Studies of the electronic structure of metallocene-based second-order nonlinear optical dyes

This paper describes a simple orbital picture for understanding the optical transitions and the second-order nonlinear optical response of metallocene -based chromophores of the form metallocene-(pi-bridge)-acceptor, and exper imental studies to test this model. From a combination of UV photoelectron spectroscopy, cyclic voltammetry, and density functional calculations, it i s deduced that the three highest occupied orbitals are little perturbed fro m the parent metallocenes, that the HOMO-3 is a pi-orbital delocalized betw een the metallocene cyclopentadienyl ring and the unsaturated bridge, and t hat the LUMO is acceptor based. The lowest energy transition in the UV/visi ble/near-IR spectra of these compounds is assigned to a metal-to-acceptor t ransition, while the higher energy transition is attributed to a transition to the acceptor-based LUMO from the delocalized HOMO-3 orbital. The variat ions in oscillator strength can be rationalized by considering the low-ener gy transition as borrowing intensity from the high-energy transition. Stark spectroscopy confirms that large dipole moment changes are associated with both transitions, as expected from our assignment. These dipole moment cha nges indicate that, according to the perturbation theory-derived expression for the first hyperpolarizability, beta, both transitions contribute signi ficantly to the observed optical nonlinearity.