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 experimental studies to test this model. From a combination of UV photoelectron spectroscopy, cyclic voltammetry, and density functional calculations, it is deduced that the three highest occupied orbitals are little perturbed from the parent metallocenes, that the HOMO-3 is a pi-orbital delocalized between the metallocene cyclopentadienyl ring and the unsaturated bridge, and that the LUMO is acceptor based. The lowest energy transition in the UV/visible/near-IR spectra of these compounds is assigned to a metal-to-acceptor transition, while the higher energy transition is attributed to a transition to the acceptor-based LUMO from the delocalized HOMO-3 orbital. The variations in oscillator strength can be rationalized by considering the low-energy 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 changes indicate that, according to the perturbation theory-derived expression for the first hyperpolarizability, beta, both transitions contribute significantly to the observed optical nonlinearity.