Metal-to-Ligand Charge Transfer Excited-State ν(CO) Shifts in Rigid Media

Abstract

The results of ν(CO) infrared measurements on the ground and metal-to-ligand charge transfer (MLCT) excited states of fac-[ReI(4,4‘-X2bpy)(CO)3(4-Etpy)]+ (X = H, CH3, and CO2Et; 4-Etpy is 4-ethylpyridine) in both CH3CN and poly(methyl methacrylate) (PMMA) films at 298 K are reported. In PMMA, the shifts in ν(CO) between the excited and ground states (+18 to +68 cm-1) are noticeably less than in solution (+33 to +88 cm-1). Our work was stimulated by the observation by Turner and co-workers (Chem. Commun. 1996, 1587−1588) that ν(CO) excited-to-ground-state shifts for fac-[Re(bpy)(CO)3Cl] are much less in a rigid butyronitrile/propionitrile glass at 77 K than in CH3CN at 298 K. For the Re complexes, a single correlation exists between excited-state ν(CO) shifts and the MLCT ground-to-excited-state energy gap (E0) regardless of whether the energy gap is changed by varying the substituent X or the medium. The substituent and rigid medium effects appear to have a common orbital origin arising from π*(4,4‘-X2bpy•-)−π*(CO) mixing. This provides the orbital basis for mixing higher lying dπ(Re)−π*(CO) MLCT states with the emitting dπ(Re)−π*(4,4‘-X2bpy•-) MLCT excited state(s).