Experimental determinations of the dynamics of photoinduced electron transfer from rubrene to duroquinone in three solvents, dibutyl phthalate, diethyl sebacate, and cyclohexanone are presented. Measurements of the donor (rubrene) fluorescence decays were made with time-correlated single-photon counting. The data are analyzed using recent theoretical developments that include important features of the solvent, i.e., the effects of finite molecular volume on local solvent structure and on the mutual donor-acceptor diffusion rates. Inclusion of the liquid radial distribution function (rdf) in the theory accounts for the significant variation of the acceptor concentration near a donor. Because the concentration of acceptors near a donor is substantially greater than the average concentration used in a featureless continuum liquid model, incorporating the rdf is necessary to properly analyze experimental data. Hydrodynamic effects, which slow the rate of donoracceptor approach at short distance, are important and are also included in the theoretical analysis of the data. The data analysis depends on a reasonable model of the rdf. A hard-sphere liquid rdf is shown to be sufficiently accurate by comparing model electron-transfer calculations using a hard-sphere rdf and an rdf from neutronscattering experiments reported in the literature. A method is presented to obtain the hard-sphere parameters needed to calculate the rdf. The method uses a self-consistent determination of the hard-sphere radius and diffusion constant and the solvent self-diffusion constant calculated from the Spernol and Wirtz equation. The Marcus form of the distance-dependent transfer rate is used. For the highest viscosity solvent (dibutyl phthalate), a unique set of the Marcus transfer parameters is obtained. For lower viscosity solvents, the transfer parameters are less well defined, but information on the distance and time dependence of charge separation is still acquired. These experiments, combined with the theoretical analysis, yield the first realistic description of through-solvent photoinduced electron transfer.
Photoinduced electron transfer between N,N-dimethylaniline (DMA) and octadecylrhodamine B (ODRB) is studied on the surfaces of three alkyltrimethylammonium bromide micelles: dodecyl-(DTAB), tetradecyl-(TTAB), and hexadecyltrimethylammonium bromide (CTAB). The DMA and ODRB molecules are localized at the micelle surface. Time-resolved fluorescence and fluorescence yield data are presented and analyzed with the theoretical methods of ref 1. Lateral diffusion of the molecules over the micelle surfaces is included. Although the three micelles are structurally similar, pronounced differences in the electron-transfer kinetics are observed, with the overall amount of electron transfer increasing with alkyl chain length for the same DMA surface packing fraction. This result is attributed to differences in the solvent reorganization energy, possibly due to varying extents of water penetration into the headgroup regions of the three micelles. As the surfactant chain length increases, the solvent reorganization energy is reduced, resulting in faster electron transfer.
The coupled processes of intermolecular photoinduced forward electron transfer and geminate recombination between donors (rubrene) and acceptors (duroquinone) are studied in two molecular liquids: dibutyl phthalate and diethyl sebacate. Time-correlated single-photon counting and fluorescence yield measurements give information about the depletion of the donor excited state due to forward transfer, while pump-probe experiments give direct information about the radical survival kinetics. A straightforward procedure is presented for removing contributions from excited-state-excited-state absorption to the pump-probe data. The data are analyzed with a previously presented model that includes solvent structure and hydrodynamic effects in a detailed theory of through-solvent electron transfer. Models that neglect these effects are incapable of describing the data. When a detailed description of solvent effects is included in the theory, agreement with the experimental results is obtained. Forward electron transfer is well-described with a classical Marcus form of the rate equation, though the precise values of the rate parameters depend on the details of the solvents' radial distribution function. The additional experimental results presented here permit a more accurate determination of the forward transfer parameters than those presented previously.1 The geminate recombination (back transfer) data are highly inverted and cannot be analyzed with a classical Marcus expression. Good fits are instead obtained with an exponential distance dependence model of the rate constant and also with a more detailed semiclassical treatment suggested by Jortner.2 Analysis of the pump-probe data, however, suggests that the geminate recombination cannot be described with a single solvent dielectric constant. Rather, a time-dependent dielectric constant is required to properly account for diffusion occurring in a time-varying Coulomb potential. A model using a longitudinal dielectric relaxation time is presented. Additionally, previously reported theoretical results 3 are rederived in a general form that permits important physical effects to be included more rigorously.
Solvent-solute reaction path curvature effects on energy transfer corrections to the solute reaction rate Solvent effect on the electron transfer rate and the energy gap law AIP Conf.A previously developed statistical mechanical theory describing photo-induced electron transfer and geminate recombination in liquid solutions has been modified to account for realistic finite-volume solvent effects. This work introduces physically important effects caused by the solvent which fundamentally affect the rates and spatial distribution of charge transfer events. The finite volume of solvent molecules gives rise to a nonuniform distribution of particles around an electron donor, which is incorporated into the theory by a two-particle radial distribution function ͑rdf͒. The Percus-Yevick solutions for the rdf can give numerically useful values for the solvent structure, g(R) although any form of g(R) can be used with the method. The nonuniform particle distribution significantly affects the electron transfer rates and the distribution of ion pairs formed by forward electron transfer, particularly at short times. In addition, finite solvent size affects the rate of relative diffusion between any donor-acceptor pair. These ''hydrodynamic effects'' slow down the interparticle diffusion rates when near contact, resulting in a major change in the long time behavior of photoexcited electron transfer systems. This work formally introduces the mathematical modifications to charge transfer theory necessary to account for the solvent structure and hydrodynamic effect and illustrates the results with model calculations. These calculations show that analysis of experiments with theories that do not include the rdf and hydrodynamic effects can result in significant errors in the interpretation of data.
We report here the direct measurement of intra-tRNA distances during annealing of the tRNA primer to the HIV RNA genome. This key step in the initiation of retroviral reverse transcription involves hybridization of one strand of the acceptor arm of a specific lysine tRNA to the primer binding site on the RNA genome. Although the mechanism of tRNA unwinding and annealing is not known, previous studies have shown that HIV nucleocapsid protein (NC) greatly accelerates primer/template binary complex formation in vitro. An open question is whether NC alone unwinds the primer or whether unwinding by NC requires the RNA genome. We monitored the annealing process in solution by using f luorescence resonance energy transfer (FRET). Distance measurements demonstrate unequivocally that the tRNA acceptor stem is not substantially unwound by NC in the absence of the RNA genome, that is, unwinding is not separable from hybridization. Moreover, FRET measurements show that both heat-and NC-mediated annealing result in an Ϸ40-Å increase in the separation of the two ends of the tRNA acceptor arm on binding to the template. This large increase in separation of the two ends suggests a complete displacement of the nonhybridized strand of the acceptor stem in the initiation complex.
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