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Chromonic liquid crystalline phases John Lydon

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Faculty of Biological Sciences, University of Leeds, Leeds, UK Published online: 22 Nov 2011.

To cite this article: John Lydon (2011) Chromonic liquid crystalline phases, Liquid Crystals, 38:11-12, 1663-1681, DOI: 10.1080/02678292.2011.614720 To link to this article: http://dx.doi.org/10.1080/02678292.2011.614720

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Liquid Crystals, Vol. 38, Nos. 11–12, November–December 2011, 1663–1681

INVITED TOPICAL REVIEW Chromonic liquid crystalline phases John Lydon* Faculty of Biological Sciences, University of Leeds, Leeds, UK

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(Received 3 June 2011; final version received 5 August 2011) Chromonic systems are lyotropic mesophases formed by soluble aromatic compounds. The basic structural units in these systems are stacks of molecules (rather than individual molecules or micellar assemblies). There are two common chromonic phases: a more dilute phase consisting of a nematic array of columns (the N phase) and a more concentrated phase in which the columns lie in a hexagonal array (the M phase). Chromonic phases are formed by a range of compounds, including drugs, dyes and nucleic acids, typically with three or four fused aromatic rings. They have distinctive optical textures and characteristic multi-peritectic phase diagrams (in contrast to the multi-eutectic phase diagrams of conventional amphiphiles). Many commercial dyes have proved to be chromonic, but the ability to form liquid crystalline phases has been incidental to their use. However, recent studies have shown that the combination of self-ordering, ease of alignment, sensitivity to changing conditions and additives, coupled with their optical and electro-optical properties, gives these systems unique and potentially valuable properties. It is expected that these will lead to a new generation of applications. It is predicted that there will be increased emphasis on the development of a range of sophisticated devices, either produced from, or actually incorporating chromonic phases. These include polarisers, optical compensators, light-harvesting devices, and biosensors for medical diagnosis. Keywords: liquid crystals; chromonic phases; dyes; organic electronics; biosensors

1. Introduction 1.1 Early history The history of chromonic phases has been charted in a number of reviews over the last 20 years [1–6]. The story falls into three parts. The first consists of isolated reports of novel patterns of liquid crystalline phase formation, dating from 1915 when Sandquist described what we would now recognise as the unmistakeable optical texture, of a chromonic N phase, in aqueous solutions of a phenanthrene sulphonic acid [7]. Later reports, mainly in the dye literature, by Balaban and King [8], Gaubert [9], Jelley [10] and Scheibe [11] pictured molecules aggregated like ‘piles of pennies’ or ‘stacks of cards’. The second phase concerned the detailed study of the anti-asthmatic drug, disodium cromoglycate (DSCG) (Figure 1) using mainly a combination of optical and X-ray diffraction techniques [12–20]. At this stage there were many misconceptions about the structures of the mesophases, as models were sought to explain them in terms of more familiar systems – and there were misleading references in the literature to discotic nematics, smectic patterns of assembly and to the formation of micelles. The final phase was the realisation that chromonic phases are actually widespread amongst drugs, dyes, nucleic acids and similar water-soluble, aromatic compounds and *Email: [email protected] ISSN 0267-8292 print/ISSN 1366-5855 online © 2011 Taylor & Francis http://dx.doi.org/10.1080/02678292.2011.614720 http://www.tandfonline.com

that they have a characteristic distinctive pattern of structures and properties. The investigation of the mesophases of DSCG forms a key part of the story and the development (and the development of this drug by Altounyan is itself an epic story of medical research) [12, 13]. The drug was marketed in large quantities by Fisons (under the trade names INTAL (from inhibition to allergy) in Britain and Chromolyn in the USA. When it was discovered that this compound formed two novel liquid crystalline phases with water, it seemed that the presence of what appeared to be unique medical properties together with what appeared to be unique mesogenic properties might be significant. We now appreciate that neither of these was unique, but at that time, it was suspected that knowledge of the mesophase structures might throw some light on the mode of action [14]. Fisons therefore commissioned a detailed study by the McCrone Research Institute in London, which sought help from Norman Hartshorne (one of the most senior British optical microscopists of the time). The optical textures (Figure 2), phase diagram (Figure 3) and X-ray diffraction patterns were investigated [15–19]. Hartshorne noted the characteristic nematic schlieren texture of the more dilute mesophase and accordingly labelled it the N phase. He also noted that the more

