Lumichrome tautomerism in alcohol-water mixtures: Effect of carbon chain length and mole fraction of alcohols

Abstract

Alcohol-water mixtures are important solvent systems, widely used in many applications. In this study, the intriguing water induced tautomerism of the fluorescent chromophore, Lumichrome (LC), has been explored to reveal interesting changes in the solvent environments of alcohol-water mixtures of methanol (MeOH), ethanol (EtOH), n-propanol (PrOH) and tert-butanol (t-BuOH), as a function of the carbon chain length/alkyl group size and concentrations of the alcohols. Since LC remains in the alloxazine form in pure aqueous medium, the appearance of emission from the isoalloxazine tautomer in alcohol-water mixtures provides compelling evidence that addition of alcohol disrupts the H-bonded network structure of bulk water to facilitate the availability of isolated water molecules in the solvent medium, which can form the required LC-water precursor complex for conversion of the alloxazine form to isoalloxazine form. Interestingly, two estimated parameters for LC tautomerism, namely, Rlong, which is the relative emission contribution of the isoalloxazine form in the fluorescence decay traces of LC (and hence a measure of the population of isoalloxazine form in the solution), and τlag, which is the lagtime involved in the evolution of the isoalloxazine emission band in the time-resolved area normalized emission spectra of LC (and hence a measure of the ease with which the LC-water precursor complex is formed), show a non-monotonic dependence on the mole fraction of alcohols (χalcohol) in the mixed solvent systems. The unique variation of Rlong and τlag with χalcohol, indicates the occurrence of three composition zones in the alcohol-water mixtures, which we correlate with characteristic transformations in the microscopic structures of the solvent mixtures. Our results reveal that the ability of the alcohols to disrupt the H-bonded network structure of bulk water increases with increasing size of their hydrophobic alkyl groups, following the trend, t-BuOH > PrOH > EtOH > MeOH.