Tuning the structure and photocatalytic performance of tri-s-triazine-based polyimides through temperature-controlled imidization

Abstract

Tri-s-triazine-based polyimides (TPIs) represent a promising class of photocatalysts for both energy and environmental applications due to their high stability, ease of fabrication, visible-light response, and intrinsic donor–acceptor (D–A) system. However, systematic studies on TPI-based photocatalysts, particularly addressing synthesis conditions and charge-transfer mechanisms, remain insufficiently explored, restricting further performance optimization. Herein, two distinct TPIs were synthesized at different temperatures via the thermal imidization of melem with two conventional polyimide anhydride precursors, pyromellitic dianhydride (PMDA) or 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTCDA). The results demonstrate that the polymerization degree and photocatalytic activity of the TPIs exhibit similar trends, both being dependent on the nature of the anhydride comonomer and reaching optimal properties within a narrow temperature window between the melting point and the onset of anhydride degradation. Moreover, the anhydride unit strongly influenced the charge dynamics of TPIs. Melem-NTCDA showed facilitated D–A charge transfer; however, limited diffusion between the six-membered imide and naphthalenic rings created a higher density of trap sites, thereby restricting the generation of reactive species. On the other hand, Melem-PMDA showed significantly higher photocatalytic efficiency in degrading methyl orange and the herbicide 2,4-D due to its extended conjugation, which reduces charge recombination and promotes efficient radical generation. This work underscores both the potential of TPIs for environmental photocatalysis and the significance of synthesis conditions in organic photocatalysts, providing valuable insights into their structure–activity relationship.