J O U R N A L O F C H E M I S T R Y Materials Feature Article Structure formation of functional sheet-shaped mesogens Dietmar Janietz* Research Group T hin Organic L ayers, Potsdam University, Kantstr. 55, D-14513, T eltow, Germany The combination of anisometric sub-units with an additional intramolecular functionality has frequently resulted in the creation of supramolecular systems not only able to form thermotropic mesophases due to their anisotropic molecular shape but also capable of structure formation resulting from their amphiphilic properties and/or from non-covalent intermolecular interactions with complementary components.This article summarizes diVerent examples of this interplay of structure formation tendencies based on functionalized sheet-like liquid crystals. The formation of thermotropic liquid crystalline phases is Mesophase structures usually formed by flat predominantly caused by an anisometric molecular shape1 sheet-shaped molecules and phase manipulation whereas amphiphilic molecules characterized by combined by doping with electron acceptors hydrophilic and lipophilic groups may exhibit lyotropic mesophases in the presence of a solvent2 or two-dimensional It was discovered in 1977 that hexa-n-alkanoyloxybenzene supramolecular assemblies at an air–water interface.3 Yet the derivatives exhibit mesophases with a columnar structure,10 shape of a single molecule is not the only structure controlling and since then a wide variety of liquid crystalline compounds factor.Form-anisotropic aggregates giving rise to mesomorphic of quite diVerent chemical structures have been prepared structure formation can also be formed by specific intermolecu- possessing a flat or nearly flat rigid core surrounded by a lar interactions between identical or diVerent individual mol- specific number of peripheral long chain alkyl substituents ecules.Such attractive interactions may, for example, arise (three to twelve). from ionic structures,4 dipole–dipole interactions,5 hydrogen Several types of mesophases formed by those sheet-like bonding6 or charge transfer eVects.7 molecules have been identified which diVer with respect to the The combination of anisometric sub-units with an additional state of order11 (Fig. 2). The mesophase exhibiting the lowest intramolecular functionality, furthermore, oVers a powerful state of order is the nematic discotic (ND) phase, well estabtool to design molecular architectures which are characterized lished, e.g.for radial multialkynylbenzene derivatives,12 in by the fact that their self-organization results from a complex which the planes of the flat molecules are oriented more or interplay and/or competition of diVerent factors and driving less parallel to each other giving rise to a preferred orientational forces of structure formation.In this way, by creating multi- order of the short molecular axis. functional chemical primary structures, a much greater scope In contrast to the common ND phase, the nematic columnar is oVered for control and manipulation of supramolecular (Ncol) phase is characterized by a columnar stacking of the assemblies than can be achieved with monofunctional molecules.However, these columns do not form two-dimenmesogens. In the case of rod-like mesogens an additional functional sub-unit can be incorporated either along or perpendicular to the main molecular axis.8 The flat molecular geometry of disclike or, more generally, sheet-like9 compounds allows two possibilities to be combined with an additional intramolecular function (Fig. 1). The functional sub-unit can be fixed at one or more positions at the periphery of the molecule via flexible spacers (A) or it can be an integrated part of the rigid central molecular core (B). This paper is concerned with examples of functionalized sheet-like systems of both general structure types A and B, given schematically in Fig. 1, which are not only able to form thermotropic mesophases due to the anisotropic molecular shape but also capable of structure formation resulting from amphiphilic properties and/or from non-covalent intermolecular interactions with complementary components, the latter giving rise to a manipulation or an induction of columnar liquid crystalline phases.However, it is far beyond the scope of the present paper to give a comprehensive overview of this topic. Fig. 1 Multifunctional supramolecular systems by intramolecular