FEATURE ARTICLE Thin film formulations of substituted phthalocyanines Michael J. Cook School of Chemical Sciences, University of East AngEia, Norwich, UK NR4 7TJ Phthalocyanines are macrocyclic compounds which possess a variety of properties of potential importance in advanced technologies. The article considers the development of phthalocyanine derivatives for deposition as thin films by the Langmuir- Blodgett method, spin-coating and self-assembly. Important aspects in this area of research are the development of good quality films which contain a high and reproducible degree of molecular order and the deposition of films constructed with different types of molecular packing. Attention is drawn to the potential of purpose-designed non-uniformly substituted amphiphilic derivatives.A number of these exhibit liquid-crystal behaviour as the bulk material and this appears also to be manifested in some of the films. The potential of examples of the films as the detecting component in gas sensors is commented upon. Phthalocyanines (Pcs) possess some remarkable properties 24 23 which render them important commercial commodities. In particular, their intense blue/green colours and stability towards heat, acids and bases have ensured their extensive use as pigments and dyes. Furthermore, examples are used as catalysts for a number of industrial processes.' However, they 7 N I 25P22also possess some intriguing electrical, photophysical and redox properties and attention is being focused increasingly 77on exploiting them within an impressive range of advanced 3ebl-M-NDtechnologies, some potential, some already realised.2 Thus 16 there is extensive research into the applications of phthalocyan- ..ines in electr~photography,~ optical data storage sy~tems,~,~ 4 NyNyN 75 gas sensing device^,^,^ photovoltaic cells,' fuel cells,' and in electrochromic displays." Phthalocyanines are also recognized as having excellent potential in the photodynamic therapy of certain types of Many of these exciting develop- ments have been reviewed in articles elsewhere. Much of the fascination of research into phthalocyanines lies in the fact that the properties of the compounds, and these include the solid-state properties of various formulations as discussed later, can be tuned through relatively straightforward chemistry or fabrication procedures.On the chemical front, there is extraordinary potential for ringing the changes by varying the atom(s) at the centre of the macrocycle and by introducing substituents onto the ring system. Some 70 elements have been inserted into the central cavity. By far the most common derivatives are those of the 1:1 type, but small monovalent ions such as the proton and Li' form 2: 1 ion:macrocycle complexes and large ions, e.g. those of the rare-earth metals, form 1 :2 complexes. The introduction of substituents on the ring system raises the number of known or feasible derivatives almost beyond bounds. Axial ligation to a five- or six-coordinate atom at the centre of the macrocycle offers one option, but particular attention has been focused on the 16 available sites on the benzenoid rings.These sites fall into two categories, the so-called peripheral (2,3,9,10,16,17,23,24) and non-peripheral (1,4,8,11,15,18,22,25) positions (Fig. 1). Substituents are intro- duced either by substitution reactions on the preformed macro- cycle or, more commonly these days, through the use of appropriately substituted precursors, especially phthalonitrile derivative^.'^ Substituents provide the prime means of solubil- izing the ring system in either aqueous media or organic solvents and offer a useful way of tuning the wavelength of the visible region absorption band, the Q-band. Importantly, cer- tain substituents, in particular long aliphatic chains, can pro- mote discotic liquid-crystal beha~iour.'~ The most common 9 10 Fig.1 Phthalocyanine (Pc) ring system and part of the atom num- bering system. The 1,4,8,11,15,18,22,25 positions are referred to as the non-peripheral sites, the 2,3,9,10,16,17,23,24 positions as the peripheral sites. M can be H,H or one of up to ca. 70 elements of the periodic table. The article concentrates on derivatives of H,Pc and the metal(]]) derivatives, the MPcs. molecular packings adopted by such phthalocyanines in their mesophases are of the columnar type whereby the planar molecules stack like piles of coins. The columns are thought to be stabilised by interactions between the aromatic cores but are mobile once the side chains have 'melted'.Polymorphism of Unsubstituted Phthalocyanines A number of the applications of Pcs referred to above utilise the compounds in the solid state and this clearly drives much of the research into the properties of their solid-state formu- lations. Often, these properties are a function of the molecular packing for which there is a wealth of data, particularly for the unsubstituted ring system as its metal-free form, H,Pc, and when metallated with the common divalent metals to give the MPcs. Indeed, the patent literature refers to at least ten polymorphic forms of these derivatives, of which the a, /?and x-forms are perhaps the best known.I5 There are some ambi- guities concerning the extent of the differences between some of the other forms16 and further complications arise through subtle variants of the a-form.17 What is firmly established is J.Muter. Chem., 1996, 6(5), 677-689 677 that in sublimation experiments, the conditions and the sub- strate onto which the material is deposited can affect the packing significantly;" an unusual illustration of the impor- tance of conditions is provided by the growth of one of the newest polymorphs of CuPc during experiments in micro- gravity on the Orbiter space sh~ttle.'~-~' Crystals of the P-modification, normally regarded as the thermodynamically most stable form, can be grown by subli- mation at high temperature under an inert atm~sphere.~ X-Ray diffraction studies of single crystals of various transition-metal phthalocyanines reveal that the molecules stack in columns with the planes of the molecules tilted with respect to the column axe^.'^'^^ The direction of the tilt alternates from column to column to give the classic herring-bone arrangement found in a number of aromatic molecules.The normal of the plane of the phthalocyanine ring typically makes an angle of ca. 48" to the column axis. Films of the a-modification can be obtained by sublimation under high vacuum with collection on a substrate at room temperature. A single crystal X-ray diffraction study of the a-form2' of PtPc and electron diffraction studies of vacuum- condensed films of Cu, Co, Ni, Fe, Ni, Pt and the metal-free derivatives2' show that the tilt angle of the rings relative to the column axis is ca.26", much less than that in the @-form. Spectacular images have been obtained using high-resolution transmission electron microscopy, see for example the earlier review in this journal by Kobayashi and Isoda.22 These reveal the herring-bone structure but also show areas of disorder arising, for example, where the tilt in one column is in the wrong direction, i.e. in the same sense as those of the two adjacent columns. The a-form undergoes dimorphic change into the p-form, a transition facilitated by heat and exposure to solvent^.^^,^^ Grinding the @-form converts it into the a-f~rm,~' while neat milling of the a-form provides one of the methods for obtaining the x-polymorph.26 Each form gives rise to a characteristic absorption-band envelope in the visible region, known as the Q-band.The band structure is more complex than that observed in the solution phase where non-aggregated MPcs give rise to a single main band assigned to the doubly degenerate transition alu-eg. For metal-free complexes, the lower symmetry of the system lifts the degeneracy and the Q-band is split into two components. In the solid state, however, the spectra of MPcs and H2Pc are broadened through exciton coupling effects which also lead to shifts in the band positions. These are dependent upon molecu- lar packing.16 Thus, the p-form of metal-free phthalocyanine has a band envelope with the two most intense components centred at ca. 660 and 700 nm, while the a-form shows maxima at ca.600 nm and a lower intensity band at ca. 690 nm, A,,, varying somewhat according to the formulation and particle size.26,27 In contrast, the x-form has absorptions in the 560-660 nm and 780-800 nm regions.26 These differences are important with regard to the use of the compounds as pigments and, in higher technologies such as optical data storage and electrophotography, for choosing materials for matching laser or LED o~tput.