Excited State Proton Transfer of a Versatile Fluorescent Probe in Different Reverse Micelles: An Overview

  • Sharmistha Dutta Choudhury Bhabha Atomic Research Centre
  • Haridas Pal
Keywords: prototropism, tautomer, fluorescence, reverse micelle, ionic liquid, water pool


Proton transfer reactions are ubiquitous in chemical and biological systems. In this article we describe the excited state prototropic behavior of 7-hydroxyl-4-methyl coumarin (7H4MC) in different kinds of reverse micelles. The dye, 7H4MC, is interesting because depending on the solvent conditions it can exist in the neutral, anionic or tautomeric forms, in its excited state. These prototropic forms have characteristic emission spectral features, which makes 7H4MC an excellent fluorescence reporter of aqueous microenvironments. In nonpolar solvents, 7H4MC exists in the neutral form in both the ground and the excited states. In aqueous medium, the dye exists in the neutral form in the ground state but is converted quantitatively to the anionic form in the excited state. In ionic reverse micelles, both the anionic and tautomeric forms evolve simultaneously from the excited neutral form of the dye, with increasing w0 values (w0= [H2O]/[surfactant]). However, the anion form is more favoured in cationic reverse micelles compared to anionic reverse micelles, due to additional electrostatic stabilization of the anion in the former system. In nonionic reverse micelles, although both anionic and tautomeric forms of the dye are observed on increasing the w0 values, their contributions are quite low due to the absence of a well-defined water pool in these systems. The excited state prototropism of 7H4MC in aqueous mixtures of 1-alkyl-3-methylimidazolium ionic liquids (ILs) resembles that in the conventional cationic reverse micelle systems, and reveals that reverse micelle-like water pools are formed in IL-water mixtures as well.


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