Improving the photocatalytic activity of porphyrins for hydrogen evolution from water splitting through combined effects inspired by meso-carboxyfuryl substitution

Abstract

Porphyrins have been widely used in designing photocatalyst systems. However, finding an effective strategy to improve the intrinsic photocatalytic activity of porphyrin molecules remains a challenge. Here, two new porphyrins, meso-tetra(5-carboxyfuryl)porphyrin (H₂TCFP) and meso-tetra(5-carboxythienyl)porphyrin (H₂TCTP), were successfully synthesized and compared with meso-tetra(m-carboxyphenyl)porphyrin (H₂TmCPP), to clarify the influence of the organic unit bridging the carboxyl acid group with the porphine ring on the photocatalytic activity of porphyrin for H₂ evolution from water splitting. The H₂ evolution rates of 10.8 mmol g⁻¹ h⁻¹ observed for H₂TCFP and 9.7 mmol g⁻¹ h⁻¹ for H₂TCTP are both larger than 8.0 mmol g⁻¹ h⁻¹ observed for H₂TmCTP under optimal conditions. The best photocatalytic activity observed for H₂TCFP was investigated through experiments and density functional theory (DFT) calculations and was interpreted in terms of the combined effect of several key factors of the entire photocatalytic process, including the high absorptivity of UV light, high separation efficiency of photogenerated carriers, and low Gibbs energy required for associated water splitting. Meanwhile, the concentration-dependent performance of photocatalytic activity was analyzed and attributed to the aggregation and light-bleaching effect exerted on porphyrins.