Computational Study of Charge-Transfer Dynamics in the Carotenoid-Porphyrin-C60 Molecular Triad Solvated in Explicit Tetrahydrofuran and Its Spectroscopic Signature

We investigated the charge-transfer dynamics between distinctive excited states of a carotenoid-porphyrin-C-60 molecular triad in tetrahydrofuran solvent. Our approach combines all-atom molecular dynamics simulations with an explicit solvent and electronic-state-specific force fields with a recently proposed hierarchy of approximations based on the linearized semiclassical method. The validity of the second-order cumulant approximation, which leads to a Marcus-like expression for the rate constants, was established by comparing the rate constants calculated with and without resorting to this approximation. We calculated the rate constants between the porphyrin-localized pi pi* state, porphyrin-to-C-60 charge-transfer state, and carotenoid-to-C-60 charge-separated state for the bent and linearly extended conformations. In agreement with our earlier finding, the charge separation was found to occur via a two-step mechanism, where the second step is switched on by the bent-to-linear conformational change. By comparing the rate constants calculated for a flexible and a rigid triad molecule, while allowing the solvent molecules to fluctuate, we showed that the charge-transfer process is driven by the solvent, rather than by the triad's intramolecular degrees of freedom. We further calculated the triad's amide I stretch frequency distributions and found them to be highly sensitive to the electronic state, thereby demonstrating the possibility of monitoring charge-transfer dynamics in this system via UV-vis/IR pump-probe spectroscopy.