Polymerization-Enhanced Photocatalysis for the Functionalization of C(sp3)–H Bonds

A series of conjugated polymetric photocatalysts (CPPs) based on 1,2,3,5-tetrakis(carbazol-9-yl)4,6-dicyanobenzene (4CzIPN) were synthesized via nucleophilic substitution reaction and Sonogashira–Hagihara coupling. Among the as-prepared CPPs (CPP1-CPP6), CPP3 was selected as the optimized heterogeneous photocatalyst for visible-light-driven functionalization of C(sp3)–H bonds to synthesize 87 final products with yields up to 99%. The heterogeneous photocatalyst CPP3 is easily recovered from the reaction with excellent retention of photocatalytic activity, which can be reused at least for five times. Time-dependent density functional theory (TD-DFT) calculations demonstrate that the photocatalysis performance of CPP3 is superior to the corresponding monomer 4CzIPN because of the enhanced intersystem crossing (ISC) process. Spin–orbit coupling calculations prove that the rate constant of ISC outcompetes that of reverse intersystem crossing (RISC). The rapidly generated and long-lived triplet state T1 is considered to be a more accessible excited state than the singlet-state S1 generated from either direct excitation or RISC. The successful application of this polymerization-enhanced photocatalysis strategy should guide a metal-free approach to the rational design of CPP-type heterogeneous photocatalysts to address the dilemma of homogeneous photocatalysts in sustainability, stability, and recyclability.