rod-shaped conformation in the C(7)MN(10) 1.466(11) C(8)MC(7)MN(10) 118.6(7) crystalline state due to the bent hydrazone backbone C(1)MC(13) 1.446(8) N(2)MC(1)MC(13) 125.3(6) (MCHNNSNHM) as examined from the molecular structures N(19)MC(20) 1.478(11) C(6)MC(5)MC(4) 120.3(6) (see Table 1).The electron-withdrawing group, the 4-nitrophe- N(19)MC(16) 1.341(8) C(20)MN(19)MC(16) 124.5(7) nyl ring and the electron-donating group, the donor-substituted N(19)MC(22) 1.532(17) C(20)MN(19)MC(22) 116.7(6) C(8)MC(9) 1.370(10) C(16)MN(19)MC(22) 117.7(8) C(20)MC(21) 1.474(13) C(7)MC(8)MC(9) 121.7(6) † Atomic coordinates, thermal parameters, and bond lengths and C(16)MC(17) 1.421(10) N(19)MC(20)MC(21) 112.5(9) angles have been deposited at the Cambridge Crystallographic Data C(16)MC(15) 1.398(11) N(19)MC(16)MC(17) 122.0(7) Centre (CCDC). See Information for Authors, J.Mater. Chem., 1997, C(18)MC(13) 1.399(9) N(19)MC(16)MC(15) 122.1(7) Issue 1. Any request to the CCDC for this material should quote the C(18)MC(17) 1.365(9) C(17)MC(16)MC(15) 115.9(6) full literature citation and the reference number 1145/44. C(13)MC(14) 1.365(11) C(13)MC(18)MC(17) 121.7(6) O(11)MN(10) 1.242(11) C(1)MC(13)MC(18) 123.4(6) O(12)MN(10) 1.188(11) C(1)MC(13)MC(14) 120.1(6) Table 4 Bond lengths and angles for BTNPH C(14)MC(15) 1.364(10) C(18)MC(13)MC(14) 116.5(6) C(22)MC(23) 1.354(19) C(16)MC(17)MC(18) 121.3(7) bond length/A ° bonds angle/° C(22)MQ(1) 1.011(90) C(4)MC(9)MC(8) 118.0(6) C(23)MQ(1) 0.852(96) C(13)MC(14)MC(15) 123.4(7) BrMC(4) 1.911(12) C(1)MSMC(4) 89.5(5) C(7)MN(10)MO(11) 115.9(8) SMC(1) 1.734(9) C(12)MC(13)MC(8) 120.3(9) C(7)MN(10)MO(12) 120.8(8) SMC(4) 1.714(12) C(1)MC(2)MC(3) 111.5(10) O(11)MN(10)MO(12) 123.2(8) C(13)MC(12) 1.367(13) C(12)MC(11)MN 119.4(9) C(16)MC(15)MC(14) 121.1(8) C(13)MC(8) 1.417(13) C(12)MC(11)MC(10) 122.2(8) N(19)MC(22)MC(23) 112.1(15) C(2)MC(1) 1.365(15) NMC(11)MC(10) 118.4(8) N(19)MC(22)MQ(1) 142.3(55) C(2)MC(3) 1.379(18) C(1)MC(5)MN(6) 119.9(9) C(23)MC(22)MQ(1) 38.9(54) C(11)MC(12) 1.375(13) SMC(1)MC(2) 112.0(8) C(22)MC(23)MQ(1) 48.2(59) C(11)MN 1.488(12) SMC(1)MC(5) 121.1(7) C(22)MQ(1)MC(23) 92.9(78) C(11)MC(10) 1.375(14) C(2)MC(1)MC(5) 126.9(9) C(5)MC(1) 1.417(14) C(13)MC(12)MC(11) 118.7(9) C(5)MN(6) 1.318(13) C(13)MC(8)MC(9) 118.2(8) 4-nitrophenylhydrazone crystals, DANPH,12 ACNPH12 and C(8)MC(9) 1.391(12) C(13)MC(8)MN(7) 117.6(8) C(8)MN(7) 1.342(12) C(9)MC(8)MN(7) 124.1(9) MTTNPH13 that were reported previously will be discussed C(9)MC(10) 1.349(13) C(8)MC(9)MC(10) 121.4(9) (Table 1).C(4)MC(3) 1.326(18) BrMC(4)MS 117.6(7) NMO(2) 1.210(12) BrMC(4)MC(3) 129.7(10) NMO(2) 1.219(14) SMC(4)MC(3) 112.7(9) Experimental N(6)MN(7) 1.364(12) C(2)MC(3)MC(4) 114.4(12) The two new hydrazone derivatives, 4-diethylaminobenzal- O(2)MNMC(11) 119.4(9) O(2)MNMO(2) 123.2(9) dehyde 4-nitrophenylhydrazone, DEANPH, and 5-bromothio- C(11)MNMO(2) 117.4(9) phene-2-carbaldehyde 4-nitrophenylhydrazone, BTNPH, were C(5)MN(6)MN(7) 119.5(8) synthesized according to the previous procedure.13 All the C(11)MC(10)MC(9) 119.2(8) samples used for physical measurements including X-ray C(8)MN(7)MN(6) 119.5(8) structure determination were grown from acetonitrile.J. Mater. Chem., 1997, 7(10), 2021–2026 2023Fig. 2 The hydrogen bonded lambda-shaped