Fully Conjugated D-A COFs with Thiadiazole and Thiazolothiazole Linkers: Tailoring Electronic Structures for Efficient Metal-Free and Pt-Assisted Hydrogen Evolution

Abstract

Developing efficient photocatalysts for both metal-free and noble-metal-assisted systems is essential for sustainable hydrogen production. Herein, we report the rational design and synthesis of two fully π-conjugated donor-acceptor (D-A) covalent organic frameworks (COFs), Py-CN-BZTD and Py-CN-TZTZ, constructed via cyano-vinylene linkages using pyrene-based donor cores with thiadiazole (BZTD) and thiazolothiazole (TZTZ) electronacceptor linkers. These COFs exhibit high crystallinity, excellent chemical stability, and tunable electronic structures. Py-CN-BZTD demonstrates a narrower bandgap (2.06 eV) and superior charge separation efficiency, achieving a remarkable metal-free hydrogen evolution rate (12,200 μmol g⁻ 1 h⁻ 1 )-outperforming Py-CN-TZTZ (7,652 μmol g⁻ 1 h⁻ 1 ) under visible light. This is attributed to its enhanced light absorption, better hydrophilicity, and more efficient charge transport. Remarkably, upon introducing 1 wt% Pt as a co-catalyst, Py-CN-TZTZ exhibits a dramatic enhancement, reaching an outstanding hydrogen evolution rate of 61,577 μmol g⁻ 1 h⁻ 1 , which is among the highest reported values for COF-based photocatalysts to date, surpassing Py-CN-BZTD (26,016 μmol g⁻ 1 h⁻ 1 ). Mechanistic studies, including EPR with DMPO and TEMPO spin-trapping combined with DFT calculations, reveal that Pt effectively compensates for the less efficient intrinsic charge separation in Py-CN-TZTZ by facilitating electron extraction, enhancing charge transfer, and suppressing recombination. This work demonstrates how heteroatom linker engineering and band structure tailoring synergistically control photocatalytic performance, offering valuable insights into the design of highly efficient COFs for both metal-free and Pt-assisted solar hydrogen production.