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

Abstract

We investigated the charge-transfer dynamics between distinctive excited states of a carotenoid–porphyrin–C60 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 ππ* state, porphyrin-to-C60 charge-transfer state, and carotenoid-to-C60 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.

Photoinduced Charge Transfer Dynamics in the Carotenoid–Porphyrin–C60 Triad via the Linearized Semiclassical Nonequilibrium Fermi’s Golden Rule

“…This has already been observed for Marcus theory rate constants in Ref. 44. However, the initial rate coefficient in the rigid bent triad is higher than that in the flexible bent triad, which can be traced back to structural differences in the equilibrated ground state.…”

“…33,43 Recently, we used the LSC E-FGR to estimate the CT rate constants between the three excited states of the carotenoid-porphyrin-C 60 (CPC 60 ) molecular triad solvated in explicit tetrahydrofuran (THF). 44 It was found that the CT rate constants are strongly dependent on the conformation of the triad, and that CT in the linear conformation is faster than in the bent conformation. 15,44 It was also found that CT is driven by the solvent DOF, rather than by the intramolecular triad DOF.…”

Benchmarking Quasiclassical Mapping Hamiltonian Methods for Simulating Electronically Nonadiabatic Molecular Dynamics

“…Electronically nonadiabatic dynamical processes constitute an important class of inherently quantum-mechanical chemical phenomena that range from electronic energy and charge transfer to photochemistry. [1][2][3][4][5][6][7][8][9][10][11][12] Electronically nonadiabatic dynamics also underlie the functionality of many technologically-and biologically-relevant systems ranging from photovoltaic devices [12][13][14][15] to the photosynthetic reaction center. [16][17][18] The prohibitive computational cost of quantum-mechanically exact simulations of electronically nonadiabatic dynamics in complex molecular systems [19][20][21][22] has led to the development of a wide variety of approximate approaches, 23 including the Ehrenfest method, 24 surface hopping methods, [25][26][27][28][29][30][31][32][33][34][35][36] the mixed quantum-classical Liouville method, [37][38][39][40][41][42][43]…”

Direct Non-adiabatic Simulations of the Photoinduced Charge Transfer Dynamics

“…[5][6][7] Like the natural process, there are many artificial systems in which the photoinduced charge transfer plays a crucial role. [8][9][10][11][12][13][14][15] Examples include PICT in dye-sensitized solar cells, [8][9][10][11][15][16][17] ultra-fast charge-transfer in organic photovoltaic systems, [12][13][14]18,[18][19][20][21][22][23][24][25] photocatalytic electron/hole transfer in "colloidal quantum dot-organic molecule complex" interfaces, [26][27][28] and photoinduced proton-coupled electron transfer. [29][30][31] Understanding the detailed charge transfer dynamics will provide valuable mechanistic insights and design principles for next-generation photocatalytic devices, and profoundly impact energy production and catalysis.…”

“…Our approach uses the fewest switches surface hopping algorithm 32,33 to capture the influence of nuclear vibrations on the electronic nonadiabatic transitions, and the single-particle timedependent Kohn-Sham (TDKS) approach 8,34,35 to describe the quantum mechanical state of the transferring electron. With this approach, we investigate the non-adiabatic PICT dynamics in phthalocyanine dimer/fullerene system 12,18,36 and Carotenoid-Porphyrin-C 60 (CPC) triad system, 13,19,22,23,[37][38][39] which are model systems for understanding the CT dynamics in organic photovoltaics. Our results are in excellent agreement with both experimental measurements and highlevel (yet expensive) theoretical calculations.…”