DIPOLE-MOMENT AND OPTICAL ANISOTROPY STUDIES OF MESOGENIC TWIN COMPOUNDS, ALPHA,OMEGA-BIS(4,4'-CYANOBIPHENYL)OXY]ALKANES

Dipole moments and optical anisotropies of alpha,omega-bis[(4,4'-cyanobiphenyl)oxy]alkanes NCPhPhO(CH2)(n)- OPhPhCN (CBA-n) with n = 9 or 10 have been determined in 1,4-dioxane. In the latter measurements, the molecular optical anisotropy gamma(2) was determined by using a depolarized Rayleigh apparatus equipped with a Fabry-Perot interferometer to facilitate the exclusion of the collision-induced contributions. The compounds studied are composed of two mesogenic groups joined by a flexible spacer and known to form nematic Liquid crystals when heated in the bulk state. In contrast to the configurational characteristics observed in the Liquid crystalline state, the (isotropic) solution properties studied in this work exhibited only a moderate odd-even trend with the number of flexible methylene units n; dipole moment mu = 6.67 (n = 9) and 6.76 D (n = 10), optical anisotropy gamma(2) 1089 (n = 9) and 1362 Angstrom(6) (n = 10) are all values measured at 25 degrees C. The temperature coefficient of the dipole moment d In mu(2)/dT was found to be positive for both compounds. The corresponding values of the optical anisotropy d ln gamma(2)/dT exhibited a small odd-even effect. These results have been satisfactorily interpreted within the framework of the rotational isomeric state approximation. The relative orientation of the two terminal mesogenic groups was elucidated for given conformations, and spatial distribution curves were constructed for the individual molecular systems. By the use of the molecular parameters thus established in the isotropic solution, mean-square values of the optical polarizability (gamma(2)) were further estimated for the ensemble of the conformers adopted in the nematic state. The bimodal character of the orientational distribution was shown to be responsible for the odd-even effect observed in the isotropic as well as in the anisotropic states.