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O

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Figure 1. Molecular structure of the anti-asthmatic drug, disodium cromoglycate. This was the first chromonic mesogen to be studied in detail. At first it was suspected that the V-shape of this molecule might be a crucial factor in the aggregation into columns and hence in mesophase formation. However, the discovery of the chromonic phases of other drugs (such as RU31156 shown in Figure 6), which had a single fused aromatic ring system, indicated that this was not the case [20, 25, 26].

concentrated phase formed a distinctive ‘herringbone texture’, similar in appearance to that formed by the ‘middle phase’ of conventional amphiphile systems. Accordingly, Hartshorne labelled this the M phase.

Temperature (°C)

100 Isotropic

80 60 40 20

N 10

20

30

40

50

60

M Concentration (wt%)

Figure 3. Phase diagram of the chromonic DSCG/water system. This was the first chromonic phase diagram to be recorded [14, 15]. Its multi-peritectic form later proved to be the standard ‘classical’ pattern for chromonic systems. Single mesophase regions are shown shaded. Two-phase regions are striped.

1.2 The ‘classical’ picture of chromonic phases

50μm

M

The re-examination of the DSCG/water system led to the realisation that the structural units in both mesophases are columns of stacked molecules [17]. In the N phase these lie in a nematic array, and in the M phase, they lie on a hexagonal lattice (Figure 4). Chromonic phases can be regarded as the lyotropic analogues of the thermotropic discotic columnar phases. In both cases, mesophase formation is dictated by the formation of stacks of multi-ring aromatic molecules. And in both cases, these stacks lie in a fluid continuum. In columnar phases this is a sea of flexible alkyl chains, and in chromonics, it is water. In virtually every aspect, the structures and properties of chromonics are distinct from those of the mesophases formed by conventional amphiphiles of

N

Figure 2. Optical textures of the chromonic N and M phases (crossed polars with a red1λ plate). Optical micrograph showing the N/M boundary of a DSCG/water system at room temperature, with the concentration increasing upwards. The letters N and M, used to designate the two principal types of chromonic phase, originated in the appearance of these two optical textures – the letter ‘N’ from the similarity of this schlieren texture to that shown by thermotropic nematic systems and the letter ‘M’ because similar ‘herringbone’ textures had been previously seen in the ‘middle’ phases of conventional amphiphile systems.

N

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Figure 4. Chromonic N and M phases. In both mesophases the molecules are stacked in columns. In the N phase the columns lie in a nematic array with no positional ordering. In the M phase the column lie in a hexagonal array.

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Liquid Crystals the soap/detergent/phospholipid type. The molecules have rigid cores rather than flexible aliphatic chains. The molecules are not ‘polar’ (in the sense that the solubilising groups are not confined to a specific hydrophilic end but occur around the periphery of the molecules). There is no chromonic analogue of the critical micelle concentration. Chromonic molecules begin to aggregate even in very dilute solution and columns of increasing length form as the solution is concentrated. And there is no analogue of the Krafft point (the temperature at which alkyl chains become too rigid to allow micelle formation). The thermodynamic driving force for the formation of chromonic stacks appears to be significantly different to that which causes conventional amphiphile molecules to cluster together in micelles. Measurements of thermodynamic parameters indicate that it is enthalpic rather than entropic. In terms of the molecular interactions, this seems to imply that there is a significant force of attraction between the π-orbitals of the aromatic rings of adjacent molecules, and that it is not simply the interactions between water molecules which force the solute molecules to segregate. Calculations of the energy involved in stacking one ring on top of another suggest values of the order of 5–10 kB T. This would appear to be a reasonable value. Clearly it must be larger than kB T otherwise any aggregates would be shaken apart by thermal motion. On the other hand, the temperatures of the mesophase → isotropic transitions indicate that it can not be very much greater. The molecules aggregate into stacks even in the dilute isotropic solution. It is only at extreme low concentrations (say 1.5 μm) for the

