Low temperature Raman study of a liquid crystalline system 4-decyloxy benzoic acid (4DBA)

A new low temperature solid modification in 1-isothiocyanato-4-(trans‑4-propylcyclohexyl)benzene (3CHBT) probed by Raman spectroscopy and quantum chemical calculations

Raman spectra of 1-isothiocyanato-4-(trans 4-propylcyclohexyl)benzene (3CHBT) were studied in the region, 1450-2300cm^-1 at twenty two different temperatures in the range, 83-293K in cooling and heating cycles. All bands in this region were critically evaluated in term of linewidth, peak position and relative intensity. Raman bands at ~2085cm^-1 and ~2120cm^-1 shows clear evidence of a solid modification through anomaly in temperature dependence peak positions and linewidths variation in the temperature range 173-203K. A detailed analysis of the variation of the linewidth and peak position of the two component bands leads to the conclusion that the molecular/dimer arrangement in crystalline packing changed between 173K and 203K. This solid modification was also analyzed at the molecular level. The ~2085cm^-1 and ~2120cm^-1 bands were corresponded as parallel and anti-parallel dimers of 3CHBT, which are identified as dimer I (D₁) and dimer II (D₂), respectively. The structures of both the dimers have been optimized by quantum chemical calculations employing density functional theoretic (DFT) methods.

Mesomorphic, micro-Raman and DFT studies of new calamitic liquid crystals; methyl 4-[4-(4-alkoxy benzoyloxy)benzylideneamino]benzoates

The mesomorphic properties of newly synthesized homologous series of calamitic liquid crystals; methyl 4-[4-(4-alkoxy benzoyloxy)benzylideneamino]benzoates, H(2n+1)C(n)OC6H4COOC6H4C(H)=N C6H4COOCH3; n = 6, 8, 10, 12, 14, 16 (MABBAB-n) containing ester and Schiff base groups as linker have been studied by temperature dependent micro-Raman study, differential scanning calorimetry (DSC) and polarizing optical microscopy (POM). All members of this series exhibit enantiotropic smectic A (SmA) mesophase with oily streak and focal conic textures. Analyses of Raman marker bands of phenyl rings, Schiff base and ester groups of MABBAB-10 confirm the phase transitions. The Raman study also gives an evidence of breaking of weak intermolecular hydrogen bonds associated with ester groups and formation of new hydrogen bonds through CN bond at Cr→SmA phase transition. The monomer and dimer were optimized and vibrational assignment of MABBAB-10 was also done with density functional theoretical (DFT) technique to understand the experimental results.