CONFORMATIONAL DEPENDENCE OF THE QUADRATIC HYPERPOLARIZABILITIES OF A SERIES OF PUSH-PULL DIARYL ACETYLENES - AN EXPERIMENTAL AND COMPUTATIONAL INVESTIGATION

A combination of experimental and computational data are necessary to investigate the nonlinear optical response of push-pull diaryl acetylenes. We have found, by using the AM l semi-empirical method, that the molecules exist as conformational mixtures in solution. The effect of the distortion of the diaryl acetylene backbone on the molecular conjugation has been studied by using a finite field MNDO method. Results indicate that even when the two phenyl rings are set orthogonally to each other, the donor-acceptor interaction is still a dominant contribution to the molecular quadratic hyperpolarisability. We have also related the measured EFISH nonlinearity to an average calculated hyperpolarisability, taking into account the contribution of all conformations present in solution.