Madan Kumar Singh

Dielectric relaxation studies in n-(4-octyloxy-2-hydroxybenzylidene)-4-carbethoxyaniline

Temperature dependent dielectric spectroscopy of a newly synthesized compound N-(4-octyloxy-2-hydroxybenzylidene)-4-carbethoxyaniline, having a wide temperature range (˜70°C) Smectic A (SmA) phase has been carried out in the frequency range 100u2009Hz–10u2009MHz. Dielectric permittivity and loss in the homeotropic ( and ) and planar ( and ) alignments of the molecules have been determined. Relaxation frequencies corresponding to the rotation of the molecules about their short axes have been found to lie below 1u2009MHz and exhibit Arrhenius behaviour. Relaxation frequencies corresponding to the rotation of the molecules about their long axes are expected to lie above 10u2009MHz. Dielectric anisotropy (Δε′u2009=u2009 u2009−u2009 ) has been found to be negative throughout the range of the SmA phase.

Studies of the mesomorphic behavior of bivalent carbohydrate amphiphiles

A homologous series of bivalent glycolipids constituted of 1,3-bis-O-(β-D-glucopyranosyl)-2-O-alkyl glycerol, bearing two sugar head groups and one alkyl chain on a glycerol backbone, was synthesized. The mesomorphic behaviour was investigated with the aid of polarizing optical microscopy, differential scanning calorimetry and X-ray diffraction methods. The homologous members exhibit a smectic A phase, consistently with a partial bilayer structure. From experimental d-spacing and computationally derived length of the molecules, it is suggested that a partial interdigitation of the molecules is present in the bilayer structure.

Crystal structures and thermal analyses of alkyl 2-deoxy-α-D-arabino-hexopyranosides

The crystal structures of alkyl 2-deoxy-alpha-d-arabino-hexopyranosides, with the alkyl chain lengths from C(8) to C(18), are established by the single crystal X-ray structural determination. The even-alkyl chain length derivatives crystallized orthorhombic, with space group P2(1)2(1)2(1), whereas the odd-alkyl chain length derivatives crystallized monoclinic, with space group P2(1). The sugar moieties retained a (4)C(1) chair conformation and the conformation of the alkyl chains was all-trans. The molecules formed a bilayer structure, in which alkyl chains were interdigitated. The hydrogen bonds, originating from the sugar moieties, were observed in adjacent layers and also within the same layer, resulting in the formation of infinite chains. The alkyl chains arranged parallel to each other and formed planar structures. The thermal properties of the alkyl 2-deoxy glucosides were analyzed further. It was observed that none of the derivatives exhibited mesomorphism. This study establishes that the absence of the hydroxyl group at C-2 of the sugar moiety results in a non-mesogenic nature of the alkyl 2-deoxy-alpha-d-glycosides, as opposed to the profound mesogenic nature of the normal alkyl glycosides.

Tuning the thermotropic properties of liquid crystalline p-substituted aroylhydrazones

The synthesis and mesomorphic properties of forty substituted aroylhydrazones with peripheral mono-, di- and tri- alkoxy chains derived from a p-amino aroylhydrazone core are described. The compounds with two side chains exhibited a smectic A phase, while the compounds with six soft alkoxy side chains at symmetrical positions formed a rectangular columnar mesophase. The structures of these mesophases were confirmed by differential scanning calorimetry (DSC) analysis, polarized optical microscopy (POM) and powder X-ray diffraction (XRD) studies. Raman studies with the help of density functional theory on some of the mesogenic members have been performed to understand the changes in the intermolecular interactions during phase transitions. A structure–property relationship has been deduced, and mesogenic properties are found to be dependent on the chain length, density and position of the alkoxy chains around the molecular core.

Dielectric anisotropy and relaxation studies of the homologous series of N -(4- n -alkyloxy-2-hydroxy-benzylidene)-4-carbethoxyaniline

Dielectric properties of the homologous series of newly synthesized nonchiral compounds N-(4-n-alkyloxy-2-hydroxy-benzylidene)-4-carbethoxyaniline, (n = 6, 8, 10, 12) having wide temperature range (∼60°C) smectic A (SmA) phase, have been studied by the impedance spectroscopy in the frequency range of 100 Hz to 1 MHz. Measurements have been carried out for two principal alignments (planar as well as homeotropic) of the SmA phase. Dielectric anisotropy (Δε′ = ϵ∥u2009−u2009ϵ⊥) for all the members of the series has been found to be negative for the whole temperature range of SmA phase. Magnitude of the dielectric anisotropy (|Δε′|) has been found to decrease with the number of alkyl chains. Relaxation frequencies corresponding to the rotation of the individual molecules about their short axes, lie below 1 MHz and obey the Arrhenius law by which activation energies have been determined. However, the relaxation frequencies corresponding to the rotation of the molecules about their short axes apparently lie above 10 MHz.

