TWISTED INTRAMOLECULAR CHARGE-TRANSFER DYNAMICS IN POLAR-SOLVENTS

The excited state electron transfer and time-dependent fluorescence (TDF) for twisted intramolecular charge transfer (TICT) molecules in polar solvents are studied theoretically. This class of reaction involves critical solvent stabilization of the charge-separated state with a large dipole moment compared with the less polar, locally excited state; it is also accompanied by a significant solute geometry distortion compared with vacuum, i.e. from a planar to a twisted configuration. A theoretical framework for TICT dynamics is reported and illustrated throughout for a model dimethylaminobenzonitrile (DMABN) solute in acetonitrile solvent. By employing a two-valence-bond-state description for the solute in a dielectric continuum solvent for illustration, a two-dimensional free energy surface is obtained in terms of the solute torsional angle 0 and a solvent coordinate s that gauges the non-equilibrium solvent orientational polarization. The TICT rate constant and TDF are analyzed via the minimum free energy solution phase reaction path. It is found that the variations of the activation free energy with the solvent polarity and the overall free energetics of the reaction correlate well with Hammond postulate behavior and experimentally observed trends for activated TICT reactions.