Dynamics of ground state absorption spectra in donor-acceptor pairs with ultrafast charge recombination.

A theoretical approach to simulation of the transient spectra in molecular systems with ultrafast photoinduced nonradiative electronic transitions is developed. The evolution of the excited and ground state populations as well as the nonradiative transitions between them are calculated in the framework of the stochastic multichannel point-transition model involving the reorganization of the medium and the intramolecular high frequency vibrational modes. Simulations of transient spectra of donor-acceptor pairs excited in the charge-transfer band that are accompanied by ultrafast charge recombination into the ground state demonstrate a possibility of positive band appearance in the transient absorption spectrum caused by those systems in the ground state, which returned there from the excited state.

U-shaped temperature dependence of rate constant of intramolecular photoinduced charge separation in zinc-porphyrin-bridge-quinone compounds.

The multichannel stochastic point transition model of photoinduced electron transfer from both a vibrationally unrelaxed and a relaxed states involving the vibrational relaxation in donor-acceptor pairs has been elaborated. The U-shaped temperature dependencies of the rate constants of the intramolecular photoinduced charge separation from both the vibrationally unrelaxed and the relaxed states observed in Zn-porphyrin-bridge-quinone compounds in 2-methyltetrahydrofuran solvent have been reproduced in the framework of the proposed model that accounts for the temperature dependencies of the charge separation free energy gap and the medium reorganization energy. This modeling has allowed uncovering the mechanism of such a variation of the rate constant with the temperature.

Dynamics of charge separation from second excited state and following charge recombination in zinc-porphyrin-acceptor dyads.

Intramolecular charge separation from the second singlet excited state of directly linked Zn-porphyrin-imide dyads and following charge recombination into the first singlet excited and the ground states has been investigated in the framework of a model incorporating four electronic states (the first and the second singlet excited, the charge separated, and the ground states) as well as their vibrational sublevels. Kinetics of the transitions between these states are described in terms of the stochastic point-transition approach involving reorganization of a number of high frequency vibrational modes. The influence of the model parameters (the number of high frequency vibrational modes, the magnitude of the reorganization energies of the medium and the high frequency intramolecular vibrations, the solvent polarity) on the kinetics of population of the second and first singlet excited states as well as the charge separated state has been investigated.

What factors control product yield in charge separation reaction from second excited state in zinc-porphyrin derivatives?

Intramolecular charge separation from the second singlet excited state of directly linked Zn-porphyrin-imide dyads and following charge recombination into the first singlet excited state has been investigated in the framework of a model involving three electronic states (the first and the second singlet excited and charge separated states) as well as their vibrational sublevels. Kinetics of the transitions between these states are described in terms of the stochastic point-transition approach. The influence of the model parameters (free energy change of charge separation, magnitude of the reorganization energies of the medium and the high frequency intramolecular vibrations, the rate of relaxation of the medium and the intramolecular high frequency vibrational mode) on the kinetics of population of both the charge separated and the first singlet excited states has been explored.

Numerical simulations of ultrafast charge separation dynamics from second excited state of directly linked zinc-porphyrin-imide dyads and ensuing hot charge recombination into the first excited state.

A model of the intramolecular charge separation from the second singlet excited-state of directly linked Zn-porphyrin-imide dyads and following charge recombination into the first singlet excited-state has been constructed and investigated. The model incorporates three electronic states (the first and the second singlet excited and charge separated states) as well as their vibrational sublevels. Dynamics of the transitions between these states are described in the framework of the stochastic point-transition approach.

Solvent and spectral effects in the ultrafast charge recombination dynamics of excited donor-acceptor complexes.

The charge recombination dynamics of excited donor-acceptor complexes consisting of hexamethylbenzene (HMB), pentamethylbenzene (PMB), and isodurene (IDU) as electron donors and tetracyanoethylene (TCNE) as electron acceptor in various polar solvents has been investigated within the framework of the stochastic approach. The model accounts for the reorganization of intramolecular high-frequency vibrational modes as well as for the solvent reorganization. All electron-transfer energetic parameters have been determined from the resonance Raman data and from the analysis of the stationary charge transfer absorption band, while the electronic coupling has been obtained from the fit to the charge recombination dynamics in one solvent.

Effect of high-frequency modes and hot transitions on free energy gap dependence of charge recombination rate.

The charge recombination (CR) dynamics of geminate ion pairs formed by excitation of the ground-state donor-acceptor complexes in polar solvent have been investigated within the framework of stochastic approach. It is shown that for low exergonic reactions these dynamics critically depend on the reorganization energy of intramolecular high-frequency mode. Even moderate reorganization energies (0.