Madhushree Mal, D. Mandal
2021
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0
Influential Citations
3
Citations
Journal
Journal of Molecular Liquids
Abstract
Abstract The photophysics of Alloxazine was studied in Acetic Acid – Water solvent systems which included neutral water (pH = 7), pure acetic acid, and their binary mixtures. Alloxazine was found to exhibit tautomerization into Isoalloxazine over a time-scale which depended on the nature of the solvent. While the process was too fast in neutral water to be detected with a ~ 100 ps time-resolution, it was drastically inhibited on addition of small quantities of acetic acid, displaying a remarkably long time-constant ~700 ps. However, further addition of acetic acid gradually decreased the time-constant, until it finally reached ~280 ps in pure acetic acid. Quantum chemical calculations indicated that the tautomerization occurred via Excited-state Double Proton Transfer (ESDPT) mechanism in cyclic intermolecular H-bonded complexes of Alloxazine with the solvent molecule (water or acetic acid). This involved reorganization of the H-bonded structures in the S1 energy surface, causing effective transfer of the N(1) bound proton of Alloxazine to its N(10) position, producing the Isoalloxazine tautomer. The H-bond reorganization of the complexes were strongly coupled with the solvation dynamics of the solvent concerned, and this helped to explain the large divergence of the tautomerization time-constants in the different solvents and solvent mixtures used in this study.