MLCT excited states and charge delocalization in some ruthenium-ammine-polypyridyl complexes

The ab initio calculations of polypyridine pi*-orbital energies are the basis for assignment of the lowest energy, highest intensity metal-to-ligand charge transfer (MLCT) transitions in simple ammine-polypyridine-ruthenium(II) complexes. A gaussian analysis of the absorption and emission spectra of these complexes enables the evaluation of reorganizational energies for the vertical MLCT transitions from component bandwidths and from apparent vibronic progressions. The observed bandwidths are about half of the widths expected in the limit of no metal-ligand mixing. The excited state-ground state mixing coefficient, alpha(DA), is inferred to be about 0.3 in [Ru(NH3)(4)bpy](2+) based on this observation and a perturbation theory argument. These estimated reorganizational energies are combined with the observed ambient Stokes shifts to determine that the excited state electron exchange energy, K-e, is small (600-1200 cm(-1) for 2,2' bipyridine complexes; similar to1500 cm(-1) for 2,3-bis-(2-pyridyl)pyrazine complexes), but significant. This and the observation that the N-H stretching frequency increases as the vertical MLCT energy (or alpha(DA)) decreases suggests that there is significant charge delocalization in these complexes. (C) 2002 Published by Elsevier Science B.V.