Dual fluorescence of 1-hydroxy-substituted Nile Red dye in the red and near-infrared spectral range: Excited-state proton transfer along intramolecular hydrogen bond

Photophysical properties of 1-hydroxy-substituted Nile Red dye proved to be entirely different from those of its 2-hydroxy isomer. Strong hydrogen bonding between the 1-HO-moiety and the heterocyclic nitrogen of the molecule made deprotonation with bases very difficult but facilitated photoinduced tautomerization via intramolecular proton transfer. Dual fluorescence was found in all solvents and the relative intensity of the two bands markedly changed with the polarity as well as hydrogen bond donating power of the medium. The quantum yield for both the normal fluorescence and the tautomer emission went through a maximum as a function of the ET(30) solvent polarity parameter with highest values obtained in dichloromethane.