H. T. Srinivasa

Anticlinic smectic C phase in new and novel five-ring hockey stick-shaped compounds

The synthesis of a new homologous series of novel five-ring compounds composed of hockey stick-shaped molecules derived from 4-hydroxybiphenyl-3-carboxylic acid which exhibit an interesting sequence of phase transitions is presented. Uniaxial nematic and smectic A phases, anticlinic smectic C phase and an unidentified smectic phase at a lower temperature than the latter have been observed. This is perhaps the first example of hockey stick-shaped compounds exhibiting anticlinic smectic C phase directly from the isotropic phase. Polarised light optical microscopy, differential scanning calorimetry, X-ray diffraction measurements and electro-optical switching studies were carried out to identify the mesophases.

New pyrimidine-based photo-switchable bent-core liquid crystals

The first examples of liquid crystalline pyrimidine-based photo-switchable bent-core monomers incorporating azobenzene as side arms linked with terminal double bonds as polymerizable functional groups are synthesized and characterized. Polarizing optical microscopy (POM), differential scanning calorimetry (DSC) and X-ray diffraction analysis reveal that the bent-shaped lower homologue compounds are crystalline in nature whereas higher homologue compounds display the stable enantiotropic B6 phase. They exhibit fast photoisomerization effects in solution and relatively slow photoisomerization effects in liquid crystal cells. In solution both trans–cis and cis–trans occur at around 3 s and 200 s, respectively, whereas in solids they occur at around 10 s to 200 min. These are some of the first examples of azobenzene liquid crystals which exhibit very fast switching properties in solutions.

Synthesis and Liquid Crystal Property of New Fluoro Coumarin Carboxylates

New liquid crystalline 4-alkoxyphenyl-coumarin-3-carboxylates 6a–e, 7a–g, 8a–e, and 9a–e were prepared by reacting various coumarin-3-carboxylic acids 5a–d with 4-(alkoxy) phenols 4a–g in the presence of 1(3-dimethylaminopropyl-3-ethylcarbodiimide/dimethyl amino pyridine (EDCI/DMAP) as a coupling agent. The structures of the new coumarin derivatives were confirmed by spectral analysis and the liquid crystalline property was established by polarizing optical microscope and by differential scanning calorimetric techniques. The diethyl amine and morpholine were taken as electron-donating and –CF3 as electron-withdrawing groups at the seventh position of the coumarin-3-carboxylic acids to check the mesomorphic property in all new 4-alkoxyphenyl-coumarin-3-carboxylates. Among them, only 4-alkoxyphenyl-7-triflouromethyl-coumarin-3-carboxylates 7a–g exhibited liquid crystalline SmA phase.

Symmetric 3,5-Pyrazole and Isoxazole Heterocycles Comprising a Bent Core Unit: Synthesis and Mesomorphic Characterisation

The synthesis and characterisation of some new liquid crystalline (LC) heteroaromatic compounds containing the five-membered pyrazole/isoxazole rings is reported. Some of the compounds exhibited enantiotropic LC properties. The transition temperatures and LC textures of the mesophases were determined using polarised optical microscopy (POM), differential scanning calorimetry (DSC) and X-ray studies. The five-membered heterocyclic compounds with the pyrazole core exhibit smectic C (SmC) phase, whereas isoxazoles show SmC and nematic (N) phases.

Synthesis of Banana-Shaped Liquid Crystals for Photoswitching Properties

Three banana-shaped monomers containing naphthalene as central units and azobenzene in side arms with terminal alkenes were synthesized and characterized. DSC, polarizing optical microscopy, and X-ray diffraction measurements prove that all compounds exhibit B6 phase. The trans-azobenzene showed high-intensity π–π* transition at about 365 nm and low-intensity n–π* transition at about 450 nm. The strong photoisomerization behavior in solutions of the molecules displayed that trans to cis isomerization takes 45 sec whereas reverse process takes about 28 h and 30 min. This long thermal back relaxation is useful for creation of optical image storage.

New symmetric azobenzene molecules of varied central cores: Synthesis and characterisation for liquid crystalline properties

ABSTRACT A new series of azobenezene liquid crystals (LCs) was designed and synthesised by known and straightforward methods. The central connecting cores of the molecules varied viz., benzene/naphthalene/biphenyl. The molecular structures were confirmed by infrared, UV, nuclear magnetic resonance spectroscopic techniques. The LC texture and thermal phase behaviours were investigated by polarising optical microscopy and differential scanning calorimetry. Smectic B phase was found in biphenyl-derived azobenzene ester. The effect of central core on LC properties was investigated. The decreased angle at central connecting core suppressed the melting points, phase transition temperatures without losing LC properties, whereas, in moderate angle compounds LC properties were suppressed. The compounds with increased angle at central core favours rich mesomorphism. Graphical Abstract

New mesogenic compounds possessing a biphenyl ester and ether moiety comprising 1,3-dimethylbarbituric acid: synthesis, characterisation and mesomorphic studies

ABSTRACT New substituted derivatives of 5-vinyl-1,3-dimethylbarbituric acid were synthesised and evaluated for liquid crystal properties. Two sets of molecules were prepared. One end of all the molecules possesses the 1,3-dimethylbarbituric core. The first set comprises biphenyl ethers, 4a–n and the second set biphenyl esters, 5a–g. Liquid crystalline properties were investigated by POM and DSC techniques. All the compounds exhibited enantiotropic smectic A and nematic mesophases. The LC properties were found to depend on the spacer and terminal alkoxy- chain and alkoxy- ester moiety of the molecules. Smaller alkyl chain members showed a smectic phase, while higher alkyl chain members showed a nematic phase. GRAPHICAL ABSTRACT

Novel banana-discotic hybrid architectures.

Summary Here we present the design and synthesis of novel banana-discotic dimers and banana-bridged discotic dimers. The chemical structures have been characterized by spectral techniques and elemental analysis. The thermal behaviors of the compounds have been investigated by polarizing optical microscopy and differential scanning calorimetry. None of these synthesized compounds exhibit any liquid crystalline property probably because of the incompatibility of the bent-core with the discotic core.

Effects of chain branching and lateral fluorine substitution on mesomorphism of cholesteryl benzoates

The mesogenic cholesteryl 4′-alkoxyphenyl-4-carboxylates possessing terminal normal/branched/saturated/unsaturated alkyl chains with laterally ortho/meta substituted electronegative fluorine atom are described. All the homologues exhibited enantiotropic mesomorphism. Smectic A phase, chiral nematic, blue phase (BP) and TGBA phases were observed in different homologues. All the novel compounds were characterised by spectroscopic and elemental analysis. Thermal investigations and mesophase characterisations for all the compounds were carried out by the combination of DSC, POM and X-ray analysis. The effects of the various terminal normal/branched/saturated/unsaturated alkyl chains and the position of the substituted fluorine atom with its structurally related compounds have been discussed.

Synthesis and characterisation of some new chalcone liquid crystals

ABSTRACT A series of new chalcones with four aromatic rings is synthesised and characterised. The chemical structures of chalcones are evaluated by Fourier transform-infrared spectroscopy, elemental analysis, 1H and 13C nuclear magnetic resonance spectroscopic techniques. The compounds were investigated for liquid crystalline properties using differential scanning calorimetry and polarising optical microscopy with hot stage. The results indicate that the formation of mesophase type is dependent on the alkyl chain length at one end of the molecule. Compounds 9a–d with relatively shorter chains show SmA and nematic mesophases, whereas 9e–g exhibit SmC and SmA mesophases. Compound 9h having a cyano group at one end, exhibit SmA and nematic mesophases. Graphical Abstract