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7.2 The production of aligned films

Figure 18. Intercalation in DNA. The sugar/phosphate chains of DNA are wound round the column loosely enough to allow intercalation of a guest molecule. In order to accommodate the alien molecule, the chain must unwind, allowing two bases to move apart and making room for the intercalating to molecule to enter the stack [42].

Figure 19. Stylised representation of the ‘melting’ of DNA. This occurs when a solution of double-stranded DNA is heated and the double helices separate into two single strands. This occurs at a temperature in the 60–80o C range (with the precise value depending on the composition and sequence of the bases). There is an appreciable endotherm, easily detectable with a scanning calorimeter [42]. Two events occur synergistically – the breaking of the hydrogen bonds that pair the bases and the loosening of the stacking interactions between the bases in each strand by thermal vibrations.

individual banding to be resolved, have a characteristic ‘fingerprint’ texture (where the spacing of the bands corresponds to half of the pitch). For low concentrations of dopant, there is a linear relationship between concentration and twist. Lee and Labes developed this technique as an assay for small quantities of optically-active water-soluble compounds and they have measured the ‘helical twisting power’ of a number of compounds [58].

Thermotropic nematic and smectic phases are extensively used in display devices and it is usually a straightforward matter to produce planar, (homogeneous) tilted or perpendicular (homeotropic) alignment as required. In contrast, the tilted and homeotropic alignment of chromonic aggregates has proved to be difficult. All the work on the production of aligned films and aligned samples in cells described below concerns material with a planar arrangement (where the column axes lie parallel to the surface of the film or the substrate plates). The only report describing the production of homeotropically aligned of chromonic material appears to be that of Nazarenko et al. [59], who found that, this state was metastable – and in time, the director re-aligned to become tangential. Many N and M phases can be aligned into a homogeneous state (in the optical sense) and then dried down to produce an aligned solid phase with the same level of ordering This appears to be a property more or less unique to chromonic systems. In general, the mesophases of both lyotropic and thermotropic systems produce crystalline solids with lower symmetry when they are solidified (by cooling or allowing the solvent to evaporate). They do not retain the structures of the parent mesophases. Chromonic N and M phases are usually as easy to align as thermotropic phases and a variety of similar methods have proved effective. Simply spreading the material unidirectionally over a cleaned surface with a blade can give surprisingly good alignment. Materials for X-ray diffraction study are often sufficiently well-ordered by the flow alignment incurred as the sample tube is filled and no further treatment is required. Samples for optical investigation can be aligned on prepared substrates and the use of a unidirectionally buffed layer of polyimide is a convenient and widely-used technique. The use of cells with prepared substrate surfaces, coupled with a careful filling technique, can give order parameters in the 0.8–0.95 range – producing material sufficiently well-oriented for most purposes. Highly aligned bands of solid dye, for use as polarising filters, can be produced by drying down the mesophase on micro-grooved substrates. The other obvious approaches – electric and magnetic fields, zone refining and solvent zone refining (where the film is slowly moved across the surface of a dish of volatile solvent) – have all been found to be effective [2]. Polarisers are optical devices which filter out the component of an incident beam with vibrations in a particular plane (by removing vibrations in other directions). Over the decades, a number of a number

