Acidic photosensitizers promote the efficient photooxygenation of aromatic substrates catalyzed by anthraquinone-2-carboxylic acid through proton-coupled electron transfer and particle aggregation effect

Abstract

This article reports that acidic photosensitizers can significantly enhance the photooxidation of aromatic substrates catalyzed by anthraquinone-2-carboxylic acid (AQ-COOH) through proton-coupled electron transfer and particle aggregation effects, and PhSO₃H exhibits the best improvement effect for the photooxidation of toluene. Under ambient conditions and visible light irradiation, 41.9 % toluene conversion and 49.0 % benzoic acid (BC) selectivity can be achieved. The introduction of benzenesulfonic acid (PhSO₃H) can enhance the conversion of toluene to 70.1 % and significantly improve the BC selectivity, which likely originates from the particle aggregation and proton-coupled electron transfer (PCET) effect caused by the π-π interaction between AQ-COOH, PhSO₃H and aromatic substrates. The PCET mechanism was successfully established by introducing PhSO₃H into the AQ-COOH-based photooxidation system, resulting in efficient photooxidation of diverse aromatic substrates. The primary photo-generated active species involved in the photooxidation of toluene are superoxide free radicals (O₂^·−), photogenerated electrons (e⁻), oxygen vacancies (h⁺) and singlet oxygen (¹O₂), according to quenching experiments and EPR spectrum characterizations. The current photooxidation methodology exemplifies the O₂-based selective oxidation of inert hydrocarbons under mild circumstances, aligning with the principles of green chemistry.