Effect of temperature and viscosity on intramolecular charge-transfer fluorescence of a 4-p-dimethylaminostyrylpyridinium salt in protic solvents

Abstract

The steady-state fluorescence of a 4-p-dimethylaminostyrylpyridinium salt (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. Possible origins of species responsible for the red-side emission are discussed: (i) excitation at 360 nm, followed by twisted intramolecular charge transfer (TICT) and (ii) excitation at 469 nm of a different ground state with intramolecular charge transfer (ICT).

The effect of temperature 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 reorientation relaxation for both CT bands corresponds to the viscosity energy of activation of the medium. Nearly identical activation energies for both CT bands suggest that the corresponding emitting species have the same origin.