~ Molecular packing also proves to be an important factor in conductiometric gas sensing applications. There have been many studies of the effects of doping sublimed films with both electron donor gases and, more especially, with oxidising gases such as NO,. Exposure of the films to the gases modulates the semiconductivity of the system.Donor-acceptor inter-actions are thought to facilitate charge-carrier generation, thus raising conductivity and providing a basis for an analytical probe. Comparative studies have shown that the electrical properties of films of different polymorphic forms differ for various metallated phthalocyanine~.~-~.~~ New Derivatives, New Formulations While substituents on the Pc ring can advantageously tune some of the system's electronic characteristics, they may also lead to new types of packing. This in turn expands the range and variety of properties of the macrocycle in the solid state. However, there is sometimes a price to pay in that the extraordinary thermal stability of the unsubstituted ring com- pounds may, in part, be sacrificed.Indeed, few substituted phthalocyanines have been satisfactorily sublimed without decomposition. On the other hand, the solubility in organic solvents conferred by substituents opens the way for the deposition of phthalocyanines as films by methods very differ- ent from sublimation. Three developments will be considered here, of which the main one is the application of the Langmuir- Blodgett (LB) technique, a methodology which has attracted much attention over the last 20 years and which was first applied satisfactorily to phthalocyanines in the early 1980s. The second development has been the exploitation of spin- coating methods. The procedure is much simpler and more convenient than the LB method, but there is less control over film thickness.The third development, one which gives ultra- thin films, is the newest and involves generating a self-assembled monolayer (SAM) chemically bonded to a substrate surface. Major challenges in these areas are the reproducible deposition of well ordered films and the design and develop- ment of derivatives which lead to different types of molecular packing. Langmuir-Blodgett Films The Langmuir-Blodgett procedure, recently reviewed in this journal,29 was devised in the 1930s and offers the possibility of constructing ultrathin films of organic molecules by transfer- ring molecular monolayers at an air-water interface onto a substrate, one monolayer at a time. The full experimental procedure is in two parts.In the first part, a small amount of the compound of interest, typically an amphiphilic material having both a hydrophilic headgroup and a hydrophobic tail, is transferred onto the surface of high-purity water as a dilute solution in an organic spreading solvent. The trough containing the water is fitted with a movable barrier and, once the solvent has evaporated, the barriers are gradually closed to reduce the surface area, A, available to the organic molecules. As the surface area is reduced, the molecules, originally well separated in the so-called two-dimensional gas phase, are compressed together, sometimes through a two-dimensional liquid or expanded state, until they form a close-packed monolayer.Ideally the monolayer is well ordered and this is frequently the case with amphiphilic molecules. Once the close-packed state is achieved, further compression causes a substantial increase in the surface pressure, n, until a point is reached where the molecular monolayer loses its structure and col- lapses. All these changes are monitored as a surface press- ure-area plot, the n-A isotherm. Release of the barriers at a compression point prior to collapse should lead to the reverse behaviour. Indeed, the profile of the n-A isotherm obtained during the compression-decompression cycle is an important indicator of the behaviour of the material as a monolayer. From a knowledge of the amount of material transferred onto the water surface and the area it occupies in the close-packed state, it is possible to calculate the mean area occupied per molecule, A,.This provides an indication of how the molecules are packed in the compressed monolayer. The second component of the LB experiment is the transfer of the molecular monolayer onto a solid substrate at a surface pressure corresponding to the close-packed state. The most common procedure is the vertical dipping method whereby the substrate, aligned normal to the water surface, is lowered through the monolayer which may then become attached to it. A second monolayer may then be deposited onto the first as the substrate is withdrawn from the water. This type of transfer is said to be Y-type and builds up a 'head-to-head', 'tail-to-tail' bilayer assembly.Alternative modes of deposition 678 J. Muter. Chem., 1996,6(5), 677-689 are X-type where transfer occurs only on the downstroke and Z-type with transfer only on the upstroke. During the transfer of material from the water surface onto the substrate, the barriers are closed to maintain a constant surface pressure. The second, less common, method of deposition is the hori- zontal lifting method. This involves lowering the horizontally aligned substrate such that it just touches the surface. A monolayer becomes attached and the substrate is lifted off. The first reported studies of phthalocyanines at an air-water interface appeared in the 1930s and were rather unpromising, functionalities. These include aryloxy particularly tetrac~mylphenoxy,~and alkoxy groups,41 and amide~.~~.~~Examples, illustrative rather than comprehensive, of compounds which have been deposited as films, and referred to in the text that follows, are gathered in Fig.2. Films of tri- and tetra-substituted phthalocyanines frequently give rise to a visible region absorption spectrum showing an intense band which is blue-shifted with respect to that observed in the solution-phase spectrum, see for example Fig. 3 which shows the spectrum of an LB film of asy-CuPc.4' There is The often also a weaker shoulder, more or less at the solution FePc and MgPc failing to form a stable m~nolayer.~~~~~ resurgence of interest in LB films of phthalocyanines com- menced in the early 1980s with results reported by Roberts' group at Durham.The group transferred Li,Pc, one of the more organic-solvent-soluble derivatives of the unsubstituted ring, onto the water surface where hydrolysis to H,Pc was expected. The resultant material was then deposited as an LB film.32 Secondly, and arguably more significantly, they initiated research into monolayer and deposition studies of organic- solvent-soluble substituted phthalocyanines and in particular the tris-N-isopropylaminomethylderivative, Pam-CuPc, and the tetra-tert-butyl derivatives, the ttb-MP~s,~',~~ the structures of which are shown in Fig. 2. The Durham group was among the first to identify the potential of the films as the active component of a conductiometric NO, gas sensor,33 and to show their use within electroluminescent diodes34 and a bistable Subsequently, there has been research on both unsubstituted and substituted phthalocyanines with the aim of producing films containing a good extent of molecular ordering and/or films for particular applications.Work on films of unsubstituted compounds has included annealing at 300°C a film of H,Pc phase A,,,. The spectrum is thus unlike those for the a-, /I-and x-polymorphs of the unsubstituted phthalocyanines dis- cussed earlier and signifies that a different type of molecular packing has been achieved. In particular, the appearance of the blue-shifted band can be attributed to exciton effects within a cofacial columnar structure.47 The extent of the blue shift reflects the length of the column.For example, LB films of AmPc1 give spectra in which A,,, varies between 608 and 614 nm depending upon deposition conditions; a theoretical treatment indicates that the 608 nm band corresponds to a stack containing 14.4 molecules.46 The spectra often show dichroism. That is to say, the absorbance differs when the films are interrogated with normal incident light polarized with the electric field vector, E, first orthogonal, I,and then parallel, 11, to the dipping direction, d. The transition moments for the two transitions giving rise to the Q-band are polarized in the plane of the ring and the observed transition will be strongest when the plane of the ring is perpendicular to the substrate and aligned with the direction of E.No dichroism is expected if the rings are lying with their planes parallel to the substrate surface. Nor will deposited under conditions used by Roberts' gro~p.