STOCHASTIC-MODEL FOR SOLVENT-ASSISTED INTRAMOLECULAR CHARGE-TRANSFER

(N,N-Dimethylamino)benzonitrile (DMABN) and related aromatic donor-acc eptor compounds show dual fluorescence emission in polar solvents. The static and dynamic features of the spectra are strongly affected by t he polarity and viscosity of the medium. A successful model was first proposed by Grabowski et al., by using a phenomenologic kinetic scheme . According to this interpretation, the excited singlet state undergoe s an adiabatic intramolecular electron transfer. Two metastable states are assumed to interconvert by a torsional motion, which provides a n atural reaction coordinate for the electron transfer (ET) process. In this work we discuss a stochastic model which extends the simple kinet ic picture to a continuous description. The dynamics of interconversio n is described as a diffusional process coupled to a solvent polarizat ion coordinate. Decay to the ground state is included in the form of a sink term depending upon instantaneous conformation. The model provid es a satisfactory description of all static and dynamic fluorescence s pectral features available from experiments. The Grabowski scheme is d erived from the continuous model in the case of a relatively high barr ier between interconverting metastable states. Agreement between theor etical simulations and observed experimental spectra supports the orig inal hypothesis based on intramolecular electron-transfer involving di stinct conformers.