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Liquid Crystals of types have been evolved. Victorian optics depended heavily on the Nicol prism, but since the 1920s, Polaroid film, invented by Edwin Land, has been the most widely used. This consists of an aligned array of small iodine crystals in a plastic matrix. It is expensive to produce to the extent that the most costly parts of a liquid crystal display are often the Polaroid sheets. There is therefore considerable commercial pressure to produce cheaper alternatives. It is beginning to look as if drying down chromonic solutions will offer a commercially viable method of producing aligned dye films. Although highly oriented and more or less monodomain films have been produced by alignment on buffed polyimide surfaces, industrial scale samples, produced by direct shear alignment of the mesophase, have appreciable levels of disorder, which must reduce their effectiveness. In

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this context, the defects in molecular alignment of chromonic films are of more than trivial interest. In recent investigations, Kaznatcheev et al. [60] used soft X-ray spectromicroscopy to study the textures of films prepared by drying down films of the two isomeric sulphonated heterocyclic mesogens (Y1O4 and Y1O5) shown in Figure 20. In both cases, they found a high level of in-plane alignment, with the column axes lying parallel to the plane of the substrate. The average alignment of column axes was along the direction of shearing, but there was a sinusoidal undulation of the director, giving rise the familiar tiger skin texture of diffuse bands, perpendicular to the shear vector, when the samples were viewed between crossed polars. (There were also occasional criss-cross patterns resulting from the presence of disclinations and domain walls.)

either/or (HO3 S)

O

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N

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N

SO3H

N

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Figure 20. Sinusoidal undulations of the director (the chromonic column axis) in films of dried-down, shear-aligned dyes and the corresponding molecular structures of the two isomeric mesogens examined. The broad arrow indicates the direction of shear of the mesophase (before the sample is allowed to dry-down). Kaznatcheev et al. [60].

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When viewed between crossed polars, undulating patterns of this kind give the pattern of diffuse bands described as a ‘tiger skin’ texture. Aligned striated patterns of this kind are frequently encountered in samples of chromonic N phase prepared in a variety of ways, and held in tubes and flat cells and spread as surface films. They are also found in thermotropic discotic systems. It would appear, therefore, that the creation of this pattern of regular undulation of the director represents an easy way for phases composed of flexible stacks to relieve asymmetric stress that has built up in the system. The two mesophases examined gave films with qualitatively similar striated textures. However, there were appreciable quantitative differences. The principal distinction was the level of undulation, with the angular variation of the in-plane director for Y104 being twice that for the isomeric Y105. This can be taken to illustrate the level of sensitivity of the bulk properties of the chromonic mesophase to changes in the molecular structure; in this case, presumably the elastic constant for bending. It will be interesting to see how easily this can be demonstrated with computer simulations. Polarising filters made from dried-down chromonic dye mesophases are only effective over the range of absorption wavelengths of the particular dyes used. However, this may not be a serious problem in view of the co-miscibility of many chromonic dyes in the N and M phases. Tam-Chang et al. have described the use of a mixture of two chromonic perylene dyes which absorbs over almost the entire visible range [61]. 7.3 Photo-alignment One particularly promising approach for the production of aligned dye films is to dry down the mesophase on a directing layer. Orienting surfaces for this process can be prepared from photo-aligned polymers as outlined in Figure 21. The directing layer, consisting of a polyamide with dimethylaminoazobenzene side chains, is first deposited on the glass substrate by spin coating. This gives a surface film that is un-oriented. However, the azobenzene groups become aligned when irradiated with plane-polarised light and this gives a directing layer capable of aligning the N and M phases of a chromonic dye. Crucially, the molecular orientation is preserved in the solid state when the mesophase is allowed to dry down [62–64]. The vital features of most compounds used in the construction of directing layers are the azo groups. By incorporating these groups into the mesogen molecules themselves, it is possible to by-pass the separate directing layer and produce chromonic phases which can be directly aligned by polarised

Figure 21. Production of oriented dye films using a photoalignment technique [62–64]. This schematic diagram shows the principle of the photo-alignment technique. (a) The substrate surface is spin-coated with a polymer film. (b) Irradiation with a beam of plane-polarised light aligns the polymer film. (c) A solution of a chromonic dye in the Nphase is aligned by contact with the polymer film and the molecular orientation is retained as the dye is dried down.

