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Photoinduced Radical Formation from the Complexes [Re(R)(CO) 3 (4,4‘-Me 2 -bpy)] (R = CH 3 , CD 3 , Et, i Pr, Bz): A Nanosecond Time-Resolved Emission, UV−Vis and IR Absorption, and FT-EPR Study

1998; American Chemical Society; Volume: 120; Issue: 42 Linguagem: Inglês

10.1021/ja980540k

1943-2984

Cornelis J. Kleverlaan, Derk J. Stufkens, Ian P. Clark, Michael W. George, James J. Turner, Débora M. Martino, Hans van Willigen, Antonı́n Vlček,

Photochemistry and Electron Transfer Studies

Irradiation of the complexes [Re(R)(CO)3(dmb)] (R = CH3, CD3, Et, iPr, 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, iPr, or Bz this reaction proceeds with unit efficiency. The nanosecond time-resolved absorption (TA) spectra show that the long-lived (τ = 7 μs) [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 1MLCT state, may either pass a barrier of 1560 cm-1 to the dissociative 3σπ* state and decompose into radicals or decay to the ground state via an excited-state having predominant 3MLCT 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 3MLCT state has a significant admixture of a σπ* 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 3σπ* state.