Reduced graphene oxide/few-layer phosphorene binary heterojunctions as metal-free photocatalysts for the sustainable photoredox C-H arylation of heteroarenes

Abstract

Herein, we report the fabrication of few-layer phosphorene (FLP)/reduced graphene oxide (rGO) binary heterojunctions as metal-free photocatalysts for the direct C-H arylation of heteroarenes under visible light irradiation. The FLP/rGO heterojunctions were prepared by mixing the solutions of well-exfoliated rGO and FLP nanosheets in an ultrasonic bath, resulting in a well-coupled structure between rGO and FLP. Characterization revealed enhanced stability, charge separation efficiency, and extended charge transfer ability in the heterojunction compared to the pristine materials. Studying different FLP to rGO mass ratios helped to find the optimum synergy where the materials exhibited the highest photocatalytic activity, and the optimized FLP/rGO catalyst with 30% FLP yielded the desired products with the highest photocatalytic efficiency in the C-H arylation of aryl diazonium salts and heteroarenes (24 examples in total). Notably, aryl diazonium salts with electron-withdrawing groups achieved high yields in the range of 68-90%. The FLP/rGO heterojunctions were successfully applied in synthesizing dantrolene, a commercially available drug, yielding 41% yield for C-H arylation and 90% yield for subsequent synthesis. The heterojunctions demonstrated excellent reusability, maintaining high catalytic activity over five cycles with only a 6% decrease in their initial activity. Mechanistic studies suggest a plausible single electron transfer mechanism wherein photogenerated electrons are transferred from FLP/rGO to aryl diazonium salts, forming biaryl radical intermediates and subsequent products. Overall, the FLP/rGO binary heterojunctions have been demonstrated to be efficient and sustainable metal-free photocatalysts for C-H arylation reactions, showcasing a broad substrate scope and potential applications in synthetic chemistry and pharmaceutical synthesis.