Competition and interplay of various intermolecular interactions in ultrafast excited-state proton and electron transfer reactions.

The main features of the photoinduced kinetics of both ultrafast excited-state proton and electron transfer reactions that occur in the picosecond (ps) and femtosecond (fs) time domains are compared. Proton transfer (PT) reaction kinetics can be described in terms of several discrete values of rate coefficients in the form of polyexponential functions where each value of the rate coefficient can be attributed to a definite physical behavior of the reaction mechanism. In contrast, electron transfer (ET) reaction kinetics requires a consideration of a continuous distribution of rate coefficients. This difference can be related to structure of the ground-state reactant pairs for each reaction. Excited-state ET can occur at various configurations of reactant molecules and its rate reflects the fluctuations of the distances and orientations of these molecules. In contrast, excited-state PT requires preliminary formation of a ground-state H-bonded complex with definite structure where the reaction occurs after photoexcitation.

Analysis of transformations of the ultrafast electron transfer photoreaction mechanism in liquid solutions by the rate distribution approach.

Representation of the experimental reaction kinetics in the form of rate distribution is shown to be an effective method for the analysis of the mechanisms of these reactions and for comparisons of the kinetics with QC calculations, as well as with the experimental data on the medium mobility. The rate constant distribution function P(k) can be obtained directly from the experimental kinetics N(t) by an inverse Laplace transform. The application of this approach to kinetic data for several excited-state electron transfer reactions reveals the transformations of their rate control factors in the time domain of 1-1000 ps. In neat electron donating solvents two components are observed. The fastest component (k > 1 ps(-1)) was found to be controlled by the fluctuations of the overall electronic coupling matrix element, involving all the reactant molecules, located inside the interior of the solvent shell, rather than for specific pairs of reactant molecules. The slower component (1 > k > 0.1 ps(-1)) is controlled by the medium reorganization (longitudinal relaxation times, τL). A substantial contribution from the non-stationary diffusion controlled reaction is observed in diluted solutions ([Q] < 1 M). No contribution from the long-distance electron transfer (electron tunneling) proposed earlier for the excited-state electron transfer between perylene and tetracyanoethylene in acetonitrile is observed. The rate distribution approach provides a simple and efficient method for the quantitative analysis of the reaction mechanism and transformation of the rate control factors in the course of the reactions.

Microphase mechanism of "superquenching" of luminescent probes in aqueous solutions of DNA and some other polyelectrolytes.

A new approach in terms of microphase model of aqueous solutions of polyelectrolytes is proposed for explanation of a very strong quenching of luminescent probes ("superquenching") in these solutions. This phenomenon is used in literature for creation of extremely sensitive chemical and biosensors and was attributed predominantly to efficient energy or electron transfer. Microphase approach considers this phenomenon in terms of local concentrations of both the luminescent compound and of the quencher in microphase, formed by DNA and other polyelectrolytes, which can be several (4-10) orders of magnitude greater than their apparent concentrations in solution. Large local concentrations of the light absorbing centers in the microphase also provide conditions for aggregation of these centers and efficient energy transfer, which provides a significant increase in quenching constants (∼10(2)-10(5)). Microphase approach provides good quantitative description of all the features of the superquenching, new possibilities for analysis and control of kinetics of DNA reactions, and for improvement of the sensitivity of luminescent sensors. It reveals nonspecific localization of the luminescent centers and of Aun nanoparticles in different positions of DNA molecules that hinders from the simultaneous use of optical methods and electron or tunneling microscopy for the combined study of the structure of DNA.

Evolution of the reaction mechanism during ultrafast photoinduced electron transfer.

Specific features of ultrafast photoinduced electron transfer (ET) in concentrated liquid solutions and in neat electron donating solvents are discussed in terms of continuous distribution of ET rate constants, related to electron tunneling with statistical distribution of electronic coupling matrix element and distances between reactant molecules. Available data on photoinduced electron transfer in solutions for several systems are analyzed. Electron tunneling approach is shown to provide global description of nonexponential kinetics of excited states decay at various concentrations of reactant and in neat solvents.

The behavior of exciplex decay processes and interplay of radiationless transition and preliminary reorganization mechanisms of electron transfer in loose and tight pairs of reactants.

Exciplex emission spectra and rate constants of their decay via internal conversion and intersystem crossing are studied and discussed in terms of conventional radiationless transition approach. Exciplexes of 9-cyanophenanthrene with 1,2,3-trimethoxybenzene and 1,3,5-trimethoxybenzene were studied in heptane, toluene, butyl acetate, dichloromethane, butyronitrile, and acetonitrile. A better description of spectra and rate constants is obtained using 0-0 transition energy and Gauss broadening of vibrational bands rather than the free energy of electron transfer and reorganization energy. The coincidence of parameters describing exciplex emission spectra and dependence of exciplex decay rate constants on energy gap gives the evidence of radiationless quantum transition mechanism rather than thermally activated medium reorganization mechanism of charge recombination in exciplexes and excited charge transfer complexes (contact radical ion pairs) as well as in solvent separated radical ion pairs. Radiationless quantum transition mechanism is shown to provide an appropriate description also for the main features of exergonic excited-state charge separation reactions if fast mutual transformations of loose and tight pairs of reactants are considered. In particular, very fast electron transfer (ET) in tight pairs of reactants with strong electronic coupling of locally excited and charge transfer states can prevent the observation of an inverted region in bimolecular excited-state charge separation even for highly exergonic reactions.

Evidence for diffusion-controlled electron transfer in exciplex formation reactions. Medium reorganisation stimulated by strong electronic coupling.

Diffusion-controlled rates of formation were found from the temperature dependence of apparent quenching rate constants for exciplexes, when the driving force of excited-state electron transfer -0.1 < deltaG(ET)* < +0.1 eV. This is inconsistent with the conventional mechanism of electron-transfer reactions, involving preliminary reorganisation of the medium and reactants, and provides strong support for the mechanism of medium reorganisation stimulated by strong electronic coupling of locally excited and charge-transfer states.