Multiscale Simulation on a Light-Harvesting Molecular Triad

Abstract

We have investigated the effect of solvation and confinement on an artificial photosynthetic material, carotenoid-porphyrin-C(60) 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.

On the Interplay between Electronic Structure and Polarizable Force Fields When Calculating Solution-Phase Charge-Transfer Rates

“…In this paper, we investigate the effect of the aforementioned choices on the rate constants of the multiple CT processes that occur in the carotenoid–porphyrin–C 60 (CPC 60 ) molecular triad dissolved in explicit tetrahydrofuran (THF) liquid solvent (see Figure a). This system has received much recent attention as a model for understanding photoinduced CT processes similar to those occurring in photosynthetic reaction centers and organic photovoltaic devices. − In a recent paper, we calculated the rate constants for the various CT processes that occur in this system using nonpolarizable force fields (NFFs) with Mulliken partial charges obtained from time-dependent density functional theory (TDDFT) with the Baer–Neuhauser–Livshits (BNL) density functional . In this paper, we investigate the robustness of the results we obtained in ref by comparing them to results obtained by using polarizable force fields (PFFs), , using the iterative restrained electrostatic potential method (i-RESP) partial charges based on TDDFT with either BNL or B3LYP functionals.…”

“…The CT dynamics in the above-mentioned solvated molecular triad system is strongly dependent on the conformation of the triad and is much faster in the linear conformation than in the bent conformation ,,, (see the conformations illustrated in Figure b). Accounting for this calls for treating the different conformations as individual chemical species, each of which associated with a corresponding force field. , It should also be noted that CT in this system was found to be driven by the solvent degrees of freedom (DOF), rather than by the intramolecular triad DOF . Thus, solvent polarization effects are expected and their effect on the CT dynamics is of interest.…”

Confined organization of fullerene units along high polymer chains

“…Molecular dynamics simulations were performed using AMBER 11 (ref. 59) and the general AMBER force eld (GAFF) 60,61 for polymers with three different lengths of spacers, m ¼ 3 (4d), m ¼ 6 (4e), and m ¼ 12 (4f). The 2-decyl-tetradecanyl solubilizing group, which is less computationally demanding than the polystyrene side chain, was used in the simulations for all polymers.…”

The effect of structural changes on charge transfer states in a light-harvesting carotenoid-diaryl-porphyrin-C60 molecular triad

“…On the other hand, a Coulombic interaction between the negatively charged C 60 fullerene and the positively charged carotenoid tail in the excited state may induce significant structural changes which can move the carotenoid chain into a closer distance to the fullerene component. Experimental evidence for such a folded conformer comes from a study of the triad in micelle nanoreactors suspended in water, where contractions in the molecular volume of the triad were attributed to entropy changes arising from solvent movements and possible conformational changes upon photoinduced electron transfer in generating a dipole of ~110 D. 66 The conformational changes of the triad in water were studied by Cheung et al 67 using classical molecular dynamics simulations. This study has brought out that the linear structure favored in the gas phase is one of the least populated conformational states, which shows that the triad may undergo significant conformational changes in solution.…”