Preresonance Raman studies of metal-to-ligand charge transfer in (NH₃)₄Ru(2,2′-bpy)²⁺. In situ bond length changes, force constants, and reorganization energies

As a prototype for charge-transfer reactions in general, the intense metal-to-ligand charge-transfer transition occurring in Ru(NH₃)₄(bpy)²⁺ (bpy = 2,2′-bipyridine) has been examined experimentally by resonance and preresonance Raman spectroscopy and analytically by time-dependent scattering theory. To our knowledge, the present example represents the first application of the theory to charge-transfer problems. From the experiments and corresponding theory, the normal-coordinate changes accompanying the transition have been calculated. Both metal-ligand and intraligand bonds are found to distort significantly. When the distortion data are combined with the observed vibrational frequencies, a mode-by-mode assessment of the inner-shell reorganization energy is possible. Further experiments, in which the nature of the solvent is systematically varied, show that selected force constants (and therefore selected components of the internal reorganization energy) are modulated significantly (ca. 6-11%) by ligand-solvent hydrogen bonding. Finally, variations in the nature of the solvent are found to shift ground-and/or excited-state energies in such a way as to either enhance or attenuate the occurrence of net photochemistry.