com- * E-mail: zetsche@rz.uni-potsdam.de bination of anisometric and functional molecular sub-units J. Mater. Chem., 1998, 8(2), 265–274 265n-alkanoates or hexaalkoxy(acyloxy)triphenylene derivatives, form monolayers at the air–water interface with only a low compressibility.17 It is therefore obvious that those compounds lack distinct amphiphilic properties.A powerful tool towards sheet-like molecules with more pronounced hydrophilic and hydrophobic regions consists of the asymmetrical incorporation of one or two terminal polar substituents at the periphery of an extended core via flexible spacers.In this way it might be possible to combine liquid crystalline behaviour and amphiphilic self-organization within one molecule.18 Members of three families of flat molecules 1a,b, 2a–e and 3a–d are consistent with the general structure type of amphiphilic sheet-shaped compounds given schematically in Fig. 3. Fig. 2 Typical examples of thermotropic mesophases formed by low molar mass or polymeric sheet-like molecules sional lattice structures.They display a positional short range order and an orientational long range order.13 A parallel alignment of the columns results in columnar phases with a two-dimensional lattice symmetry such as columnar hexagonal (Colh), oblique (Colob; not shown in Fig. 2) and rectangular (Colr), the latter usually arising from a tilt of the average molecular plane against the column axis.Furthermore, the molecules may be arranged in a regular ordered manner or, in the case of a liquid-like ordering, aperiodically (disordered) within each column. For example, disc-like mesogens based on the triphenylene core surrounded by six alkoxy substituents usually exhibit a hexagonal columnar ordered (Colho) phase.11,14 Organic compounds containing electron donor units can be doped with acceptor molecules and it is well-established that disc-like electron-rich systems such as triphenylenes and multialkynes form charge-transfer complexes with rather flat but non-liquid crystalline electron acceptors like 2,4,7-trinitro- fluoren-9-one (TNF).15 Donor–acceptor interaction may lead to manipulations as well as the induction of columnar mesophases.The columns are then formed by mixed stacks of the flat donor molecules and the electron acceptor. Charge-transfer (CT) interaction of hexagonal columnar phase-forming hexaalkoxytriphenylenes with TNF results in a stabilization of the already existing mesophase.15b,c Even using non-mesogenic triphenylene derivatives, doping with TNF gives rise to the induction of hexagonal columnar phases.15b Nematic discotic hexaalkynylbenzene compounds were found to exhibit CT-induced Colho liquid crystalline structures15b whereas binary mixtures of non-liquid crystalline pentakis(phenylethynyl)benzene ethers and an electron acceptor were found to form nematic-columnar (Ncol) as well as ordered hexagonal columnar (Colho) structures due to the donor–acceptor interactions.13,16 Considering the donor function as part of the rigid central molecular cores that, apart from the thermal properties of the pure compounds, give rise to control of structure formation through intermolecular CT interactions it follows that radial multiynes and triphenylenes are representatives of functional sheet-like mesogens of the general structure type B (Fig. 1). H11C5O OC5 H11 H11C5O OC5 H11 O O N N N HN NH N N N O (CH2) n X R R R R (CH2) n (CH2) n R R R R R R X X R (CH2)6 OH 1a R = C5H11 b R = (CH2)6OH 2a R = C8H17 X = CO2H n = 3 b R = C10H21 X = CO2H n = 3 c R = C8H17 X = OH n = 6 d R = C6H13 X = OH n = 4 e R = C10H21 X = OH n = 4 3a R = C5H11 X = CO2H n = 10 b R = C5H11 X = OH n = 11 c R = H X = CO2H n = 10 d R = H X = OH n = 11 R Whereas the triphenylene 1a exhibits a hexagonal columnar Low molar mass sheet-like molecules bearing ordered (Colho) phase the two-fold hydroxy terminated com- peripheral polar substituents pound 1b forms a monotropic lamellar LC phase with a well defined double-layer packing of associated pairs of discs (aris- Studies on the spreading behaviour of disc-like liquid crystals have shown that aromatic core systems fitted with a specific ing from partial overlapping of the OH-containing tails) and a local columnar intralayer ordering.19 Doping of 1b with number of equal long aliphatic chains, such as benzene-hexa- 266 J.Mater. Chem., 1998, 8(2), 265–274Fig. 4 Schematical presentation of the molecular edge-on