Barrierless electronic relaxation

Hence, the experimental results on DM ABN have been discussed within the framework of the theoretical model of a barrierless electronic relaxation where the reaction is modeled by a pinhole sink on the minimum of a harmonic potential (see Section IV.I). 

The recent theoretical approaches include a theory of barrierless electronic relaxation which draws on the model of nonradiative excited state decay, and a general treatment of the effect of solvent dielectric relaxation based on the theory of optical line shapes, as well as treatments based on classical and quantum rate theories. Equation(5) does not hold for all solvents and, more generally, may be frequency-dependent. Papers by Hynes, Rips and Jortner, Sumi and Marcus, and Warshel and Hwang " contain good overviews of the theoretical developments. 

Flow does the occurrence of two fluorescing states for MK fit into the dynamic picture developed in Section IV The observed temperature dependence of the fluorescence quantum yield of MK in ethanol206 yields direct evidence that in this case, also, EBA < Ev. Recent time-resolved measurements at the Berlin Electron Storage Ring for Synchrotron Radiation (BESSY)207 support this argument The viscosity dependence of the decay of the short-wavelength fluorescence band in ethanol is consistent with an apparent value BA — 0.5Ev. Moreover, the decay is nonexponential, as would be expected for a barrierless relaxation. The lifetime of the TICT state (exponential decay) is 0.65 ns in acetonitrile at room temperature, that is, it is unusually short. 

There are a number of experimental systems for which the rate constant is higher than the frequency of longitndinal polarization relaxation. These systems indicate that here mnst be faster nuclear modes driving electron transfer. One possible sonrce is the inertial component of solvent dynamics occurring on shorter timescales than diffusive polarization relaxation. The participation of high-frequency vibrations rendering the reaction essentially barrierless is stiU another scenario. Both mechanisms would obviate any correlation of the rate constant with the difiusional solvation timescale. 

The forward electron-transfer step can occnr nnder exceptionally fast time scales—faster than nnclear relaxation. The fastest forward electron-transfer times will be realised with dyes that have excited-state surfaces above the CBM (barrierless conditions). For dyes in intimate contact (no intervening solvent or contaminant), the electron coupling can be in the 100 to 1000 cm range (100 fs-10 fs dynamics). 

The ultrafast PT, which occurs typically on time scales of 10 13-10 14 s will not be considered. Such transfers are observed in molecular systems in which the potential energy surface (PES) governing the proton motion is essentially barrierless but has different minima positions in different electronic states, so that the proton finds itself in an off-equilibrium position after electronic excitation and relaxes to the new equilibrium position. The contribution of tunneling may be disregarded and the rate of these processes does not depend very strongly on temperature. These reactions, which are of great current interest, are intensely studied by ultrafast laser spectroscopy and are reviewed elsewhere [16,17],... 

Kosower and co-workers have found the photoinduced, barrierless charge separation processes of substituted polyaromatics to be controlled by solvent relaxation behavior over a large temperature range in alcohol solvents. Heitele and Michel-Beyerle reported on the complex solvent- and temperature-dependent electron transfer fluorescence quenching in some covalently linked aromatic donor-acceptor compounds in viscous solvents. These authors have attempted a critical comparison between current theoretical models and their experimental results, and the limitations of current theoretical models are discussed. 

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