light. This approach is very versatile. It is possible to make detailed micro-patterned polymer surfaces by masking out parts of the original aligning surface and then re-radiating the exposed areas with polarised light with a different vibration direction. Micro-patterned dye films have been produced in this way, by Matsunaga et al. for use in binocular disparity stereoscopic displays [65]. As a further demonstration of the versatility of photoalignment techniques, Hahn et al. have constructed real-time holographic gratings [55]. They used a cell filled with the N phase of the azo dye, Levafix Goldgelb. This cell was placed in the light path of a holographic generating system and subjected to forced Rayleigh scattering. The holographic grating was apparent when the cell was viewed under polarised light, 7.4 Re-usable templates Re-usable template surfaces can be employed to produce aligned dried-down films of chromonic dye, by the procedure depicted in Figure 22. Tam-Chang and

Liquid Crystals

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b

c

Figure 22. Production of micropatterns of chromonic dye using the template-guided procedure developed by TamChang and co-workers [2, 66]. (a) The micro-grooved template made from polydimethyl siloxane. (b) An isotropic solution of a perylene dye is introduced between a cover slip and the template. The solution is allowed to concentrate by evaporation of water at the edges and an aligned M phase ribbon forms in each micro-groove. (In this stylised sketch, a few chromonic stacks have been drawn to indicate the molecular orientation.) (c) The template is peeled away, leaving the stripes of aligned solid chromonic dye adhering to the cover slip. Redrawn from Tam-Chang and Haung [2].

Huang used a polydimethyl siloxane templating substrate to produced an aligned film of a perylene dicarboximide dye [2, 66]. Slabs of this polymer can be produced with arrays of parallel microgrooves on their surfaces. An isotropic solution of the perylene dye is introduced between a cover slip and the template and, as the solution becomes more concentrated by peripheral evaporation, an M-phase ribbon of aligned dye molecules forms in each micro-groove. After the mesophase has dried completely, the template can be peeled off, leaving behind an array of parallel stripes of aligned solid chromonic dye adhering to the cover slip. The template can then be re-used.

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7.5 E-type polarisers A major defect, inherent in the simple twisted nematic display, is the variation of contrast with viewing angle (grey-scale inversion). In displays used by one person at a time, such as cash machines and garage fuel pumps, this not usually a problem, but for more sophisticated applications it is a serious handicap and considerable efforts have been made to remedy it. This has led to recent interest in combinations of conventional Polaroid and chromonic dye polarisers. Conventional iodine-based Polaroid type filters are said to be O-type since they allow through only the ‘ordinary’ ray. These are in contrast to polarising films made from aligned chromonic dyes which absorb the component of the electric vector perpendicular to the optic axis and allow the parallel component to pass through. Because this is the ‘extra-ordinary’ ray, these polarisers are said to be of the E-type. The two types of filter behave more or less identically for beams passing normally and conventional display devices will work with either pairs of O or pairs of E type polarisers, with little difference in their effectiveness for head-on viewing. However, for oblique viewing angles, as shown by Paukshto [67] a combination of O and E filters is more effective than either OO or EE pairs. 7.6 Biosensors Chromonic phases are water-based, appearing to be far more sensitive to the presence of water-soluble solutes than conventional amphiphile phases. This makes them uniquely suitable for the production of biosensors. Lavrentovich and others have demonstrated the potential of chromonic phases as detectors of antigens [68–70]. They constructed a cell (Figure 23) to exploit the recognition of an antigen, streptavidin, by its corresponding antibody, anti-streptavidin. The surfaces of the cell substrates are coated with aligning layers of rubbed polymer, giving a parallel, homogeneous alignment of the director in the ‘off’ state. The cell is filled with dispersion of latex beads in a chromonic N phase. The beads are coated with the specific antigen to the antibodies under investigation. Neither the isolated individual latex spheres not the antigens are large enough to affect the alignment of the mesophase in a way that can be detected optically. However, antibodies have two identical recognition sites – one on each end of the arms of the Y. They are able to link the latex spheres together, forming sizeable aggregates (>2 μm in diameter) and these are sufficiently large to disrupt the parallel director field of the surrounding mesophase. The cell contents now consist of a homogeneously aligned N phase with islands of disturbance around the aggregates of beads. These show up as light areas against a black

