Effect of molecular conformation on the mesogenic properties of a partially fluorinated nematogenic compound investigated by X-ray diffraction and dielectric measurements

Abstract

A bicyclohexyl trifluorophenyl compound has been studied in both the crystalline and the nematic phases by X-ray diffraction and dielectric methods. Difference in molecular geometry, conformation, packing, density and orientation of molecular dipole moments explains substantial increase of melting point and decrease of nematic range in the compound compared to structurally very closely related systems. The facts that the apparent length of molecules is about 1.35 times more than the most extended molecular length and the average value of the dielectric constant is about 6% less than the extrapolated isotropic phase values give conclusive evidence of existence of short-range antiparallel order in fluorinated compounds like cyanobiphenyls and isothiocyanates supporting our previous conclusion (Haldar S, Mandal PK, Prathap SJ, Guru Row TN, Haase W. X-ray studies of the crystalline and nematic phases of 4′-(3,4,5-trifluorophenyl)-4-propylbicyclohexyl. Liq Cryst. 2008;35:1307–1312). About 5 V dc bias is sufficient for switching the orientation of the molecules from homogeneous to homeotropic configuration. Dielectric anisotropy and splay elastic constant exhibit similar decreasing trend with temperature. Only one strong Debye-type dielectric absorption process, connected with reorientations of the molecules around the short axis, is observed. Dielectric spectra fitted to modified Cole-Cole function revealed that the relaxation frequency increases from 2.6 MHz to 4 MHz exhibiting Arrhenius-type behaviour. Activation energy of the thermally activated relaxation process is about 32.4 kJ/mole.