LARGE OPTICAL NONLINEARITIES WITH CONJUGATED FERROCENE AND RUTHENOCENE DERIVATIVES

Donor-acceptor polyenes of increasing length combining the ferrocene o r ruthenocene electron-donating moieties and different electron-withdr awing end groups have been synthesized in order to achieve enhanced qu adratic optical nonlinearities. These compounds exhibit two absorption bands, the less intense red-shifted one being attributable to a metal -to-ligand charge transfer (MLCT) transition, and the sharper and more intense blue-shifted one to a pi-->pi charge transfer transition. Bo th bands exhibit positive solvatochromic behavior and are redshifted w ith increasing polyenic chain length and stronger accepters. In additi on, the lowest energy band gains in relative intensity, suggesting inc reased coupling between the metal center and the acceptor. The variati on of their quadratic hyperpolarizability beta in solution has been in vestigated using the electric-field-induced second-harmonic (EFISH) ge neration technique, at 1.907 mu m in chloroform. In each series of hom ologous compounds, lengthening the conjugation path results in a marke d increase in mu beta values. This phenomenon leads to very large mu b eta values. The potent methyridene)-2,3-dihydrobenzothiophene-1,1-diox ide acceptor moiety leads to even higher quadratic nonlinearities than the effective electron-withdrawing 1,3-diethyl-2-thiobarbituric acid end group.