Synthesis, characterization, and mesomorphic investigations of diester-substituted salicylaldimines and their copper (II) complexes

ABSTRACT A novel homologous series of N-[4-[4′-n-alkoxy)benzoyloxy-2-hydroxybenzylidene)-4-carbethoxy anilines, H2n+1CnOC6H4C(O)OC6H3(OH)C(H)˭NC6H4COOC2H5 (n = 6, 8, 10, 12, 14, 16) and their copper(II) complexes have been synthesized. All these compounds have been characterized by suitable spectroscopic techniques. The mesomorphic properties of these compounds were investigated by differential scanning calorimetry (DSC) and polarizing optical microscopy (POM). The ligands exhibit wide range of enantiotropic smectic A and nematic phases as confirmed by their typical optical texture under polarizing microscope. The square planar copper(II) complexes of the ligands show only an isotropic phase at higher temperature and no mesogenic nature is observed. DFT calculations have been performed using GAUSSIAN-03 program at B3LYP level to obtain the stable electronic structure of the ligand with decyloxy chain length and its copper(II) complex.

Effect of the C-2 hydroxyl group on the mesomorphism of alkyl glycosides: synthesis and thermotropic behavior of alkyl 2-deoxy-d-arabino-hexopyranosides

A homologous series of alkyl 2-deoxy-alpha-d-arabino-hexopyranosides and alkyl 2-deoxy-beta-d-arabino-hexopyranosides were synthesized, upon glycosylation of 1-alkanols (from C8 to C18 alkanols) with ethyl 2-deoxy-3,4,6-tri-O-acetyl-1-thio-d-arabino-hexopyranoside, followed by a deprotection. The thermotropic behavior of these new types of alkyl glycosides was investigated. It was observed that the beta-anomers of these alkyl glycosides, bearing nonyl to tetradecyl alkyl chain are mesomorphic, exhibiting monotropic smectic A phase. In contrast, the alpha-anomers are all non-mesomorphic. An effort to identify the liquid crystalline behavior of binary mixtures of the alpha- and beta-anomers was undertaken and it was found that mixtures containing equimolar amounts of the anomers exhibited mesomorphic behavior. A fine balance of the hydrophilic and hydrophobic components within the molecule is also found to be important for the alkyl 2-deoxy glycosides to form the mesophase.

Synthesis, characterization and mesomorphic behaviour of lanthanide (III) 4-alkoxybenzoates

The lanthanide (III) 4-alkoxybenzoates [Ln(CnH2nu2009+u20091OC6H4CO2)3, Lnu2009=u2009La (III), Pr (III), Nd (III), Eu (III), Gd (III), Tb (III) and Dy (III) and nu2009=u20096, 8, 10, 12 and 16] have been synthesized and characterized by elemental analyses, magnetic susceptibility measurements, and IR and electronic spectroscopy. Hot-stage polarizing optical microscopy and differential scanning calorimetry have been used to investigate the mesomorphic behaviour. The chain length influences the structure and hence the thermal behaviour of these compounds. All the lanthanide complexes except decyloxy derivatives exhibit smectic A mesophase. The decyloxy-containing complexes are non-mesomorphic. The differential scanning calorimeter traces do not display the exothermic peak for all the compounds except for the hexadecyloxy derivatives, which exhibit enantiotropic smectic A phase. The influence of the lanthanide ions on the phase transition has also been clearly demonstrated.

Role of hydroxyl group on the mesomorphism of alkyl glycosides: synthesis and thermal behavior of alkyl 6-deoxy-β-d-glucopyranosides

A homologous series of alkyl 6-deoxy-beta-d-glucopyranoside amphiphiles was prepared, in an effort to identify the role of hydroxyl group in the mesomorphic behavior of alkyl glycosides. Synthesis was performed by a chlorination of the sugar moiety in alkyl beta-d-glucopyranosides with methylsulfonyl chloride in DMF, followed by a metal mediated dehalogenation to secure alkyl 6-deoxy-beta-d-glucopyranosides, wherein the alkyl chain length varied from C(9) to C(16). The mesomorphic behavior of these 6-deoxy alkyl glycosides was assessed using polarizing optical microscopy, differential scanning calorimetry and X-ray diffraction method. Whereas the lower homologues exhibited a monotropic SmA phase till sub-ambient temperatures, the higher homologues formed a plastic phase. A partial interdigitized bilayer structure of SmA phase is inferred from experimental d-spacing and computationally derived lengths of the molecules. The results were compared with those of normal alkyl glucopyranosides, retained with hydroxyl groups at C-2-C-6 carbons, and alkyl 2-deoxy-glucopyranosides, devoid of a hydroxyl group at C-2 and the comparison showed important differences in the mesomorphic behavior.