Dimeric liquid crystals with 5‐(4‐alkoxybenzoyloxy)tropone or 4‐(4‐alkoxybenzoyloxy)phenyl cores: evaluation of the tilt angles of the cores,spacers, and side chains

Two types of symmetric dimers with 5‐(4‐alkoxybenzoyloxy)tropone cores or with 4‐(4‐alkoxybenzoyloxy)phenyl cores were synthesized to evaluate the effect of the core structure and the length of the spacer on the mesomorphic properties. The former had smectic C phases whereas the latter had smectic C and F phases. Both types of dimer showed a remarkable odd–even effect on varying the spacer on the mesomorphic properties. Comparison of the thermal stability between them demonstrated that benzenoid twins are more stable than troponoid ones. The layer spacings of the smectic C phases were measured to determine the tilt angles of the core part, the spacer, and the side chains on changing the length of the spacer and the side chains. Troponoid dimers had a larger tilt angle of the core part of the smectic C phase than benzenoid twins, which lowered the thermal stability of the troponoid. Entropy changes of the smectic C phase to the isotropic liquid showed a contrast between troponoids and benzenoids. The former had smaller values and odd–even effects than the latter, which indicated that the former troponoids had a limited number of conformers in mesophases.     

Non-symmetric calamitic liquid crystal dimers containing troponoid and benzenoid units

Five types of non-symmetric calamitic dimers were synthesized to investigate the effect of the core structure and length of the spacer on mesomorphic properties. Two non-symmetric dimers containing a troponoid and benzenoid unit showed smectic A and C phases whereas the corresponding benzenoid dimers showed no mesophase. Non-symmetric dimers with a three-ring system showed smectic A and C phases with higher transition temperatures than the two-ring system. We propose packing models for these non-symmetric dimers by considering the direction of the dipole moments of the ring structures and microsegregation between the polar units and the non-polar chains.     

Roles of sigmatropic migration of a benzoyl group and intermolecular hydrogen bonding between a tropone carbonyl and an NH group on the mesomorphic properties of 2-(4-alkoxybenzoyloxy)-5-alkylaminotropones and 5-alkoxy-2-(4-alkylaminobenzoyloxy)tropo

The mesomorphic properties of 2-(4-alkoxybenzoyloxy)-5-alkylaminotropones and 5-alkoxy-2-(4-alkylaminobenzoyloxy)tropones are discussed on the basis of results obtained by X-ray crystallographic and X-ray diffraction studies, as well as temperature-dependent FTIR spectral measurements. The X-ray crystallographic analysis of 2-(4-dodecylaminobenzoyloxy)-5-tetradecyloxytropone (2f) indicated that it formed a head-to-tail dimer through an intermolecular hydrogen bond between the NH and the tropone carbonyl group. The X-ray diffraction study of compound 2f suggested that the molecules formed interdigitated bilayer smectic C phases with a tilt angle of as much as c. 40° to a layer plane. The corresponding benzenoids, however, were non-mesomorphic, with higher melting points than the troponoids. In the troponoids, sigmatropic migration of the benzoyl group weakened the intermolecular hydrogen bonding and assisted the occurrence of mesomorphic properties.     

Roles of sigmatropic migration of a benzoyl group and intermolecular hydrogen bonding between a tropone carbonyl and an NH group on the mesomorphic properties of 2-(4-alkoxybenzoyloxy)-5-alkylaminotropones and 5-alkoxy-2-(4-alkylaminobenzoyloxy)tropo

The mesomorphic properties of 2-(4-alkoxybenzoyloxy)-5-alkylaminotropones and 5-alkoxy-2-(4-alkylaminobenzoyloxy)tropones are discussed on the basis of results obtained by X-ray crystallographic and X-ray diffraction studies, as well as temperature-dependent FTIR spectral measurements. The X-ray crystallographic analysis of 2-(4-dodecylaminobenzoyloxy)-5-tetradecyloxytropone (2f) indicated that it formed a head-to-tail dimer through an intermolecular hydrogen bond between the NH and the tropone carbonyl group. The X-ray diffraction study of compound 2f suggested that the molecules formed interdigitated bilayer smectic C phases with a tilt angle of as much as c. 40° to a layer plane. The corresponding benzenoids, however, were non-mesomorphic, with higher melting points than the troponoids. In the troponoids, sigmatropic migration of the benzoyl group weakened the intermolecular hydrogen bonding and assisted the occurrence of mesomorphic properties.     

