PHOTOPHYSICS OF NILE-BLUE-A IN PROTON-ACCEPTING AND ELECTRON-DONATING SOLVENTS

Nile Blue A perchlorate (NBA) dissolved in nonaqueous and hydrogen-bonding acceptor solvents (triethylamine and dimethylaniline) exhibits a ground-state intermolecular proton-transfer reaction. In weakly hydrogen-accepting solvents, the process is enhanced when the concentration of the dye is decreased. The reaction product, Nile Blue base, regenerates the parent molecule in the excited state via an intermolecular proton-transfer reaction with the solvent. Picosecond time-resolved fluorescence measurements suggest that the excited-state intermolecular proton-transfer process and the Stokes shift dynamics occur within 30 ps. In a weaker base and stronger electron-donor solvent, N,N-dimethylaniline, the fluorescence of NBA depends on the excitation wavelength. This is explained by a very efficient fluorescence quenching due to excited-state intermolecular electron transfer and by a reorganization of the solvation shell mainly caused by the excited-state intermolecular proton-transfer reaction.