Liquid crystalline behaviour of hydrogen-bonded complexes of alkoxycinnamic acids with octyloxystilbazole

Hydrogen-bonded liquid crystalline complexes have been obtained through 1:1 (molar ratio) complexation of 4- n -alkoxycinnamic acids ( n CNA: n = 4, 8, 10, 12, where n is the number of carbons in the alkyloxy chain) and trans -4-octyloxystilbazole (8Sz). These hydrogen-bonded complexes ( n CNA8Sz) form stable mesophases. The mesomorphic range was extended by the mixing of complexes. Hexatic modification of smectic B (SmB h ), smectic C (SmC), smectic A (SmA), and nematic mesophases of these complexes were determined by a combination of X-ray diffraction and polarizing optical microscopy. Transitions between the various smectic phases were deduced from the temperature-dependent layer spacing of n CNA8Sz. The layer spacing of these complexes in the SmB h and SmA phases gradually increased with increasing alkoxy chain length. The favouring of smectic phases in these complexes is believed to originate from the increment of polarity of the mesogen by intermolecular H-bonding.     

Phase behaviour of hydrogen-bonded liquid crystalline complexes of alkoxycinnamic acids with 4,4'-bipyridine

The hydrogen-bonded liquid crystalline complexes of 4-n-alkoxycinnamic acids (nCNA: n=4-8, 10, 12, 16, 18, where n is the number of carbons in the alkoxy chain) with 4,4'-bipyridine (BPy) have been prepared and characterized. The existence of smectic C, smectic A, and nematic mesophases of these complexes was demonstrated by a combination of polarized optical microscopy and X-ray diffraction (XRD). In this H-bonded mesogenic structure, nonmesogenic BPy functions as the core unit of the mesogen through the H-bond, as confirmed by infrared spectroscopy and XRD. A general comparison of nCNABPy with the benzoic acid analogues (nOBA)_2-BPy, showed that the elongated nCNABPy mesogen behaves as a rod unit and increases the transition temperature. Smectic phases in nCNAs (5≤ n≤ 12) were induced on complexation with BPy. The favouring of smectic phases in these complexes is believed to originate from the increment of polarity of the mesogen by intermolecular H-bonding. With increasing chain length of the nCNABPy complexes the smectic C phase becomes stabilized, like conventional rod-coil molecules.     

Liquid-crystalline oxovanadium(IV) complexes accessed from bidentate [N,O] donor salicylaldimine Schiff-base ligands

A series of bis[4-(n-alkoxy)-N-(4′-R-phenyl)salicylideneiminato]oxovanadium(IV) complexes (n?=?6,?10,?14,?16,?18 and R?=?C₃H₇) were prepared and their mesogenic properties were investigated. The mesomorphic behaviors of the compounds were studied by polarized optical microscopy and differential scanning calorimetry. Ligands display SmA/SmC and unexpected nematic mesophases. The complexes bearing longer alkoxy carbon chain (n?=?10,?14,?16, and 18) showed both monotropic or enantiotropic transitions with smectic A and high ordered smectic E phases. However, the complex with shorter carbon chain length (n?=?6) showed monotropic transition with an unprecedented nematic (N) phase. A density functional theory study was carried out using DMol3 at BLYP/DNP level to obtain a stable optimized structure. A square-pyramidal geometry for the vanadyl complexes has been suggested. A ν_V=O stretching value of ～970?cm^?1 corroborated absence of any V?=?O?···?V?=?O interactions. Cyclic voltammetry revealed a quasireversible one-electron response at 0.61?V for the VO(IV)–VO(V) redox couple. Variable temperature magnetic susceptibility measurements of the vanadyl complexes suggested absence of any exchange interactions among the vanadyl spin centers.     

