Factors determining formation efficiencies of one-electron-reduced species of redox photosensitizers

Improvement in the photochemical formation efficiency of one-electron-reduced species (OERS) of a photoredox photosensitizer (a redox catalyst) is directly linked to the improvement in efficiencies of the various photocatalytic reactions themselves. We investigated the primary processes of a photochemical reduction of two series [Ru(diimine)(3)](2+) and [Os(diimine)(3)](2+) as frequently used redox photosensitizers (PS2+), by 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH) as a typical reductant in detail using steady-irradiation and time-resolved spectroscopies. The rate constants of all elementary processes of the photochemical reduction of PS2+ by BIH to give the free PS center dot (+) were obtained or estimated. The most important process for determining the formation efficiency of the free PS center dot (+) was the escape yield from the solvated ion pair [PS center dot (+)-BIH center dot (+)], which was strongly dependent on both the central metal ion and the ligands. In cases with the same central metal ion, the system with larger -Delta G(bet), which is the free energy change in the back-electron transfer from the OERS of PS center dot (+) to BIH center dot (+), tended to lower the escape yield of the free OERS of PS2+. On the other hand, different central metal ions drastically affected the escape yield even in cases with similar -Delta G(bet); the escape yield in the case of RuH2+ (-Delta G(bet) = 1.68 eV) was 5-11 times higher compared to those of OsH2+ (-Delta G(bet) = 1.60 eV) and OsMe2+ (-Delta G(bet) = 1.71 eV). The back-electron transfer process from the free PS center dot (+) to the free BIH center dot (+) could not compete against the further reaction of the free BIH center dot (+), which is the deprotonation process giving BI center dot, in DMA for all examples. The produced BI center dot gave one electron to PS2+ in the ground state to give another PS center dot (+), quantitatively. Based on these findings and investigations, it is clarified that the photochemical formation efficiency of the free PS center dot (+) should be affected not only by -Delta G(bet) but also by the heavy-atom effect of the central metal ion, and/or the oxidation power of the excited PS2+, which should determine the distance between the excited PS and BIH at the moment of the electron transfer.