Free-Energy-Gap Law for Ultrafast Charge Recombination of Ion Pairs Formed by Intramolecular Photoinduced Electron Transfer

Equivalence between inverted regions of the energy gap law and inverted regions of donor–acceptor distances in photoinduced electron transfer processes in flavoproteins

In the present work, we discuss about the relationship between the energy gap law and extended Dutton law in flavoproteins. The extend Dutton law is defined herein as the dependence of logarithmic rates (ln Rate) of photoinduced electron transfer (ET) from aromatic amino acids to excited isoalloxazine (Iso*) on donor–acceptor distances (Rcs). Both functions of ln Rate vs. negative values of the standard free energy gap and ln Rate vs. Rc display a parabolic behavior, when the ET rates are ultrafast. The negative values of the standard free energy gap at peaks of ln Rate [X_m(ES)] were obtained for FMN-binding protein, wild-type pyranose 2-oxidase, T169S (Thr169 is replaced by Ser) pyranose 2-oxidase, and medium-chain acyl-CoA dehydrogenase. The values of Rc at peaks of ln Rate [X_m(Rc)] were also obtained for these flavoproteins. The negative values of the standard free energy gap decreased with approximate linear functions of Rc. The negative values of standard free energy gap [X_m(ESRc)] at Rc = X_m(Rc) were evaluated using the linear functions of the negative standard free energy gap with Rc. The values of X_m(ESRc) were mostly in very good agreement with the values of X_m(ES). This implies that the energy gap law and the extend Dutton law are equivalent. X_m(ES) values in ET donors displaying the linear extend Dutton law with Rc were obtained by energy gap law, and then X_m(Rc) values were evaluated with the negative standard free energy gap. Thus, the obtained X_m(Rc) values were much smaller than the Rc range obtained by the method of molecular dynamics simulation. This suggests that ET processes with linear profiles of the extend Dutton law could be parabolic when Rc becomes much shorter than the Rc range obtained by the method of molecular dynamics simulation.

Relationship between EXDL and SEGL.

The Efficiency of Photoinduced Intramolecular Charge Separation from the Second Excited State: What Factors Can Control It?

The efficiency of photoinduced charge separation (CS) in electron donor-acceptor compounds is commonly limited due to fast deactivation processes, such as the excited-state internal conversion and ultrafast hot reverse electron transfer to the acceptor, charge recombination (CR). A traditional way to avoid undesired energy losses due to CR is to put the reverse electron transfer into the Marcus inverted region, thus effectively suppressing it. This method, however, is not generally applicable when considering CS from the second locally excited state because the driving force of CR to the first excited state is small, and thus charge recombination is ultrafast and efficient. In this paper, we study the kinetic features of CS/CR from the second locally excited state of the donor using a semiclassical stochastic model of electron transfer. Particular attention is paid to the CS efficiency as well as the influence of the polar environment and intramolecular high-frequency vibrational modes on the kinetics of the charge-separated state. The influence of a number of model parameters on the CS yield and the energy efficiency has been analyzed using the results of numerical simulations. Several simple practical recipes for creating molecular compounds with high CS yields have been suggested. Simulations have also revealed a strong and non-monotonous (double-humped) dependence of both the yield and energy efficiency of CS on the driving force.

Quantum yield and energy efficiency of photoinduced intramolecular charge separation

Kinetics of photoinduced intramolecular charge separation (CS) and the ensuing ultrafast charge recombination (CR) in electron-donor-acceptor dyads are studied numerically, taking into account the excitation of charge-transfer active intramolecular vibrations and multiple relaxation time scales of the surrounding polar solvent. Both energetic and dynamic properties of intramolecular and solvent reorganization are considered, and their influence on the CS/CR kinetics and quantum yield of ultrafast CS is explored. Particular attention is paid to the energy efficiency of CS, as one of the most important parameters indicating the promise of using a molecular compound as a basis for emerging optoelectronic devices. The CS quantum yield and the energy efficiency of CS are shown to depend differently on the key model parameters. Necessary conditions for the highly efficient CS are evaluated using analytic formulae for the electron transfer rates and derived from numerical simulation data. The reasons why low-exergonic CS taking place in the Marcus normal region can be much slower than CR in the deep inverted region are discussed.

Solvation and hydrogen bonding aided efficient non-radiative deactivation of polar excited state of 5-aminoquinoline

The mechanism of efficient fluorescence quenching of 5-aminoquinoline in alcoholic solvents.

The effect of solvent relaxation time constants on free energy gap law for ultrafast charge recombination following photoinduced charge separation

Ultrafast low exergonic charge recombination following photoinduced charge separation proceeds in a non-equilibrium mode and its rate constant is nearly independent of the free energy gap.