orientation of sheet-shaped amphiphiles bearing terminal polar head groups with (a) columnar ordering parallel to the surface and (b) two-dimensional nematic-discotic (ND) like arrangement. Lateral flexible side groups are not shown, only one hydrophilic function per molecule is presented.dimensional gas phase into the compressed state.21–23 In contrast to oligomers derived from 2c22 the surface pressure–area isotherms of 2a–e show only a small hysteresis during expansion of the monolayers. In the case of sheet-like multialkynylbenzene compounds only the incorporation of terminal hydrophilic substituents (compounds 3) gives rise to amphiphilic properties.Only threedimensional crystallization was described for the radial symmetrical hexakis[(4-hexyl-phenyl)ethynyl]benzene at the air–water interface.27 The p–A isotherms of the hydroxy or Fig. 3 Peripheral attachment of a hydrophilic functional sub-unit to a carboxy terminated pentaynes 3a–d show no phase transition rigid sheet-like molecular part during compression but a direct transition to a solid condensed form.26 Attributed to the presence of only one hydrophilic terminal substituent, the collapse pressure of the pentaynes 3, TNF leads to an enantiotropic charge-transfer-induced hexagonal columnar ordered (Colho) phase.20 however, is relatively low on a pure water subphase.26 The monolayer stability can be enhanced significantly either by Unlike the phthalocyanine dicarboxylic acids 2a,b, especially designed as materials for Langmuir–Blodgett (LB) film fabri- incorporation of a second polar head group as demonstrated for tetraalkynylbenzene derivatives with two neighbouring cation,21 the two-fold hydroxy substituted compound 2c exhibits an enantiotropic columnar phase with a two-dimensional hydrophilic functions attached to the ortho positions of the central benzene ring via flexible spacers28 or by supplementary hexagonal lattice symmetry as bulk material22 whereas the phthalocyanines 2d,e form monotropic columnar phases.23 interactions of the hydrophilic head groups with counterions dissolved in the subphase.29 The pentaalkynylbenzene derivatives 3a,b bearing five lateral pentyl substituents form a nematic-discotic (ND) mesophase The two-dimensional monolayer assemblies of the edge-on oriented sheet-shaped amphiphiles 1–3 can vary from a colum- on both heating and cooling; the appropriate laterally unsubstituted compounds 3c,d are only crystalline materials.24 Charge- nar stacking parallel to the surface25,27 to a nematic–discotic (ND) like arrangement;28 the latter was proved for compounds transfer interaction of the five-fold pentyl-modified pentaynes 1a,b with TNF results in the induction of hexagonal columnar 3.The monolayer arrangements presented schematically in Fig. 4, which are quite diVerent from those of classical amphi- ordered (Colho) mesophases whereas the lateral unsubstituted penaalkynes 3c,d form a CT-induced nematic columnar (NCol) philes, may not only arise from decreasing the available area per molecule during the compression process but also arise phase in mixtures with TNF.24 This behaviour agrees well with the thermal properties of radial hexaalkynylbenzene derivatives from a spontaneous aggregation of the edge-on oriented molecules immediately after spreading resulting in condensed and pentakis(arylethynyl)phenyl ethers without a terminal polar functionality.12,13,16 It indicates that it is not the incorpor- monolayer islands which are pushed together during compression.28,30 ated polar function but the lateral substitution pattern that determines the liquid crystalline structure formation of the Successful attempts to prepare LB multilayers have been reported for members of all three series of non-classical amphi- pentaynes 3a–d.All sheet-like systems 1–3 functionalized by one or two philes 1–3 (triphenylenes 1,19,20,31 phthalocyanines 2,21–23,33 pentaynes 326,29,32).polar terminal groups form monolayers on a pure water subphase.21–23,25,26 It is a common feature that, independent It is common for all amphiphiles 1–3 that dipping of a hydrophobic substrate through the compressed monolayer of the diVerent chemical structures of the flat, sheet-like cores, the average areas per molecule in the compressed monolayers results in transfer of a monolayer each time the substrate passes the surface boundary.During first immersion the first of the compounds 1–3 are less