Crystal → nematic phase transition in the liquid crystalline system 1‐isothiocyanato‐4‐(trans‐4‐propylcyclohexyl) benzene (3CHBT) probed by temperature‐dependent micro‐Raman study and DFT calculations

Abstract

Raman spectra of 3CHBT in unoriented form were recorded at 14 different temperature measurements in the range 25-55 • C, which covers the crystal → nematic (N) phase transition, and the Raman signatures of the phase transition were identified. The wavenumber shifts and linewidth changes of Raman marker bands with varying temperature were determined. The assignments of important vibrational modes of 3CHBT were also made using the experimentally observed Raman and infrared spectra, calculated wavenumbers, and potential energy distribution. The DFT calculations using the B3LYP method employing 6-31G functional were performed for geometry optimization and vibrational spectra of monomer and dimer of 3CHBT. The analysis of the vibrational bands, especially the variation of their peak position as a function of temperature in two different spectral regions, 1150-1275 cm −1 and 1950-2300 cm −1 , is discussed in detail. Both the linewidth and peak position of the (-C-H-) in-plane bending and ν(N C S) modes, which give Raman signatures of the crystal → N phase transition, are discussed in detail. The molecular dynamics of this transition has also been discussed. We propose the co-existence of two types of dimers, one in parallel and the other in antiparallel arrangement, while going to the nematic phase. The structure of the nematic phase in bulk has also been proposed in terms of these dimers. The red shift of the ν(N C S) band and blue shift of almost all other ring modes show increased intermolecular interaction between the aromatic rings and decreased intermolecular interaction between two -NCS groups in the nematic phase.

Recent advances in linear and nonlinear Raman spectroscopy. Part V

“…Vikram et al . described the crystal nematic phase transition of the liquid crystalline1‐isothiocyanato‐4‐(trans‐4‐propylcyclohexyl) benzene (3CHBT) system as probed by temperature‐dependent micro‐Raman and DFT calculations . Raman scattering and IR absorption of ammonium oxyfluoride crystals (NH 4 ) 3 WO 3 F 3 , (NH 4 ) 2 KWO 3 F 3 and Cs 2 (NH 4 )WO 3 F 3 were compared by Vtyurin et al .…”

Molecular approach to phase transitions in a calamitic ester substituted aroylhydrazone liquid crystal

“…At molecular level rotation of spacers in the mesogenic molecules, charge transfer phenomenon, changes in intra/intermolecular H‐bonding interactions, changes in molecular symmetry and others are observed during phase transitions, and therefore for better understanding, molecular techniques are needed. In LC systems due to the anisotropy of the molecular polarizability, Raman spectroscopy has a great potential to reflect the molecular changes during phase transitions in terms of some measurable parameters of marker bands . XRD, density functional theoretical (DFT) and Raman spectroscopic techniques together can be very helpful to study phase transitions at molecular level.…”

Low Temperature Raman Study of 1-Isothiocyanato-4-(trans-4-propylcyclohexyl) benzene (3CHBT)

“…In a recent study, [6] we have discussed the molecular dynamics of the crystal → nematic and nematic → isotropic transitions in 3CHBT. To the best of our knowledge, low temperature Raman study of nCHBT series is not reported as yet.…”