Unraveling the Mechanism of Photoinduced Charge Transfer in Carotenoid–Porphyrin–C₆₀ Molecular Triad

Photoinduced charge transfer (CT) plays a central role in biologically significant systems and in applications that harvest solar energy. We investigate the relationship of CT kinetics and conformation in a molecular triad. The triad, consisting of carotenoid, porphyrin, and fullerene is structurally flexible and able to acquire significantly varied conformations under ambient conditions. With an integrated approach of quantum calculations and molecular dynamics simulations, we compute the rate of CT at two distinctive conformations. The linearly extended conformation, in which the donor (carotenoid) and the acceptor (fullerene) are separated by nearly 50 Å, enables charge separation through a sequential CT process. A representative bent conformation that is entropically dominant, however, attenuates the CT, although the donor and the acceptor are spatially closer. Our computed rate of CT at the linear conformation is in good agreement with measured values. Our work provides unique fundamental understanding of the photoinduced CT process in the molecular triad

Multiscale Simulation on a Light-Harvesting Molecular Triad

We have investigated the effect of solvation and confinement on an artificial photosynthetic material, carotenoid-porphyrin-C₆₀ molecular triad, by a multiscale approach and an enhanced sampling technique. We have developed a combined approach of quantum chemistry, statistical physics, and all-atomistic molecular dynamics simulation to determine the partial atomic charges of the ground-state triad. To fully explore the free energy landscape of triad, the replica exchange method was applied to enhance the sampling efficiency of the simulations. The confinement effects on the triad were modeled by imposing three sizes of spherocylindrical nanocapsules. The triad is structurally flexible under ambient conditions, and its conformation distribution is manipulated by the choice of water models and confinement. Two types of water models (SPC/E and TIP3P) are used for solvation. When solvated by SPC/E water, whose HOH angle follows an ideal tetrahedron, the structural characteristics of triad is compact in the bulk systems. However, under a certain nanosized confinement that drastically disrupts hydrogen bond networks in solvent, the triad favors an extended configuration. By contrast, the triad solvated by TIP3P water shows a set of U-shaped conformations in the confinement. We have shown that a slight structural difference in the two water models with the same dipole moment can have great distinction in water density, water orientation, and the number of hydrogen bonds in the proximity of a large flexible compound such as the triad. Subsequently, it has direct impact on the position of the triad in a confinement as well as the distribution of conformations at the interface of liquid and solid in a finite-size system

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

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>_B<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