Flexible Units Induced Three-Dimensional Covalent Organic Frameworks with a Heteromotif Molecular Junction for Photocatalytic H2O2 Production

In recent years, significant progress has been achieved in the field of three-dimensional covalent organic frameworks (3D COFs). However, there is still a great challenge to use flexible building units to synthesize high-connectivity 3D COFs. In this work, we showed that 6 + 3 3D COFs could be constructed by inducing highly flexible planar building blocks from C3 antitriangular prismed 3D building blocks. Herein, two 3D COFs with spn topology based on flexible cyclotriphosphazene units were constructed and denoted as TAA/TAB-CTP-COF, and their redox ability and photophysical features can be finely regulated by the coupled functional group, such as benzene and triazine groups. As a result, the redox heteromotif molecular junction TAA-CTP-COF with fully exposed active sites achieved high photocatalytic production of hydrogen peroxide (H2O2) with a yield of 1041 μM h–1 without any additional photosensitizers, organic scavengers, or cocatalysts. Furthermore, in the presence of an electron donor, the production rate of H2O2 could reach up to 2221 μM h–1, exhibiting one of the best performances for photocatalytic H2O2 production in the field of crystalline materials. This work showcases the potential of using flexible building blocks in building 3D COFs and highlights the versatility and efficacy of metal-free COFs in solar energy conversion.