FREE-ENERGY AND SOLVENT DEPENDENCE OF INTRAMOLECULAR ELECTRON-TRANSFER IN DONOR-SUBSTITUTED RE(I) COMPLEXES

A comprehensive investigation of photoinduced intramolecular electron transfer (ET) in a series of six complexes of the type fac-(b)Re(I)(CO)3-D (where b is a diimine ligand and D is a dimethylaniline electron donor) is reported. Photoexcitation of the d-pi (Re) --> pi* (diimine) metal-to-ligand charge-transfer excited state initiates a sequence of forward and back ET reactions: [GRAPHICS] The driving force for forward and back ET (DELTA-G(FET) and DELTA-G(BET), respectively) is varied by changing the electron demand of the diimine ligand. Cyclic voltammetry and steady-state emission studies were carried out for each complex in three solvents (CH2Cl2, DMF, and CH3CN) to allow estimation of DELTA-G(FET) and DELTA-G(BET). The forward ET reactions are weakly exothermic (-0.5 eV < DELTA-G(FET) < -0.1 eV) and the back ET reactions are highly exothermic (-2.6 eV < DELTA-G(BET) < -1.5 eV). Rates for forward ET (k(FET)) for each of the complexes in the three solvents were determined by using time-resolved emission spectroscopy. The forward ET rate ranges from 10(7) s-1 to > 10(9) s-1 and is strongly dependent on DELTA-G(FET) and solvent polarity. The dependence of k(FET) on DELTA-G(FET) is consistent with nonadiabatic semiclassical Marcus theory. The solvent dependence of k(FET) suggests that the reorganization energy increases with solvent polarity in a manner that is consistent with the Marcus-Hush dielectric continuum model. Rates for back ET (k(BET)) were determined by using laser flash photolysis in two solvents. The back ET rate ranges from 10(7) s-1 to 5 x 10(8) s-1 and is not solvent dependent. Interestingly, k(BET) displays a weak, inverted dependence on DELTA-G(BET). Analysis of the rate data using a multimode quantum mechanical expression suggests that a possible explanation for the weak free-energy dependence may be that metal complex-based high-frequency acceptor modes are coupled to the back ET process.