An attempt to unravel the differentiable relationship between the activity and reactivity of selected cationic and anionic dye molecules towards their catalytic degradation: An experimental and theoretical study

This study investigates the degradation of cationic and anionic dyes using chalcopyrite quantum dots (QDs) under various conditions, such as sunlight, darkness, and the presence of quenchers. Depending on the type of dye and experimental conditions, dye removal efficiencies ranged from 83% to 95.6%. Scavenger tests are performed for all the dyes, yielding the order of importance of reactive oxygen species and photogenerated charge carriers as •OH > h⁺ > e⁻ > O₂•-. Out of these, h⁺ and •OH play their roles differently in the mechanism for cationic and anionic dyes. Theoretical reactivity of dyes is analyzed using conceptual density functional theory, which shows a strong correlation between experimental degradation rates and theoretical reactivity, particularly in aqueous phase. The studied electronic and structural differences between cationic and anionic dyes offer insights into their catalytic degradation mechanisms. Local quantum descriptors were used to assess dye reactivity in both gaseous and aqueous phases, identifying reactive sites for electrophilic, nucleophilic, and radical attacks within the dye structures. Key findings emphasized the impact of orbital delocalization and aromaticity on dye reactivity, noting minimal changes in cationic dyes but significant shifts in anionic dyes upon solvation. Para-delocalization index and frontier molecular orbital analysis revealed that cationic dyes undergo destabilization of HOMO and LUMO levels, reducing the HOMO-LUMO energy gap, while anionic dyes experience frontier orbital energy stabilization. The impact of orbital delocalization and aromaticity on dye reactivity is important, which could be attributed to the above fact. The study highlights the importance of ionic characteristics and solvation effects in photocatalytic dye degradation.