MLCT excitation of the complexes [Re(R)(CO),(a-diimine)] (R = Me, Et, benzyl (Bz); a-diimine = iPr-PyCa, R-DAB) results in the homolysis of the Re-R bond leading to the formation of radicals R and [Re(CO),(a-diimine)]' as primary photoproducts. The quantum yield of this photoprocess is dependent on the alkyl group used. For R = Me, the quantum yield is low and depends on the temperature and excitation wavelength, whereas for R = Et and Bz the quantum yield is near unity and independent of T and A,,,. The reaction is shown to proceed via a o(Re-R)x* excited state that is rapidly (< 20 ps) populated by a nonradiative transition from the optically excited MLCT state. Time-resolved IR and UV/Vis absorption spectra studied in the ns-ps and ps-ps time domains, respectively, show that the ox* excited state is rather long-lived (z x 250 ns) in noncoordinating solvents; the dissociation of the Re-R bond from this state is strongly accelerated by polar or coordinating solvents (rent < 20 ps). The ox* excited state is spectroscopically characterized by a (presumably ox* -+ MLCT) transition at approximately 500 nm and by CO stretching frequencies closely resembling their ground-state values. The relative energies of the MLCT and reactive cm* states, controlled by the nature of the alkyl ligand, determine the photoreactivity of the complexes.

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