Asymmetric zinc porphyrin derivatives bearing three pseudo-pyrimidine meso-position substituents and their photosensitization for H2 evolution

Abstract

Novel asymmetric zinc porphyrin derivatives (ZnPy-5 and ZnPy-6) with meso-positions bearing one benzoic acid and three pseudo-pyrimidines with two N atoms located at different positions were synthesized and utilized as sensitizers for Pt-loaded g-C₃N₄ (PCN). Compared to the analogue (ZnPy-1 bearing one benzoic acid and three phenyl meso-position substituents), ZnPy-5 and ZnPy-6 exhibit significantly enhanced photosensitization on PCN under visible light (λ ≥ 420 nm) illumination. In particular, ZnPy-5/PCN and ZnPy-6/PCN exhibit H₂ evolution activities of 418 and 585 μmol h⁻¹, corresponding to turnover numbers (TON) of 8845 and 12 381 h⁻¹, respectively. Both of these values are much better that (316 μmol h⁻¹) of ZnPy-1/PCN, which has a TON of 6687 h⁻¹. In addition, ZnPy-5/PCN and ZnPy-6/PCN show apparent quantum yields of 32.6% and 33.1% at 420 nm monochromatic light, and these are much higher than that (10.6%) for ZnPy-1/PCN. Compared with ZnPy-5, the two N atoms of the pseudo-pyrimidines in ZnPy-6 are further away from the porphyrin macrocycle, which can more effectively combine with the sacrificial reagent and g-C₃N₄, thus promoting dye regeneration and the photoexcited charge transfer for delivering better photocatalytic performance. The present results demonstrate that the number and positions of the N atoms in the peripheral substituents of the porphyrin derivatives have a great influence on the photosensitization, and that the fine-tuning of molecular structures is crucial for improving the H₂ evolution activity of the dye-sensitized semiconductors.