MOLECULAR 2ND-ORDER OPTICAL NONLINEARITIES OF METALLOCENES

The nonresonant quadratic molecular hyperpolarizabilities of several ferrocene and ruthenocene derivatives were studied by dc electric-field-induced second-harmonic generation (EFISH) experiments using fundamental radiation at 1.91-mu-m. Hyperpolarizabilities approaching that of 4-(dimethylamino)-4'-nitrostilbene (DANS) were observed, indicating that the ferrocene moiety can act as an effective donor. It was found that the dipole projections of the beta-tensors of ferrocene complexes are comparable to those of methoxyphenyl systems with similar acceptors. Thus, (E)-ferrocenyl-4-(4-nitrophenyl)ethylene (5) has a beta value of 31 x 10(-30) esu whereas 4-methoxy-4'-nitrostilbene has a value for beta of 29 x 10(-30) esu. Replacement of the iron atom with ruthenium, which has a higher ionization potential, leads to a reduction in beta relative to 5. Replacement of a cyclopentadienyl ring with a pentamethylcyclopentadienyl ring leads to increased nonlinearity.