Coupling of a new porphyrin photosensitizer and cobaloxime cocatalyst for highly efficient photocatalytic H2 evolution

Abstract

Photocatalytic hydrogen evolution (PHE) is a promising strategy to produce environmentally friendly clean energy with the use of solar power and water. For this, developing an efficient and noble metal free photocatalytic system (PS) comprising high light-harvesting and photostable photosensitizers is an important and challenging task. Herein, a new porphyrin photosensitizer ZnDC(p-NI)PP, containing two naphthalimide (NI) donor chromophores and 4-carboxyphenyl substituents is developed for PHE. Using chloropyridinecobaloxime (CoPyCl) as a cocatalyst in phosphate buffer/THF solution at pH 7.4, the homogeneous PS of ZnDC(p-NI)PP exhibits a very high hydrogen evolution rate (η_H2) of 35.70 mmol g⁻¹ h⁻¹, turnover number (TON) of 5958 and apparent quantum efficiency (AQE) of 10.01%. This performance recorded under the same conditions is significantly higher than that of the PS of ZnDCPP which lacks the NI moieties (η_H2 of 4.64 mmol g⁻¹ h⁻¹, TON of 1397 and AQE of 1.30%), the typical PS of ZnTCPP with four 4-carboxyphenyl substituent groups (η_H2 of 2.43 mmol g⁻¹ h⁻¹, TON of 562 and AQE of 1.00%), and previously reported ZnD(p-NI)PP containing only two NI chromophores (η_H2 of 3.8 mmol g⁻¹ h⁻¹, TON of 590 and AQE of 1.5%). The noticeable performance of ZnDC(p-NI)PP in PHE is attributed to the intramolecular energy transfer from the NI donor chromophore to the porphyrin acceptor that would promote the long-lived photoexcitation states. At the same time, the anionic form (–COO⁻) of 4-carboxyphenyl substituents at pH 7.4 enables a faster electron transfer from the porphyrin group to the cationic Co(III) due to electrostatic force. To the best of our knowledge, the PHE results of ZnDC(p-NI)PP represent the best one for porphyrin photosensitizers and cobaloxime PSs reported so far. This work paves a new direction for developing new porphyrin-based materials for efficient PHE through facile molecular structure engineering.