Hydrogen evolution from photocatalytic reduction of water holds promise as a sustainable source of carbon-free energy. Covalent organic frameworks (COFs) present an interesting new class of photoactive materials, which combine three key features relevant to the photocatalytic process, namely crystallinity, porosity and tunability. Here we synthesize a series of water- and photostable 2D azine-linked COFs from hydrazine and triphenylarene aldehydes with varying number of nitrogen atoms. The electronic and steric variations in the precursors are transferred to the resulting frameworks, thus leading to a progressively enhanced light-induced hydrogen evolution with increasing nitrogen content in the frameworks. Our results demonstrate that by the rational design of COFs on a molecular level, it is possible to precisely adjust their structural and optoelectronic properties, thus resulting in enhanced photocatalytic activities. This is expected to spur further interest in these photofunctional frameworks where rational supramolecular engineering may lead to new material applications.
We demonstrate photocatalytic hydrogen
evolution using COF photosensitizers
with molecular proton reduction catalysts for the first time. With
azine-linked N2-COF photosensitizer, chloro(pyridine)cobaloxime co-catalyst,
and TEOA donor, H2 evolution rate of 782 μmol h–1 g–1 and TON of 54.4 has been obtained
in a water/acetonitrile mixture. PXRD, solid-state spectroscopy, EM
analysis, and quantum-chemical calculations suggest an outer sphere
electron transfer from the COF to the co-catalyst which subsequently
follows a monometallic pathway of H2 generation from the
CoIII-hydride and/or CoII-hydride species.
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