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 pi-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 TiO2, initiated by a shallow trapping site with a low-energy level within the bulk TiO2 semiconductor (positioned 0.3-1.0 eV below the conduction band (CB) of TiO2), efficiently mitigated the endergonic ET process from the porphyrin array dye to the catalytic center. In the photolysis experiments of the TiO2-mediated hybrid system (dye + TiO2/Re(I) catalyst), the D-ZnP and T-ZnP-sensitized photocatalytic systems exhibited superior CO2 conversion activities (turnover number (TON)(CO) = 1939 for D-ZnP and 1757 for T-ZnP) compared with the monomeric Zn(II) porphyrin dye (TONCO = 837 for M-ZnP). This enhancement is attributed to the alleviation of the endothermic ET process achieved through TiO2 mediation and the excellent light-harvesting capacity facilitated by pi-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 pi-conjugated dye arrays in various photocatalytic systems.