DYNAMICS OF TWISTED INTRAMOLECULAR CHARGE-TRANSFER COMPLEXES 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 reaction class involves cr itical solvent stabilization of the charge-separated state with a larg e dipole moment compared to the less polar, locally-excited state; it is also accompanied by a significant solute geometry distortion compar ed to vacuum, i.e., from a planar to a twisted configuration. A theore tical framework for TICT dynamics is constructed and illustrated throu ghout for a model dimethylaminobenzonitrile (DMABN) solute. By employi ng a model two valence-bond state description for the solute in a diel ectric continuum solvent for illustration, we obtain a two-dimensional free energy surface in terms of the solute torsional angle theta and a solvent coordinate s that gauges the nonequilibrium solvent orientat ional polarization. The TICT rate constant and TDF are analyzed via th e minimum free energy solution-phase reaction path (SRP), which is fou nd to be strongly curved on the reactive free energy surface. For slow aprotic solvents, the SRP is mainly along the theta coordinate near t he transition state. There is little motion in the solvent coordinate s; the solvent lags the solute twisting: motion and there is nonadiaba tic nonequilibrium solvation. Near the reactant anti product states, h owever, the SRP is almost completely along s with little solute torsio nal motion. This indicates that the solvent orientational polarization fluctuation is important in initiating: the TICT reaction. By contras t, for faster solvents (on the same surface), the SRP becomes nearly p arallel to s at the transition state while it is almost along theta ne ar the reactant anti product states. Thus the reactive mode relevant f or electron transfer at the transition state changes markedly with the solvent time scale. The consequences of the SRP curvature on the TICT rate constant and its contrasts with traditional activated electron t ransfer are also discussed. The SRP analysis also shows that the dynam ics relevant for TDF vary during the course of the reaction. In partic ular, for DMABN in acetonitrile solvent, about the first 70% of the dy namic Stokes shift upon photoexcitation occurs mainly via the solute t orsional dynamics with a minor s participation, followed by the remain ing relaxation along s with a minimal theta motion near the perpendicu lar geometry. Thus the TDF dynamics probed via DMABN involve the solut e twisting dynamics as well as the solvation dynamics; the former domi nates at the early and middle stages of TDF until the latter eventuall y takes over towards the end. As the solvent becomes faster, the solve nt motions begin to participate in the TDF dynamics progressively earl ier; as a result, the TDF probes the concomitant motions of both the s olute and solvent. This picture provides a theoretical explanation for the recent experimental findings that the dynamic Stokes shift measur ed with DMABN is much faster than that with coumarin dyes (with no tor sional degrees of freedom) for the same solvents. Finally, it is found that the variations of the activation fr ee energy with solvent polar ity and overall reaction free energetics correlate well with Hammond p ostulate behavior and experimentally observed trends for activated TIC T reactions.