~~*~~ The there be dichroism if the rings are randomly ordered within annealed film showed a visible band envelope similar to that of the a-form and exhibited electrochromism. In contrast, films of ZnPc gave spectra comparable to those of the a- and x- polymorphs, depending upon the surface pressure during dipping.38*39However, research into films of substituted deriva- tives of phthalocyanines has been much more intense. While there has been a continuing interest in various metallated ttb- Pcs, there has been much research into phthalocyanines tetra- substituted, one substituent per benzenoid ring, with other X x Commund Abbreviation X = -C&NHisoC3l+, X' = H pam-CuPc tbc-CuPc toc-CuPc tdc-cu Pc Fig.2 Examples of some tri- and tetra-substituted phthalocyanines referred to in the text. The abbreviations are based on those used in the literature. the film. Dichroism is therefore an indication of some degree of anisotropic order of molecules whose planes are tilted to the substrate. It is normally reported as the dichroic ratio, R, where R=Eld:Elld. R is sensitive to three factors. These are n 500 600 700 800A/ nm Fig. 3 Optical spectrum of asy-CuPc. The red-shifted band is for the material as a solution in chloroform. The remaining plots show the blue-shifted spectrum of the compound as a Y-type LB film and are measured with polarised light at various orientations to the substrate and the dipping direction.Reproduced by permission from ref. 40. J. Mater. Chem., 1996, 6(5), 677-689 679 the tilt angle of the rings relative to the substrate surface, the angle between the molecular axis parallel to the substrate and the dipping direction and, of course, the extent of uniformity of order. This makes interpretation difficult but, nevertheless, attempts have been made to quantify the precise orientation of the rings within the films through measurements of R at different angles of incidence of the interrogating beam.40,47 Qualitatively it can be stated that for cofacial columnar packing, R is greater than 1when the molecular planes are on average perpendicular to d, i.e.the columnar axes are, on average, aligned along d. Conversely, R is less than 1 when the column axes are on average aligned predominantly orthogonal to d. Both types of packing have been observed and are illustrated by results obtained for films of the tetrabutoxycar- bony1 derivative, tbc-CuPc, and its longer chain homologues, the octyloxy and decyloxy compounds toc-CuPc and tdc-CuPc (for structures see Fig. 2). LB films of tbc-CuPc give X-ray diffraction bands corresponding tp a repeat spacing, i.e. the width of the monolayer, of 18.8A consistent with the rings standing essentially perpendicular to the substrate surface.43 The dichroic ratio for the main absorption band at 618 nm is larger than unity.Its magnitude, which rather interestingly shows a dependency on the width of the substrate, is as high as 7.3: 1 for narrower slides of ca. 10-15 mm wide. On annealing at ca. 90°C the value of R rises further to ca. 16: 1. By contrast, films of toc-CuPc and tdc-CuPc show R of the order of 1:2.4 to 1 :2.6.44 A model to account for R being less than 1 postulates that columnar stacks are formed on the water surface. On compression they align with the stack axes parallel to the surface of the partially immersed substrate. These stacks are then transferred onto the substrate. While there is clearly good evidence for some degree of molecular order in films of a number of the tri- and tetra- substituted phthalocyanines, it is likely that the materials are mixtures of isomers and this is unlikely to promote optimum ordering.Accordingly, attention has also been given to octa- substituted compounds having a pair of identical substituents on either the two peripheral or non-peripheral positions of each benzenoid ring of the macrocycle. Such materials can be synthesised by unambiguous pathways to give single isomers. The first example to be investigated was peripherally substi- tuted octakis(dodecyloxymethyl)-CuPc.48The A, value was consistent with the molecules lying with the aromatic ring flat on the water surface with the aliphatic chains directed away. The compound underwent Y-type transfer and the film showed i,,,=615 nm with R= 1.9: 1.Peripherally substituted octaal- kyl Pcs also lie flat on the water surface and can be deposited by the horizontal lifting method.49 However, their non-periph- erally substituted analogues, R8 MPcs in Fig. 4, either fail to form a surface film or else give an A, value which is inconsistent with simple monolayer behavi~ur.'~ In a further contrast, non- peripherally substituted octaalkoxy Pcs, (RO)8MPcs in Fig. 4, form better monolayers which can undergo Z-or Y-type deposition depending upon the length of the aliphatic chain, the octapentyloxy derivatives forming the most even films.51 Finally, there has been an important body of work directed at peripherally substituted octaalkoxylated rings constrained within phthalocyaninato-polysiloxanepolymers.16 These mate- rials form linear, 'shish-kebab' polymers, the rod-like molecules depositing with the long axis preferentially parallel to the substrate surface.The films are characterised by exceptional blue shifts of the visible region band to ca. 555 nm.52 Monolayer and LB Deposition Behaviour of Amphiphilic Phthalocyanines While the octa-substituted phthalocyanine derivatives des- cribed above contain regions of differing hydrophilicity and hydrophobicity, their amphiphilic character does not approach 680 J. Muter. Chem., 1996, 6(5), 677-689 X = 0-Alkyl. Abbreviation (RO), MPc X =Alkyl. Abbreviation Rs MPc Fig. 4 Examples of non-peripherally octa-substituted phthalocyanines investigated for LB film deposition.The identifiers are those used in the text. M is H,H or a metal@) ion. that of the long-chain aliphatic carboxylic acids and alcohols which are among the classic materials examined for LB film deposition work. Since the mid-1980s the Norwich group, with important contributions from Richardson's group at Bristol and the DRA laboratories at Malvern, has investigated the synthesis and LB film-forming properties of a variety of purpose-designed phthalocyanine derivatives containing both hydrophilic and hydrophobic substituent groups. Fig. 5 shows R Q-R n = 1; (HO),(RO), MPc n = 1; (H0)& MPc n =3; (HO'),(RO), MPc n =3; (HO')& MPc n = various; (HO)R7 MPc. R and R' may be the same or different. Fig. 5 Examples of amphiphilic octa-substituted phthalocyanines designed for the LB technique. The identifiers are those used in the text.some representative examples, the common feature being that the Pc ring is non-uniformly substituted; one of the four benzenoid rings bears one or two substituent groups which contain a terminal hydrophilic moiety, and the other three bear simple alkyl or alkoxy substituents. All the derivatives are octa-substituted to ensure isomeric homogeneity. They fall into two main classes, hereafter referred to as the 'alkoxy amphiphiles' and the 'alkyl amphiphiles'. The compounds provide a basis for probing how the type and lengths of the hydrophilic and hydrophobic chains and the central ion pro- mote good monolayer and deposition behaviour.It transpires that the alkyl amphiphiles differ from the alkoxy amphiphiles in that they exhibit discotic mesophase behaviour, a point which is taken up later.53*54 The compounds are prepared from the appropriately substi- tuted phthal~nitriles.