Electroabsorption Studies of Metal-to-Ligand Charge Transfer in Ru(phenanthroline)32+: Evidence for Intrinsic Charge Localization in the Initially Formed Excited State

Electroabsorption studies of ruthenium(II) tris(phenanthroline) show that a substantial change in dipole moment (|Delta| = 6.7 +/- 1 D) accompanies ground state to "singlet" metal-to-ligand charge transfer (MLCT) excited-state formation. The change is nearly identical to that reported for the 2,2'-bipyridine analogue (Oh; et al. J.Am. Chem. Soc. 1989, 111, 1130). Since both species lack ground-state dipole moments, the finite values for Delta are diagnostic of intrinsic charge localization in the excited states. The nominally triplet transition for the phenanthroline complex also involves the formation of a charge localized state. The combined results are inconsistent, therefore, with an alternative "delocalized" charge transfer excited-state interpretation suggested by time-resolved resonance Raman studies of the tris(phenanthroline) complex.