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

Kyohei Ozawa,Kei Kamogawa,Osamu Ishitani,Yusuke Tamaki,Kazuhide Koike

doi:10.1063/5.0023593

Abstract

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•+ were obtained or estimated. The most important process for determining the formation efficiency of the free PS•+ was the escape yield from the solvated ion pair [PS•+-BIH•+], 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 -ΔGbet, which is the free energy change in the back-electron transfer from the OERS of PS•+ to BIH•+, 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 -ΔGbet; the escape yield in the case of RuH2+ (-ΔGbet = 1.68 eV) was 5-11 times higher compared to those of OsH2+ (-ΔGbet = 1.60 eV) and OsMe2+ (-ΔGbet = 1.71 eV). The back-electron transfer process from the free PS•+ to the free BIH•+ could not compete against the further reaction of the free BIH•+, which is the deprotonation process giving BI•, in DMA for all examples. The produced BI• gave one electron to PS2+ in the ground state to give another PS•+, quantitatively. Based on these findings and investigations, it is clarified that the photochemical formation efficiency of the free PS•+ should be affected not only by -ΔGbet 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.

Highlights

Photochemical redox reactions have been intensely investigated for a long time
Scitation.org/journal/jcp both series, absorption bands at λmax = ∼300 nm and λmax = 450 nm– 500 nm attributed to 1π–π∗ and 1MLCT excitations were observed, only spectra of OsX2+ showed a relatively strong broad absorption band at λmax = 650 nm–750 nm. These are attributed to singlet to triplet (S-T) absorption bands, i.e., direct excitation to the 3MLCT excited state of OsX2+, which is a forbidden transition in principle of the quantum chemistry but is partially allowed owing to the strong heavy-atom effect of the central Os(II)
The quantum yield for the formation of one-electron-reduced species (OERS) strongly depends on both the central metal ion and the ligands (Table III)

Summary

Introduction

Photochemical redox reactions have been intensely investigated for a long time. In artificial photosynthesis, for example, H2 evolution1 and CO2 reduction2,3 can be initiated by a photoinduced electron transfer using a redox photosensitizer from an electron donor to a catalyst. As a typical example of photochemical reduction of the metal complexes, Fig. 7(a) shows the UV–Vis absorption changes for a DMA solution containing RuH2+ (0.3 mM) and BIH (0.1M) during irradiation [λex = 480 nm (3.7 × 10−9 E s−1)] under an Ar atmosphere using a 500 W Xe lamp with a band-pass filter, and fitting results using the spectrum of the OERS (RuH+) obtained by flow electrolysis (Fig. 5).

Results

Conclusion