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

Abstract

The absorption spectra of Ru II (NH 3 ) 5 L and Ru III (NH 3 ) 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 (π-backbonding for Ru(II) and π-bonding for the Ru(III) complexes). Other contributions, namely the dipole moment induced by the NH 3 ligands and by the surrounding solvent molecules, are also considered.