Importance of Selective Quenching of the Triplet Excited State of Thermally Activated Delayed Fluorescence (TADF) Photosensitizers in Redox-Photosensitized Reactions: Case Studies on Photocatalytic CO₂ Reduction

Redox photosensitizers exhibiting thermally activated delayed fluorescence (TADF) are widely used in the various research fields. We investigated the roles of the singlet and triplet excited states of such molecules in photocatalytic CO2 reduction. Two TADF compounds (4DPAIPN and 3DPAFIPN) were used in combination with a manganese(I) complex as a catalyst and 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH) and triethanolamine (TEOA) as sacrificial electron donors. In the case of 4DPAIPN, the quantum yield of CO2 reduction (ΦCO+HCOO-) was negatively correlated with the concentration of BIH, equaling 43.2% and 20.3% at [BIH] = 5 and 200 mM, respectively. The reduction of the singlet excited state by BIH afforded a singlet geminate ion pair, 1(4DPAIPN•-···BIH•+), which experienced a markedly faster backward electron transfer affording the ground state than the corresponding triplet, 3(4DPAIPN•-···BIH•+). The escape yield of the singlet state was approximately 10 times lower than that of the triplet state. High BIH concentrations favored the quenching of the singlet excited state and disfavored the formation of the triplet excited state, resulting in low photocatalytic efficiencies. Although the system with 3DPAFIPN exhibited a similar tendency, the maximum ΦCO+HCOO- was lower (11.9% at [BIH] = 10 mM) because of its greater oxidizing power resulting in the efficient quenching of its singlet excited state by TEOA. Based on these results, we extracted the reaction conditions and molecular designs of TADF photosensitizers suitable for constructing efficient photocatalysts, namely those minimizing the quenching of the singlet excited state and maximizing the quenching of the triplet excited state.