Large optical nonlinearities with conjugated ferrocene and ruthenocene derivatives

Donor—acceptor polyenes of increasing length combining the ferrocene or ruthenocene electron-donating moieties and different electron-withdrawing end groups have been synthesized in order to achieve enhanced quadratic 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 π → π* charge transfer transition. Both bands exhibit positive salvatochromic behavior and red-shifted with increasing polyenic chain length and stronger acceptors. In addition, the lowest energy band gains in relative intensity, suggesting increased coupling between the metal center and the acceptor. The varition of their quadratic hyperpolarizability β in solution has been investigated using the electric-field-induced second-harmonic (EFISH) generation technique, at 1.907 μm in chloroform. In each series of homologous compounds, lengthening the conjugation path results in a marked increase in μβ values. This phenomenon leads to very large μβ values. The potent 3-(dicyanomethylidene)-2,3-dihydrobenzothiophene-1,1-dioxide acceptor moiety leads to even higher quadratic nonlinearities than the effective electron-withdrawing 1,3-diethyl-2-thiobarbituric acid end group.