Rational Regulation of the Exciton Effect of Acrylonitrile-Linked Covalent Organic Framework toward Boosting Visible-Light-Driven Hydrogen Evolution

Covalent organic frameworks (COFs) exhibit relatively inefficient exciton dissociation and free charge generation, which arise from their high exciton binding energy. Here, a series of crystalline, acrylonitrile-linked COFs are fabricated to regulate exciton effects by strategies of shortening the linker length (TP-PN COF), introducing nitrogen atoms (TP-BPyN COF), and post-protonation (TP-BPyN PCOF). The temperature-dependent photoluminescence (PL) spectra and Nyquist plots reveal a decrease in exciton binding energy and charge transfer resistance of TP-PN COF, TP-BPyN COF, and TP-BPyN PCOF. Thus, an increase in free carrier generation and an extension of carrier lifetime are achieved, as demonstrated by transient photocurrents response and time-resolved fluorescence spectra (TRFS). DFT calculation reveals that low exciton binding energy and charge transfer resistance could relate to higher planarity structure. Besides, light absorption performance was improved by shortening the linker length, while the distribution density of Pt nanoparticles as H2 evolution reaction (HER) sites was significantly improved by introducing nitrogen atoms as anchor points. As a result, the optimized TP-PN COF and TP-BPyN COF show efficient photocatalytic HER rates of 10,890 and 6457 μmol g–1 h–1, respectively, improved by 13.92 and 8.26 times compared to that without modification (TP-BPN COF, 782 μmol g–1 h–1). Through a simple post-protonation strategy, the charge is redistributed and the structural distortion is reduced. Consequently, the HER rate of TP-BPyN PCOF significantly increased to 12,276 μmol g–1 h–1. Meanwhile, the HER rate of TP-BPyN PCOF was further boosted to 15,929 and 22,438 μmol g–1 h–1 by optimizing the volume fractions of the sacrificial agent and the pH of the reaction system, respectively. This work could pave the way for developing efficient organic photocatalysts via rational regulation of the exciton effect.