Evaluation of the first-order hyperpolarizability tensor of bent moleculesby means of hyper-Rayleigh scattering method

The first-order hyperpolarizabilities beta s were determined by hyper-Rayle igh scattering in two homologous series of two-dimensional bent molecules w ith alkyl and alkoxy end chains. It was found that the beta values of alkyl and alkoxy compounds are nearly the same and increase slightly with increa sing chain length from ca. 10 x 10(-30) esu to ca. 40 x 10(-30) esu. Using the beta value determined in the present study and the dihedral angle of th e bent molecule, the tensor elements of beta were also determined. The seco nd-order nonlinear susceptibilities d(33) and d(31) were estimated and comp ared with, an experimental result obtained by a bulk second-harmonic genera tion (SHG) measurement by Macdonald et al. [Phys. Rev. Lett. 81 (1998) 4408 ]. This comparison leads to a conclusion that the high SHG activity in bent molecular liquid crystal systems is attributed to highly ordered molecules possessing high hyperpolarizability in smectic layers.