Cholesterol-based dimeric liquid crystals: synthesis and mesomorphic behaviour

Chiral non-symmetric dimeric liquid crystals consisting of a cholesteryl ester moiety as chiral entity and a biphenyl aromatic core, interconnected through n-butyl (C₄) or n-pentyl (C₅) parity alkylene spacers, have been synthesized and investigated for their liquid crystalline properties. All the dimers exhibit enantiotropic mesophases. The first member of the dimers having the C₄ central spacer exhibit only the chiral nematic (N*) mesophase, while the higher homologues also show smectic A (SmA) and twist grain boundary (TGB) mesophases. The dimers of the other series containing the C₅ central spacer also have stable SmA, TGB and N* mesophases, except for the first which does not show the TGB phase. Both series of compounds show a weak odd-even effect with terminal alkyl chain substitution, while the spacer length has a marked influence on the phase transition temperatures.     

Novel chiral dimesogenic bidentate ligands and their Cu(II) and Pd(II) metal complexes

The synthesis and characterization of cholesterol-based dimesogenic bidentate ligands and their Cu(II) and Pd(II) metallomesogens are reported in detail. To understand structure-property relationships in these materials the terminal alkoxy chains and the central metal atom have been varied. Our studies reveal that chiral dimesogenic bidentate ligands with n-butyloxy chains exhibit smectic A (SmA), twist grain boundary and chiral nematic (N*) mesophases while substitution with either n -decyloxy or 3,7-dimethyloctyloxy chains also show a ferroelectrically switchable chiral smectic C (SmC*) mesophase. The metal complexes with n-butyloxy chains show only the SmA phase whereas higher chain length derivatives exhibit N* phase irrespective of the metal atom present. The ligands are thermally stable whereas their metal complexes, especially Pd(II) systems, seem to be heat sensitive. Spontaneous polarization, response time and tilt angle measurements have been carried out in the smectic C* phase of the two ligands.     

Thermal expansion of liquid crystalline amphiphilic di-block copolymer observed by simultaneous DSC-XRD

Phase transition process of PEO_m-b-PMA(Az)_n was investigated by the simultaneous DSC-XRD measurement using the synchrotron radiation facility. Four endothermic DSC peaks were observed during heating process. These DSC peaks were assigned to the melting of PEO, the transition from SmX, which is a mixture of super-cooled SmC and crystal, to SmC, from SmC to SmA, and from SmA to isotropic liquid state as determined by XRD profiles. In SmC phase, the liner expansion coefficient calculated from the spacing variation of the smectic layer distance was larger than that of the other phases. This result reflected the fact azobenzene moieties in the long-side chains of PMA(Az)_n forming the smectic layers and then they were tilted and stood up during the heating process.     

Optical and thermal properties of unsymmetrical liquid crystalline compounds based on isoxazole

The thermal and optical properties of two series of unsymmetrical liquid crystalline compounds based on an isoxazole ring are described. Of these materials, 3–7 and 8–12 displayed strong absorption at 285 nm and a weak blue fluorescence in solution at around 377 nm, even with the presence of a high conjugated core. The fluorescence quantum yields observed varied from low to moderate (Φ _F ?=?1–62%), with large Stokes shifts (60–178 nm). All compounds exhibited liquid crystalline behaviour with a wide mesomorphism temperature range and characteristic phases of calamitic compounds, among them smectic A (SmA), smectic C (SmC) and nematic (N) phases. Their phases were characterised using polarising optical microscopy and differential scanning calorimetry. In addition, with X-ray diffraction experiments, layer spacing of 33.3–40.0 Å were observed for the smectic phases.     

Small angle X-ray scattering study of chiral side chain liquid crystalline polymers in 5CB and 8CB solvents

Small angle X-ray scattering was used to examine the new chiral side chain liquid crystalline polyacrylates (P_4M and P_11M) and their mixtures (2?wt?%) in the low molar mass nematogenics 4′-n-pentyl-4-cyanobiphenyl (5CB) and 4′-octyl-4-cyanobiphenyl (8CB). Complementary data were obtained by polarizing optical microscopy. In agreement with previous studies, the mesophases of the bulk polymers show a dependence on the aliphatic spacers linking the mesogenic units to the polymer backbone. Chiral nematic and smectic A₁ phases were observed for the polyacrylates with four (P_4M) and eleven (P_11M) methylene units as spacers, respectively. In solution with 5CB and 8CB, P_4M exhibits an injected smectic phase, whereas P_11M maintains the smectic arrangement already observed in the bulk, with swollen smectic layers. In all the mixtures, layer stability was found to depend on the liquid crystal used as solvent, as well as on the temperature. At temperatures corresponding to the nematic 5CB and 8CB, the coexistence of two mesophases was observed in the mixtures. Moreover, with the liquid crystal solvents in the isotropic phase, microstructures suspended in the solvent matrix containing the liquid crystalline polymer in the smectic arrangement were detected.     

