Supramolecular photocatalysts comprising [Ru(diimine)3]2+ photosensitiser and fac-[Re(diimine)(CO)3{OC(O)OC2H4NR2}] catalyst units can be used to reduce CO2 to CO with high selectivity, durability and efficiency. In the presence of triethanolamine, the Re...
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.
Supramolecular photocatalysts, wherein redox photosensitizer (PS) and catalyst (CAT) molecules are connected to each other, have been extensively studied because of their high photocatalytic activity in both homogeneous and heterogeneous environments compared with the corresponding mixed systems of separated PS and CAT. A supramolecular photocatalyst RuC2PhC2Re, wherein [Ru(diimine)3]2+ redox PS and fac-[Re(diimine)(CO)3{OC(O)OCH2CH2N(CH2CH2OH)2}] CAT units were spatially separated by a bridging ligand p(-C2H4)2Ph consisting of 8 C–C bonds including a p-phenylene ring, was developed. Although the rate of intramolecular electron transfer of RuC2PhC2Re from one-electron-reduced Ru unit to the Re unit, which is a critical process of photocatalysis proceeding through the bond mechanism, was slower than that of RuC2Re having shorter bridging ligand with an ethylene chain, it could reduce CO2 to CO with higher durability (TON = 3880) than RuC2Re (TON = 2800). These results clearly suggest that the PS and CAT units can be separated further without lowering photocatalysis of supramolecular photocatalysts because the rate of intramolecular electron transfer is much faster, even in RuC2PhC2Re, than that of the subsequent processes in photocatalytic CO2 reduction.
Organic chromophores displaying TADF emission were coupled to a Mn(i)-complex as the catalyst and investigated as photosensitizers for CO2 reduction. Upon 470 nm LED excitation, TONCO+HCOOH > 650 and a ΦCO+HCOOH = 22.8% were obtained.
Two new supramolecular photocatalysts containing Ru(II) polypyridine units as light-harvesting photosensitizers and Re(I) polypyridine subunits as catalytic centers have been prepared. The new species, RuRe2A and Ru2ReA, contain catalytic Re(I) subunits coordinated by the preformed CO2TEOA adduct (known to be the effective catalytic subunits; TEOA is triethanolamine) and exhibit quite efficient and selective photoreduction of CO2 to CO, with outstanding TONs of 2368 and 2695 and a selectivity of 99.9% and 98.9%, respectively. Such photocatalytic properties are significantly improved with respect to those of previously studied RuRe2 and Ru2Re parent compounds, containing chloride ligands instead of the CO2TEOA adduct. Comparison between photocatalytic performance of the new species and their parent compounds allows to investigate the effect of the CO2TEOA insertion process as well as the eventual effect of the presence of chloride ions in solution on the photocatalytic processes. The improved photocatalytic properties of RuRe2A and Ru2ReA compared with their parent species are attributed to a combined effect of different distribution of the one-electron reduced form of the supramolecular photocatalysts on the Ru-subunit(s) (leading to decreased CO formation due to a poisoning ligand loss process) and on the Re-subunit(s) and to the presence of chloride ions in solution for RuRe2 and Ru2Re, which could interfere with the CO2TEOA adduct formation, a needed requisite for CO forming catalysis. These results strongly indicate the utility of preparing supramolecular photocatalysts containing preformed adducts.
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