Fe-N₂ single-atom sites unlocked ligand-to-metal charge transfer in semiconductor photocatalytic decarboxylation

The outer-sphere single-electron transfer (SET) is the main pathway for activating substrates during semiconductor photocatalysis, which is limited by energy band structure and excited-state lifetime. Under light irradiation, the inner-sphere ligand-to-metal charge transfer (LMCT) from substrate to metal species can break through the above limitations and be complementary to the SET process. However, this LMCT activation mode has rarely been involved in heterogeneous semiconductor photocatalysis. Herein, we build Fe-N₂ single atom sites on carbon nitride (Fe-N₂/CN) and achieve the photocatalytic decarboxylative Giese reaction via radical–radical cross-coupling (up to 99% yields). This Fe-N₂/CN photocatalyst has both LMCT and SET activity, which could activate carboxylic acids and electron-deficient alkenes simultaneously. The carboxylic acid could coordinate with Fe sites and convert into alkyl radical via photo-induced LMCT decarboxylation pathway. Meanwhile, the electron-deficient olefins could be activated to radical anion through single-electron reduction by electrons in the conduction band of carbon nitride. Combining the LMCT and SET strategy opens a new avenue for the design of semiconductor catalysts, expanding the scope of photocatalytic redox reactions.