Outer-Sphere Electron-Transfer Process of Molecular Donor-Acceptor Organic Dye in the Dye-Sensitized Photocatalytic System for CO₂Reduction

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 TiO₂surface for efficient photoelectron collection, is also feasible at the outer-sphere of the TiO₂surface despite the absence of chemical anchoring of the acceptor part (in the D-π-A dye) onto the TiO₂surface. 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 TiO₂particles 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 H₂production and CO₂reduction.