Electronic reactivity mapping of triphenylmethane dyes: An integrated DFT and Fukui function study of their oxidative...

Triphenylmethane dyes remain among the most persistent cationic pollutants due to their extended π-conjugation and high structural stability, which limit their susceptibility to oxidation. To rationalise their molecular reactivity and identify the electronic factors controlling their degradation behaviour, we investigated three representative dyes, Basic Blue 1 (BB1), Basic Green 4 (BG4) and Basic Violet 2 (BV2) using an integrated DFT framework. The comparative HOMO-LUMO gaps that revealed reactivity descriptors suggest that the three dyes have similar almost stability. Nevertheless, fragments containing nitrogen atoms are consistently highlighted by condensed Fukui indices and molecular electrostatic potential (MESP) maps as the major nucleophilic centers. Strong electrostatic polarization around these nitrogen atoms further supports their role as key reactive sites. Density of states (DOS) describes the distribution of electronic states as a function of energy, whereas the partial density of states (PDOS) resolves this distribution into contributions from specific atoms or molecular fragments. Fragment-resolved DOS and PDOS analysis shows significant orbital overlap of nitrogen fragments in the deep valence region, followed by clear differentiation near the frontier orbitals. This distribution explains their preferential involvement in oxidative cleavage mechanisms. Together, Fukui, MESP, and DOS/PDOS analyses outline a coherent reactivity pattern that identifies the structural regions most vulnerable to oxidative attack, offering valuable insights for predicting degradation under advanced oxidation conditions.

All quantum-chemical calculations were performed using Gaussian 16, Multiwfn 3.8 was utilized to evaluate reactivity descriptors, which included MESP surfaces, and DOS/PDOS. The LC-BLYP/6-31 +  + G (d,p) level of theory was selected after benchmarking several functional and basis sets. Solvent effects were included through the CPCM implicit solvation model with water.