Protonation of an Imine-linked Covalent Organic Framework for Efficient HO Photosynthesis under Visible Light up to 700 nm.

Covalent organic frameworks (COFs) are promising photocatalysts for HO production from water via oxygen reduction reaction (ORR). The design of COFs for efficient HO production indubitably hinges on an in-depth understanding of their ORR mechanisms. In this work, taking an imine-linked COF as an example, we demonstrate that protonation of the functional units such as imine, amine, and triazine, is a highly efficient strategy to upgrade the activity levels for HO synthesis. The protonation not only extends the light absorption of the COF but also provides proton sources that directly participate in HO generation. Notably, the protonation simplifies the reaction pathways of ORR to HO, i.e. from an indirect superoxide radical ( ) mediated route to a direct one-step two-electron route. Theoretical calculations confirm that the protonation favors HO synthesis due to easy access of protons near the reaction sites that removes the energy barrier for generating *OOH intermediate. These findings not only extend the mechanistic insight into HO photosynthesis but also provide a rational guideline for the design and upgradation of efficient COFs.