Electronic Distribution in the Metal-to-Ligand Charge Transfer (MLCT) Excited States of [(4,4‘-(X)₂bpy)(CO)₃ReI(4,4‘-bpy)ReI(CO)₃(4,4‘-(X)₂bpy)]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, CH 3 ). 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 I.5 −Re I.5 ) sites in the mixed-valence excited states formed by MLCT excitation? Application of time-resolved resonance Raman (TR 3 ) 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‘-(CH 3 ) 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 ν(CO) bands in the excited state by time-resolved infrared (TRIR) measurements is consistent with the localized description, Re II −Re I .