<i>trans</i>-β^2,3-Amino Acid-Based Supramolecular Synthons for Probing the Interrelationships between Structure, Torsion-Directed Assembly, and Isomorphism

Variations in the lattice arrangement and the tendency toward isomorphic behavior in a group of trans-β2,3 amino acid derivatives with Boc and oxazolidinone moieties at the N- and C-terminals are discussed. The substitution pattern at the α- and β positions in these systems was found to give different torsional preferences and hence different molecular organizations in their crystals. Analysis of such preferences in their azide analogs has unraveled the involvement of a relatively uncommon carbonyl−azide dipolar interaction in lattice stabilization

<i>trans</i>-β^2,3-Amino Acid-Based Supramolecular Synthons for Probing the Interrelationships between Structure, Torsion-Directed Assembly, and Isomorphism

Variations in the lattice arrangement and the tendency toward isomorphic behavior in a group of trans-β2,3 amino acid derivatives with Boc and oxazolidinone moieties at the N- and C-terminals are discussed. The substitution pattern at the α- and β positions in these systems was found to give different torsional preferences and hence different molecular organizations in their crystals. Analysis of such preferences in their azide analogs has unraveled the involvement of a relatively uncommon carbonyl−azide dipolar interaction in lattice stabilization

Structural and Electronic Properties of Bare and Capped Cd_<i>n</i>Se_<i>n</i>/Cd_<i>n</i>Te_<i>n</i> Nanoparticles (<i>n</i> = 6, 9)

Relationships between structures and properties (energy gaps, vertical ionization potentials (IPv), vertical electron affinities (EAv), and ligand binding energies) in small capped CdSe/CdTe nanoparticles (NPs) are poorly understood. We have performed the first systematic density functional theory (DFT) (B3LYP/Lanl2dz) study of the structural (geometries and ligand binding energies) and electronic (HOMO/LUMO energy gaps, IPsv, and EAsv) properties of CdnSen/CdnTen NPs (n = 6, 9), both bare and capped with NH3, SCH3, and OPH3 ligands. NH3 and OPH3 ligands cause HOMO/LUMO energy destabilization in capped NPs, more pronounced for the LUMOs than for the HOMOs. Orbital destabilization drastically reduces both the IPv and EAv of the NPs compared with the bare species. For SCH3-capped Cd6X6 NPs, formation of expanded structures was found to be preferable to crystal-like structures. SCH3 groups cause destabilization of the HOMOs of the capped NPs and stabilization of their LUMOs, which indicates a reduction of the IPv of the capped NPs compared with the bare species. For the Cd9X9 NPs, similar trends in stabilization/destabilization of frontier orbitals were observed in comparison with the capped Cd6X6 species. Also, pinning of the HOMO energies was observed for the NH3- and SCH3-capped NPs as a function of a NP size

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

Two-Electron Transfer Pathways

The frontiers of electron-transfer chemistry demand that we develop theoretical frameworks to describe the delivery of multiple electrons, atoms, and ions in molecular systems. When electrons move over long distances through high barriers, where the probability for thermal population of oxidized or reduced bridge-localized states is very small, the electrons will tunnel from the donor (D) to acceptor (A), facilitated by bridge-mediated superexchange interactions. If the stable donor and acceptor redox states on D and A differ by two electrons, it is possible that the electrons will propagate coherently from D to A. While structure–function relations for single-electron superexchange in molecules are well established, strategies to manipulate the coherent flow of multiple electrons are largely unknown. In contrast to one-electron superexchange, two-electron superexchange involves both one- and two-electron virtual intermediate states, the number of virtual intermediates increases very rapidly with system size, and multiple classes of pathways interfere with one another. In the study described here, we developed simple superexchange models for two-electron transfer. We explored how the bridge structure and energetics influence multielectron superexchange, and we compared two-electron superexchange interactions to single-electron superexchange. Multielectron superexchange introduces interference between singly and doubly oxidized (or reduced) bridge virtual states, so that even simple linear donor–bridge–acceptor systems have pathway topologies that resemble those seen for one-electron superexchange through bridges with multiple parallel pathways. The simple model systems studied here exhibit a richness that is amenable to experimental exploration by manipulating the multiple pathways, pathway crosstalk, and changes in the number of donor and acceptor species. The features that emerge from these studies may assist in developing new strategies to deliver multiple electrons in condensed-phase redox systems, including multiple-electron redox species, multimetallic/multielectron redox catalysts, and multiexciton excited states

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