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
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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 <i>ab initio</i> 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 (<i>k</i><sub>B</sub><i>T</i>). 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