Intra- and intermolecular interactions in mesogenic cyano compounds

Abstract

In order to elucidate the intra- and intermolecular interactions, FTIR spectra were measured on dilute solutions of CCl₄ (0.01 mol l⁻¹) and bulk states from crystals to isotropic liquids for mesogenic 4-cyano-4′-alkylbiphenyls (nCB), 4-cyano-4′-alkoxybiphenyls (nOCB), 4-cyanophenyl 4-alkoxybenzoates (CPnOB), and 4-alkoxyphenyl 4-cyanobenzoates (nOPCB) series. CN stretching vibration frequencies in dilute solutions are slightly but definitely different—0.5 cm⁻¹ higher for nCB than for nOCB and 0.8 cm⁻¹ higher for nOPCB than for CPnOB—giving a direct evidence of the different intramolecular interaction of molecules. The frequency difference in mesophases is larger than that of molecules in solutions, 1.0 and 1.5 cm⁻¹ for cyanobiphenyls and cyanoesters, respectively. The difference is related to the different extents of molecules overlapping, leading to the conclusion that the larger electron densities of CN groups in nOCB and CPnOB led to the larger overlapping of core moieties. A relationship between the wavenumbers and arrangements of CN groups in crystals is pointed out.