Spectral properties of BADAN in solutions with different polarities

Spectral characteristics (absorption spectra, excitation and emission spectra of solvatochromic dye 6-bromo-acetyl-2-dimethylamino-naphtalene (BADAN) were studied in solvents with different polarities. Comparison of BADAN fluorescence excitation spectra corrected for the primary inner filter effect and absorption spectra of this dye allowed to conclude that the long-wavelength absorption band of BADAN is determined by the superposition of two BADAN tautomeric forms having different conformation (planar and non-planar) and different distribution of electronic density. The analysis of BADAN excitation and emission spectra has shown that in non-polar solvents (hexane) fluoresces only one dye form which absorbs in more short-wave part of the spectrum than other conformer. In polar solvents (acetonitrile, mixture of acetonitrile/water) both forms fluoresce. It is concluded that the fluorescence of the dye in hexane and short-wavelength component of BADAN fluorescence at excitation in short-wavelength region of absorption spectrum are due to the locally excited state of the planar BADAN molecule. Fluorescence of BADAN in acetonitrile at excitation in long-wavelength part of long-wavelength absorption band and long-wavelength component of BADAN fluorescence spectrum in acetonitrile at excitation in short-wavelength region of absorption spectrum are due to non-planar BADAN conformers with internal charge transfer state.