~~ The ether-linked phthalonitrile precur- sors are the easiest to prepare, requiring simple alkylation of dicyanohydroquinone. However, the dialkylated phthaloni- triles have a substitution pattern not readily afforded by simple substitution reactions of the benzene ring. Accordingly, these have been synthesized by ring construction using Diels-Alder reactions. Two general methods have been employed and are shown in Scheme 1. The one using furan as precursor is better suited for the incorporation of chains bearing a hydrophilic headgroup, whereas the other, using thiophene, is more con- venient for access to the simpler dialkyl phthal~nitriles.~~.'~ The amphiphilic phthalocyanines are obtained by base- catalysed condensation of a 9: 1 mixture of the appropriate pair of phthalonitrile precursors.Normally, the hydrophilic headgroups are protected during the cyclisation. The ratio of 9 :1, in favour of the phthalonitrile containing the hydrophobic chains, minimises the formation of other cross-products. Indeed, the two main products are the required non-uniformly substituted derivative and the corresponding R8 H2Pcs or (RO)8 H2Pcs. These are readily separated by column chroma- tography and can be easily converted into the corresponding M" metallated derivatives by reacting the metal-free phthalocy- anine with the appropriate metal salt.The alkoxy amphiphiles show better monolayer behaviour t 1 t NCLCNTCN TCNI 1(IV) (HO)R, MPcs Rg MPCS (HO),& MPcs. (HO'),& MPcs or (HO,C),& MPcs Scheme 1 Preparation of amphiphilic octa-alkylated phthalocyanines. (i) BuLi/RBr; (ii) BuLi/X(CH,),Br (X is protected OH or COzH group);(iii) sodium perborate; (iv) base; (v) 150"C;(vi) LiOPe/PeOH, followed by metal insertion as required. (Double arrows signify multiple steps. OTHP refers to an OH functionality protected as its tetrahydropyranyl derivative.) than the simpler (RO), MPcs.~~There is much less hysteresis in the n-A isotherm and the compressed monolayers are more stable.Fig. 6(a) and (b) compare the n-A isotherms obtained for an example of each series. The intermediate (less steep) region in the n-A isotherm for the alkoxy amphiphile, [Fig. 6(b)], may correspond to the liquid-expanded region or a phase in which the molecules are reorganising from a flat- on to a more perpendicular arrangement during compression. As the surface area is reduced further the molecules are compressed up towards the vertical, although the values for A, are sometimes larger than expected for molecules fully perpendicular to the water surface. Monolayer behaviour is marginally better for the metallated species and the isopen- tyloxy compounds give somewhat more stable monolayers than their straight-chain isomers.The length of the hydrophilic chain appears not to be a factor in controlling monolayer behaviour. However, the longer hydrophobic-chained com-pound, (H0)2(C7H150)6 H,Pc, behaved less well. The n-A isotherm is of the same general form as those of the other alkoxy amphiphiles but the surface pressure starts to rise at a higher surface area per molecule and above 25 mN m-' the film collapses. The alkyl amphiphiles having two hydrophilic headgroups show excellent monolayer behavio~r.~~,'~-'~ Those with only one hydrophilic headgro~p~~.'~ behave less well but are still much better than their simple R8 MPc analogues discussed earlier. Typically, the (HO)2R6 H2Pcs and (H@C)&, H,Pcs and their Cu" derivatives show well defined transitions from the two-dimensional gas phase to the condensed phase, the latter being characterised by a near vertical region in the isotherm, e.g.Fig. 6(c). Value? for A, in the compressed state are of the order of 120-135 A2, consistent with the molecules standing more or less perpendicular to the water surface. There is no evidence for an intervening state between the two-dimensional gas and compressed state and a further indication of the surface behaviour is provided by an X-ray reflectivity study of a monolayer of (H02C)2(C8H17)6 H,Pc.~' This revealed that the molecules are essentially perpendicular even in the uncompressed film, the monolayer film breaking up during decompression, probably to form islands and then single molecules.The contrasting behaviour of the alkyl amphiphiles and the alkoxy amphiphiles, especially the behaviour of isosteric pairs, i.e. compounds from the two series having the same number of linking atoms in the chains, illustrates the importance of the more hydrophobic alkyl chains and, arguably, the beneficial effect of the mesogenic properties conferred by these chains. The alkyl amphiphiles are examples of amphitropic materials,61 i.e. they possess two properties which lead to self-assembly; in this case, their capacity to form a mesophase and their amphi- philic character. It seems plausible that during compression, the molecules on the water surface may adopt the behaviour observed in the liquid-crystal state wherein the aromatic cores align in columns.There are also differences in the behaviour of the alkoxy amphiphiles and the alkyl amphiphiles in the LB deposition e~periment.'~However, only (HO)2(C7H&)6 H2Pc, the com- pound which showed poor monolayer behaviour, failed to deposit. Otherwise, the transfer ratios ranged from 0.7 to 1.0 for the alkoxy amphiphiles, generally better than for the (RO), MPcs, and 0.9-1.0 for the alkyl amphiphiles, a remarkable improvement over the R8 MPc derivatives which could not be deposited at all. These results clearly demonstrate the impor- tance of introducing formal amphiphilic character into the structure. The alkoxy amphiphiles tend to give wetted films which need to drain between dips. The straight-chain pentyloxy derivatives, (HO)2(C5H,,O)6 MPcs, undergo Y-type deposition during the first dipping cycle followed by Z-type deposition on subsequent cycles.Chain branching improves the deposition such that (HO)~(~SO-CSH,,O)~ MPcs and (H0')2(i~~-C5H110)6 J. Muter. Chem., 1996, 6(5),677-689 681 30 2s a0 15 LO S 0 0 I so \ lo0 I-1SO 200 1 2so -1 (b) h so 1m 2so0 area per moIecuIe/A* Fig. 6 n-A Isotherms showing the variation of the surface pressure as the area occupied by the surface molecules is varied The area is first reduced and then expanded (a) isotherm for (C,H,,O), H2Pc, (b)isotherm for (H0)2(~~~-C5H110)6HzPc [less hysteresis than (a)], (c) isotherm for (H0)2(C10H21)6H2Pc which shows minimal hysteresis and an absence of an intermediate 'two-dimensional liquid phase' Adapted by permission from ref 54 MPcs consistently undergo Y-type deposition to form even green filmss4 The evenness of the transfer process is evident from the linear increase of the visible region absorbance with the number of dips62 Members of all four series of alkyl amphiphiles behave especially well and, indeed, much better than the alkoxy amphiphiles All exhibit Y-type deposition to give very even films with little apparent wetting after emersion Low-angle X-ray diffraction studies of films of alkoxy amphi- philes give no Bragg peaks, indicating little or no layer ordering However, in some cases Kiessig fringes are observed which indicate evenness of the film and analysis of the data provides a measure of the film thickness Dividing this by the number of Y-type dips provides a measure of the thickness of film deposited per dip, ip the case of the alkoxy amphiphiles this is typically 20-23 A 54 Films of alkyl amphiphiles give different results The films show two or three Bragg peaks corresponding to spacings for the Y-deposited bilayers of 25 A for (HO)2(C6H13)6 H,Pc and then ranging from ca 40A to 43 5 A for the longer-chain analogues The estimated distance, w, from the hydrophilic groups to the end of fully extended alkyl chain! on the opposite side of the molecules is between 23 and 25 A depending upon the number of carbon atoms in the alkyl chains Clearly, the measured value for the bilayer spacing is less than 2w, indicating that the molecules are to some extent tilted and/or there is interdigitation of the chains of molecules in adjacent layers The markedly smaller bilayer spacing in the film of (H0)2(C6H13)6 H2Pc cannot be explained simply in terms of the length of the substituent chains It points to a different packing type Whether or not the rings are tilted can be assessed by IR spectroscopic techniques, proven tools for probing orientations of molecules within thin film assemblies supported on appro- priate substrates Various experiments are possible and several are complementary As an illustration, in a normal incidence transmission spectrum of a film deposited on an IR-transparent substrate such as silicon, the electric field vector interacts with transition moments which are parallel to the substrate surface On the other hand, when reflection-absorption IR spec-troscopy (RAIRS) is used to probe a film supported on a metal surface such as gold, only those vibrational modes having a component normal to the substrate surface will absorb radiation Phthalocyanine thin films are ideally suited for IR analysis, not least because the rigidity of the planar Pc nucleus ensures that the directions of out-of-plane and in- plane transition dipoles associated with the nucleus are all well defined This facilitates analysis of the orientation of the molecules with respect to the interrogating beam Debe's 64 and others6' have explored the application of RAIRS for studies of sublimed films of unsubstituted derivatives, probing both the mean orientation of the rings relative to the substrate and the degree of order, and there have been several studies of phthalocyanine LB films 66 IR techniques demon- strate conclusively the contrasting molecular packings in LB films of the alkoxy and alkyl amphiphiles 69 The transmission and RAIR spectra for films of (H0')2(i~~-CsH110)6 CuPc are very similar, which suggests that either there is little anisotropy in the orientation of the molecules relative to the substrate, or the molecules are tilted at 45" The absence of layer spacing referred to earlier suggests that the first interpretation is more likely IR experiments with the alkyl amphiphiles give rather different results Fig 7 compares the KBr disc spectrum for (H02C)2(C10H21)6H2Pc, the normal incidence transmission spectrum of the LB film on silicon and the RAIR spectrum of the LB film on gold The band at ca 760 cm-I is assigned to an out-of-plane vibration (arising from either an aromatic C-H or the macrocyclic ring deformation) and therefore its dynamic dipole moment is polarised perpendicular to the plane of the macrocycle The band is strong in the transmission spectrum and virtually absent in the RAIR spectrum This is consistent with the molecules being oriented with the plane of 682 J Mater Chem ? 1996, 6(5), 677-689 I1830 1700 1570 l't't0 1310 1180 lOS0 920 $90 660 v/ cm-' Fig.7 (a) IR absorbance spectrum of a pressed pellet of (H0,C)z(CloHzl)6H,Pc in KBr.(b)RAIR spectrum of an LB film of (Ho,C),(c1,&~1)6 H,Pc on a gold surface. (c) Normal incidence transmission spectrum of an LB film of (Ho~C)~(c&~1)6H,Pc supported on silicon. Reproduced (with modification) by permission from a diagram in ref. 69. the ring perpendicular to the substrate surface. Similar IR studies have been performed on the films of the (HO),R, MPcs and have yielded the same conclusions. Further experiments have given information about aniso-tropic packing of molecules within a molecular layer.69Fig. 8 shows the normal incidence transmission spectra of the LB film of (Ho&),(cloH,1)6 H,Pc on silicon with the electric field vector polarised parallel and perpendicular to the dipping direction. The 760 cm-l band is very intense in the former but is much diminished in the latter.This shows that the mean orientation of the molecules is one where the angle p between the normal to the plane of the ring and the dipping direction, d, is significantly less than 45". The dynamic dipole moment for an out-of-plane vibration at an angle p to d will have components perpendicular and parallel to d. The ratio of the absorption intensities Z,,and ZI, about 5, is given by cos2 P/sin2 P, whence /3 is ca. 25". The films of the alkoxy amphiphiles and alkyl amphiphiles __c Directton of the dynamic dipole 01 the ca 760cm ' bandq.-Molecular planed1 d is the dippng direction n Ii 1 if90 1660 I530 lkOO 1270 Ilk0 ldl0 880 750 620 v km-' Fig.8 The anisotropic alignment of the (H02C)2(C10H21)6H,Pc mol-ecules in the molecular monolayers can be detected by polarised IR beam experiments. The plots show the transmission IR spectrum of an LB film of (H02C)z(CloHzl)6H,Pc supported on silicon with the electric field vector of the beam polarised parallel (top spectrum) and perpendicular (bottom spectrum) to the dipping direction. The inset diagram shows a representation of the mean orientation, ct, of the rings to the dipping direction. The ratios of the relative intensities of the band at ca. 760 cm-' indicate that p, the mean angle of the normal to the ring to the dipping direction, is ca. 25". Reproduced by permission from ref. 69. give rise to three distinct types of visible region absorption band envelope.54Examples are shown in Fig.9 and each differs from the spectral band shape depicted in Fig. 3, demonstrating that the in-layer molecular packing is quite different from that for the tri-and tetra-substituted compounds discussed earlier. The band shape shown in Fig. 9(a) is typical of the spectra exhibited by LB films of the alkoxy amphiphiles. The more 10 500 0 450 I 0 400 4 A:350 i 0 300 -0 250 -O *0° 1 0 000l050I-dd-4 550.00 600 00 650.00 700 00 750.00 000 00 500 00 050 00 1 000 j t 0 900 - (b 1 A 0.800 - 0.700 - P) 0.600 C g 0 500 fna m 0.400 0.300 0.200 0 100 0.000 550 00 600 00 650 00 700.00 750.00 800 00 500 00 850 00 1 1 I 10 500 t 1 0 60 -(c) 0 400 -0 350 -0 300 -0 250 -0 200 -0 10001501 o.ow1 nJ\ I0 000 550 00 600 00 650 00 700 00 750 00 800 00 500 00 850.00 A/nm Fig.9 Visible region spectra of LB films on glass slides. (a)(HO),(iso-C5Hllo)6 cupc; (b) (H0)2(C6H13)6 H2Pc; (c) (H0)2(C9H19)6 cupc.Spectra are recorded with the electric field vector polarised (A) perpendicular to the dipping direction, E, and (B) parallel to the dipping direction, El,.The third line (C) in each plot corresponds to the solution phase spectrum which is shown for comparison. Adapted by permission from ref. 54. J. Mater. Chem., 1996, 6(5),677-689 683 intense component appears at ca 755 nm for metal-free denva- tives and at ca 740nm for copper derivatives These are broader and red shifted relative to the band positions in the spectra of the compounds in solution A red shift is to be expected for an assembly of phthalocyanine molecules in which the adjacent rings within a columnar structure are offset, ze the molecules are not aligned fully cofacially Some spectra, notably those of films where the alkoxy group is isopentyloxy, show dichroism suggesting that there is some degree of aniso- tropic alignment with the molecules showing a preference for alignment with their planes perpendicular to the dipping direction The two types of spectral band envelope exhibited by films of the alkyl amphiphiles are illustrated in Fig 9(b) and (c) The former is observed for LB films of (HO),(C,H,,), H,PcS4 and (HO),(iso-C,H,,), H,Pc 58 The main feature of the spectrum is the relatively narrow red-shifted absorption which again is consistent with non fully cofacially aligned stacking of mol- ecules within a columnar assembly There is dichroism, with a higher dichroic ratio than that observed for the films of any of the alkoxy amphiphiles Fig 9(c) shows the type of band structure exhibited