Schiff base linked ferrocenyl complexes for second-order nonlinear optics

Abstract

A series of substituted ferrocenyl compounds where one of the cyclopentadienyl rings is linked to an aromatic Schiff base, have been synthesized and analyzed for their second-order nonlinearity (β). Two photon fluorescence corrected β, of these complexes correlates well with the electron withdrawing nature of the substituted benzene ring. The well-known two-state model has been invoked to rationalize the observed values of the first hyperpolarizability, β, of these complexes. The metal to ligand charge transfer (MLCT) transition dominates their second-order response. These compounds form charge transfer (CT) complexes with acceptors such as iodine, p-chloranil (CA), 2, 3-dichloro-5, 6-dicyano-1, 4-benzoquinone (DDQ), tetracyanoethylene (TCNE), and 7, 7, 8, 8-tetracyanoquinodimethane (TCNQ). The CT complexes exhibit much higher second-order response. A series of bisferrocenyl complexes where two ferrocene moieties are linked through the same aromatic Schiff base spacer has also been synthesized and characterized. The β values of the bisferrocenyl complexes and their CT counterparts are much higher than the corresponding monoferrocene complexes. In all these compounds there is a strong resonant contribution to β due to the MLCT transition around 532 nm. The dispersion free hyperpolarizability, β₀ of these complexes have also been calculated using the two-state model.