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Polariser Alignment layer Homogeneously aligned N phase Alignment layer Polariser

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Antibodies

Antigen/antibody complex

Antigen

Figure 23. Design for a biosensor. This sketch shows the cell produced by Lavrentovich and co-workers to demonstrate the potential of chromonic phases as biosensors [68–70]. The surfaces of the substrates in contact with the mesophase are coated with aligning layers of rubbed polymer, giving a parallel homogeneous alignment. Latex beads, coated with the antigen dispersed in a chromonic N phase, were introduced. When the corresponding antibodies are added, the complexes formed are large enough to disrupt the director field and show up as light areas on a dark background (but the individual components are too small to do this). The high degree of specificity of the antigen/antibody bonding should make this an extremely accurate diagnostic tool. Redrawn from Shiyanoskii et al. [69].

background when the cell is viewed between crossed polars. The disruption extends some distance into the mesophase surrounding each assembly, amplifying the effect and the light areas are appreciably larger than the aggregates themselves. The binding between an antibody and an antigen is highly specific, and if a workable hand-held device could be perfected, it could become of major importance in preventing the spread of epidemics.

8.

Complex chromonics

Over the last five or six years, there has been increasing interest in the properties of what we could call ‘complex chromonics’, i.e. compounds with self-ordering features in addition to the aromatic rings which give rise to π– π stacking. Such additional features could be lengths of hydrophobic alkyl chain or segments of hydrophilic units, enabling the molecules to function as both chromonic and amphiphilic mesogens. Two examples are described below. The first involves mesogens with both hydrophobic and hydrophilic pendant chains. The second concerns a mixed system with

dendrimer beads cemented together with a layer of chromonic mesphase. In both cases, novel structures and potentially useful properties have emerged. 8.1 Graphitic nanotubes The self-ordering properties of a complex chromonic coronene derivative have been used by Aida and co-workers [71] to produce ‘extremely defect-free’ graphitic nanotubes. The large polyaromatic ring of this molecule carries two hydrophobic alkyl chains and two hydrophilic, polyethyleneoxide chains. A sequence of interactions leads to the self-assembly of nanotubes. Firstly, the stacking of the aromatic cores and segregation of the aliphatic chains creates long bilayer ribbons. These wind spontaneously into helices which then fuse to produce hollow tubes. These have 3 nm thick walls and an internal diameter of 14 nm, which makes them an order of magnitude larger than conventional carbon nanotubes. Since they are open-ended, both the interior and exterior surfaces are exposed to the aqueous sub-phase giving a very large chemically accessible surface. This, together with the conductivity and redox properties of the graphitic core, is seen as a significant step towards the construction of the next generation of devices utilising ‘molecular electronics’. This bottom–up way of constructing functional nanotubes contrasts with the more common ‘top–down’ approach (where a conventional carbon nanotube is constructed and then chemically modified) and may well prove to be more versatile. 8.2 Novel vesicles Figure 24 shows the formation of novel water-soluble vesicles as described by Gröhn et al. [72]. These consist of two layers of cationic dendrimer beads held in a layer of the chromonic dye. Note that the chemistry of these structures is totally different to that of classic amphiphile vesicles. There are π–π attractions linking the dye molecules in stacks and electrostatic interactions between the anionic dye stacks and the cationic dendrimers. There are no alkyl chains and no conventional hydrophobic interactions. These vesicles have novel and potentially useful properties for encapsulating water-soluble guest molecules. 9. Organic electronics The operational lifetime of ‘conventional’ solid state electro-optic devices is usually limited by imperfections in the long-range structure. Charge carriers migrate readily across ordered domains but tend to become trapped at grain boundaries, and their accumulation eventually brings the operation of the device