than the area requirement for the central molecular parts lying flat on the water surface. monolayer is transferred so that the hydrophobic rigid cores face the substrate surface.A second monomolecular layer with However, the collapse areas agree quite well with an edge-on arrangement27 of the compounds 1–3 with a molecular orien- the disc-shaped cores in the opposite direction is formed on top of the first monolayer during withdrawal of the substrate tation of the plane of the rings more or less perpendicular to the water (Fig. 4). [Fig. 5(a)].The edge-on orientation of the amphiphiles at the air–water interface is preserved during the formation of the The peripheral attachment of just one or two hydroxy groups to the triphenylene core (compounds 1a,b) leads to LB films. This Y-type deposition gives rise to a bilayer packing of the molecules perpendicular to the substrate (head-to-head monolayers with higher collapse pressures compared to those of symmetrically substituted members such as hexapentyloxy- and tail-to-tail arrangement of molecular monolayers).Further structural characteristics of LB films made from triphenylene.25 The behaviour of the hydroxy substituted phthalocyanines compounds of the series 1–3 other than those mentioned above are not uniform. For example, the mesomorphic double-layer 2c–e at the air–water interface is very similar to the structurally related two-fold carboxy terminated compounds 2a,b.During structure of the two-fold hydroxy terminated triphenylene 1b is preserved during the formation of the LB multilayers with compression a sharp increase in the surface pressure–area isotherms is observed indicating a transition from the two- a columnar ordering parallel to the solid support and the J.Mater. Chem., 1998, 8(2), 265–274 267Fig. 5 Langmuir–Blodgett (LB) multilayers of sheet-shaped molecules asymmetrically incorporating hydrophilic substituents. (a) Fabrication of the LB films; (b) schematic presentation of the two-dimensional structure for the single component LB film derived from the amphiphilic pentayne 3b (refs. 26,29). periodicity of the rectangular lattice relative to the normal determined by the distance between next nearest pairs of edge- Fig. 6 Modes of structure formation of functional sheet-shaped meso- on oriented molecules.19 The bilayer spacings observed for the gens as an outcome of amphiphilic properties, anisometric molecular LB films of the phthalocyanines 2a,b are less than the calculated shape and intermolecular donor–acceptor interactions molecular dimensions that might be explained by a tilt of the planes of the molecules from the normal or by an interdigitation of chains in adjacent layers.21 The pentaynes 1a,b form Y-type bilayers with edge-on orientation of the discs.The main molecular axis of the molecules is tilted against the normal of the surface.The flexible molecular segments of neighbouring molecules are interdigitated26,29 (Fig. 5). A hexagonal layer packing perpendicular to the surface proved to be possible in the LB films of the laterally unsubstituted pentaalkynyl carboxylic acid 3c without lateral substituents.26,30 Fig. 6 illustrates the richness of supramolecular assemblies of functionalized disc-like mesogens such as 1–3 arising from the combination of a hydrophilic sub-unit with a flat hydrophobic anisometric core, the latter oVering the opportunity to be combined with an additional donor function.Functional polymers bearing disc-shaped side groups Compared with calamitic polymers, the variety of polymers incorporating disc-shaped units, predominantly based on the triphenylene core, is rather limited.Triphenylene main chain polymers are usually either non-mesomorphic or exhibit hexagonal columnar (Colho) phases.34 Attaching triphenylene groups to polysiloxane, polyacrylate, polymethacrylate or polyester backbones, respectively, gives rise to triphenylene side chain Fig. 7 Molecular architecture of functional polymers attached with polymers which, depending on the chemical nature of the main sheet-shaped side groups chain and/or the spacer length, were found to be amorphous or to form columnar mesophases with either hexagonal or rectangular lattices.35 Doping of non-mesogenic triphenylene have been described capable of forming highly ordered LB multilayers.19,20,31 polymers induces either nematic-columnar (Ncol; side chain polymers) or hexagonal columnar ordered (Colho; main chain An eVective approach towards controlling supramolecular structures at interfaces as well as in the mesomorphic bulk polymers) mesophases.36 Apart from investigations on thermal properties a few examples of polymeric triphenylene derivatives state involves the attachment of sheet-like sub-units to a 268 J.Mater. Chem., 1998, 8(2), 265–274functional hydrophilic backbone having the tendency to form associated structures via complementary hydrogen bonding.This requires the combination of anisometric structural elements with a flexible polymer chain and with an additional functional sub-unit. This concept outlined schematically in Fig. 7 was realized recently by the synthesis of oligomers characterized by the attachment of either triphenylene (4a,b) or pentaalkyne (5a,b) side groups to amino substituted 1,3,5-triazine moieties in the backbone.37 Fig. 8 Model for molecular bilayer arrangement of the triphenylene oligomers 4 in LB multilayers and in the mesomorphic bulk state; flexible lateral alkyl chains are not shown These restrictions disturb closest face-to-face intracolumnar packing and favour the interdigitation of triphenylene groups belonging to diVerent backbones.38b With the exception of the laterally unsubstituted pentayne 5b the triazine oligomers exhibit an enantiotropic mesophase in the bulk.The structure displayed by the oligomers with disc-like side groups corresponds to a smectic A like arrangement, which is highly surprising.37 The magnitude of the layer dimensions indicates that no single-layer arrangement takes place but rather a double-layer one (Fig. 8). The double-layer structure is stabilized by the formation of hydrogen bonds between the aminotriazine segments of the polymer backbone. It is thus apparent that the interactions between the backbone segments frustrate the structure preferred by the disc-like units.The molecular dimensions of the LB films and the doublelayer spacing within the mesomorphic bulk state of the triphenylene oligomer 4a are very close. This implies that the layer structure in the bulk LC state is preserved during the formation of the LB films on the solid substrate although the two mechanisms of structure formation are quite diVerent.38a Doping of the triphenylene oligomer 4b with the acceptor TNF results in the induction of a rectangular columnar (Colrd) mesophase.No such strong eVect was found for the doped compound 4a characterized by the longer spacer. The meso- OC5H11 OC5H11 O H11C5O (CH2)m O N N N NH HN (CH2)6 OC5H11 H11C5O n 4a m = 11 b m = 6 O (CH2)11 O R R R R N N N NH HN (CH2)6 5a R = C5H11 b R = H R n genic organization of mixtures from 4a and TNF remains a lamellar layer structure.Thus, the mesophase arrangements of All oligomers 4 and 5 form stable monolayers when spread the donor–acceptor complexes are a function of the spacer on the water surface.37 The collapse areas of the triphenylene connecting the triphenylene side groups with the main chain.37 oligomers 4 and of the oligomeric pentaynes 5 are close to However, the rectangular lattice spacings (4b/TNF) as well as those of the respective hydroxy- or carboxy-terminated monthe double-layer dimensions (4a/TNF) indicate that the struc- omers 1 and 3.This indicates an edge-on orientation of the ture formation of the binary mixtures result from both, inter- hydrophobic triphenylene or pentaalkyne side groups for each molecular charge-transfer interactions and intermolecular triazine oligomer 4 and 5 while the amino substituted triazine hydrogen bonding.rings serve as anchor groups at the water surface. It follows that the structures displayed by the functional The amphiphilic triphenylene 4a, furthermore, has been triazine based oligomers 4 and 5 are the result of a delicate reported to form LB films with a bilayer packing of edge-on balance of interactions which may compete with each other oriented triphenylene groups (Y-type deposition) and with (Fig. 9).interdigitation of the flat cores.38 The structural model given in Fig. 8 displays columnar in-plane packing where alternate triphenylene group belongs to a diVerent backbone and the Covalently linked donor–acceptor twin mesogens orientations of adjacent disc-shaped groups are alternating.based on flat electron-rich donor sub-units This type of columnar structure is caused by spacial restrictions imposed by exceeding the distance between chemical attach- Charge-transfer