Donor–acceptor engineering in conjugated polymer photocatalysts: thieno[3, 2-b]thiophene-dibenzothiophene sulfone copolymers for noble-metal-free visible-light hydrogen evolution

Abstract

The rational design of D–A type polymeric photocatalysts using triphenylamine (TPA) as the electron donor (D) and a benzothiadiazole derivative (BTDO) as the electron acceptor (A) is investigated. To optimize photocatalytic hydrogen evolution, we systematically enhanced the electron-donating capability by incorporating oligothiophene linkers of different lengths, monothiophene (T), bithiophene (DT), and terthiophene (TT), between the TPA and BTDO units. Comprehensive characterization revealed that the number of thiophene rings significantly affects both the photophysical and electrochemical properties of the polymers, thereby directly affecting photocatalytic activity. The results demonstrate that introducing thiophene oligomers significantly improves performance, with the terthiophene (TT) linker proving exceptionally advantageous. The TPA-TT-BTDO polymer exhibits a substantially broadened visible light absorption range, a reduced bandgap, and most critically, highly efficient suppression of photogenerated charge carrier recombination. This enhancement is attributed to the unique coplanar conformation enforced by the TT unit, which minimizes the dihedral angle between TPA and BTDO, facilitating intramolecular charge transfer. TPA-TT-BTDO achieved an outstanding photocatalytic hydrogen evolution rate of 101.2 mmol h⁻¹ g⁻¹ under visible light (λ ≥ 420 nm) at 10 °C without noble metal co-catalysts. Remarkably, its activity further increased to 125 mmol h⁻¹ g⁻¹ under natural sunlight irradiation. Systematic analysis revealed that longer thiophene chains induce a bathochromic shift, diminish PL intensity, yet improve photoelectrochemical properties and charge transfer efficiency. This work validates the molecular design strategy of precisely controlling oligothiophene chain length to modulate the band structure and electronic properties of D–A polymers. The superior performance of TPA-TT-BTDO stems synergistically from its extended light harvesting, efficient charge separation, and rapid interfacial transfer. These findings may provide crucial insights and practical guidance for developing high-efficiency polymeric photocatalysts through targeted molecular engineering.