Nematic-isotropic transition temperature and dipole-dipole interaction. 4-X-substituted phenyl 4-(4-Y-substituted benzylideneamino)-benzoates

The nematic-isotropic transition temperature of 4-X-phenyl 4-(4-Y-benzylide neamino)benzoates, where the terminal groups X and Y are CN, CH3O, NO2, Cl, Br, N (CH3)(2), CH3, F, or CF3, was examined. The transition temperature d ifference between the two series, in which X or Y=CH3O and F, respectively, decreases in this order: CF3 > NO2 > Cl = Br = F > CN > CH3 > CH3O > N (CH 3)(2). Thus, the groups are almost in the order of substituent constants ex cept for the CN group, suggesting that the dipole-dipole interaction contri butes significantly to the nematic thermal stability and also that the effe ctive dipole moment of the molecule bearing CN and CH3O groups is markedly reduced in the mesophase from that of the free molecule because of the anti -parallel molecular association.