Covalent organic frameworks with high quantum efficiency in photocatalytic hydrogen evolution: mediating charge separation

Compared with inorganic semiconductors, the difficulty of exciton dissociation is one of the main reasons for the lower photocatalytic activity of organic semiconductors. In this work, we report that the charge carrier lifetime is dramatically prolonged by incorporating a suitable donor-acceptor (β-ketene-CN) pair to a covalent organic framework nanosheet (CN-CON). CN-CON showed remarkably high apparent quantum efficiency up to 82.6% at 450 nm in photocatalytic H2 evolution, superior to all the COFs reported so far. The charge carrier kinetic analysis and femtosecond transient absorption spectroscopy characterizations verified that CN-CON had intrinsically lower exciton binding energies and hence longer-lived charge carriers than the corresponding CON without CN unit. This work provides an excellent model for gaining insight into the nature of ultrashort-lived active species in polymeric organic photocatalysts.