Photocatalytic Activity of Ionic Carbon Nitrides is Governed by Cation‐Modulated Dark Exciton Dynamics

Abstract Ionic polymeric carbon nitrides, poly(heptazine imides) (PHIs), have emerged as promising photocatalysts, yet the relationship between their structure, exciton dynamics, and activity has remained elusive so far. Here, a direct link is established between photocatalytic activity, spectroscopic properties, and theoretical analysis of structural effects on exciton behavior in water‐soluble PHIs with different cations. Using steady‐state and time‐resolved emission spectroscopy alongside ultrafast transient absorption spectroscopy performed in the absence and presence of hole and electron quenchers, two distinct excitonic relaxation pathways are uncovered: sub‐100 ps decay dominated by exciton recombination and shallow‐trap states, and sub‐ns dynamics associated with deep‐trap assisted recombination. Notably, ethanol accelerated the sub‐100 ps decay via hole quenching, while the deep traps are unaffected by ethanol. The spectroscopic results show that the photocatalytic activity in H ₂ O ₂ production (CsPHI << NaPHI < KPHI) correlates with exciton lifetimes, whereby the theoretical analysis reveals that the observed modulation of exciton lifetimes is primarily related to different dark exciton dynamics governed by changes in interlayer interactions due to the altered structural corrugation in the presence of various cations. This work establishes a unified structure–dynamics–activity relationship in PHIs, offering new design guidelines for PHI‐based photocatalytic materials.