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)]PF 6 were recorded in 4:1 (v:v) EtOH−MeOH and CH 2 Cl 2 −MeOH mixtures (bpy = 2,2‘-bipyridine; 4-Etpy = 4-ethylpyridine). The temperature-dependent emission of [Ru(bpy) 3 ](PF 6 ) 2 and the emission spectra of [(bpy) 2 Ru(bpyc-NHMe)](PF 6 ) 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 (ℏω M ) 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 ℏω M and solvent reorganizational energy dependencies on medium properties and temperature.