Electronic properties of poly (triazine imide) distance-controlled of the NH and BH substitutions and their impacts on the photocatalytic descriptors

Context The development of efficient photocatalysts for water splitting using carbon-nitrogen materials, such as poly (heptazine imide) and poly (triazine imide), shows promise due to the stability, cost, and visible light absorption of these materials. However, their low light absorbance and high recombination (low charge transfer) pose key challenges that limit their application and require further development. One way to address these limitations is through dual functionalization, which is tailored to increase absorbance and charge transfer. In this context, we characterize dually modified poly(triazine imide) (PTI) systems in which one fragment contains a nitrogen site functionalized with a hydrogen atom (herein referred to as NH functionalization), while another fragment contains a nitrogen atom replaced by a BH group (herein referred to as BH substitution). In particular, we investigate how the spatial distance between these modified sites modulates the electronic properties of the PTI systems. The results reveal a clear exponential dependence of these properties on substitution distance, reflecting the short-range nature of electronic interactions in π-conjugated systems. Furthermore, a detailed thermodynamic analysis indicates that only structures with short substitution distances satisfy the rigorous potential requirements for driving both hydrogen and oxygen evolution reactions. These findings provide critical microscopic insights for the rational design and structural optimization of PTI-based materials for enhanced photocatalytic performance. Methods Geometry optimizations and vibrational frequency analyses were performed using DFT with the ωB97X-D functional, which should effectively describe local and charge- transfer excitations in conjugated organic structures, combined with the def2-SV(P) basis set. Vertical excitations were calculated using TD-DFT with the same methodology. We evaluated ionization energy, charge-transfer character, electrophilicity (Δω ± ), and the ε 3 stability index. Regression models were used to evaluate the relationship between substitution distance and the electronic properties of the studied structures.