interactions of two individual molecules each incorporating either a donor or an acceptor function, in the ments of the neighbouring sheet-like cores along the backbone.J. Mater. Chem., 1998, 8(2), 265–274 269Fig. 10 Covalently linked charge transfer twin mesogens based on flat donor and acceptor sub-units spacers of diVerent length.39 Keeping the spacer length constant, structural modifications were performed at the lateral sphere of the pentayne units of compounds 7 in order to influence the magnitude of the molecules and thus the eYciency Fig. 9 Interplay of diVerent driving forces to control the structure of expected intermolecular charge transfer interactions.40 The formation of the 1,3,5-triazine based triphenylene and pentaalkyne twin molecules 8 incorporate an asymmetric carbon as an oligomers 4 and 5 additional intramolecular functionality.41 The mesophase structure of the triphenylene based CT-twin compounds 6 is characterized by an arrangement of the case of sheet-shaped triphenylene ethers or radial multialkynylmolecules in columns in such a way that mixed stacks occur.benzene derivatives as the donor molecules, give rise to a Each column is connected in one direction with two neighbour- manipulation or an induction of columnar mesophases. The ing columns chemically via the flexible spacers.The intercolum- columnar phases, then, are usually of the hexagonal or nematicnar packing has been described as possessing an orthorhombic columnar type. symmetry with ab (for compound 6a) or in case of compound However, the components need not necessarily be derived 6b as displaying either an orthorhombic lattice with tilted from separated molecules but from mesogens which incorporplanes of the discs or as a hexagonal two-dimensional ate both donor and acceptor functions into a single molecule.structure.39 Such an approach consists of a chemical linkage of a flat The donor–acceptor molecules 7 based on sheet-like penta- anisometric phase forming moiety with an acceptor functional alkynes exhibit a rectangular columnar phase with a=b and sub-unit via a flexible spacer.This concept has been realized with a high intracolumnar periodicity resulting from closely by coupling of electron-rich triphenylene (6) or pentayne (7,8) face-to-face arranged alternating donor and acceptor moieties units and electron-poor trinitrofluorenones (Fig. 10). of the molecules.41 Whereas the molecules in the columns are connected through charge-transfer interactions, the chemical linkage of the donor and acceptor sub-units facilitates the intercolumnar packing.These special features give rise to a three-dimensional order, at least in the case of compound 7a41 (Fig. 11). Thus, it is not the chemical nature of the donor molecular moieties but the linkage with an intramolecular acceptor functionality that dominates the structure formation of the charge-transfer twin mesogens 6 and 7, e.g.the formation of rectangular symmetries of intercolumnar packing instead of common hexagonal or nematic-columnar. A further approach towards control of mesomorphic structures of sheet-shaped CT-twin mesogens arises from chirality as an additional intramolecular function. Preliminary structure investigations give rise to the conclusion that the twin compounds 8 incorporating an asymmetric carbon exhibit a nematic-columnar mesophase with a helical twisting of the columns.41,42 O OC5H11 OC5H11 H11C5O OC5H11 H11C5O (CH2)6 O (CH2)2 O O N NO2 NO2 NO2 O R R R (CH2)11 O X O N O NO2 Y NO2 NO2 R R 6a n = 0 b n = 1 7a–c X = (CH2)2 Y = H R = H (7a), R = CH3 (7b) R = C5H11 (7c) 8a–c X = C*HCH3 Y = NO2 R = H (8a), R = CH3 (8b) R = C5H11 (8c) n The triphenylene based CT-twin molecules 6 are charac- Fig. 11 Structure model of the rectangular columnar mesophase of the pentayne based donor–acceptor twin mesogens 7 terized by connecting the donor and acceptor moieties via 270 J. Mater. Chem., 1998, 8(2), 265–274Functional heterocyclic azacoronands and 1,3,5- triazines Suitably substituted N-acylated macrocyclic oligoamides, e.g. 9–12, diVering in the heterocyclic ring size as well as in the number of nitrogen atoms incorporated into the saturated central cores have been found to exhibit thermotropic hexagonal columnar mesophases.43 The central cavity in the columns led to these phases being described as tubular.43a Fig. 12 Schematic presentation of the side-on arrangement of sheetlike amphiphiles