Picosecond emission anisotropy decays of dimethyl aminobenzonitrile

Picosecond polarized fluorescence experiments on DMABN (4-(dimethyl amino) benzonitrile) in polar solvents and toluene are reported. From the detected polarized fluorescence components emission anisotropy histograms are constructed and compared with the synthetic data simulated for the energy levels scheme in DMABN. For the short-wavelength emission of DMABN vibronic coupling between the close-lying S1 and S2 energy levels occurs and leads to a vibronically mixed polarization of the S1 (FB) fluorescence. Low initial values of the emission anisotropy decays for the long-wavelength (FA) emission are observed. This observation can be explained by an assumption that the internal twisting of the amino group in DMABN changes the angular orientation of DMABN and by an assumption that the emission dipole moment in the FA emission band is not parallel to the absorption dipole moment in the S2 (1La) absorption band.