Outer-Sphere Electron-Transfer Process of Molecular Donor–Acceptor Organic Dye in the Dye-Sensitized Photocatalytic System for CO₂ Reduction

In dye-sensitized photocatalytic (DSPC) systems, the introduction and effective operation of the precious-metal free photosensitizing components are the critical issues which affect their practical applications. In this study, it was first found that the photosensitization process of a donor−π–acceptor (D−π–A)-type dye, which was designed to be immobilized onto a TiO2 surface for efficient photoelectron collection, is also feasible at the outer-sphere of the TiO2 surface despite the absence of chemical anchoring of the acceptor part (in the D−π–A dye) onto the TiO2 surface. Two ethyl-protected D−π–A dyes, namely, (E)-2-cyano-3-(5-(5-(4-(diphenylamino)phenyl)thiophen-2-yl)thiophen-2-yl)acrylate (1-Et) and ((E)-ethyl-3-(4-(7-(5′-(4-(bis(9,9-dimethyl-9H-fluoren-2-yl)amino)phenyl)-[2,2′-bithiophen]-5-yl)benzo[c][1,2,5]thiadiazol-4-yl)phenyl)-2-cyanoacrylate) (2-Et), were prepared and investigated to elucidate the photoinduced electron transfer (PET) mechanism of the non-anchored D−π–A dye in solution. From serial fluorescence and anion absorption quenching experiments, we found that the reductively quenched [D−π–A dye]•– in solution efficiently can transfer the photoexcited electrons toward solid TiO2/Re(I) catalytic particles (dye•– → |TiO2/Re(I) catalyst) through the outer-sphere electron transfer (OSET) process. The success of the collisional OSET process is attributed to the long lifetime of the solution-phase [D−π–A dye]•– species, which sufficiently can overcome the intrinsically inefficient heterogeneous electron transfer (ET) kinetics at the interface between the dye•– in solution and the dispersed TiO2 particles and can decrease the dependency on the charge-transfer reorganization energy. The effectiveness of the OSET process was verified by the efficient photocatalytic CO2-to-CO conversion activities of binary [a turnover number (TON) of 330–470 for ∼8 h] by the photosensitization of the free [D−π–A dye]•–, which are comparable to those of the dye-anchored ternary analogues (D−π–A dye/TiO2/Re(I) catalyst, a TON of 194–391 for 8 h) based on the conventional inner-sphere ET (ISET) process at the early stage of the photoreaction. The two-way photosensitization processing (which considers both OSET and ISET) of organic dyes can be a major strategic advantage in conventional dye-sensitized solar cells and DSPC systems for H2 production and CO2 reduction.