with a polar central core surrounded by a certain number of hydrophobic alkyl chains Fig. 13 1,3,5-Triazines as central parts of functional sheet-shaped molecules with open-sided cores ethers, radial multiynes) as well as in the case of ‘hollow’ core systems (azacrowns) gives rise to manipulations of as well as the induction of columnar mesophases due to specific intermolecular interactions (charge-transfer complex formation, metal complex formation).However, these functionalities do not N N N N N N X R X R X R X R X R X R 9 X = CO; R = 10 X = CO; R = 11 X = CO; R = OC12H25 OC14H29 OC10H21 OC10H21 N N X R X R N N X R X R 12 X = CO; R = OC10H21 OC10H21 allow side-by-side interactions with a complementary component.In contrast to macrocyclic amides such as compounds 9–12 Such an approach may consist of ‘open-sided’ core systems alkyl-substituted azacrowns show no mesophase behaviour due having the capacity to form columnar mesophases as single to an increased conformational flexibility of the ring system. components but also enabling control of structure formation However, complexation of transition metal salts by highly by a peripheral attack of a second component to the inner flexible benzyl substituted cyclic amines bearing a certain (functional) core region (Fig. 13). number of peripheral long alkoxy chains (for example, 10 and 12; X=CH2 instead X=CO) gives rise to the induction of columnar liquid crystalline phases.44 Coordination to the metal ions (e.g. Cu2+, Ni 2+ or Co3+) imposes the desired conformational rigidity of the functional central macrocyclic rings.In this way, the sheet-like molecular geometry of azacrowns and the complex formation properties of macrocyclic ligands, leading to specific host–guest systems, can be combined.44a The polar central core of cyclic azacoronands fitted with long lipophilic hydrocarbon chains gives rise to azamacrocycles with distinct amphiphilic properties.Beside the benzoyl substituted compounds 9 and 10 certain acylated43b,45 and alkylated46 hexacyclene and cyclam derivatives have been shown to form stable monolayers at the air–water interface. The arrangement of either amine or amide derivatized azacrowns, in general, is the same at the interface.46 Within the monolayer the molecules adopt an orientation in which the hydrophilic OR OR N H N N N N RO RO N H OR OR H 13a R = C10H21 R = C12H25 R = C16H33 b c macrocycle lies flat on the water surface (side-on arrangement).In the condensed state the hydrophobic alkyl chains are Following this concept, recently the 2,4,6-triarylamino-1,3,5- triazines 13 have been prepared bearing six long peripheral oriented more or less perpendicular to the interface in a closepacked all-trans conformation (Fig. 12). alkoxy chains. The melamines 13 form enantiotropic columnar mesophases Combining a flat anisometric molecular shape with an intramolecular functionality located in the central core region although they are characterized by a lack of inherent molecular planarity.47 In the case of the heterocyclic mesogens 13a,b the in the case of ‘closed’ aromatic core systems (triphenylene J.Mater. Chem., 1998, 8(2), 265–274 271columns are arranged in a hexagonal array with an aperiodic Furthermore functional units embedded in the central cores, in certain cases, give rise to an induction of mesomorphic intracolumnar stacking of the molecules (Colhd). Further elongation of the lateral alkyl chain length results in a major properties by metal complexation of azacoronands whereas molecular recognition due to intermolecular hydrogen bonding structural change.Compound 13c exhibits the rarely observed ordered rectangular columnar (Colro) phase. allows control of the hexagonal lattice constants as well as variation of the two-dimensional lattice type of the columnar- The mesomorphic triarylmelamines are characterized by a heterocyclic 1,3,5-triazine core with a three-fold substitution phase-forming triarylmelamines (Fig. 14). with secondary amino groups promoting attractive interactions with complementary functional molecules via intermolecular hydrogen bonding (side-by-side attack). Outlook Binary mixtures of the melamine 13a with non-mesogenic The few examples discussed here may show that it is possible 3,5-dialkoxy substituted benzoic acids exhibit a hexagonal to combine molecular sub-units of a flat sheet-like anisometric columnar disordered (Colhd) phase at least at an