Vinylene-Linked Covalent Organic Frameworks (COFs) with Symmetry-Tuned Polarity and Photocatalytic Activity

The polarity of a semiconducting molecule affects its intrinsic photophysical properties, which can be tuned by varying the molecular geometry. Herein, we developed a D-3h-symmetric tricyanomesitylene as a new monomer which could be reticulated into a vinylene-linked covalent organic framework (g-C54N6-COF) via Knoevenagel condensation with another D-3h-symmetric monomer 2,4,6-tris(4 '-formyl-biphenyl-4-yl)-1,3,5-triazine. Replacing tricyanomesitylene with a C-2v-symmetric 3,5-dicyano-2,4,6-trimethylpyridine gave a less-symmetric vinylene-linked COF (g-C52N6-COF). The octupolar conjugated characters of g-C54N6-COF were reflected in its scarce solvatochromic effects either in ground or excited states, and endowed it with more promising semiconducting behavior as compared with g-C52N6-COF, such as enhanced light-harvesting and excellent photo-induced charge generation and separation. Along with the matched energy level, g-C54N6-COF enabled the two-half reactions of photocatalytic water splitting with an average O-2 evolution rate of 51.0 mu mol h(-1) g(-1) and H-2 evolution rate of 2518.9 mu mol h(-1) g(-1). Such values are among the highest of state-of-the-art COF photocatalysts.