Mesomorphic property of 2,5-dibenzoyloxy-, 5-benzoylamino-2-benzoyloxy-, and 2,5-dibenzoylamino-tropones with mono-, di-, and tri-alkoxyl groups on the benzoyl groups and their benzenoid derivatives

Mesomorphic properties of three-ring systems such as 2,5-dibenzoyloxytropones, 5-benzoylamino-2-benzoyloxytropones and 2,5-dibenzoylaminotropones with 4-alkoxy, 3,4-dialkoxy, and 3,4,5-trialkoxy groups on the benzoyl groups were investigated together with those of the corresponding benzenoids. Derivatives with two monoalkoxybenzoyl groups showed nematic and smectic A and C phases. Troponoid tetracatenars with two dialkoxybenzoyl groups had hexagonal columnar phases except for troponoids with two ester-connecting groups, whereas the corresponding benzenoids with two dialkoxybenzoyl groups were non-mesomorphic. All troponoid hexacatenars with two trialkoxybenzoyl groups formed hexagonal columnar phases. With the exception of benzenoid hexacatenars with two ester-connecting groups, the benzenoid hexacatenars showed hexagonal and tetragonal columnar phases. These mesomorphic properties were discussed from the standpoint of the difference of the core structure and the connecting group, where the amide-connecting group played a role to induce and enhance mesomorphic properties through hydrogen bonding.     

Dimeric liquid crystals with two troponoid mesogenic moieties: effect of the direction of the carbonyl group on the mesomorphic properties

Three twin types of troponoid liquid crystals, in which the directions of the tropone carbonyl groups are different from each other, were prepared to establish their thermal properties. Symmetrical type I dimers, in which the two tropone carbonyl groups are directed inwards, had monotropic smectic C phases, whereas two other symmetrical type IIa and IIb dimers, whose troponoid dipoles are directed outwards, were not mesomorphic except for one type IIa compound having two long terminal chains and a short alkylene spacer. Unsymmetrical type III dimers had smectic A phases. Among the dimers, unsymmetrical type III dimers had the highest clearing point when the number of atoms of the inner and outer spacers was fixed. X-ray diffraction studies of a type III dimer, in which the number of the atoms of the inner spacer was odd, showed that the molecules form an interdigitated layer structure. The binary system between type I and type IIb dimers showed an induced enantiotropic smectic A phase, in which the dipole moments of the tropone rings were cancelled.     

Dimeric liquid crystals with two troponoid mesogenic moieties: effect of the direction of the carbonyl group on the mesomorphic properties

Three twin types of troponoid liquid crystals, in which the directions of the tropone carbonyl groups are different from each other, were prepared to establish their thermal properties. Symmetrical type I dimers, in which the two tropone carbonyl groups are directed inwards, had monotropic smectic C phases, whereas two other symmetrical type IIa and IIb dimers, whose troponoid dipoles are directed outwards, were not mesomorphic except for one type IIa compound having two long terminal chains and a short alkylene spacer. Unsymmetrical type III dimers had smectic A phases. Among the dimers, unsymmetrical type III dimers had the highest clearing point when the number of atoms of the inner and outer spacers was fixed. X-ray diffraction studies of a type III dimer, in which the number of the atoms of the inner spacer was odd, showed that the molecules form an interdigitated layer structure. The binary system between type I and type IIb dimers showed an induced enantiotropic smectic A phase, in which the dipole moments of the tropone rings were cancelled.     

Mesomorphic properties of monocyclic troponoids with a semi-fluorinated side group

Eight types of monocyclic troponoids with alkoxy, acyloxy, and semi-fluorinated side chains were synthesized to investigate their thermal behaviour. They showed smectic A phases. The troponoids with a fluorinated side chain had higher transition temperatures than the corresponding non-fluorinated compounds.     

The intercalated smectic A phase. The liquid crystal properties of the α-(4-cyanobiphenyl-4'-yloxy)-ω)-(4-alkyloxycinnamoate)alkanes

A novel system of non-symmetric dimers containing 4-n-alkyloxy-substituted cinnamic acid and cyanobiphenyl groups has been studied. Two series were prepared: in one the flexible spacer was varied in length while the spacer was fixed. The spacer length has a profound influence on the nematic-isotropic transition temperature of these materials and a large odd-even effect is observed for the series. The terminal chain also plays a significant role in determining the liquid crystal phase behaviour: a smectic A phase is exhibited for the ethyl and propyl homologues, in addition to a nematic phase; this smectic phase vanishes for intermediate chain lengths but then reappears for the nonyl and decyl members of the series. X-ray diffraction has revealed the structure of the smectic A phase for the ethyl homologue to be intercalated, whereas that for the decyl compound is interdigitated. The existence of the intercalated smectic A phase has previously been explained in terms of a charge-transfer interaction between unlike mesogenic groups. However, for the non-symmetric liquid crystal dimers described here this specific interaction appears unlikely and we discuss, therefore, other possible mechanisms for the formation of intercalated smectic phases.     

