β-ketoenamine based covalent heptazine polymers with significantly superior photocatalytic activity to g-C3N4

The rational design of new covalent heptazine polymers (CHPs) is one of the current hot and challenging topics in the field of photocatalytic materials, due to its great potential prospects, limited available monomers, and reaction types. Herein, two novel CHPs (MT and MTA) were successfully prepared by the Michael addition-elimination reaction of melem with β-ketoenamine monomer 1,3,5-tris(3-dimethylamino-1-oxoprop-2-en-yl)benzene (TDOEB) and 1,3,5-tris(4-(3-dimethylamino-1-oxoprop-2-en-yl)phenyl)triazine (TDOEPTA), respectively. Experiments combined with density functional theoretical calculations revealed that β-ketoenamine fragments bridging greatly improved the conjugation of MT and MTA, and its light absorption was significantly expanded to over 600 nm. The donor-acceptor structure between melem and β-ketoenamine moieties led to a significant reduction of exciton binding energy (E_b), promoting the dissociation of excitons and oxygen activation, and thus exhibiting superior photocatalytic activity for bromination reaction, [3+2] cycloaddition reaction, selective oxidation of phenyl C–H bond and thioether with both conversion and selectivity more than 99%. This work opens up a new approach for the synthesis of highly efficient CHPs and expands their applications in photocatalytic organic synthesis.