Stochastic model for solvent-assisted intramolecular charge transfer

(N,N-Dimethylamino)benzonitrile (DMABN) and related aromatic donor-acceptor compounds show dual fluorescence emission in polar solvents. The static and dynamic features of the spectra are strongly affected by the 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 undergoes an adiabatic intramolecular electron transfer. Two metastable states are assumed to interconvert by a torsional motion, which provides a natural reaction coordinate for the electron transfer (ET) process. In this work we discuss a stochastic model which extends the simple kinetic picture to a continuous description. The dynamics of interconversion is described as a diffusional process coupled to a solvent polarization coordinate. Decay to the ground state is included in the form of a sink term depending upon instantaneous conformation. The model provides a satisfactory description of all static and dynamic fluorescence spectral features available from experiments. The Grabowski scheme is derived from the continuous model in the case of a relatively high barrier between interconverting metastable states. Agreement between theoretical simulations and observed experimental spectra supports the original hypothesis based on intramolecular electron-transfer involving distinct conformers.