Short-lived intermediates in photochemistry of an OsCl62- complex in aqueous solutions.

Two mechanisms of OsIVCl62- photolysis were studied by means of quantum chemical calculations in gas and aqueous phases. The difference between these mechanisms is in the nature of the possible Os(iv) key intermediates (KI). According to calculations, the intermediate is an OsIVCl5- complex of square pyramidal coordination geometry. The calculations do not give an opportunity to make an unambiguous choice between the triplet and quintet multiplicities of OsIVCl5-. The calculated CASSCF/IMCP-SR1 transition energies for 5OsIVCl5- are lower than for 3OsIVCl5-, while the calculated XMC-QDPT2/SBKJC spectra for the triplet state are in better agreement with the experimental absorption spectrum of the KI than for the quintet state.

Primary photophysical and photochemical processes for hexachloroosmate(iv) in aqueous solution.

The photoaquation of the OsIVCl62- complex was studied by means of stationary photolysis, nanosecond laser flash photolysis and ultrafast kinetic spectroscopy. The OsIVCl5(OH)2- complex was found to be the only reaction product. The quantum yield of photoaquation is rather low and wavelength-dependent. No impact of redox processes on photoaquation was revealed. The total characteristic lifetime of the process is about 80 ps. Three intermediates were recorded in the femto- and picosecond time domains and assigned to different Os(iv) species. The nature of intermediates and possible mechanisms of photoaquation are discussed.

Reorganization of intramolecular high frequency vibrational modes and dynamic solvent effect in electron transfer reactions.

The possibility of the multichannel stochastic model to adequately describe all principal regularities observed in thermal electron transfer kinetics has been demonstrated. The most important are as follows: (i) the model predicts the solvent controlled regime in the Marcus normal region and its almost full suppression in the Marcus inverted region as well as a continuous transition between them in the vicinity of the activationless region; (ii) the suppression of dynamic solvent effect (DSE) is principally caused by the reorganization of high frequency vibrational modes; (iii) an additional factor of the DSE suppression stems from fast solvent relaxation component; (iv) in the inverted region, the multichannel stochastic model predicts the apparent activation energy to be much less than that calculated with Marcus equation. The exploration of the multichannel stochastic model has allowed one to conclude that the reorganization of high frequency vibrational modes can (i) raise the maximum rate constant above the solvent controlled limit by 2 and more orders of magnitude, (ii) shift the rate constant maximum to larger values of the free energy gap, and (iii) approach the electron transfer kinetics to the nonadiabatic regime.

Nonequilibrium phenomena in charge recombination of excited donor-acceptor complexes and free energy gap law.

The charge recombination dynamics of excited donor-acceptor complexes in polar solvents has been investigated within the framework of the stochastic approach. The model involves the excited state formation by the pump pulse and accounts for the reorganization of a number of intramolecular high-frequency vibrational modes, for their relaxation as well as for the solvent reorganization following nonexponential relaxation. The hot transitions accelerate the charge recombination in the low exergonic region and suppress it in the region of moderate exothermicity. This straightens the dependence of the logarithm of the charge recombination rate constant on the free energy gap to the form that can be fitted to the experimental data. The free energy dependence of the charge recombination rate constant can be well fitted to the multichannel stochastic model if the donor-acceptor complexes are separated into a few groups with different values of the electronic coupling. The model provides correct description of the nonexponential charge recombination dynamics in excited donor-acceptor complexes, in particular, nearly exponential recombination in perylene-tetracyanoethylene complex in acetonitrile. It appears that majority of the initially excited donor-acceptor complexes recombines in a nonthermal (hot) stage when the nonequilibrium wave packet passes through a number of term crossings corresponding to transitions toward vibrational excited states of the electronic ground state in the area of the low and moderate exothermicity.