Phenoxazine-based covalent triazine framework for photocatalytic aerobic hydroxylation of arylboronic acids to phenols†

Abstract

The development of photoactive triazine-containing donor–acceptor (D–A) polymers to achieve enhanced photocatalytic activity for organic transformations has generated intense interest. Herein, a phenoxazine-based covalent triazine framework (NP-CTF) was prepared via facile one-step trimerization of phenoxazine bearing nitrile groups in the presence of trifluoromethanesulfonic (TfOH). The resulting D–A polymer NP-CTF displayed good thermal and chemical stability, wide light absorption ability and a slightly negative conduction band. The NP-CTF could efficiently activate O₂ into reactive oxygen species (singlet oxygen (¹O₂) and superoxide radical anion (O₂˙⁻)). As expected, this metal-free NP-CTF was successfully employed as a heterogeneous catalyst for photocatalytic aerobic hydroxylation of arylboronic acids to phenols in an air atmosphere under irradiation of white light-emitting diodes and even under natural sunlight. Notably, the NP-CTF exhibited remarkable photocatalytic activity with a phenol yield of 98.2%, which at least exceeded 30% than that of commercially available catalysts (TiO₂ and g-C₃N₄) under the same reaction conditions. Moreover, the NP-CTF could be easily recycled using a simple separation procedure and reused at least ten times without obvious loss of photocatalytic activity, suggesting excellent stability and reusability. The current work potentially provides a universal approach to prepare D–A dyad CTF-based photocatalysts and suggests a promising and sustainable photocatalytic protocol to obtain phenols from arylboronic acids.