Protonated Imine-Linked Covalent Organic Frameworks for Photocatalytic Hydrogen Evolution.

Covalent organic frameworks (COFs) have emerged as an important class of organic semiconductors and photocatalysts for the hydrogen evolution reaction (HER)from water. To optimize their photocatalytic activity, typically the organic moieties constituting the frameworks are considered and the most suitable combinations of them are searched for. However, the effect of the covalent linkage between these moieties on the photocatalytic performance has rarely been studied. Herein, we demonstrate that donor-acceptor (D-A) type imine-linked COFs can produce hydrogen with a rate as high as 20.7 mmol g-1  h-1 under visible light irradiation, upon protonation of their imine linkages. A significant red-shift in light absorbance, largely improved charge separation efficiency, and an increase in hydrophilicity triggered by protonation of the Schiff-base moieties in the imine-linked COFs, are responsible for the improved photocatalytic performance.