Yusuke Tamaki

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 (Φ_CO+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 Φ_CO+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.

fac-[Mn^I(diimine)(CO)₃(L)]^0/+ has attracted significant attention as a catalyst for the photocatalytic reduction of CO₂. However, in such photocatalytic systems, the photoexcitation of Mn complexes and reaction intermediates induces their decomposition, which lowers the durability of these systems. In this study, we clarified the primary process whereby the Mn complex catalyst decomposes during the photocatalytic reaction. Based thereupon, we successfully constructed a highly durable photocatalytic system, of which the turnover number of formate (TON_HCOO-) exceeded 1700 when fac-[Mn^I(bpy)(CO)₃((OC(O)OC₂H₅N(C₂H₅OH)₂) (Mn-CO₂-TEOA) as the catalyst, [Os^II(4,4'-dimethyl-bpy)(5,5'-dimethyl-bpy)₂]²⁺ (Os) as the photosensitizer, and 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH) as the reductant were used in conjunction with irradiation at λ_ex ≥ 620 nm. In contrast, for the same photocatalytic system, irradiation at λ_ex ≥ 480 nm lowered the TON_HCOO- to less than 60. The significant difference in the durability of the photocatalytic system arises from the dependence of the Mn(0)-Mn(0) dimer [Mn⁰₂(bpy)₂(CO)₆] (Dim-Mn), an intermediate produced during the photocatalytic reaction, on the wavelength of the irradiated light for its photoreactivity. That is, the irradiation of Dim-Mn at λ_ex ≥ 620 nm selectively induces splitting of the Mn-Mn bond to produce [Mn⁰(bpy)(CO)₃] (Mn^•) and, contrary to this, splitting of the Mn(0)-CO bonds and further decomposition processes are induced by irradiation at λ_ex ≥ 480 nm.

While the quantum yields of photosensitiser-derived one-electron-reduced species (OERSs) significantly impact the overall efficiencies of various redox-photosensitised photocatalytic reactions, the primary factors that influence them remain unclear. In this study, we systematically compared the photochemical formation quantum yields for OERSs associated with Ru(II) and Os(II) tris-diimine, cis, trans-[Re^I(diimine)(CO)₂(PR₃)₂]⁺, and cyclometalated Ir(III) complexes in the presence of the same 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH) reductant. The reduction potentials of the excited metal complexes, the heavy-atom effects of the central metal ions, and the oxidation potentials and charges of their OERSs were examined, which reveals that the driving force for photoinduced electron-transfer is the most important factor that determines the quantum yields associated with photochemical OERS formation. For complexes with higher oxidation power in their excited states, the formation quantum yield of OERSs divided by the quenching efficiency of the excited state by BIH is greater. This finding suggests that a higher photoinduced electron-transfer exergonicity promotes electron transfer over larger excited-complex/BIH distances, which in turn enables more-efficient separation of the resulting OERSs and one-electron-oxidised BIH species.

This highly efficient hybrid photocatalytic system can quantitatively convert CO₂ to formate and produce 0.4 M of formate solution without additional concentration enrichment.

The whole mechanism of photocatalytic CO₂ reduction to CO by a Ru(ii)–Re(i) supramolecular photocatalyst was successfully clarified using time-resolved infrared spectroscopy, UHPLC analysis, electrochemical measurements, and DFT calculations.

[This corrects the article DOI: 10.1021/acscatal.3c01407.].

Organic chromophores displaying TADF emission were coupled to a Mn(i)-complex as the catalyst and investigated as photosensitizers for CO₂reduction. Upon 470 nm LED excitation, TON_CO+HCOOH> 650 and aΦ_CO+HCOOH= 22.8% were obtained.