A model of the distance dependence of photoinduced donor-acceptor electron transfer in DNA is presented that includes the distance dependence of the solvent reorganization energy and free energy in the heterogeneous DNA environment. DNA is modeled as a low dielectric region that represents the base stack and two regions with more moderate dielectric properties that represent the DNA backbone. The DNA is surrounded by a high dielectric medium, which represents water. Model calculations show the importance of including the reorganization energy and the free energy change and illustrate the differences between the inhomogeneous model and homogeneous single dielectric constant calculations using standard Marcus theory. Calculations are performed for comparison to published experimental work (Science 1997, 277, 673; 1 J. Am. Chem. Soc. 1992, 114, 3656 2 ). Fits to one set of data 2 permit the previously reported distance dependence to be separated into an electronic contribution and solvent reorganization energy and free energy contributions. For the other set of data, 1 inclusion of the solvent reorganization energy and free energy distance dependences in the analysis of the overall distance dependent data suggest that the Marcus form of the distance dependent rate constant including the Marcus reorganization energy is not consistent with the data.

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.

Photoinduced intermolecular electron transfer between Rhodamine 3B and N,N-dimethylaniline has been studied in a series of seven liquids: acetonitrile, ethanol, propylene glycol, and mixtures of ethanol, 2-butanol, ethylene glycol, propylene glycol, and glycerol. In each liquid, the donor and acceptors have different diffusion constants and experience distinct dielectric properties. Ps time-dependent fluorescence measurements and steady-state fluorescence yield measurements were made and analyzed using a detailed statistical mechanical theory that includes a distance-dependent Marcus rate constant, diffusion with the hydrodynamic effect, and solvent structure. All solvent-dependent parameters necessary for calculations were measured, including dielectric constants, diffusion constants, and redox potentials, leaving the electronic coupling unknown. Taking the distance-dependence of the coupling to be ␤ϭ1 Å Ϫ1 , data were fit to a single parameter, the coupling matrix element at contact, J 0. The theory is able to reproduce both the functional form of the time-dependence and the concentration-dependence of the data in all seven liquids by fitting only J 0. Despite the substantial differences in the properties of the experimental systems studied, fits to the data are very good and the values for J 0 are very similar for all solvents.

Theories are presented for calculating the solvent reorganization energy and the free energy change which occur in photoinduced donor/acceptor electron transfer at the surface of micelles. The theories are based on the Marcus theory for spherical reactants in a dielectric continuum. The micelle is modeled with regions of differing dielectric properties, representing the micelle core, the headgroup region, and the surrounding water. The free energy change accompanying electron transfer can be calculated from redox measurements made in bulk liquids. The theories are applied to previously published photoinduced intermolecular electron-transfer data between octadecylrhodamine B (ODRB) and N,N-dimethylaniline (DMA) molecules.1 The ODRB and DMA molecules are located in the surface region of three different types of surfactant micelles: dodecyl-, tetradecyl-, and cetyl-trimethylammonium bromide (DTAB, TTAB, and CTAB, respectively). The data show an increased rate of electron transfer with increasing micelle radius. Application of the new theory to the electron-transfer data along with information provided by neutron scattering experiments show that the headgroup regions of the three micelles have different dielectric constants because water penetration into the headgroup regions decreases as the surfactant length increases.