Polymerization temperature-driven trade-off between optical and structural properties in polyimide photocatalysts for efficient persulfate activation toward organic pollutant degradation

Polyimide (PI) , a metal-free conjugated polymer, holds great potential for persulfate (PDS) activation under visible light owing to its tunable electronic structure and chemical stability. However, achieving a rational balance between optical absorption, charge separation, and surface accessibility remains challenging. Here, we systematically investigate the effect of polymerization temperature (250–400 °C) on the optical, structural, and electronic properties of PI and establish its correlation with photocatalytic activity toward Acid Orange 7 (AO7) degradation. The optimized catalyst, PI-325, exhibits an appropriate bandgap of 2.69 eV, a surface area of 4.09 m²/g, and achieves 98.9 % AO7 degradation within 80 min under visible-light irradiation, demonstrating excellent activity, selectivity, and reusability. Mechanistic investigations combining electron-paramagnetic-resonance detection and radical-quenching experiments reveal that visible-light excitation promotes electron transfer from PI to both O 2 and PDS, generating SO 4 • ⁻ and O 2 •- as the dominant reactive oxygen species. Furthermore, Fukui-function analysis identifies the azo (–N = N–) bond of AO7 as the primary reactive site for radical attack, supporting the experimentally observed degradation pathway in which AO7 is transformed into low-toxicity carboxylic acids via sequential oxidation and ring-opening reactions. This integrated experimental-theoretical study demonstrates that polymerization-temperature modulation provides an effective route to optimize the conjugation, crystallinity, and defect structure of PI for visible-light-driven PDS activation, offering new insights into the structure–activity–mechanism relationship in sustainable, metal-free photocatalytic systems.