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Liquid Crystals

Figure 24. Artificial vesicles made from dendritic nanospheres, cemented together with a layer of the chromonic dye, Acid Red27 (shown in black). Redrawn from Gröhn et al. [72].

to a halt. It has been suggested that the more mobile and flexible nature of liquid crystalline phases gives them ‘self-healing’ properties which could overcome this problem. Accordingly, the electro-optical properties of π-stacked aromatics have been the focus of increasing attention over the last few years [73–78]. Techniques have been developed for the thermalquenching of aligned samples of thermotropic columnar phases to produce organic semiconductors [79, 80]. However, there are two problems inherent in this approach [81]. Firstly, it is not easy to produce large monodomain areas of aligned thermotropic mesophases. Secondly, the properties of such films are, as one would expect, highly temperature-dependent. Attention has therefore turned to the use of aligned chromonic dyes, since it is relatively easy to produce large areas of aligned chromonic mesophase by shearing, and the structure is preserved when the films are dried down [50, 53, 60, 82, 83]. In recent investigations, Nazarenko et al. have described the production of aligned dye films with promising semi-conducting properties [84]. The chromonic N phase of the perylene dye, Violet 20, was shear-aligned by either spin-coating or with an applicator rod and was dried down at room temperature. The film produced was highly dichroic and birefringent, and had highly anisotropic conductivity. Note that the initial nematic state of the dye solution appears to be essential and that films deposited from the more dilute, isotropic solution had no long-range order. These preliminary studies are promising and it would appear that large-scale productions of complex electronic circuits using ink jet printers should be possible. Current work is focused on the role of

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residual water in films, which appears to be a major factor limiting the charge transport. Of the dozens of chromonic dyes under investigation, it is perhaps reckless to predict which will become the choice mesogens for organic electronics in the future. However, perylene derivatives appear particularly promising. As long ago as 1913, they attracted the attention of the dye industry because they form stable, brilliantly coloured pigments and dyes. In recent years, attention has centred on their electro-optical properties [2, 28–30, 61, 85–87]. These mesogens appear to hold considerable promise. They are, in general, both thermally and photochemically stable and mixtures have been used to produce broad-spectrum polarising films [2]. They possess an impressive range of potentially useful properties (dichroism, fluorescence, semiconductance and photoconductance) and are expected to prove effective chromophores in a wide range of applications (reprographical processes, fluorescent solar collectors, photovoltaic devices, dye lasers and molecular switches). 10. The future In the 1950s, it was the production of synthetic detergents that caused a quiet revolution in the world’s kitchens and bathrooms. Two decades later, liquid crystal display (LCD) devices in watches and calculators began a second revolution. If a volume with the same title as this is produced in 50 years time, I consider it probable that it will record a revolution based on chromonics even more widespread and profound than either of these. Acknowledgements I am heavily indebted to Professor R.J. Busby and Professor Gordon Tiddy for their encouragement and advice over many years. I thank Professor Mark Wilson for advice and permission to include a frame from his computer simulations of the N phase of Edicol Sunset Yellow and Dr Jim Henderson for help in interpreting the results of the coarsegrain modelling of sticky-ended cylinders. I am pleased to acknowledge the major contributions of Dr Jane Turner to the pioneering dye mesophase studies and the central role of Professor Terri Attwood in the initial characterisation of chromonic systems. Finally, I wish to put on record my gratitude to the anonymous referee for help and detailed critical advice.

References [1] Lydon, J.E. J. Mater. Chem. 2010, 20, 10071–10099. [2] Tam-Chang, S-W.; Huang, L. Chem Commun. 2008, 1957–1967. [3] Lydon, J.E. In Handbook of Liquid Crystals: Demus, D., Goodby, J., Gray, J.W., Spiess, H.-W., Vill, V., Eds.; Wiley-VCH: Weinheim, 1998; Vol. 2B, pp. 981–1007.

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