Mechanism of an alkyl-dependent photochemical homolysis of the Re-alkyl bond in [Re(R)(CO)(3)(alpha-diimine)] complexes via a reactive sigma pi* excited state

MLCT excitation of the complexes [Re(R)(CO)(3)(alpha-diimine)] (R = Me, Et, benzyl (Bz); alpha-diimine = iPr-PyCa, R'-DAB) results in the homolysis of the Re-R bond leading to the formation of radicals R(.) and [Re(CO)(3)(alpha-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 (10(-2)) 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 lambda(exc). The reaction is shown to proceed via a sigma(Re-R)pi* 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-mu s and ps-mu s time domains, respectively, show that the sigma pi* excited state is rather long-lived (tau approximate to 250 ns) in noncoordinating solvents; the dissociation of the Re-R bond from this state is strongly accelerated by polar or coordinating solvents (tau(sigma pi)* < 20 ps). The sigma pi* excited state is spectroscopically characterized by a (presumably sigma pi --> 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 sigma pi* states, controlled by the nature of the alkyl ligand, determine the photoreactivity of the complexes.