Construction of Covalent Organic Frameworks with Partly Condensed Networks for Photocatalytic Hydrogen Evolution

Covalent organic frameworks (COFs) are typically designed by topologically directed condensation reactions to form the desired networks containing the respective vertex and edge linker units. In general, the building blocks in the frameworks are always complementarily and fully connected to the frameworks. In this work, we showed the design and synthesis of two COFs, TPE-TPB-A and TPE-TPB-B, which were constructed from the same monomers. Both exhibited good crystallinity and similar BET specific surface area but different XRD patterns. Combined with structure simulation and experimental data, TPE-TPB-A was determined to be connected by a normal fully bonded [4 + 4] pathway, while TPE-TPB-B was constructed via an unusual [4 + 2] pathway with unreacted dangling formyl groups. More importantly, partly condensed TPE-TPB-B with exposed formyl groups exhibited more favorable physical/chemical properties than fully condensed TPE-TPB-A for photocatalytic hydrogen evolution (PHE), such as improved hydrophilicity and better charge separation and transportation. Remarkably, the average PHE rate of TPE-TPB-B was measured to be more than 10 times higher than that of TPE-TPB-A under visible-light irradiation with platinum as a cocatalyst and ascorbic acid as a sacrificial agent. These results open up possibilities toward partly condensed COFs for various applications.