Electronic distribution in the metal-to-ligand charge transfer (MLCT) excited states of [(4,4'-(X)(2)bpy)(CO)(3)Re-I(4,4'-bpy)Re-I(CO)(3)(4,4'-(X)(2)bpy)](2+) (X = H, CH3). Application of time-resolved infrared and resonance Raman spectroscopies

Ground- and excited-state resonance Raman and infrared spectra (354.7 nm excitation, in acetonitrile at 298 K) have been measured for [(4,4'-(X)(2)bpy>(Co)(3)Re-I(4,4'-bpy)Re-I(CO)(3)(4,4'-(X)(2)bpy)](2+)(bpy is 2,2'-bipyridine; 4,4'-bpy is 4,4'-bipyridine; X = H, CH3). The lowest lying excited states in these molecules are metal-to-ligand charge transfer (MLCT) in character. The spectra provide answers to two questions: Is the acceptor ligand for the excited electron 4,4'-bpy or 4,4'-(X)(2)bpy? Are there localized (Re-II-Re-I) or delocalized (Re-1.5-Re-1.5 5) sites in the mixed-valence excited states formed by MLCT excitation? Application of time-resolved resonance Raman (TR3) spectroscopy demonstrates that 4,4'-bpy is the acceptor in the lowest lying MLCT excited state(s) of [(dmb)(CO)(3)Re(4,4'-bpy)Re(CO)(3)(dmb)](2+)* (dmb = 4,4'-(CH3)(2)bpy) and that there is an equilibrium between 4,4'-bpy and bpy as acceptor in [(bpy)(CO)(3)Re(4,4'-bpy)Re(CO)(3)(bpy)](2+*). For both complexes, the appearance of two sets of overlapping v(CO) bands in the excited state by time-resolved infrared (TRIR) measurements is consistent with the localized description, Re-II-Re-I.