Efficient Photosensitization of π-Extended Oligomeric Porphyrin Pigments through TiO₂-Mediated Outer-Sphere Electron Transfer in Photochemical CO₂ Reduction Systems

This study describes an effective approach for addressing the challenges associated with low-lying excited states in π-extended porphyrin oligomeric dyes ([ZnP]-[ZnP] ( D-ZnP ) and [ZnP]-[ZnP]-[ZnP] ( T-ZnP ), where ZnP = Zn(II)-porphyrin unit), which create an energy barrier in the electron transfer (ET) process from the oligomeric dye to the catalytic center and hinder the maximal utilization of the superb visible-light-harvesting ability of oligomeric porphyrin arrays in dye-sensitized photocatalysis (DSPC). The outer-sphere ET from the porphyrin array dyes to TiO 2, initiated by a shallow trapping site with a low-energy level within the bulk TiO 2 semiconductor (positioned 0.3–1.0 eV below the conduction band (CB) of TiO 2 ), efficiently mitigated the endergonic ET process from the porphyrin array dye to the catalytic center. In the photolysis experiments of the TiO 2 -mediated hybrid system (dye + TiO 2 /Re(I) catalyst), the D-ZnP and T-ZnP -sensitized photocatalytic systems exhibited superior CO 2 conversion activities (turnover number (TON) CO = 1939 for D-ZnP and 1757 for T-ZnP ) compared with the monomeric Zn(II) porphyrin dye (TON CO = 837 for M-ZnP ). This enhancement is attributed to the alleviation of the endothermic ET process achieved through TiO 2 mediation and the excellent light-harvesting capacity facilitated by π-extension. In contrast, the photolysis results for the mixed homogeneous system (dye + Re(I) catalyst) indicated relatively lower conversion performances of the D/T-ZnP -sensitized homogeneous systems due to the uphill photoinduced ET between components caused by the low-lying lowest unoccupied molecular orbital (LUMO) levels of porphyrin array dyes ( E red1 1/2 = −1.30, −1.18, and −1.14 V (vs SCE) for M-ZnP, D-ZnP, and T-ZnP, respectively). The proposed strategy provides a useful tool for achieving efficient photosensitization of long π-conjugated dye arrays in various photocatalytic systems.