Spatial modulation of triphenylamine donors in covalent organic frameworks for enhanced photocatalytic hydrogen evolution

Abstract

Incorporation of electron donor (D) and acceptor (A) units into covalent organic frameworks (COFs) has attracted considerable attention owing to their remarkable potential for highly efficient photocatalytic hydrogen (H₂) evolution from water. Triphenylamine (TPA) units, with inherent electron-donating and redox-active characteristics, facilitate efficient intramolecular electron transfer. However, systematic and strategy-oriented investigations into their role in photocatalytic H₂ evolution remain insufficient. In this study, for the first time, a series of D–π–A covalent organic frameworks (COFs) were precisely synthesized by regulating the number of TPA donor units (n = 0, 1, and 2) to clarify their structure–activity relationships in photocatalytic H₂ evolution. The photocatalytic H₂ evolution activity follows the order of PY-BFQ-COF < PY-TPA-COF < PY-(TPA)₂-COF, which positively correlates with the amount of TPA donor units. Specifically, PY-(TPA)₂-COF exhibits an excellent H₂ evolution rate of 17.93 mmol g⁻¹ h⁻¹ under visible-light irradiation in water, which is 2.1 and 2.7 times higher than those of PY-TPA-COF and PY-BFQ-COF, respectively. Combined experimental and theoretical analyses reveal that increasing TPA donor units improves photocatalytic performance by enhancing both electronic and physical properties. This strategy modulates electron distribution and enhances framework polarization, thereby facilitating charge transfer dynamics, while also yielding a larger specific surface area and improved light-harvesting capability. These factors synergistically boost the activity by providing more active sites and enhancing photon utilization. Furthermore, PY-(TPA)₂-COF still maintains considerable photocatalytic activity in seawater, demonstrating promising potential for practical applications. This study provides fundamental molecular-level insights into the rational design of high-performance COF photocatalysts through systematic TPA donor engineering strategies.