molecular assemblies found in the crystalline solids of DEANPH, DANPH, ACNPH, BTNPH and MTTNPH. The dotted line represents the hydrogen bond. For clarity, the view is not along a crystal axis.hetero(aromatic) ring that are attached to the hydrazone and the amino proton, were found to be the key and common characteristics of all these acentric crystal structures (Fig. 2). backbone generally show a good co-planarity. This is an essential conformation for an eYcient delocalization of p- Although the O,N distances (>3.0 A ° ) and the O,H bond lengths (>2.0 A ° ) are relatively long, implying relatively weak electrons along the entire hydrazone skeleton and thus ensures the desirable solid-state properties such as large macroscopic hydrogen bonds, the co-operative eVect of the hydrogen bond network is believed to play an important role in stabilizing nonlinear optical properties.The optimized molecular geometries of all these hydrazone derivatives, calculated by the the assembly.In addition, these lambda-shaped molecular assemblies stack up with a diVerent extent of oVset from the AM1 semi-empirical method, are in a good agreement with those found in the crystalline state with an exception of the adjacent assemblies at a van der Waals distance (>3.5 A ° ) forming two-dimensional acentric layers as shown in Fig. 3. conformation of the MNHM part. The conformation of MNHM in the optimized geometry is pyramidal. On the other Finally, for those molecules packing in space group Pca21, i.e. DEANPH, ACNPH and MTTNPH, the two-dimensional hand, the solid-state structures show an excellent planarity of the hydrazone bridge. The calculated dipole moments are very acentric layers arrange in an anti-parallel fashion giving rise to the three-dimensional crystal structure shown in Fig. 4. large for all these 4-nitrophenylhydrazone derivatives. The dipole orientations are strongly dependent on the molecular Since the molecules do not lie parallel to the planes of a unit cell, this leads to a net dipole along the crystallographic c axis conformation and not always directed along the molecular plane as shown in Table 1.as a resultant polar axis. In contrast, the two-dimensional acentric layers of BTNPH and DANPH (space group Cc) pack As revealed from the single crystal X-ray structures, the packing modes of all the investigated 4-nitrophenylhydrazone in a parallel manner resulting in the crystal structures as shown in Fig. 5. Similar to the previous case, as the molecules do not derivatives are very similar and only fall into two noncentrosymmetric space groups: Pca21 and Cc.Impressively, lie parallel to the planes of a unit cell, the crystallographic aaxis and c-axis are polar axes of these packings, respectively. the one-dimensional, lambda-shaped molecular assemblies, which are composed of the glide plane-related molecules and The angles, hp, between the dipole moment of a molecule and the resultant polar direction of the hydrazone crystals are held together by the hydrogen bonds between the nitro oxygen 2024 J.Mater. Chem., 1997, 7(10), 2021–2026Fig. 3 The relative arrangement of the two lambda-shaped molecular assemblies of DEANPH and BTNPH in the crystalline states. The dotted line represents the hydrogen bond.For clarity, the