Effect of a lateral substituent on the mesomorphic properties of ferroelectric side chain liquid crystalline polysiloxanes

The synthesis of side chain liquid crystalline polysiloxanes containing oligooxyethylene spacers and ( S )-2-methylbutyl 4-\[(4-oxybiphenyl-4-yl)carbonyloxy]-3-fluorobenzoate mesogenic side groups is presented. Differential scanning calorimetry, optical polarizing microscopy and X-ray diffraction measurements reveal liquid crystalline properties for all synthesized monomers and polymers. All three precursor olefinic monomers reveal cholesteric and smectic A phases. The olefinic monomer which contains two oligooxyethylene units in the spacer is the only one which reveals a twist grain boundary A phase and a blue phase, besides the cholesteric and smectic A phases. All three polysiloxanes present enantiotropic smectic A and chiral smectic C phases. The mesomorphic behaviours of the monomers and polymers are compared with those of the corresponding monomers and polymers without the lateral fluoro substituent. The results seem to demonstrate that incorporating a lateral fluoro substituent in the mesogenic cores of the monomers affects not only the mesophase thermal stability, but also the nature of the mesophases formed. However, incorporating a lateral fluoro substituent in the mesogenic cores of the polymers affects only the thermal stability of the mesophases formed. The lateral fluoro substituent has a more profound effect on the mesomorphic behaviour for the monomers than that for the polymers.     

Liquid crystal dimers possessing chiral rod-like anisometric segments: synthesis, characterization and electro-optic behaviour

Several new optically active liquid crystal dimers comprising pro-mesogenic cholesterol and a chiral diphenylacetylene (tolane) segment, covalently linked in an end-to-end fashion through a flexible spacer, have been synthesized and investigated for their mesomorphic behaviour with the aid of optical, calorimetric and X-ray diffraction studies. Five unsymmetrical dimers, designed on the basis of recent work, involve molecular structural variations of the tolane mesogenic entity with a view to stabilizing a wide thermal range smectic A (SmA) phase featuring the electroclinic effect. Three different chiral chains, namely, (S)-1-methylheptyloxy, (S)-2-methylbutyloxy, (3S)-3,7-dimethyloctyloxy, with or without polar (nitro or fluoro) lateral substituents, were incorporated, while keeping the length (C₆) of the spacer constant. As expected, all the dimers exhibited a SmA phase. A few also showed chiral nematic (N*) and/or twist grain boundary and/or chiral smectic C (SmC*) phases. Remarkably, some of these oligomesogens, upon melting, had a stable SmA phase over a wide thermal interval (100-150°C); this state seems to be stable for a long period of time. Electro-optic studies, including optical tilt angle as well as temporal response as a function of temperature, were carried out in the SmA phase. The SmC* phase was also investigated for its electrical switching and optical tilt angle, as well as spontaneous polarization as a function of temperature. These studies showed that the mesophase response to an applied field is weak and is independent of variations in the dimer investigated.     

Effects of benzoyl group substituents on the mesomorphic properties of 5-alkoxy-2-benzoylaminotropones

Three types of 5-alkoxy-2-benzoylaminotropones, containing an electron-donating group and seven types of derivatives with electron-withdrawing groups on the benzoyl group, were prepared in order to study the thermal ranges of the mesophases exhibited. The troponoid amides had higher transition temperatures than the corresponding troponoid esters and benzenoid amides. From the ¹H NMR spectroscopic measurements and X-ray crystallographic analysis of 5-butoxy-2-(4-methoxybenzoylamino)tropone, it was observed that the benzoyl carbonyl group faced to the H-3 of the tropone ring to form an intramolecular hydrogen bond between the tropone carbonyl and the amide NH groups. The intramolecular hydrogen bonding of the troponoid amides made the molecules flat, inducing strong π–π intermolecular interactions between head-to-tail dimers and so reduced the possibility of intermolecular hydrogen bonding between the NH group and the carbonyl group of neighbouring molecules so decreasing melting points. Electron-donating groups enhanced the appearance of nematic phases while electron-withdrawing groups promoted smectic A phases.     

