TWISTED INTRAMOLECULAR CHARGE-TRANSFER DYNAMICS IN POLAR-SOLVENTS

The excited state electron transfer and time-dependent fluorescence (T DF) for twisted intramolecular charge transfer (TICT) molecules in pol ar solvents are studied theoretically. This class of reaction involves critical solvent stabilization of the charge-separated state with a l arge dipole moment compared with the less polar, locally excited state ; it is also accompanied by a significant solute geometry distortion c ompared with vacuum, i.e. from a planar to a twisted configuration. A theoretical framework for TICT dynamics is reported and illustrated th roughout for a model dimethylaminobenzonitrile (DMABN) solute in aceto nitrile 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 tor sional angle 0 and a solvent coordinate s that gauges the non-equilibr ium 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 t he solvent polarity and the overall free energetics of the reaction co rrelate well with Hammond postulate behavior and experimentally observ ed trends for activated TICT reactions.