Excited state intramolecular proton transfer of 2-Aryl,3-Hydroxybenzo[g]quinolones in surfactant supramolecular assemblies

Abstract

The 2-Aryl 3-Hydroxybenzo[g]quinolones (2Ar3HBQ) are a relatively new family of ESIPT fluorophores distinguished by their visible absorption spectra, which makes them potential ratiometric probes for biological media. In the present work, we found that 2Ar3HBQ residing as guest fluorophores inside biomimetic host systems like surfactant supramolecular assemblies like micelles and reverse micelles, exhibit dual emission attributable to ESIPT. Ratiometric analysis of the two emission peaks corresponding to the normal (N*) and tautomer (T*) species indicated that the ESIPT rates depend strongly on the nature of the host supramolecular assembly. For example, the ESIPT rate in reverse micelles containing no water was found to be similar to that in weak H-bonding and low polar solvents like acetonitrile. However, addition of water leads to the formation of water nano-droplets inside the reverse micelles, with concomitant sharp decrease of ESIPT rates. In each of the cases studied, the major factor determining the ESIPT rate appears to be the competition between intramolecular H-bonding within the guest fluorophore on the one hand, and intermolecular H-bonding involving the guest fluorophore and water molecules in its local environment, on the other hand.