Solvent and pH-sensitive Fluorescence Response of Alloxazine

Abstract

Abstract The fluorescence response of Alloxazine was found to be sensitive to solvent properties like polarity and H-bonding capacity, which also played a significant role in controlling the incidence of Alloxazine – Isoalloxazine photo-tautomerization. The latter was strongly promoted in polar, protic solvents like alcohols, via an N(1)→N(10) excited state proton transfer mechanism involving Alloxazine : solvent (1:2) intermolecular H-bonded complexes, similar to that proposed earlier for 7,8-Dimethylalloxazine. Even for Alloxazine in aqueous solutions over a wide range of acid/base strengths from pH\u202f=\u202f2 to pH\u202f=\u202f13, the fluorescence was found to be always dominated by two emissive species, which were identified as the Alloxazine – Isoalloxazine couple, irrespective of the pH of the solution, and of the nature of the ionic equilibrium prevailing at that pH. The results in aqueous pH solutions could be explained by a mechanism where the photo-tautomerization was resolved into two independent dynamics involving deprotonation and protonation of the N(1) and N(10) atoms of Alloxazine, respectively.