Semiconducting 2D Triazine-Cored Covalent Organic Frameworks with Unsubstituted Olefin Linkages

Establishing an sp²-carbon-bonding pattern is one of the efficient accesses to various organic semiconducting materials. However, the less-reversible carbon-carbon bond formation makes it still challenging to spatially construct a well-defined organic framework with π-extended two-dimensional (2D) structure through solution process. Here, a Knoevenagel condensation approach to synthesize two new 2D covalent organic frameworks (COFs) connected by unsubstituted carbon-carbon double bond linkages through activating the methyl carbons of a 2,4,6-trimethyl-1,3,5-triazine monomer is presented. The resulting sp²-carbon-linked triazine-cored 2D sheets are vertically stacked into high-crystalline honeycomb-like structures, endowing this kind of COF with extended π-delocalization, tunable energy levels, as well as high surface areas, regular open channels, and chemical stabilities. On the other hand, their microfibrillar morphologies allow for the facile manipulation of thin films as photoelectrodes without additive. Accordingly, such kinds of COF-based photoelectrodes exhibit photocurrents up to ∼45 μA cm^-2 at 0.2 V vs RHE as well as rapid charge transfer rates, in comparison with imine-linked COF-based photoelectrodes. In addition, both COFs are applicable for conducting photocatalytic hydrogen generation from water splitting by visible-light irradiation.