Optically Biaxial,Re‐entrant and Frustrated Mesophases in Chiral,Non‐symmetric Liquid Crystal Dimers and Binary Mixtures

Sixteen optically active, non‐symmetric dimers, in which cyanobiphenyl and salicylaldimine mesogens are interlinked by a flexible spacer, were synthesized and characterized. While the terminal chiral tail, in the form of either (R)‐2‐octyloxy or (S)‐2‐octyloxy chain attached to salicylaldimine core, was held constant, the number of methylene units in the spacer was varied from 3 to 10 affording eight pairs of (R & S) enantiomers. They were probed for their thermal properties with the aid of orthoscopy, conoscopy, differential scanning calorimetry and X‐ray powder diffraction. In addition, the binary mixture study was carried out using chiral and achiral dimers with the intensions of stabilizing optically biaxial phase/s, re‐entrant phases and important phase sequences. Notably, one of the chiral dimers as well as some mixtures exhibited a biaxial smectic A (SmA_b) phase appearing between a uniaxial SmA and a re‐entrant uniaxial SmA phases. The mesophases such as chiral nematic (N*) and frustrated phases viz., blue phases (BPs) and twist grain boundary (TGB) phases, were also found to occur in most of the dimers and mixtures. X‐ray diffraction studies revealed that the dimers possessing oxybutoxy and oxypentoxy spacers show interdigitated (SmA_d) phase where smectic periodicity is over 1.4 times the molecular length; whereas in the intercalated SmA (SmA_c) phase formed by a dimer having oxydecoxy spacer the periodicity was found to be approximately half the molecular length. The handedness of the helical structure of the N* phases formed by two enantiomers was examined with the aid of CD measurements; as expected, these enantiomers showed optical activities of equal magnitudes but with opposite signs. Overall, it appears that the chiral dimers and mixtures presented herein may serve as model systems in design and developing novel materials exhibiting the apolar SmA_b phase possessing D_2h symmetry and nematic‐type biaxiality.     

Liquid crystalline properties of dissymmetric molecules part X: The effect of a terminal nitro group on molecular arrangement in smectic phases in three aromatic ring systems with two ester groups

The smectic A (SmA) layer structure has been examined by X-ray diffraction for four isomeric systems: 4-nitrophenyl 4-(4-alkoxybenzoyloxy)benzoates 1, 4-alkoxyphenyl 4-(4-nitrobenzoyloxy)benzoates 2, 4-(4-alkoxybenzoyloxy)phenyl 4-nitrobenzoates 3, and 4-alkoxyphenyl 4-nitrophenyl terephthalates 4. The phase transition behaviour and layer structure of the SmA phases are notably influenced by the relative orientation of the two ester groups to the terminal groups, by alkoxy chain length, and by temperature. The SmA phase for compounds 1 and 3 has a mainly ‘monolayer’ arrangement, where the layer spacing is 2-5 Å longer than the molecular length. The SmA phase for compounds 2 and 4 consists of a mixed layer structure having monolayer, bilayer, and partially bilayer arrangements, the distribution depending on the relative orientation of the ester groups, the alkoxy chain length, and temperature. It is supposed that the diversity of the SmA phase is related to the polar interaction around the nitro or the nitrobenzene moieties through the smectic layers and reversal of the ester groups causes a subtle change in the entire molecular shape.     

