Low‐Energy Photons in Radical Chemistry: From Deep‐Red to Near‐Infrared Radical Generation for Organic Synthesis

ABSTRACT Highly reactive radical intermediates enable bond formations that are often inaccessible through conventional ionic processes. Visible‐light irradiation has emerged as a mild and sustainable energy input for generating such radical species. Most photo‐induced radical generation methods have relied on photocatalysts or substrates that absorb high‐energy light. Increasing attention has been directed toward photochemical reactions driven by deep‐red (DR) to near‐infrared (NIR) light through the development of chromophores and catalytic systems capable of harvesting low‐energy photons. Compared with short‐wavelength light‐driven processes, NIR light‐mediated reactions offer distinct advantages, including enhanced penetration through biological tissues and reduced unwanted excitation of intermediates, thereby suppressing side reactions. Nevertheless, the intrinsically low energy of NIR photons imposes fundamental challenges, limiting the scope of accessible transformations. This Review summarizes recent advances in photochemical transformations enabled by DR‐ to NIR‐absorbing catalysts and compounds, with a particular focus on strategies for the efficient utilization of low‐energy photons. The discussion is organized according to four distinct activation modes: single‐photon absorption catalysis, triplet–triplet annihilation upconversion, two‐photon absorption (TPA) catalysis, and direct excitation. Through this mechanistic framework, we highlight key molecular design principles that have driven progress in this rapidly evolving field and outline remaining challenges and future opportunities for low‐energy‐light‐driven radical chemistry.