Electroabsorption spectroscopy of molecular inorganic compounds

Electroabsorption spectroscopy is known to report directly on the changes in dipole moment and molecular polarizability accompanying electronic excited state formation. Because ground-state/excited-state dipole moment changes can be equated with effective one-electron transfer distances, experimental electroabsorption spectroscopy holds exceptional promise as a methodology for investigating light-induced charge transfer processes within inorganic systems. A survey of the available studies, including metal-to-ligand charge transfer, ligand- to-metal charge transfer and localized and delocalized ‘intervalence’ charge transfer studies, is presented. Also surveyed are electroabsorption studies aimed at illuminating selected molecular nonlinear optical responses. A general observation from electroabsorption studies has been that experimentally determined one- electron transfer distances are less than simple geometric descriptions would predict. Mononuclear transition-metal systems have proven to be good models for unravelling the complicating effects of ground-state localization and many-electron polarization. The capability of electroabsorption spectroscopy to resolve fundamental questions relating to electronic localization and delocalization has been highlightedin a series of studies in bridged dinuclear and tris(diimine) systems. The available electroabsorption data have also been used to reassess a number of molecular charge-transfer related parameters such as the electronic coupling energies and solvent reorganization energies. Very recent studies of putative octupolar complexes and donor-acceptor porphyrinic structures have highlighted the utility of electroabsorption spectroscopy for evaluating second-order nonlinear optical response mechanisms and for providing detailed information about state- specific contributions to overall molecular hyperpolarizabilities.