Activation of Carbonyl Oxygen Sites in β-Ketoenamine-Linked Covalent Organic Frameworks via Cyano Conjugation for Efficient Photocatalytic Hydrogen Evolution.

2D covalent organic frameworks (COFs) are drawing intense attention in heterogenous photocatalysis due to their porous, crystalline, and tailor-made structures. For highly efficient solar-to-chemical energy conversion, revealing and modulating active centers in the skeletons of COFs are of great importance but encounter severe challenges. Herein, it is demonstrated that cyano conjugation on a typical β-ketoenamine-linked COF via aldehyde-imine Schiff-base condensation contributes to an enhanced stable photocatalytic H2 -evolution rate of 1.8 mmol h-1 g-1 (λ > 420 nm) with a superior apparent quantum yield of 2.12% at 420 nm, compared to pristine COFs. Both experimental results and density functional theory calculations disclose that the cyano conjugation can efficiently improve photoinduced charge separation and effectively decrease the energy barrier for H-intermediate generation on the carbonyl oxygen sites of the functionalized COFs. These findings present a precise organic functionalization strategy to optimize active centers on COF-based photocatalysts for the practical applications.