Photochemical Reversibility of Ring-Closing and Ring-Opening Reactions in Diarylperfluorocyclopentenes

Time dependent density functional theory (TDDFT) is used to study the important factors that control the photoisomerization of diarylperfluorocyclopentenes. The calculations are carried out for free molecules and for diarylperfluorocyclopentenes perturbed by gold atoms. Potential energy surfaces for the cyclization reaction are obtained for the ground state and for the excited states involved in the photoswitching. Analysis of the computed UV/vis spectra, the excitation energies, and the spatial distribution of the frontier orbitals of both unperturbed and perturbed molecules give an inside view of the ring opening and the ring closing. The bonding interaction in the unoccupied orbials is considered to be the driving force for the photochemical cyclization while the antibonding interaction significantly hinders the reaction. The obtained theoretical results are in good agreement with the experimental data and provide an explanation of the one-directional and bidirectional photoswitching of diarylperfluorocyclopentenes attached to gold surface

Orbital Control of the Conductance Photoswitching in Diarylethene

Diarylethenes are photosensitive π-conjugated molecules whose application to various molecular devices is expected. The molecular and electronic structures of diarylethenes are switchable upon photoirradiation with their reversible structural isomerization. Site-specific electron transport phenomena through a diarylethene molecule, 1,2-di(2-methyl-1-naphthyl) perfluorocyclopentene, are studied by using the nonequilibrium Green’s function method combined with the Hückel molecular orbital method (NEGF−HMO) and density functional theory (NEGF−DFT). On the basis of the orbital symmetry rule, the conductance of the diarylethene is predicted to be efficiently switchable when the C3 and C10 atoms are appropriately connected with electrodes. Transmission spectra, spatial distribution of the MPSH (molecular projected self-consistent Hamiltonian) states, and I−V curves are obtained from DFT calculations. These results obtained from the higher-level DFT calculations are consistent with the prediction based on the qualitative frontier orbital analysis at the HMO level of theory. The computed current for the closed-ring form of the 3-10 connection is about 3 orders of magnitude high compared with those for other connections. The phase, amplitude, and spatial distribution of the frontier orbitals play an essential role in designing the electron transport properties through the photoswitching system

Conductance through Short DNA Molecules

The conductance through short DNA molecules connected to gold electrodes is studied with density functional theory and nonequilibrium Green’s function method combined with density functional theory. The anchoring of the molecules to the electrodes is investigated, and in addition to the covalent S−Au bond, weak interactions between the aromatic heterocyclic bases and the electrodes are found. These weak interactions are important for the electron transport through DNA molecules. A tunneling mechanism is suggested, and the conductive properties of the nucleotides in a metal−molecule−metal junction are compared. Different four-nucleotide DNA sequences are investigated. A significant value for the current, 20 pA, is calculated for 1.5 V applied bias for a DNA sequence consisting of guanine and cytosine nucleotides. It is shown that adenine-thymine nucleotide pairs introduce potential barriers for the electron transport and therefore significantly decline the conductance. The obtained results are compared with recent experimental observations (Nanotechnology 2009, 20, 115502) and confirm the possibility for electron transport through DNA molecules as well as provide an explanation for the reduced conductance through DNA sequences, which contain adenine-thymine nucleotide pairs. The results are compared with a previous theoretical study, performed with the extended Hückel method (ChemPhysChem 2003, 4, 1256), which reports low conductance for DNA molecules. The difference in the conclusions is due to the applied bias self-consistent field calculations used in the recent study, which take into account the changes of the transmission probabilities with the bias

Photoswitching of Conductivity through a Diarylperfluorocyclopentene Nanowire

The optical photoswitching of conductivity of a diarylperfluorocyclopentene nanowire is investigated using Green's function method combined with density functional theory. A model closer to the real molecular electronic device is considered with relaxation of the molecular geometry under the interaction with external electric field. The ratio of conductance for the closed- and open-ring forms is on the order of magnitude 102. The influence of the HOMO−LUMO gaps and the spatial distributions of frontier molecular orbitals on the quantum transport through the molecular wire is investigated

Curvature Effect in Polydimethylsiloxane Interaction with CO₂. Insights from Theory

