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, and 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.