Triphenyltriazine-Substituted Zinc Porphyrin on TiO₂ with Competitive Electron Transfer for Dye Photodegradation

A typical dye-sensitized semiconductor system prefers a cascade electron transfer to gain efficient injection and to avoid charge recombination, whereas this design protocol might be an unfit photocatalysis domain since the radical species are no longer bare photoelectrons. To check the hypothesis, a zinc porphyrin sensitizer with the triphenyltriazine (TPTZ) substituent 3TPTZ-ZnP was designed, along with a triphenylamine (TPA) analogue 3TPA-ZnP for comparison. Density functional theory (DFT) calculations suggested that the TPTZ substituents on 3TPTZ-ZnP form a more planar structure with no significant coupling to the porphyrin core. When coated onto TiO₂, enhanced light absorption was achieved by 3TPTZ-ZnP. This increased light-harvesting ability did not lead to a greater photocurrent of 3TPTZ-ZnP/TiO₂ as compared to 3TPA-ZnP/TiO₂, which is due to a competitive electron transfer pathway from the porphyrin to the TPTZ groups, besides the electron injection to TiO₂. Nonetheless, 3TPTZ-ZnP/TiO₂ exhibited superior photodegradation performance compared to 3TPA-ZnP/TiO₂. This is attributed to the planar conjugated structure with more nitrogen content of TPTZ, which promotes π-π interactions with aromatic dyes and allows more superoxide radicals (•O₂^-) to be generated on the surface. Further experiments also confirmed that the main free radicals in the reaction were photogenerated holes (h⁺) and •O₂^-.