Mesomorphic properties of orthopalladated complexes of laterally substituted azobenzene derivatives, 2-alkoxycarbonyl-4-(4-ethoxyphenylazo)phenyl 4-methoxybenzoates

Homologous series of 2-alkoxycarbonyl-4-(4-ethoxyphenylazo)phenyl 4-methoxybenzoates (L) and their orthopalladated μ-chloro dimers (Pd₂Cl₂L₂) have been synthesized and their thermotropic phase transition behaviour has been characterized. The ligands, L, are three-ring nematogens with their central rings substituted in the 2-position by n-alkyl chains of varied length. The thermal stability of the nematic phases decreases rapidly with increasing side chain length and then levels off as the series is ascended. In every case the mesomorphic phase appears only over a small temperature range (less than 20° C). The orthopalladated species also proved to be nematogenic. Melting and clearing points rise on the average by 87° C and 126° C, respectively, and thus the temperature ranges of the nematic phases are enlarged considerably upon metallation. In relation to the observed stabilization of the nematic phases, particularly of higher homologues, a lath-like molecular structure, in which the side chains of the ligands fill up the clefts of the core part, has been deduced for a Pd2Cl2L2 homologue from its solution ¹H-NMR spectrum.     

High strength disclinations in the nematic and smectic states of 2,5-bis(4-alkoxybenzoyloxy)- p -benzoquinones

High strength disclinations were observed in both nematic and smectic phases exhibited by members of the 2,5-bis(4-alkoxybenzoyloxy)- p -benzoquinone series, beyond 4- n -pentoxy. The shorter members of the series show only a nematic mesophase, the highest members only a smectic mesophase, while intermediate members exhibit both types of mesophase. These high strength disclinations seen in a single component system are very stable due to energy dispersion and the high viscosity of the smectic state. All the high strength disclinations are negative in sign.     

Effects of benzoyl group substituents on the mesomorphic properties of 5-alkoxy-2-benzoylaminotropones

Three types of 5-alkoxy-2-benzoylaminotropones, containing an electron-donating group and seven types of derivatives with electron-withdrawing groups on the benzoyl group, were prepared in order to study the thermal ranges of the mesophases exhibited. The troponoid amides had higher transition temperatures than the corresponding troponoid esters and benzenoid amides. From the ¹H NMR spectroscopic measurements and X-ray crystallographic analysis of 5-butoxy-2-(4-methoxybenzoylamino)tropone, it was observed that the benzoyl carbonyl group faced to the H-3 of the tropone ring to form an intramolecular hydrogen bond between the tropone carbonyl and the amide NH groups. The intramolecular hydrogen bonding of the troponoid amides made the molecules flat, inducing strong π-π intermolecular interactions between head-to-tail dimers and so reduced the possibility of intermolecular hydrogen bonding between the NH group and the carbonyl group of neighbouring molecules so decreasing melting points. Electron-donating groups enhanced the appearance of nematic phases while electron-withdrawing groups promoted smectic A phases.     

On the structure of binary mixtures of non-symmetric liquid crystal dimers: the alpha-(4-cyanobiphenyl-4-yloxy)-omega(4-alkylanilinebenzylidene-4-oxy) alkanes

Liquid crystal dimers, in which two mesogenic groups are linked by a flexible spacer, exhibit a rich smectic polymorphism for both symmetric and non-symmetric dimers which differ in the nature of the mesogenic groups. For example, smectic phases having monolayer, interdigitated and intercalated structures have been discovered. We have extended our studies of such systems to binary mixtures in an attempt to understand the origin of the different phase structures at the molecular level. The dimers studied include non-symmetric systems differing in the parity of the spacer and in the length of the terminal chains; for comparison we have also studied a mixture of symmetric dimers differing solely in the parity of the spacer. We have constructed the phase diagrams for the various mixtures and found that for certain systems the smectic phases exhibited by either one or both components can be destroyed. To investigate the local structure of the nematic phase for mixtures in which a smectic A phase is eliminated from the phase diagram we have determined their orientational order using NMR and ESR spectroscopy. To provide more direct information on the local structure an X-ray diffraction study was undertaken on certain of the mixtures.     

