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

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.

Dynamics of Photoinduced Intramolecular and Intermolecular Electron Transfers in Ligand-Conjugated Ir(III)–Re(I) Photocatalysts

“…As exhibited in Figure b, the EAS obtained from the fits clearly show that the 3 MLCT state (A, 3 [Ir + -BL – ]-Re ) evolves into the IET state (B, Ir + -[BL-Re] – ) followed by an additional C state appearing on a slower time scale of 1.2–1.4 ns (Figure b). Note that the evolutions of EAS for A → B and its time constants are similar to those observed in the TA spectra of Ir-BL-Re (Figure b), except that more intense and broad absorption appears near >600 nm in the TA spectra of the Ir-BL-Re + BIH samples, presumably due to the absorption of BIH radicals generated by pump pulse . This indicates that the same IET process occurs in the presence of BIH.…”

“…Note that the evolutions of EAS for A → B and its time constants are similar to those observed in the TA spectra of Ir-BL-Re (Figure 2b), except that more intense and broad absorption appears near >600 nm in the TA spectra of the Ir-BL-Re + BIH samples, presumably due to the absorption of BIH radicals generated by pump pulse. 29 This indicates that the same IET process occurs in the presence of BIH.…”

Highly Functional Dinuclear CuI-Complex Photosensitizers for Photocatalytic CO2Reduction

“…We reported that the higher quantum yield of photochemical formation of the OERS of [Ru(X 2 bpy) 3 ] 2+ and [Os(X 2 bpy) 3 ] 2+ (X 2 bpy = 4,4′-X 2 -2,2′-bipyridine) using an electron donor was obtained using the complex with higher oxidation power in the excited state. 16 It was proposed that the stronger oxidation power of the excited complex should induce longer distance between the reduced complex and the oxidized electron donor in a solvent cage, which can suppress the speed of the backelectron transfer reaction. We can assume that the stronger oxidation power of the excited Cu2CF 3 ph than those of Cu2Hph and Cu2Bph, which is supported by the fact that the quenching rate constant k q is larger in the case of Cu2CF 3 ph than those of the others (Table 1), caused the highest quantum yield of formation of the OERS of Cu2CF 3 ph in these three complexes as well.…”

“…Although we have not been able to clarify the reason, yet, we can propose one possible reason: the strongest oxidation power of the excited state of Cu2CF 3 ph . We reported that the higher quantum yield of photochemical formation of the OERS of [Ru­(X 2 bpy) 3 ] 2+ and [Os­(X 2 bpy) 3 ] 2+ (X 2 bpy = 4,4′-X 2 -2,2′-bipyridine) using an electron donor was obtained using the complex with higher oxidation power in the excited state . It was proposed that the stronger oxidation power of the excited complex should induce longer distance between the reduced complex and the oxidized electron donor in a solvent cage, which can suppress the speed of the back-electron transfer reaction.…”

Controlling Spin‐Correlated Radical Pairs with Donor–Acceptor Dyads: A New Concept to Generate Reduced Metal Complexes for More Efficient Photocatalysis

“…In addition to photoredox catalysis, the M À generation is important for many photochemical carbon dioxide reduction [12,[81][82][83] and hydrogen production [13,[84][85][86][87][88][89][90][91][92] mechanisms, demonstrating that the new mechanism introduced in this manuscript has severalp ossible application areas. Our study on the heavily underexplored inherente ffi-ciency of photoinduced electron transfer events might trigger furtherq uantitative investigations that could contribute to a better understanding of how to use photons moree fficiently, [93] which could ultimately result in more sustainability in photochemistry. [94] Elucidating the interplay of spin states, heavy atom effects and inherentr eaction (rather than emission quenching) efficienciess hould therefore be very important for the photochemistry of the future.…”