Electroabsorption spectroscopy of charge-transfer states of transition-metal complexes .2. Metal-to-ligand and ligand-to-metal charge-transfer excited states of pentaammineruthenium complexes

The absorption spectra of Ru-II(NH3)(5)L and Ru-III(NH3)(5)L (L is an aromatic N-heterocycle or nitrile) complexes in 50:50 glycerol-water glasses at 77 K (D-s = 3.9) are a function of the applied field in the 10(6)-10(7) V/m range. Analysis of the spectra in terms of the Liptay equations yields ground-excited state dipole-moment differences ranging from 4 to 37 D, depending upon the nature of L. The measured dipole moment differences, particularly those for the MLCT transitions, are much smaller than the values estimated from a simple consideration of the electron-transfer distances. The discrepancy between the observed and naive dipole-moment estimates arises mainly from the multielectron nature of the response to excitation. Good agreement is obtained with the predictions of a model which includes refinement of the effective electron-transfer distance, the shift in the valence electron distribution in the excited state, and the effects of electron delocalization (pi-backbonding for Ru(II) and pi-bonding for the Ru(III) complexes). Other contributions, namely the dipole moment induced by the NH3 ligands and by the surrounding solvent molecules, are also considered.