Self‐Assembly of Imidazolium‐Based Rodlike Ionic Liquid Crystals: Transition from Lamellar to Micellar Organization

By using aryl‐amination chemistry, a series of rodlike 1‐phenyl‐1H‐imidazole‐based liquid crystals (LCs) and related imidazolium‐based ionic liquid crystals (ILCs) has been prepared. The number and length of the C‐terminal chains (at the noncharged end of the rodlike core) and the length of the N‐terminal chain (on the imidazolium unit in the ILCs) were modified and the influence of these structural parameters on the mode of self‐assembly in LC phases was investigated by polarizing microscopy, differential scanning calorimetry, and X‐ray diffraction. For the single‐chain imidazole derivatives nematic phases (N) and bilayer SmA₂ phases were found, but upon increasing the number of alkyl chains the LC phases were lost. For the related imidazolium salts LC phases were preserved upon increasing the number and length of the C‐terminal chains and in this series it leads to the phase sequence SmA–columnar (Col)–micellar cubic (Cub_I/Pm3n). Elongation of the N‐terminal chain gives the reversed sequence. Short N‐terminal chains prefer an end‐to‐end packing of the mesogens in which these chains are separated from the C‐terminal chains. Elongation of the N‐terminal chain leads to a mixing of N‐ and C‐terminal chains, which is accompanied by complete intercalation of the aromatic cores. In the smectic phases this gives rise to a transition from bilayer (SmA₂) to monolayer smectic (SmA) phases. For the columnar and cubic phases the segregated end‐to‐end packing leads to core–shell aggregates. In this case, elongation of the N‐terminal chains distorts core–shell formation and removes Cub_I and Col phases in favor of single‐layer SmA phases. Hence, by tailoring the length of the N‐terminal chain, a crossover from taper‐shaped to polycatenar LC tectons was achieved, which provides a powerful tool for control of self‐assembly in ILCs.     

The influence of terminal chlorine on the mesomorphic properties of new thiobenzoates containing two and three benzene rings

New liquid crystalline chloro‐substituted thioesters containing two and three benzene rings have been synthesized. 4‐Chlorophenyl 4‐n‐alkoxythiobenzoates and 4‐chlorophenyl 4‐n‐alkoxybenzoyloxy‐4′‐thiobenzoates are referred to as nO.SCl and nO.OSCl, respectively, where n varies from 4 to 16 for nO.SCl, from 4 to 10 for nO.OSCl and denotes the number of carbon atoms in the alkyl chain. Their mesomorphic properties were investigated by means of polarizing optical microscopy, differential scanning calorimetry, transmittance light intensity and X‐ray diffraction measurements. The nO.SCl homologous series possesses smectic A (SmA) and nematic (N) phases for n?=?4, 5, 6 while higher homologues have only an enantiotropic SmA phase. Those from the nO.OSCl homologous series have enantiotropic N and SmA phases and higher transition temperatures. The range of the N phase decreases, and of the SmA increases, with the elongation of the alkoxy chain in the nO.OSCl homologous series. The effect on mesomorphic behaviour of terminal alkoxy chain lengthening and replacement by chlorine on the other side of the molecule is discussed.     

Mesogenic cinnamates with a substituted ethyl terminal chain*

Two new mesogenic homologous series of liquid crystalline cinnamates with substituted ethyl tails, β‐methoxyethyl [4‐(4′‐n‐alkoxycinnamoyloxy)benzoates (I) and β‐chloroethyl [4‐(4′‐n‐alkoxycinnamoyloxy)benzoates (II), have been synthesized and characterized by a combination of elemental analysis and standard spectroscopic methods. In series I, lower‐chain members exhibit nematic mesophase, middle members exhibit enantiotropic nematic as well as smectic A (SmA) mesophases, whereas higher members exhibit only an enantiotropic SmA mesophase. In series II, methoxy to n‐butyloxy derivatives exhibit a monotropic nematic mesophase. The SmA mesophase commences from n‐propyloxy derivative as monotropic and persists up to the last member synthesized. The mesomorphic properties of present series were compared with each other and with a structurally related mesogenic homologous series to evaluate the effects of substituted ethyl tail and cinnamoyloxy central linkage on mesomorphism.     

