Photoinduced intramolecular charge transfer of sodium 4-(N,N-dimethylamino)benzenesulfonate

A new dual fluorescent N,N-dimethylaniline derivative, sodium 4-(N,N-dimeth ylamino)benzenesulfonate (SDMAS), is reported. In SDMAS, the electron accep tor is linked to the phenyl ring via a sulfur atom at the para-position of the electron donor. It was found that SDMAS emits dual fluorescence only in highly polar solvent water but not in organic solvents such as formamide, methanol and acetonitrile. In organic solvents only a single-band emission at ca.360 nm was observed in the short wavelength region. The dual fluoresc ence of SDMAS in water was found at 365 and 475 nm, respectively. Introduct ion of organic solvent such as ethanol, acetonitrile, and 1,4-dioxane into aqueous solution of SDMAS leads to blue-shift and quenching of the long-wav elength emission. Measurements of steady-state and picosecond time-resolved fluorescence indicate that the long wavelength fluorescence is emitted fro m a charge transfer (CT) state that is populated from the locally excited ( LE) state, with the latter giving off the short wavelength fluorescence. Th e fact that a highly polar solvent is required to bring out the dual fluore scence suggests that the CT process of SDMAS has a high activation energy ( E-a). In supporting this assumption the time-resolved fluorescence measurem ents give an E-a of 15.35 kJ . mol(-1). It was assumed that the participati on of the sulfur atom d-orbital in the conjugation of sulfonate group with phenyl ring and the strong twisting and inverting of the dimethylamino plan e relative to the phenyl ring could be the reasons for the high activation energy. A molecular configuration change upon charge transfer in water was suggested for SDMAS based on the thermodynamic data. SDMAS reported here re presents the example of the dual fluorescent amine substituted aromatic sul fonate.