EFFECT OF TEMPERATURE AND VISCOSITY ON INTRAMOLECULAR CHARGE-TRANSFERFLUORESCENCE OF A 4-P-DIMETHYLAMINOSTYRYLPYRIDINIUM SALT IN PROTIC SOLVENTS

The steady-state fluorescence of a 4-p-dimethylaminostyrylpyridinium s alt (1) has been studied in aqueous solutions. An orange emission band close to 600 nm arose from two excitation bands, at 360 and 469 nm. P ossible origins of species responsible for the red-side emission are d iscussed: (i) excitation at 360 nm, followed by twisted intramolecular charge transfer (TICT) and (ii) excitation at 469 nm of a different g round state with intramolecular charge transfer (ICT). The effect of t emperature and solvent viscosity on the fluorescence quantum yield of both charge transfer (CT) bands of 1 has been investigated. The Debye- Stokes-Einstein hydrodynamic model gave an accurate description of the reorientation dynamics of the fluorescent probe. At low viscosities, the quantum yield of the charge-transfer fluorescence was linear with solvent viscosity. The activation energy of the thermally activated re orientation relaxation for both CT bands corresponds to the viscosity energy of activation of the medium. Nearly identical activation energi es for both CT bands suggest that the corresponding emitting species h ave the same origin.