Fluorenone-based covalent organic frameworks with efficient exciton dissociation and well-defined active center for remarkable photocatalytic hydrogen evolution

Precisely modulating both the fine structures and exciton behaviors of covalent organic framework (COF)-based photocatalysts for efficient photocatalytic hydrogen evolution is still a great challenge. Herein, we report two COFs, fluorenone-based COF (FOO-COF) and fluorenyl-based COF (FO-COF), for photocatalytic H2 evolution. The results demonstrated that FOO-COF achieved a remarkable stability and hydrogen evolution rate of 119.1 mmol/h/g, surpassing those of many previously reported COF-based photocatalysts, thus resulting in an apparent quantum efficiency up to 20.5% at 435 nm. The joint observations from temperature-dependent photoluminescence, the delayed fluorescence and phosphorescence spectroscopy confirm that FOO-COF shows a stronger carrier separation and migration ability, much lower exciton binding energy, and faster dissociation speed of singlet and triplet excitons than FO-COF. More importantly, the femtosecond time-resolved transient absorption spectra (fs-TAS) revealed the great difference between the two COFs in electron trapping and exciton dissociation after Pt loading. Furthermore, theoretical calculations unveiled the feasibility of CO groups in the LUMO sites of FOO-COF as active electronic centers for anchoring the photodeposited Pt cocatalysts, due to its much lower Gibbs free energy of hydrogen adsorption and more negative charge density. We believe that the introduction of CO groups in the FOO-COF can act as both a dominant electronic collection center and partial active sites, which improve the D-A polarizability and boost the generation, separation, migration, and utilization of photogenerated carriers in the FOO-COF for achieving remarkable photocatalytic hydrogen evolution. The insights into the molecular-level mechanism in this work may guide the significance of the rational design of COF structures for advanced photocatalytic hydrogen evolution.