by longer-chained homologues of the (HO),R, H,Pc and (HO,C),R, H,Pc series and their Cu" derivatives This is very different from that in Fig 9(b) and confirms that the type of packing is sensitive to the length of the alkyl chains There are two main absorption bands centred at ca 635 and 768 nm, and two lower intensity transitions, one to the red and one to the blue of the 635 nm band Spectra of the films of these types of compound all show dichroism such that R is > 1 for the 635 nm band and < 1for the 768 nm band, the magnitude of R at the two wavelengths varying slightly for films of different compounds The evident splitting of the visible region transition into red-shifted and blue-shifted components and the opposite dichroism for the two bands is indicative of Davydov splitting (exciton splitting) arising from translationally non-equivalent molecules within the 'unit cell' An example of such packing is the 'herring-bone' structure referred to earlier Interestingly, this type of Davydov split absorption is also observed for the horizontally lifted LB films of the peripherally substituted octaalkyl phthalocyanines 49 Other longer-chained alkyl amphiphiles with Ni as the central atom show a similar band structure and dichroism but the exciton splitting of the band into the red- and blue-shifted components is lower, indicating some differences in packing or spacing within the columnar assembly Spin-coated Films The technique of spin-coating or spin-casting involves the evaporation of a drop of a solution of a compound in an organic solvent on a substrate rotating at speeds of ca 2000rpm As the solution is spread by centrifugal force, the solvent evaporates leaving the solute as a 'spin-coated' film Films of some tetra-substituted neopentyloxy derivatives obtained in this way have been shown to have a lower percentage area coverage of crystallites than a simple evapor- ated film7' Spin-coated films of AmPc1 (see Fig 2 for the structure) have been compared with the LB film of the same material (see Langmuir-Blodgett Films, earlier) The spin- coated film shows a blue-shifted band at 603 nm, attributed to a columnar stacking of 18 7 molecules, cf 14 4 for the LB film4546 However, the electrical properties of the two films suggest that overall the spin-coated film may be less well ordered, the LB film shows a conductivity of an order of magnitude higher 71 The solubility of the R8 MPcs in solvents such as THF and especially toluene renders these compounds also suitable for spin-coating They form particularly even films and there is no evidence of crystallites on viewing the film under a microscope There is, moreover, good evidence for molecular self-assembly 684 J Muter Chern , 1996, 6(5), 677-689 into an ordered film structure insofar as X-ray reflectivity studies reveal diffractions attributable to layering of molecules within the films The spacings are of the order of 17-18 A, depending upon the compound deposited The values are lower than the effective diameters of theSompounds in their discotic columnar mesophases (ca 22-23 A) which may reflect a more compressed structure in which the side chains are either less fully extended or more interdigitated with those of adjacent molecules Alternatively, it may indicate some tilting of the planes of the molecules from the perpendicular to the substrate 72 Remarkably, the visible region spectra reveal very similar band envelopes to those of LB films of the corresponding alkyl amphiphiles having the same length of alkyl chains Thus the spectra of the spin-coated films of (C6H13)8 H,Pc and (CgH1-)8 H,Pc (the latter shown in Fig 10) show the same band shapes as the LB film spectra of (H0),(C,Hl3), H,Pc and (HO),(C8Hl7), H,Pc, respectively A spin-coated film of a 1 1 mixture of (C6H13)8 H,Pc and (C8H17)8 H,Pc gives a film having the same spectrum as the former 73 These experiments point to the fact that the length of the alkyl chains is quite crucial in controlling film structure in both spin-coated and LB films Subsequent studies showed that this applies quite generally to the solid-state formulation of these materials When crystals of the different R8 MPcs are pressed onto a glass slide to give a thin smeared film, they give rise to spectra very similar to those of the corresponding spin-coated film Furthermore, when the crystals are smeared in one direction only, the spectrum shows dichroism 74 Yet another indication that spin-coating generates molecular packing which approxi- mates to that in the crystal state, at least over short range, is forthcoming from the observation that cooling a thin liquid layer of (C8H17)8 H,Pc on a glass slide produces a crystallised film having the same spectral characteristics as the spin-coated film 75 At the present time, the precise type of packing in the films and, indeed, the crystal state remain to be determined, to date only one crystal-structure determination has been achieved, that of (C6H13)8 H,Pc This reveals that the molecules 1 coo , .1 0 0 0 0 -0 500 400 1 ' c 1 1 000 J -f--1[a):;. , 0 200 0 000 50 100 150 170 TI% Fig. 10 A, Spectra of the spin-coated film of (C8H17)6H,Pc at tempera- tures which correspond to different states (a) 50°C the crystal state, (b) 90 "C the columnar mesophase of rectangular symmetry, (c) 145 "C, the columnar mesophase of hexagonal symmetry, and (d) 160°C the isotropic liquid phase B, Plot of the change of absorbance at 714 nm plotted as the temperature of the film is raised, (a)-@) as in A The fall in absorbance on heating the materlal in the liquid state is due to material flowing down the slide are aligned in columnar stacks in which the planes are tilted with respect to the column axis, i.e.they are not fully cofacially stacked. The tilts in the column are in the same sense in adjacent columns as in a J-sta~k.~, Compounds with longer chains, i.e. those which give spectra for the solid state showing the red- and blue-shifted bands, have been obtained only as fibrous needles, unsuited for single-crystal X-ray studies. Examples of the alkyl amphiphiles also give very even spin- coated film^.^^,^, Low angle X-ray reflectivity measurements of a film of (H0'),(C8H17), H,Pc show six sharp Bragg peaks which index as the 001, 002 and 004 reflections fr:m two different repeat units.77 The spacings are 37 and 44A.Thus there are two different types of packings co-existing and the magnitudes of the d spacings somewhat surprisingly indicate that these are both 'bilayers', implicating hydrogen-bonding participation during the self-assembly process. The correspond- ing LB film of the compougd contains a bilayer structure with a repeat spacing of 43.5 A. Despite these differences in the packing of the two films, they give the same spectral band shape in the visible region, showing that the feature of the packing which gives rise to the exciton splitting is retained. From this it may be inferred that the exciton splitting arises through interaction between molecules within the same mol- ecular layer rather than in adjacent layers.Thermally Induced Molecular Reorganisations in Films As both the R, MPcs, R, H,Pcs and the alkyl amphiphiles exhibit thermotropic liquid-crystal behaviour as crystalline materials, it is to be expected that the molecular assemblies within the films might undergo molecular reorganisations on heating. This is indeed the case.78 The visible region spectra of the spin-coated films of examples of the R, MPcs, R, H,Pcs and the (HO),R, H,Pc series undergo sharp changes at or close to the mesophase transition temperatures of the bulk material. These are fully reversible provided the films are not heated into the liquid phase where the evenness is lost. Spectra of a spin-coated film of (C8H17), H,Pc, at temperatures corre- sponding to different phases, are shown in Fig.10A. Fig. 10B shows the change in absorbance at 714nm with temperature, the marked changes corresponding to the transition from the crystal state, through two mesophases, to the liquid phase. Heating LB films of the alkyl amphiphiles produces more