ELECTRONIC AND 2ND-ORDER NONLINEAR-OPTICAL PROPERTIES OF CONFORMATIONALLY LOCKED BENZYLIDENEANILINES AND BIPHENYLS - THE EFFECT OF CONFORMATION ON THE FIRST HYPERPOLARIZABILITY

The first hyperpolarizability beta and electronic properties of N,N-di methylamine and/or nitro-substituted benzylideneanilines and biphenyls are compared with those of conformationally locked model compounds, v iz. identically substituted 2-phenyI-3,3-dimethyl-3H-indoles and fluor enes, respectively. Although PM3 semi-empirical calculations indicate that a small to moderate increase in beta can be achieved by locking t he pi-systems in a planar conformation, experimental results show that the actual gain is either small or negligible. This mainly finds its origin in relatively narrow electronic absorption bands of the conform ationally locked compounds, which are related to their rigidity. Moreo ver, single crystal X-ray structures of the donor-acceptor substituted 3H-indoles demonstrate that the presence of a saturated bridge does n ot necessarily lead to a more planar structure. Furthermore, it is sho wn that the difference in beta of the two N,N-dimethylamino-nitro-subs tituted benzylideneaniline isomers is of electronic rather than of con formational origin.