Metal-to-ligand charge transfer excited-state v(CO) shifts in rigid media

The results of v(CO) infrared measurements on the ground and metal-to-ligand charge transfer (MLCT) excited states of fac-[Re-I(4,4'-X(2)bpy)(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 v(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, 15871588) that v(CO) excited-to-ground-state shifts for fac-[Re(bpy)(CO)(3)Cl] 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 v(CO) shifts and the MLCT ground-to-excited-state energy gap (Eo) 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 pi*(4,4'-X(2)bpy(circle-))-pi*(CO) mixing. This provides the orbital basis for mixing higher lying dpi(Re)-pi*(CO) MLCT states with the emitting dpi(Re)-pi*(4,4'-X(2)bpy(circle-)) MLCT excited state(s).