Solvent-Dependent Dynamics of the MQ^•→Re^II Excited-State Electron Transfer in [Re(MQ⁺)(CO)₃(dmb)]²⁺

The Re→MQ⁺ MLCT excited state of [Re(MQ⁺)(CO)₃(dmb)]²⁺ (MQ⁺ = N-methyl-4,4‘-bipyridinium, dmb = 4,4‘-dimethyl-2,2‘-bipyridine), which is populated upon 400-nm irradiation, was characterized by picosecond time-resolved IR and resonance Raman spectroscopy, which indicate large structural differences relative to the ground state. The Re→MQ⁺ MLCT excited state can be formulated as [Re^II(MQ^•)(CO)₃(dmb)]²⁺. It decays to the ground state by a MQ^•→Re^II back-electron transfer, whose time constant is moderately dependent on the molecular nature of the solvent, instead of its bulk parameters:  formamides ∼ DMSO ∼ MeOH (1.2−2.2 ns) < THF, aliphatic nitriles (3.2−3.9 ns) << ethylene-glycol ∼ 2-ethoxyethanol (4.2−4.8 ns) < pyridine (5.7 ns) < MeOCH₂CH₂OMe (6.9 ns) < PhCN (7.5 ns) < MeNO₂ (8.6 ns) <<< CH₂Cl₂, ClCH₂CH₂Cl (25.9−28.9 ns). An approximate correlation was found between the back-reaction rate constant and the Gutmann donor number. Temperature dependence of the decay rate measured in CH₂Cl₂, MeOH, and BuCN indicates that the inverted MQ^•→Re^II back-electron transfer populates a manifold of higher vibrational levels of the ground state. The solvent dependence of the electron transfer rate is explained by solvent effects on inner reorganization energy and on frequencies of electron-accepting vibrations, by interactions between the positively charged MQ⁺ pyridinium ring and solvent molecules in the electron-transfer product, that is the [Re(MQ⁺)(CO)₃(dmb)]²⁺ ground state.