Syntheses of asymmetric zinc porphyrins bearing different pseudo-pyridine substituents and their photosensitization for visible-light-driven H2 production activity

A series of asymmetric zinc porphyrin (ZnPy) derivatives bearing different external substituents were synthesized and used to sensitize Pt-loaded graphitic carbon nitride (Pt/g-C3N4) for photocatalytic H-2 production. Among them, ZnPy-1 has one benzoic acid and three phenyls as peripheral substituents, while ZnPy-2, ZnPy-3, and ZnPy-4 contain one benzoic acid and three pseudo-pyridines with different N-atom positions. The experimental results indicate that the pseudo-pyridine substitution for the phenyls in ZnPy-1 lead to enhanced photosensitization with an order of ZnPy-1 < ZnPy-2 < ZnPy-3 < ZnPy-4 under visible light (lambda > 420 nm) irradiation. ZnPy-4-sensitized Pt/g-C3N4 (ZnPy-4-Pt/g-C3N4) exhibits the best average H2 production activity of 524 mu mol h(-1) with an extremely high turnover number (TON) of 11 089 h-1, which is much higher than that (328 mu mol h(-1)) of ZnPy-2-Pt/g-C3N4 with a TON of 6942 h-1. Also, ZnPy-4-Pt/g-C3N4 shows a much higher apparent quantum yield (AQY) of 32.3% than that (11.5%) of ZnPy-2-Pt/g-C3N4 under 420 nm monochromatic light irradiation. The different N-atom positions in the pseudo-pyridines result in different interactions of the ZnPy dyes with a sacrificial reagent, which then strongly influences the photoactivity for H-2 production. The present results demonstrate the molecular engineering aspect of ZnPy dyes in which fine-tuning of molecular structures is crucial for improving the photo-catalytic H-2 production activity of dye-sensitized semiconductors.