Structure property and mesomorphic behaviour of S-shaped liquid crystal oligomers possessing two azobenzene moieties

A new series of symmetrical S-shaped oligomers 4,4?-bis(ω-2-(ω-[(bromophenyl)diazenyl]-alkoxy)phenoxy)hexylbiphenyl consisting of two different spacers (inner -(CH₂)₆- and outer -(CH₂)_n-) have been synthesised. Their physical, thermal and texture observation over various transition temperatures are reported. The outer spacers for these compounds vary from n = 4 to n = 9. The oligomers with even number of members exhibit monotropic phase in which the compound with n = 4 shows nematic (N) phase whilst those with members n = 6 and 8 exhibit N and smectic A (SmA) phases. However, the homologs with odd number of members display enantiotropic phase in which the compound with n = 5 exhibits N and smectic phases whereas the members with n = 7 and 9 are predominantly smectogenic. The temperature range of N phase for even-numbered member decreased with elongation of the outer spacer. The smectic phase stability among the members in the present series increases when the outer spacer n is increased from 5 to 8.     

Creation of a nematic phase in mixtures of smectic A1 phases

Abstract

Phase diagrams were determined for binary mixtures consisting of two 5-n-alkyl-2-(4'-isothiocyanatophenyl)-1,3-dioxane compounds (n-DBT) or 4'-isothio-cyanatophenyl 4-(trans-4'-n-decylcyclohexyl)benzoate and n-DBT. All compounds investigated have monolayer smectic A phases. A nematic phase in the upper temperature range and a nematic gap between two smectic regions also were observed, with the smectic layer spacing ratio, d/d', of 1.23 and 1.87 respectively. The variation of the enthalpy of transition with mixture composition in relation to changes of layer spacing ratio are also discussed for these systems.     

First liquid crystalline cuneane‐caged derivatives: a structure–property relationship study

The synthesis and phase behaviour of the first cuneane derivatives having liquid crystalline properties are reported. This new class of liquid crystalline materials is the homologous series of bis[4‐(n‐alkoxyphenyl)]cuneane‐2,6‐dicarboxylates. They were synthesized by two methods: either by isomerization of the previously prepared bis[4‐(n‐alkoxyphenyl)]cubane‐1,4‐dicarboxylates or by direct esterification of the cuneane‐2,6‐dicarboxylic acid. Enantiotropic nematic phases were observed for the first four homologues. The higher homologues exhibited enantiotropic smectic A phases. The bis[4‐(n‐octylphenyl]cuneane‐2,6‐dicarboxylate exhibited the SmA phase at a lower temperature than its octyloxy analogue. The mesophases were investigated and established by polarizing optical microscopy, differential scanning calorimetry and X‐ray diffraction. Quantum chemical calculations suggest elongated structure for these new liquid crystalline compounds.     

Heterocyclic pyridine-based liquid crystals: synthesis and mesomorphic properties

A series of new calamitic liquid crystals, 4-{[(pyridin-4-yl)methylidene]amino}phenyl 4-alkoxybenzoates comprising a heterocyclic (pyridine) and two phenyl rings core system, terminal alkoxy chain, imine and ester linkers were synthesised and characterised. This series consists of nine members wherein the members differ by the length of alkoxy chain (C_nH_2n+1O–, where n = 2, 4, 6, 8, 10, 12, 14, 16, 18). Spectral analysis results were in accordance with the expected structure. Their thermotropic behaviours were studied by using differential scanning calorimetry, optical polarising microscopy and powder X-ray diffraction techniques. A single mesophase (nematic) was observed for the first three members of the series (n = 2, 4 and 6). As the alkoxy chain increased to n = 8 and n = 10, the nematic phase appeared together with an additional smectic A (SmA) phase. When moving from n = 12 until the highest members (n = 18), the nematic phase disappeared and these compounds only exhibited a single mesophase (SmA).     

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.     

New Architecture of Liquid‐Crystalline Polymers from Fluorinated Vinylcyclopropanes

A new class of fluorinated polymers was prepared by radical ring‐opening homopolymerization of vinylcyclopropane monomers with a perfluorinated (CF₂)_nF chain (n = 6, 8, or 10). The polymers were in fact copolymers composed of 1,5‐linear and cyclobutane isomer units, the relative content of which depended on n. Surprisingly, they formed liquid‐crystalline mesophases (SmB_d and/or SmA_d), which was attributed to phase separation of the incompatible fluorocarbon and hydrocarbon components of the repeat unit.     

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.