Modeling Charge Transfer Reactions by Hopping between Electronic Ground State Minima: Application to Hole Transfer between DNA Bases

Abstract

In this paper, we extend the previously described general model for charge transfer reactions, introducing specific changes to treat the hopping between energy minima of the electronic ground state (i.e., transitions between the corresponding vibrational ground states). We applied the theoretical–computational model to the charge transfer reactions in DNA molecules which still represent a challenge for a rational full understanding of their mechanism. Results show that the presented model can provide a valid, relatively simple, approach to quantitatively study such reactions shedding light on several important aspects of the reaction mechanism.

Unveiling the Excited State Dynamics of Indole in Solution

“…Note, however, that all such unperturbed excited state minima correspond to three different energy minima for the aqueous indole electronic first excited state (the perturbed electronic first excited state): i.e., in each minimum well (conformational basin) the perturbed first electronic excited state corresponds to either the L b , πσ*, or L a state, respectively. 25 In previous papers, 17,18 we derived in detail a general model for treating diabatic state transitions, focusing specifically on charge transfer reactions 17 modeled via using the unperturbed electronic states as the proper diabatic states. In this paper, we apply the same theoretical−computational model using the L b , πσ*, and L a unperturbed electronic states as the appropriate diabatic states to reconstruct the kinetics of the perturbed electronic first excited state relaxation (energy transfer) in aqueous indole.…”

“…In the last equations, χ e corresponds to the electronic adiabatic fraction as obtained at the first crossing encountered by the reactive trajectories and ξ i 2 = |⟨φ R,0 0 |φ P,i 0 ⟩| 2 is the squared (unperturbed) vibrational state overlap of the R (ground) and P (ith) vibrational states (see Figure 3), as obtained by assuming for the R and P diabatic states identical vibrational modes and frequencies with wave functions of corresponding modes differing only for their minimum energy positions. 17,18 Therefore, when considering the first crossing for all the reactive trajectories (providing different values of χ e ), we obtain by averaging over all the crossing trajectories…”

“…A A , with such a Gaussian range including the diabatic energy crossing, i.e., = 0 defining the first crossing encountered by the R A trajectories (note that the R A subscript of the angle brackets indicates that averaging is performed in the R A equilibrium ensemble). In fact, considering that within each TR we have a virtually unidirectional flux, we obtain 18…”

“…For example, the exact mechanism of the excited electronic state relaxation (energy transfer) in polar solvents, such as water is still a matter of debate. This paper will exploit the general model for treating electronic state transitions, focused specifically on charge transfer and intersystem crossing reactions, developed by our group in the past, 17,18 to address the above-mentioned relaxation of the excited electronic states of aqueous indole, focusing on the thermodynamics and kinetics of the deactivation processes at the basis of the fluorescence properties of this model molecule. Such an approach has provided accurate results at significantly less computational expense when compared to the more commonly used strategies relying on the calculation of an ensemble of QM/MM trajectories.…”

“…This paper will exploit the general model for treating electronic state transitions, focused specifically on charge transfer and intersystem crossing reactions, developed by our group in the past, , to address the above-mentioned relaxation of the excited electronic states of aqueous indole, focusing on the thermodynamics and kinetics of the deactivation processes at the basis of the fluorescence properties of this model molecule. Such an approach has provided accurate results at significantly less computational expense when compared to the more commonly used strategies relying on the calculation of an ensemble of QM/MM trajectories. − In fact, the high cost of this latter approach usually limits its applicability to the picosecond time scale, making it unsuitable for the current study.…”