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'-cyano biphenyl)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 depola rized 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 fle xible spacer and known to form nematic Liquid crystals when heated in the bulk state. In contrast to the configurational characteristics obs erved in the Liquid crystalline state, the (isotropic) solution proper ties studied in this work exhibited only a moderate odd-even trend wit h 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) an d 1362 Angstrom(6) (n = 10) are all values measured at 25 degrees C. T he temperature coefficient of the dipole moment d In mu(2)/dT was foun d to be positive for both compounds. The corresponding values of the o ptical anisotropy d ln gamma(2)/dT exhibited a small odd-even effect. These results have been satisfactorily interpreted within the framewor k of the rotational isomeric state approximation. The relative orienta tion of the two terminal mesogenic groups was elucidated for given con formations, and spatial distribution curves were constructed for the i ndividual molecular systems. By the use of the molecular parameters th us established in the isotropic solution, mean-square values of the op tical polarizability (gamma(2)) were further estimated for the ensembl e of the conformers adopted in the nematic state. The bimodal characte r of the orientational distribution was shown to be responsible for th e odd-even effect observed in the isotropic as well as in the anisotro pic states.