Protonation of an Imine-linked Covalent Organic Framework for Efficient H 2 O 2 Photosynthesis under Visible Light up to 700 nm.

Covalent organic frameworks (COFs) are promising photocatalysts for H 2 O 2 production from water via oxygen reduction reaction (ORR). The design of COFs for efficient H 2 O 2 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 H 2 O 2 synthesis. The protonation not only extends the light absorption of the COF but also provides proton sources that directly participate in H 2 O 2 generation. Notably, the protonation simplifies the reaction pathways of ORR to H 2 O 2 , i.e. from an indirect superoxide radical ( O 2 • - ${{O}_{2}^{\bullet -}}$ ) mediated route to a direct one-step two-electron route. Theoretical calculations confirm that the protonation favors H 2 O 2 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 H 2 O 2 photosynthesis but also provide a rational guideline for the design and upgradation of efficient COFs.