layers in the foreground are represented by the stick model and the layers in the background are represented by the cylindrical model. Fig. 4 Crystal packing of DEANPH in a unit cell. The arrow represents the permanent dipole calculated by the AM 1 semi-empirical calculations. listed in Table 1. Interestingly, all these hydrazone derivatives show very favourable chromophoric arrangements for large nonlinear optical eVects such as SHG, especially MTTNPH, which even has the optimal chromophoric orientation for highly eYcient nonlinear optical eVects.In addition to the large molecular hyperpolarizabilities,12,13 such a highly favourable chromophoric arrangement can explain the very strong activities of these hydrazone crystals in the powder test.Conclusions In summary, the donor-substituted (hetero)aromatic aldehyde 4-nitrophenyl-hydrazone derivatives show an overwhelmingly high propensity for a non-centrosymmetric packing in spite of their large permanent dipoles. In view of the five strongly SHG active single crystal X-ray structures, the hydrogen bonded Fig. 5 Crystal packing of BTNPH in a unit cell.The arrow represents lambda-shaped molecular assemblies provide a common and the permanent dipole calculated by the AM 1 semi-empirical calculations. fundamental basis for all these non-centrosymmetric structures. J. Mater. Chem., 1997, 7(10), 2021–2026 2025Organic Molecules and Crystals, ed. D. S. Chemla and J. Zyss, Because of the preferential lambda-shaped arrangement of the Academic Press, Orlando, 1987, p. 227. chromophores, the crystals formed are especially suitable and 7 J. Jacques, A. Collet and S. H. Wilen, Enantiomers Racemates and optimized for nonlinear optical eVects. As a result, the hydrogen Resolutions, Wiley, New York, 1981. bonded lambda-shaped packing motif is a very useful design 8 S. R. Marder and J. W. Perry, Adv.Mater., 1993, 5, 804.tool for engineering acentric crystals. 9 M. C. Etter, G. M. Frankenbach and D. A. Adsmond, Mol. Cryst. L iq. Cryst., 1990, 187, 22. 10 C. B. Aakero�y and K. R. Seddon, Chem. Soc. Rev., 1993, 397. This work was supported in part by the Swiss National Science 11 M. S. Wong, F. Pan, V. Gramlich, C. Bosshard and P. Gu� nter, Foundation. We thank Rolf Spreiter for the determination of Adv.Mater., 1997, 9, 554. the angles hp. 12 C. Serbutoviez, C. Bosshard, G. Kno� pfle, P. Wyss, P. Pre�tre, P. Gu� nter, K. Schenk, E. Solari and G. Chapuis, Chem. Mater., 1995, 7, 1198. 13 M. S. Wong, U. Meier, F. Pan, C. Bosshard, V. Gramlich and References P. Gu� nter, Adv.Mater., 1996, 8, 416. 1 G. R. Desiraju, Crystal Engineering, Elsevier, Amsterdam, 1989. 14 M. S.Wong, C. Bosshard, F. Pan and P. Gu� nter, Adv.Mater., 1996, 8, 677. 2 G. M. J. Schmidt, Pure Appl. Chem., 1971, 27, 647. 15 M. J. S. Dewar, E. G. Zoebisch, E. F. Healy and J. P. Stewart, 3 G. R. Desiraju, Angew. Chem., Int. Ed. Engl., 1995, 2311. J. Am. Chem. Soc., 1985, 107, 3902. 4 V. A. Russell and M. D. Ward, Chem. Mater., 1996, 8, 1654. 5 J. C. MacDonald and G. M. Whitesides, Chem. Rev., 1994, 2383. 6 J. F. Nicoud and R. J. Twieg, in Nonlinear Optical Properties of Paper 7/02339A; Received 7th April, 1997 2026 J. Mater. Chem., 1997, 7(10), 2

ISSN:0959-9428    

DOI:10.1039/a702339a

出版商:RSC    

年代:1997

数据来源: RSC