N‐Aryl Acridone Derivatives as Effective Catalysts for Energy Transfer Reactions

Organic photocatalysts capable of promoting high-energy triplet energy transfer (EnT) remain limited, particularly for demanding [2+2] photocycloaddition reactions. In this study, we report the use of readily accessible N-aryl acridones as efficient visible-light-driven organic photocatalysts for challenging EnT processes. Although largely unexplored in photocatalysis, acridone derivatives exhibit photophysical properties suitable for sensitizing substrates with high triplet energies. Using N-Boc-indole-2-methyl ester and coumarin as representative high-triplet-energy substrates, we demonstrate that N-aryl acridones effectively mediate [2+2] cycloadditions under mild conditions via an energy-transfer mechanism. The modular acridone scaffold enables straightforward structural modification through N-functionalization, allowing systematic tuning of electronic and steric parameters to optimize catalytic performance.