Schiff base linked ferrocenyl complexes for second-order nonlinear optics

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 $(\beta)$. Two photon fluorescence corrected \beta, 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, \beta, 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 \beta 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 \beta due to the MLCT transition around 532 nm. The dispersion free hyperpolarizability, $\beta_0$ of these complexes have also been calculated using the two-state model