Atomically Engineered Acridine Derivatives Serve as Metal‐Free and Self‐Sensitized Catalysts for Solar‐Driven CO2 to Formic Acid with High‐Efficiency and Near‐Perfect Selectivity

Achieving efficient and selective light‐driven CO2 conversion to formic acid is a significant scientific challenge, particularly when utilizing purely organic, metal‐free, and earth‐abundant element‐based molecule photocatalysts. Herein, we first reported the discovery of acridine derivatives (DADN, PXZN, PTZN) as new‐type, metal‐free, self‐sensitized molecule catalysts that enabled exceptional performance in solar‐driven CO2 reduction to formic acid. Notably, the atomically engineered sulfur‐containing heterocycle PTZN demonstrated unprecedented formate yield rate of 47.8 mmol g‐1 h‐1, and > 99% selectivity in a photocatalytic system using 1,3‐dimethyl‐1H‐benzo[d]imidazol‐3‐ium (BI+) as proton and electron relay. The superior activity of PTZN was revealed to arise from its synergistic combination of strong CO2 binding affinity (‐0.195 eV), prolonged charge‐separated states (11 ns), and robust CO2 electronic coupling (2.51 eV). Comprehensive studies including in‐situ electron spin resonance, in‐situ infrared, and transient absorption spectroscopy unambiguously unveiled a direct single electron transfer process from the excited singlet‐state acridine derivatives to CO2, generating CO2∙‐. Moreover, a hydrogen atom transfer process utilizing in‐situ generated BIH as a hydrogen atom carrier enabled the conversion of CO2∙‐ to formic acid. This work establishes the first demonstration of a sequential proton‐electron transfer mechanism in acridine‐based photocatalysis, resolving long‐standing challenges in proton and electron delivery during CO2 activation.