Optical nonlinearities of organometallic structures: Aryl and vinyl derivatives of ferrocene

With an objective to understand the nonlinear optical properties of organometallic structures, various aryl and vinyl derivatives of ferrocene were synthesized and their nonlinear optical properties were investigated by using degenerate four-wave mixing. The molecular second hyperpolarizability γ increases strongly with the length of the conjugated π-electron system. The results show that the effective conjugation is determined predominantly by the length of the aryl-vinyl system; the contribution from the ferrocenyl group is less significant. The d-d resonance of the metal in the ferrocene unit does not appear to make an important contribution to optical nonlinearity. The experimental results on ferrocene are compared with those from a recent theoretical study using semiempirical calculations. Although a qualitative agreement with the theoretical result is found, the experimental value of γ determined by our method is about 4 times larger. Possible sources of such discrepancies are discussed.