complex behaviour. An analysis of the behaviour of the LB films of a series of (HO),R6 H,Pcs indicates that the films of the monotropic liquid crystals, i.e. materials which develop a discotic mesophase only during cooling, differ from those of the enantiotropic liquid crystals, which exhibit a mesophase during both heating and cooling. Films of the former series tend to melt at temperatures very close to those of the bulk material; those of the latter show gradual molecular reorganis- ations at temperatures in the ranges corresponding to the mesophase of the bulk materials and melt below the tempera- ture required to melt the bulk material into the liquid phase.Fig. 11 shows the variation of the absorbance at a set wave- length with change of temperature for the two types of film of (HO)2(CgH1,), H,Pc. The transitions evident for the spin- coated film correspond to the phase transitions of the bulk material; for the LB film there is a change in the spectrum but this occurs more gradually over a much broader temperature. The LB film also has a lower melting point into the liquid phase. When the LB films of the (HO),R, H,Pc materials are cooled from a temperature below their melting temperatures they show broadly reversible behaviour, giving room tempera- ture spectra similar to the spectra of the freshly deposited film.Sometimes there are small shifts of the two principal bands and enhanced di~hroism.~~,~~ Similar behaviour is also exhibited by the LB film of I. 100 1.000 8 0.900 C f!0.800 $13 a 0.700 0.600 0.500 0.400 -j 80 100 120 TIT Fig. 11 Plots showing the change of absorbance at 707 nm on heat- ing: -, a spin-coated film; ----, a LB film of (H0)2(C9H19)6H2Pc. The drop in absorbance on heating the latter at high temperature corresponds to the melting of the film. (H02C)2(CloH21)6H,Pc. However, films of two of the shorter- chain materials of this series behave differently insofar as heating generates a new molecular assembly which is retained The LB film of (H02C)2(C8H17)6 on ~ooling.'~,~~ H,Pc, for example, gives rise to a spectrum very similar to that for the spin coated film of (C&t17)8 H,Pc [Fig.lO(u)]. The LB film assembly undergoes a molecular reorganisation at 127.5 "C which results in a change to one very similar to that for the columnar hexagonal mesophase of (C,H17)6 H,Pc [Fig. 1o(c)]. The new band shape is retained on cooling though there is a small (5 nm) blue shift of the individual transjtions. There is also a reducFion in the bilayer spacing from 41 A in the original film to 36 A in the heat-treated film.56 The changes in the spectrum imply that the molecules in the film have adopted a similar packing to that in the columnar hexagonal mesophase; that this is retained on cooling is perhaps attributable to stabilisation of this packing through hydrogen bonding involv- ing the carboxylic acid groups.Spin-coated Films of More Complex Mesogenic Pht halocyanines The alkyl amphiphiles are ideally functionalised to form more complex structures, the hydroxy function, for example, of the (HO)R7 MPcs and (HO),R6 MPcs providing a means of linking the phthalocyanine moiety to other units, not least to further phthalocyanine rings. Organic-solvent-soluble deriva- tives retaining mesophase-forming behaviour have indeed been prepared and include liquid-crystalline ferrocenyl-phthalocyan- ines,,' and ester-linked liquid-crystalline dimers, trimers" and main-chain polymers,82 e.g. Fig.12. The spin-coated film of the ferrocenyl-phthalocyanine is smooth and even and has a Q-band absorption very similar to that of the spin-coated film of the (HO)R7 MPc precursor, suggesting that the ferrocenyl unit has not disrupted the packing significantly. The film undergoes reversible changes on heating and cooling" and the behaviour of the ferrocenyl group in these different environ- ments is under examination. The dimers and trimers as bulk materials generate a meso- phase on heating which is retained on cooling to room temperature when the compounds become highly viscous.81 Both series are poor materials for LB deposition but give very good spin-coated film^.^^,'^ X-Ray reflectivity profiles from films of examples of a dimer and a trimer show well defined Kiessig fringes implying uniform thickness.They also show a feature in the profile consistent with either a rectangular or hexagonalo columnar 'lattice' with an intercolumnar distance of ca. 23 A. The band structures in the visible region spectra J. Muter. Chem., 1996, 6(5), 677-689 685 Fig. 12 Structures of examples of liquid-crystalline ferrocenyl-phthalocyanines and phthalocyanine dimers and tnmers which deposit well as spin- coated films of the films for the dimer and trimer are broadly similar to that exhibited by the alkyl amphiphiles in their mesophase range Thus, as the solvent evaporates during the spinning process, the molecules appear to assemble to give a room-temperature packing which is quite different from the room- temperature packing of the simpler materials Self-assembled Monolayer Films A fast developing area of thin film research is the formation of a self-assembled monolayer (SAM) at the surface of an appropriate substrate 29 These ultrathin films, one molecule thick, are chemically bonded to the surface and are thus potentially more robust than LB or spin-coated films Typical methodologies include the reactions of thiols and disulfides at gold or silver surfaces and of trichlorosilylalkyl derivatives on glass or silicon Much of the fundamental work has been performed on simple functionalised alkanes2' but now there is increasing attention given to depositing SAMs of compounds with properties which are potentially more interesting with regard to applications The alkyl amphiphiles can be used as precursors to derivatives appropriate for SAM deposition through conversion of the hydroxy function into the corre- sponding thiol or disulfide This is achieved by routine chemis- try and these derivative^^^ form good SAM films on gold surfaces The films are best characterised by FTIR techniques Fig 13 shows the C-H stretching region of the RAIR spec- trum obtained for the SAM film obtained using a disulfide derivative 85 Attachment of a phthalocyanine SAM film to the surface of a silicon wafer and a glass slide has been achieved using non-uniformly substituted phthalocyanines bearing one or two trichlorosilylalkyl groups 86 The compounds, in solu- tion, react with the surfaces of substrates rendered hydrophilic pnor to deposition Deposition onto glass enables the films to be monitored by visible region spectroscopy which shows an absorption band with an absorbance of ca 0004 for the monolayer Applications in Gas Sensing Devices Gas sensing continues to be a particularly promising area for exploiting phthalocyanine thin films in practical devices The first demonstration by Roberts' group of the effectiveness of an LB film of Pam-CuPc as a conductiometnc NO, gas sensor has been followed by a number of other studies33 Typically, the film is deposited onto a substrate bearing interdigitated electrodes and exposed to pulses of low concentrations of gases mixed with air Among the many interesting results obtained, it has become apparent that there are significant differences in behaviour between LB films of substituted phthalocyanines and sublimed films of unsubstituted phthalocyanines For example, relative to the latter, LB films of tetraaryloxy deriva- tives show a lower response to electron-acceptor gases but a higher response to electron-donor gases In one instance, the conductivity of an LB film of copper tetra-4-( 2,4-di-tert-amyl- phenoxy)phthalocyanine has been shown to be insensitive to 200ppm of NO, and SO, whereas there is a significant response upon exposure to ammonia87 Films of the alkyl amphiphiles, on the other hand, show a good response to NO, in the range 1-5 ppm and limited response to ammonia up to concentrations of 25 ppm 88 89 These concentrations are within the occupational hygiene ranges prescribed by the UK's Health & Safety Executive (HSE) Typical output from