Application of Transient Resonance Raman Spectroscopy to the Structure of a Photoinduced Electron-Transfer Intermediate

Transient resonance Raman spectra have been acquired for the chromophore-quencher complexes fac-[(bpy)-Re{sup I}(CO){sub 3}(MQ{sup +})]{sup 2+} and cis-[(bpy){sub 2}Os{sup II}(CO)(MQ{sup +})]{sup 3+} (MQ{sup +} is monoquat, N-methyl-4,4{prime}=bipyridinium cation; bpy is 2,2{prime}-bipyridine) following metal-to-ligand charge-transfer excitation. The transient spectra verify that bpy{sup {sm_bullet}{minus}} {yields} MQ{sup +} intramolecular electron transfer occurs ({tau} < 7 ns) with the excited electron residing ultimately on the monoquat ligand. By comparison to vibrational data for biphenyl, 4,4{prime}-bipyridine, N,N{prime}-dihydro-4,4{prime}-bipyridinium dication, and methyl viologen (paraquat) and their corresponding, one-electron reduced forms, it can be inferred that, in the MQ{sup +}-based MLCT excited state, the electron is delocalized over the entire ligand and the two pyridyl rings assume a coplanar geometry.