Crystal Integrity Suppresses Bulk Trapping in Potassium Poly(Heptazine Imide) for Selective Polyester Photoreforming

Bulk defects in organic semiconductor photocatalysts intercept photogenerated charges before interfacial reaction, constraining activity and selectivity. Here, seeded molten-salt recrystallization yields potassium poly(heptazine imide) (PHI-K) nanocrystals with reduced cyano content and enhanced lattice coherence for polyester photoreforming. Density functional theory and transient absorption spectroscopy identify cyano-derived occupied states as bulk hole traps. Defect suppression preserves observable hole-polaron absorption and extends the average exciton-associated absorption decay time from 69.52 to 984.72 ps, consistent with reduced trap-assisted carrier loss. Under air mass 1.5 global (AM 1.5G) irradiation, crystalline PHI-K photoreforms alkaline-pretreated poly(glycolic acid) and poly(lactic acid) into glyoxylic and pyruvic acids with respective target-product carbon yields of 95.2% and 92.5%. Corresponding H₂ evolution rates reach 42.6 and 26.0 mmol g^-1 h^-1, with apparent quantum yields of 50.8% and 30.1% at 400 nm, respectively. In situ infrared spectroscopy, isotope effects, and carrier kinetics support a pathway in which holes drive two-electron oxidation of surface-bound α-hydroxy species while electrons reduce protons on Pt. These findings establish crystal integrity as an organic semiconductor materials principle that couples bulk charge retention to selective bond conversion in conjugated photocatalysts.