equimolar shape with an additional intramolecular functionality in vari- ratio of the components. The intercolumnar distances are a ous ways.Functional disc-like systems can result that combine function of the length of the alkoxy groups of the acid structure forming tendencies due to amphiphilic properties and component.48 A 3,4-dialkoxy substitution pattern of the benthose arising from an anisometric molecular geometry within zoic acid gives rise to a change of the columnar mesophase one molecule. The intramolecular function, furthermore, may structure of the melamine 13a from a hexagonal to a rectanguallow control of and/or induction of supramolecular (columnar) lar lattice (Colrd) in binary mixtures with the aromatic acids.48 structures by non-covalent interactions with complementary Equimolar mixtures of the melamines 13 and 4-alkoxybenzoic components.However, those intramolecular functionalities acids exhibit a hexagonal columnar disordered (Colhd) strucpresented here usually do not facilitate reversible control of a ture.49 In the case of the melamine 13a the hexagonal lattice supramolecular assembly once obtained either in single compo- constants increase with increasing number of methylene groups nent or mixed multicomponent systems. They might therefore of the para-alkoxy substituted benzoic acid component.49 The be considered as functions of the first generation.appearance of a calamitic phase characteristic of 4-alkoxyben- Reversible control of columnar structures, such as gener- zoic acids in their pure state, due to a dimerization of the ation, manipulation and destruction, might be possible by acids, is not observed.Hence, associations with the aminotriazcombining a disc-shaped molecular part, probably already ines 13 completely frustrate the tendency of the aromatic acids incorporating a function of the first generation, with an to form dimers. additional reversibly switchable sub-unit (function of the Furthermore, the triarylmelamines 13 are characterized by second generation). a polar central heterocyclic core and long non-polar alkoxy Compounds 14 are the first representatives of CT-triple side chains.Amphiphilic properties arise from this combination molecules consisting of a sheet-like pentayne donor and a of diVerent structure elements and the triazines 13 form TNF based acceptor which are covalently linked via a rod- monolayers on the water surface. Similarly, as found for like sub-unit incorporating an azobenzene moiety (Fig. 15). amphiphilic azamacrocycles a molecular side-on arrangement The compounds exhibit a nematic mesophase at elevated is observed with the amino modified triazine ring as the most temperatures and form a glassy state at room temperature.41 hydrophilic molecular part lying flat47 (Fig. 12). It seems possible to combine here the structure formation of Thus, azamacrocycles and 1,3,5-triazines bearing long alidisc- like donor–acceptor twin mesogens with the ability of the phatic side chains are two classes of sheet-like molecules possessing both mesomorphic and amphiphilic behaviour.azo group present in the spacer to be switched by light. Fig. 14 Aspects of mesomorphic structure formation of functional azacrowns and triarylmelamines 272 J.Mater. Chem., 1998, 8(2), 265–274O R R R (CH2)11 R R O CO N N O CO (CH2)2 O N NO2 O2N O2N 14 R = H, CH3 B. Kohne, K. Praefcke, H. Ringsdorf, J. H. WendorV and R. Wu� stefeld, Adv. Mater., 1990, 2, 141; (c) M. Ebert, G. Frick, C. Baehr, J. H. WendorV, R. Wu� stefeld and H. Ringsdorf, L iq. Cryst., 1992, 11, 293. 16 K. Praefcke, D. Singer, M.Langner, B. Kohne, M. Ebert, A. Liebmann and J. H. WendorV, Mol. Cryst. L iq. Cryst., 1992, 215, 121. 17 O. Albrecht, W. Cumming, W. Kreuder, A. Laschewsky and H. Ringsdorf, Coll. Polym. Sci., 1986, 264, 659. 18 H. Ringsdorf, B. Schlarb and J. Venzmer, Angew. Chem., 1988, 100, 117. 19 O. Karthaus, H. Ringsdorf, V. V. Tsukruk and J. H. WendorV, L angmuir, 1992, 8, 2279. Fig. 15 Donor–acceptor triple mesogens incorporating an additional 20 V. V. Tsukruk, J. H. WendorV, O. Karthaus and H. Ringsdorf, calamitic functional sub-unit L angmuir, 1993, 9, 614. 21 (a) M. J. Cook, M. F. Daniel, K. J. Harrison, N. B. McKeown and A. J. Thomson, J. Chem. Soc., Chem. Commun., 1987, 1148; The author appreciates very much the colleagues and co- (b) N. B. McKeown, M. J. Cook, A.J. Thomson, K. J. 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