Medium Dependence of Intramolecular Vibrational Modes Coupled to MLCT Transitions in Metal Polypyridyl Complexes

The temperature and solvent dependent emission spectra of fac-[(bpy)Re(CO)3(4-Etpy)]PF6 were recorded in 4:1 (v:v) EtOH−MeOH and CH2Cl2−MeOH mixtures (bpy = 2,2‘-bipyridine; 4-Etpy = 4-ethylpyridine). The temperature-dependent emission of [Ru(bpy)3](PF6)2 and the emission spectra of [(bpy)2Ru(bpyc-NHMe)](PF6)2 (bpyc-NHMe = 4-methyl-4‘-(N-methyl carboxamido)-2,2‘-bipyridine) in 10 solvents were also analyzed for comparison purposes. The data reveal a significant dependence of the effective vibrational quantum spacing (ℏωM ) with the energy difference between the ground and excited states (E 0) as the medium properties are systematically varied. Initial attempts to explain this dependence in terms of a medium induced modulation of the metal-to-ligand charge transfer energy were not fully successful. That model is based on the relationship between the energy gap, the extent of charge transfer, and excited state distortions. A more satisfactory explanation of the experimental data was achieved by a model that includes hydrogen bonding interactions between the CO groups of the Re complex and a protic solvent. The inclusion of extended hydrogen-bonding interactions between the solute and the solvent are required to fully explain ℏωM and solvent reorganizational energy dependencies on medium properties and temperature.