Multiscale Simulation of the Ground and Photo-Induced Charge-Separated States of a Molecular Triad in Polar Organic Solvent: Exploring the Conformations, Fluctuations, and Free Energy Landscapes

Abstract

The approach of a multiscale simulation that combines quantum chemical calculations, classical molecular dynamics simulations, and statistical physics to integrate the information of the electronic states of a conformationally complex molecule into its structural distribution over an ensemble was performed to understand the influence of a polar organic solvent on the structural stability of carotene-porphyrin-fullerene molecular triad in both the ground and the photoinduced charge-separated excited states. The excited states of the triad were computed with the ab initio quantum chemical calculations using the algebraic diagrammatic construction through the second order correction (ADC(2)) method and the time-dependent density functional theory (TDDFT). The replica exchange molecular dynamics was used for the enhanced sampling of the ensemble in order to explore the phase space of the ground state and the photoinduced charge-separated excited state of triad in explicit tetrahydrofuran (THF) solvent. An importance sampling was strategically employed to bridge the gap between the two computational methods that aim to explore distinct realms of dynamics. We analyzed the free energy landscape, the structural fluctuations, the solvent arrangements, the static dielectric constant, and the interactions between the triad and the solvent molecules. The analysis of the free energy landscape of the triad indicates that the charge-separated excited state of the triad is thermodynamically stable in a linearly extended geometry, while the ground-state triad explores several extended and bent conformations that are populated in the local free energy minima separated by low free energy barriers at an order of thermal fluctuation (k(B)T). The stabilization of a linearly extended structure of the charge-separated excited state triad is predominantly due to the solvation interactions (van der Waals and electrostatic interactions) between the triad and the THF molecules as well as the interactions within the THF molecules. Differences in the charge distribution on a molecular triad induce slight changes in the dielectric property of THF near the triad. Our study suggests that by measuring the differences in a dielectric response of solvent near the triad, it is possible to provide insight into the population of the charge-separated electronic state of the triad relative to that of the ground state.

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

“…The model also includes electronic coupling coefficients between the excited electronic states (assumed constant within the Condon approximation and calculated via the fragment charge difference (FCD) method). The partial atomic charges for THF were adopted from ref , where they were generated via AM1-BCC . The electronic excitation energies, partial charges, and electronic coupling coefficients for the triad were adopted from ref , where they were obtained via time-dependent density functional theory (TD-DFT) using a range-separated hybrid (RSH) functional designed to yield accurate energetics for the CT states.…”

New tools for the systematic analysis and visualization of electronic excitations. I. Formalism

“…10 However, also the potential of this latter methodology has already been exploited by a number of researchers studying organic electronics and photobiology. [16][17][18][19][20] A unified formalism is present through the use of one-particle density matrices (1DM) and transition density matrices, a common ground, which allows for a clear comparison between the different methods and suggests extensions leading to a wide variety of routines for population analysis, as well as, orbital and density plotting, which are provided in this implementation. To our knowledge, a similarly comprehensive analysis toolbox has not been made available so far, let alone in a correlated ab initio framework.…”

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

“…As such, Marcus theory provides a robust, flexible, and convenient platform for calculating CT rate constants in complex condensed-phase systems. ,− However, Marcus theory cannot account for the effects caused by the nonequilibrium nature of the initial state in photoinduced CT. Such effects are expected to be important when the timescale of relaxation to thermal equilibrium on the donor PES is similar to or longer than the timescale on which the electronic transition occurs. , …”

“…Such effects are expected to be important when the timescale of relaxation to thermal equilibrium on the donor PES is similar to or longer than the timescale on which the electronic transition occurs. 35,36 In contrast, the nonequilibrium Fermi's golden rule (NE-FGR) is designed to account for effects caused by the nonequilibrium nature of the initial state. 37−42 Furthermore, combining the NE-FGR with the linearized semiclassical (LSC) approximation has made it possible to apply the NE-FGR to complex condensed-phase systems described by allatom anharmonic Hamiltonians.…”