experiments performed at the HSE laboratories on devices constructed from LB films of the alkyl amphiphiles is shown as Fig 14 Changes in current flow are monitored over time The baseline current is established by measuring the film current in clean air over two minutes The film is then exposed to the analyte gas, in air as carrier, for two minutes and this is followed by a two minute recovery time in clean air The latter cycle is then repeated four times using either the same or different concentrations of gas The first two plots of Fig 14 show the results of five exposures of a film of (HO),(C,H,,), H2Pc to 3 ppm NO, followed by five exposures to another gas The remaining plots show part of a series of repetitions of this ten-exposure cycle There is encour- aging selectivity with no response to chlorine or CO, and limited responses to ammonia, H2S and SO, in the occu-pational hygiene ranges Importantly, the devices continue to respond to NO, after exposure to the other gases and their operation is not diminished by exposure to air with 40% relative humidity, a problem sometimes encountered with films of unsubstituted phthalocyanines It seems plausible that the hydrophobic alkyl chains serve to protect the film from moisture The anisotropic deposition of the molecules in the LB experiment which gave rise to the dichroism in the visible regon and FTIR spectra discussed earlier is also manifested in the conductivity measurements Thus there is a different level of response to NO, gas when LB films of (H0)2(C,H,3)6 H,Pc are deposited with the dipping direction perpendicular and parallel to the long arms of the electrodes [Fig 15(a)] The larger response, by a factor of ca 7 [Fig 15(b)] arises in the former case where it is supposed that the axes of columns within the molecular layers are predominantly perpendicular to the long arms of the electrodes The results are thus consistent with conduction occurring along the columns 89 There is clearly scope for further work in this area which may turn up surprising effects indeed, preliminary results have shown that films of an alkoxy amphiphile show an unexpected conductiometric response to toluene Future Prospects and Developments Much has been achieved in developing thin films of substituted phthalocyanines in the dozen or so years following the Durham 686 J Mater Chem , 1996, 6(5),677-689 -.0074 I 3800 3600 3400 3200 3000 2800 2600 2400 2200 v/cm-' Fig.13 Part of the RAIR spectrum of a phthalocyanine SAM film on a gold surface. The film was obtained by immersing the gold-coated glass slide into a solution of the disulfide derivative shown. The three C-H bands are assigned as follows: 2856cm-', CH, symmetric stretch; 2927 cm-', CH, asymmetric stretch; 2968 cm-', CH, in-plane asymmetric stretch.Adapted by permission from ref. 85. Air Gas Air Gas Air Gas Air Gas Air Gas Air +#+it+t++G+ 8 10-82 ?!= 3 0 t0 li~/secS (a) five sequenhal pulses of 3ppm NO2 A c ?! -3 0 0 0 t Timdsecs (b) five sequential pulses of 0.625ppm Cl2 8.104 sz= 30 0 0 t Timdsecs (c)five sequential pulses of 3ppm NO2 8.,lo-*2 -?! -50 0 0 t TirneJ/seCs (d) fwe sequential pulses of 5Oppm CO O 0 t 0 Timelsecs (e) five sequential pulses of Bppm NO2 1320 1320 1320 1320 a1320 Fig. 14 Sequential exposure of an LB film of (H0),(CSH13), H,Pc deposited onto interdigitated electrodes (Fig. 15) on a glass substrate. The plots show the changes in current flow during exposure.The curved line in the second plot arises from the continued 'recovery' of the film from the final exposure to NO, in the first plot. Reproduced with permission from ref. 89. group's first publications on phthalocyanine LB deposition. The present article has reviewed some, but by no means all, of the contributions. What is clear is that the field has attracted scientists from a number of disciplines with progress in syn- thetic chemistry leading to a range of novel compounds which can be deposited to produce LB films with a satisfactory degree of molecular order. Furthermore, films can now be constructed with different molecular packings and there is increased knowl- edge of how substituents can be used to control the packing.Undoubtedly, there is scope for examining the extent to which the central metal or metalloid atom can influence packing, as it surely must. Reports of spin-coating of phthalocyanine films in the open literature are relatively few, but the convenience of the technique has attracted the attention of industrial and applied groups. Furthermore, the reports that are available suggest that appropriate compounds can give rather well lppm NO2 Pppm NO2 Bppm NO2 4ppm NO2 5ppm NO2A&$irp%r~;~;\; 0 t 1320 Time/secs Fig. 15 Schematic representation of an array of interdigitated elec- trodes patterned onto the glass substrate for use in the LB experiment. Lower plot: results for 30 layer films of (HO),(C,H,,), H,Pc exposed to 1, 2, 3, 4 and 5 ppm NO, for films dipped (a) perpendicular, and (b) parallel to the arms of the electrodes.Reproduced with permission from ref. 89. J. Muter. Chem., 1996,6(5), 677-689 687 ordered films, albeit of a lesser calibre than LB films However, it is the third type of film discussed in this article, the self- assembled monolayer, which seems likely to attract more immediate attention inasmuch as SAM research in general is receiving a high profile at this time In the three areas of film research discussed here, there is a particular need for further 20 21 22 23 24 C J Brown, J Chem SOC A, 1968,2494 M Ashida, N Uyeda and E Suito, Bull Chem SOC Jpn, 1966, 39,2616 T Kobayashi and S Isoda, J Muter Chem , 1993,3,1 J H Beynon and A R Humphnes, Trans Faraday SOC, 1955, 51,1065 P T Cardew and R J Davey, Proc R SOC London Ser A, 1985, studies to identify unambiguously the finer details of the packings obtained to date, as yet there have been rather few applications of advanced microscopies to the problem A survey of the potential applications of the three types of film was beyond the scope of the present article but a few 25 26 27 28 398,415 F Iwatsu, J Phys Chem, 1988,92, 1678 J H Sharp and M Lardon, J Phys Chem, 1968,72,3230 R 0 Loutfy, Can J Chem, 1981,59,549 S Dogo, J-P Germain, C Maleysson and A Pauly, Thin Solid Films, 1992,219,244,251 observations are offered nevertheless The exploitation of 29 R H Tredgold, J Muter Chem, 1995,5,1095 See also A Ulman, phthalocyanine films in gas sensors is likely to remain a key area of research Emphasis may well change from con-ductiometnc to optical sensors and, indeed, there are a number of laboratories already devoting effort towards devising sensing devices exploiting various optical effects, e g the surface plas- 30 31 32 An Introduction to Ultrathin Organic Films from Langmuir- Blodgett to Self-assembly, Academic Press, San Diego, 1991 A Hughes, Proc R SOC London Ser A, 1936,155,710 A E Alexander, J Chem SOC ,1937,1813 S Baker, M C Petty, G G Roberts and M V Twigg, Thin Solid Films, 1983,99, 53 mon resonance phenomenon In principle, optical sensors have 33 S Baker, G G Roberts and M C Petty, IEE Proc Part I Solid- various advantages over conductiometric sensors, especially where there are fire hazards in the environment where the sensor is to be used There is a need, however, in both the conductiometric and the optical sensing areas for more empha- sis to be given to their marketability, perhaps by focusing on their incorporation into sensing units already available Redox behaviour in thin films of phthalocyanines deposited onto electrodes is an ongoing area of research which will continue to attract attention Electrochromism has already been men- tioned in passing in the text and the extensive progress which has been made to date, especially on the rare-earth-metal 34 35 36 37 38 39 40 State Electron Devices, 1983, 130,260 J Batey, M C Petty, G G Roberts and D R Wight, Electron Lett, 1984,20,489 G G Roberts, M C Petty, S Baker, M T Fowler and N J Thomas, Thin Solid Films, 1985, 132, 113 H Yamamoto, T Sugyama and M Tanaka, Jpn J Appl Phys, 1985,24, 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