Red light mediated energy transfer catalysis via stable organic radicals

Photocatalysis is a versatile tool for chemical synthesis but has largely relied on high energy ultraviolet (UV) or blue light excitation. Recently, efforts have been made to conduct photocatalysis with lower-energy (red light) excitation. It offers key advantages, such as improved selectivity with fewer side reactions, greater biocompatibility, and deeper light penetration through reaction media. However, current red light photocatalysts often require rare and expensive metals, have limited reaction scope, or are difficult to prepare. Herein, we report a stable organic radical, TTM-3PCz, as a red-light-absorbing energy transfer photocatalyst. The radical consists of a tris(2,4,6-trichlorophenyl)methyl moiety that is readily synthesized from inexpensive precursors. TTM-3PCz can be optically excited to a doublet state because of its open-shell character. This excited doublet state directly populates the reactive triplet excited state of various substrates via Dexter-type doublet-to-triplet energy transfer. We show that TTM-3PCz promotes a broad range of reactions, including photooxidation, cyclization, [2 + 2] cycloaddition, and isomerization. Its performance is on par with traditional rare-metal containing photocatalysts excited with high-energy light. We demonstrate stable organic radicals as a powerful new class of air-tolerant, rare-metal-free photocatalysts for red light mediated transformations.