Modulating exciton dissociation and charge transfer by extending π-conjugation in thiazole-based COFs to boost photocatalytic hydrogen evolution

Covalent organic frameworks (COFs) have emerged as promising candidates for photocatalytic hydrogen evolution due to their tailorable structures and tunable optoelectronic properties. Recently, although significant research efforts have focused on developing high-performance COF photocatalysts, the rational modulation of charge carrier dynamics through precise molecular structure regulation remains a challenge. In this study, we addressed this by systematically tuning π-conjugation extension employing bithiazole, benzobisthiazole and naphthobisthiazole as building blocks. Theoretical calculations and experimental characterization revealed that extending the π-conjugation significantly enhances light absorption and facilitates the separation and transport of photogenerated charge carriers. As a result, under the conditions of 0.5 M AA as the sacrificial agent, 3 wt% Pt loading and 2 mg of catalyst, the naphthobisthiazole-based COF (NTz-TP-COF) exhibited the most efficient photocatalytic performance, achieving a superior hydrogen evolution rate of 92.3 mmol g⁻¹ h⁻¹ under visible-light irradiation, which is substantially higher than those of its bithiazole-based (Tz-TP-COF, 4.7 mmol g⁻¹ h⁻¹) and benzobisthiazole-based (BTz-TP-COF, 29.15 mmol g⁻¹ h⁻¹) counterparts, as well as most previously reported thiazole-based COFs. Moreover, platinum photodeposited on NTz-TP-COF was found to exist predominantly in the metallic Pt⁰ state and to exhibit a uniform particle-size distribution, confirming the presence of abundant active sites for hydrogen evolution. These findings highlight the critical role of π-conjugation engineering in optimizing photocatalytic performance and provide a rational design strategy for highly efficient COF photocatalysts.