Photoinduced radical formation from the complexes [Re(R)(CO)3(4,4′-Me2-bpy)] (R = CH3, CD3, Et, iPr, Bz):: A nanosecond time-resolved emission, UV-Vis and IR absorption, and FT-EPR study

Irradiation of the complexes [Re(R)(CO)(3)(dmb)] (R = CH3, CD3, Et, Pr-i, or Bz; dmb = 4,4'-dimethyl-2,2'-bipyridine) into their visible absorption band gives rise to a homolytic cleavage of the Re-R bond with formation of the radicals [Re(CO)(3)(dmb)](.) and R-.. In the case of R = Et, Pr-i, or Bz this reaction proceeds with unit efficiency. The nanosecond time-resolved absorption (TA) spectra show that the long-lived (tau = 7 mu s) [Re(CO)(3)(dmb)](.) radicals are formed within the 7 ns laser pulse. The CH3 complex photodecomposes with a quantum yield of only 0.4. The time-resolved UV-vis and IR absorption spectra reveal that this complex, after excitation into a (MLCT)-M-1 state, may either pass a barrier of 1560 cm(-1) to the dissociative (3)sigma pi* state and decompose into radicals or decay to the ground state via an excited-state having predominant (MLCT)-M-3 state character. Qualitative potential energy diagrams are presented for the two types of complexes. In a glass at 80 K and in a low-temperature solution (T < 195 K), the methyl complex is photostable, and this allowed us to study its excited-state properties with time-resolved absorption, emission, and IR spectroscopies. According to these spectra the lowest-excited (MLCT)-M-3 state has a significant admixture of a sigma pi* character. Finally, nanosecond time-resolved FT-EPR spectra were recorded of the CH3 and CD3 radicals produced by irradiation of [Re(CH3/CD3)(CO)(3)(dmb)]. The spectra of the CH3 radical exhibit a pronounced low-field emission/high-field absorption pattern due to an ST0 radical pair mechanism (RPM) CIDEP effect. The occurrence of this ST0 RPM confirms that the radicals are formed from an excited state, having spin-triplet character, in agreement with the proposed mechanism involving a reactive (3)sigma pi* state.