Synthesis and study of 4-(4'-n-alkoxybenzoyloxybenzoyl)-4'-n-butoxyanilides

Thirteen compounds with ester and amide linkages were synthesized and their mesogenic properties evaluated. Methyl to n-propyl derivatives exhibit nematic phases, n-butyl to n-decyl derivatives exhibit smectic and nematic mesophases, whereas n-dodecyl to n-octadecyl derivatives exhibit only smectic phases. All the smectic homologues exhibit smectic C phases. Middle members of the homologous series exhibit polymorphism of smectic mesophase. A plot of transition temperatures versus number of carbon atoms in the alkoxy chain reveals an odd-even effect for nematic-isotropic transition temperatures. Nematic-isotropic and smectic-cholesteric thermal stabilities of the prepared compounds (series I) are higher compared to those of previously reported compounds, series A, B and C. The results indicate that a simple reversal of a central linkage has a dramatic effect on the appearance of smectic mesophase in a homologous series. The structures of the synthesized compounds were characterized using elemental analysis, thin-layer chromatography and spectral data.     

Mesogenic Unsymmetric dimers containing cholesteryl ester and tolane moieties

Among unsymmetric oligomesogens, chiral dimers formed by connecting a cholesteryl ester fragment with various aromatic mesogenic cores through a polymethylene spacer have been attracting much attention due to their remarkable thermal behaviour. In particular, dimers containing a diphenylacetylene segment having an alkoxy chain have shown interesting mesomorphic behaviour. In view of this a new series of unsymmetric dimers consisting of a diphenylacetylene moiety having an alkyl chain and a cholesteryl ester unit joined through a paraffinic spacer have been synthesized and their liquid crystalline properties characterized. The lengths of the central methylene spacer (C₃, C₄, C₅ and C₇) as well as that of the alkyl chain (n-butyl, n-pentyl, n-hexyl and n-heptyl) have been varied to establish structure-property relationships. These investigations have revealed that all the dimers exhibit smectic A, twist grain boundary and chiral neamtic (N*) phases with the exception of one of the dimers for which only the N* phase was observed. Some differences in the mesomorphic properties of the unsymmetric dimers containing odd or even parity methylene spacers have been observed. The majority of dimers having an even (C₄) parity paraffinic spacer show a blue phase while the dimers with odd (C₃, C₅ and C₇) parity spacers exhibit the chiral smectic (SmC*) phase. In some cases, the SmC* phase exists well below (-60°C) and above room temperature.     

Liquid crystalline properties and dependence of transition temperatures on the length of the flexible alkylene spacer of symmetric dimers composed of bent-core units

Three new series of symmetric dimers composed of bent-core molecules connected by a flexible alkylene spacer have been synthesized and their mesomorphic properties studied. The effect of varying the length of the terminal chains for fixed odd and even spacers, as well as varying the spacer length for a fixed terminal chain length, on the mesomorphic properties has been investigated. Remarkably, all the dimers investigated show the same type of mesophase, which has been characterized as a Col_obP_F phase, on the basis of X-ray diffraction and electro-optical measurements. It is found that for a fixed spacer length, the thermal range of the mesophase increases with increase in the number of methylene units in the terminal chain. On the other hand, interestingly an odd-even effect is observed when the length of the flexible spacer is varied; dimers having an odd number of methylene units possess higher clearing points.     

Synthesis and thermal behaviour of salicylaldimine-based liquid crystalline symmetrical dimers

A series of symmetrical dimers consisting of salicylaldimine moieties connected by flexible alkylene central spacer via ether linkages has been synthesized. In order to validate the empirical rule suggested by Date et al. to account for the smectic behaviour of such dimers, the chain length of the terminal alkoxy chain has been kept constant (C₈) while the number of methylene units in the central spacer was varied from C₃ to C₁₁. Another aim of the present investigation was to understand structure-property relationship in these dimers in which the salicylaldimine mesogenic segment has been used for the first time in dimers. The mesomorphic behaviour of these dimers was evaluated using optical microscopy and differential scanning calorimetry and the structure of some of the mesophases has been further investigated with the help of X-ray diffraction. Our studies reveal that the dimers consisting of 3 to 8 methylene units in the flexible spacer show only smectic (smectic C and smectic A) phases. For the dimers containing 4, 6 and 8 methylene units in the central spacer, a unique filament growth pattern has been observed in the smectic A phase while cooling from the isotropic phase. The dimers containing of C₉ to C₁₁ methylene groups exhibit the nematic phase in addition to smectic modifications. This observation indicates that when the terminal chains are shorter than the spacer, the tendency to form smectic phases is not fully extinguished but is perhaps reduced.