Manipulation of Band Gap and Hydrophilicity in Vinylene-Linked Covalent Organic Frameworks for Improved Visible-Light-driven Hydrogen Evolution by End-Capping Strategy

•Hydroxyaldehydes with variable –OH positions are end-capped into vinylene COF skeleton.•Different substituted groups of aldehydes are end-capped into vinylene COF skeleton.•End-capping groups tune the band gap, hydrophilicity and morphology of pristine COF.•The introduction of -pOH greatly improves the photocatalytic H2 evolution activity.•The end-capping strategy efficiently reduced the energy barrier for the H* formation.