E. V. Dolotova

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.

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.

Transient Exciplex Formation Electron Transfer Mechanism

Transient exciplex formation mechanism of excited-state electron transfer reactions is analyzed in terms of experimental data on thermodynamics and kinetics of exciplex formation and decay. Experimental profiles of free energy, enthalpy, and entropy for transient exciplex formation and decay are considered for several electron transfer reactions in various solvents. Strong electronic coupling in contact pairs of reactants causes substantial decrease of activation energy relative to that for conventional long-range ET mechanism, especially for endergonic reactions, and provides the possibility for medium reorganization concatenated to gradual charge shift in contrast to conventional preliminary medium and reactants reorganization. Experimental criteria for transient exciplex formation (concatenated) mechanism of excited-state electron transfer are considered. Available experimental data show that this mechanism dominates for endergonic ET reactions and provides a natural explanation for a lot of known paradoxes of ET reactions.

Activation Enthalpy and Entropy for the Decay of 9-Cyanophenanthrene Exciplexes

Activation parameters were studied for the decay of 9-cyanophenanthrene exciplexes with some weak electron donors (the Gibbs energy of electron transfer ΔG*et ranging from –0.02 to –0.09 eV), which displayed fairly high emission in both nonpolar and polar aprotic solvents. It was shown that the activation enthalpy of decay for the exciplexes is low, while the activation entropy reaches –(100–150) J mol–1 K–1, which is consistent with the two possible decay mechanisms: by dissociation into free radical ions or by intersystem crossing into the triplet state.

Competition of concatenated and thermally activated medium reorganization in photoinduced electron transfer reactions

Two mechanisms of photoinduced electron-transfer reactions, the classical mechanism of the preliminary thermally activated reorganization of a medium and reactants (at relatively weak electronic interaction between reactant molecules) and the mechanism of intermediate formation of exciplexes by concatenated medium reorganization correlated with charge displacement (at relatively strong electronic interaction between the reactants) are compared.

Similarities and Dissimilarities between Partial- and Full-Charge Transfer Exciplexes

Exciplexes with partial charge transfer generated from 9-cyanophenanthrene and methoxybenzenes were studied in solvents of different polarity. The parameters of the electronic structure and the thermodynamic parameters of the exciplexes were determined from the spectral data and the temperature dependence of the fluorescence quantum yields. It was shown that more accurate description of the experimental data requires the repulsion energy between the molecules of the exciplex components to be taken into consideration in the model of correlated polarization of an exciplex and its environment. The comparison of the electronic structure and the properties of the exciplexes with partial and full charge transfer showed that, as the extent of charge transfer decreases, the contribution of exchange interaction into the exciplex stabilization increases and the role of medium polarity and intermolecular repulsion simultaneously decreases. Owing to these factors, the exciplex energy is substantially lower than the energy of a locally excited state even at a positive difference between the energies of the charge-transfer and locally excited states in a vacuum.

Mechanism of Exciplex Decay: The Quantum Yields and the Rate Constants of Radical Ion Formation from Exciplexes with Partial Charge Transfer

The dynamics of exciplex and radical ion formation was studied in donor–acceptor systems with G*et > –0.1 eV. It was shown that the quenching of excited singlet states of aromatic molecules by electron donors in polar solvents led to the formation of radical ions via exciplex dissociation resulting to complete charge separation. Intersystem crossing and internal conversion into the ground state (back electron transfer) compete with this process. The quantum yields and the rate constants of the radical ion formation were measured.

Peculiarities and paradoxes of photoinduced electron transfer reactions

Many chemical reactions involve the electron transfer stage. The kinetics of photoinduced electron transfer reactions is commonly considered in terms of either the transition state theory as preliminary thermally activated reorganization of the medium and reactants (necessary for degeneracy of the electronic levels of the reactants and the products) or nonradiative quantum transitions, which do not require preliminary activation and are observed in the exoergic region. A new approach to the kinetics of such reactions that has been proposed recently considers a substantial reduction of the barrier in the contact reactant pair due to strong electronic interaction and takes into account the intermediate formation of a charge transfer complex. This approach has explained many well-known important features of electron transfer reactions that are inconsistent with the first two theories.