In this study we employ density functional theory to investigate the binding interaction between polydimethylsiloxane and CO2 for application in gas separation membranes. The binding strength has been studied systematically as a function of the monomer conformational rotations in the polymer chain. Our work identified major differences between the CO2 interaction with the helical conformation and the linear conformation of polydimethylsiloxane polymer chains. We have further estimated dependence between the CO2 binding strength and the polydimethylsiloxane polymer chain curvature by systematically evaluating the CO2 binding to cyclic polydimethylsiloxane oligomers. The enhanced CO2 interaction with helical chains and cyclic oligomers was attributed to cooperative, confinement effects, and local electron density distribution at the Si–O–Si fragments. The binding modes were identified using vibration frequency analysis

Orbital Views of the Electron Transport through Polycyclic Aromatic Hydrocarbons with Different Molecular Sizes and Edge Type Structures

In this work electron-transport properties of π-conjugated polycyclic aromatic hydrocarbons with different molecular sizes and edge type structures are investigated. The applicability of a derived concept for orbital control of electron transport (J. Am. Chem. Soc. 2008, 130, 9406) is tested on larger hydrocarbons in order to estimate its predictive power for different types of compounds. Favorable connections for effective electron transport in π-conjugated systems with weak coupling between the molecules and electrodes are predicted on the basis of the orbital symmetry rule by looking at the phase and amplitude of the frontier orbitals. Qualitative predictions based on frontier orbital analysis are compared with density functional theory calculations for realistic molecular junctions with strong covalent bonds between a molecule and two gold electrodes. Obtained results are in good agreement with the orbital symmetry rule predictions, which makes the frontier orbitals’ analysis a powerful tool in electron transport studies in π-conjugated polycyclic aromatic hydrocarbons

Immobilizing Metal Nanoparticles on Single Wall Nanotubes. Effect of Surface Curvature

One to several nanometer-size nanoparticles possess supreme catalytic activity for a variety of important synthetic reactions compared to larger particles and bulk surfaces. However, a significant drawback is the catalyst durability as small, active nanoparticles tend to merge to form larger, less active nanocolloids. Tailoring the nanoparticle–surface support interaction could provide a means to limit nanoparticle mobility and thus prevent aggregation. In this study, we demonstrate the stabilization of fine-metal nanoparticles on nanotube surfaces by manipulation of surface curvature. Systematic density functional theory calculations of a large variety of nanoparticle–nanotube complexes revealed that the nanoparticle–nanotube binding interaction depends on, and can be controlled by, the surface curvature. Thus, an effective mechanism is demonstrated for the immobilization of small metal clusters with high catalytic activity on support surfaces. Furthermore, we provide experimental verification of our theory by comparing the aggregation of palladium nanoparticles decorating carbon nanotube and graphene surfaces as a function of time. Our theoretical predictions and experimental observations provide fundamental understanding to the physics of nanoparticle–support interaction and demonstrate how tailoring the support geometry can improve the durability of high-performance nanocatalysts

Theoretical Study of Donor−π-Bridge−Acceptor Unimolecular Electric Rectifier

The electrical rectifying properties of a single-molecule nanowire from the type donor−π-bridge−acceptor are investigated by means of the nonequilibrium Green's function method, combined with density functional theory (NEGF−DFT). The investigated nanowire is an oligo-1,4-phenylene ethylene with π-donor and π-acceptor groups attached on opposite sides of the molecule. The donor and acceptor wires are separated by a π-bridge, in contrast to the Aviram−Ratner rectifier, which is a donor−σ-bridge−acceptor diode. A model more similar to the real molecular electronic device is considered with relaxation of the molecular geometry, under the interaction with external electric field, taking into account its influence on the electronic properties of the nanowire. An asymmetric current−bias (I−V) diagram is observed, with a conductance ratio of 7. The analysis of the spatial distribution of frontier orbitals, the highest occupied molecular orbital−lowest unoccupied molecular orbital ( HOMO−LUMO) gaps, and the transmission spectra give an inside view of the observed results

Orbital Views of the Electron Transport in Molecular Devices

Extended π-conjugated molecules are interesting materials that have been studied theoretically and experimentally with applications to conducting nanowire, memory, and diode in mind. Chemical understanding of electron transport properties in molecular junctions, in which two electrodes have weak contact with a π-conjugated molecule, is presented in terms of the orbital concept. The phase and amplitude of the HOMO and LUMO of π-conjugated molecules determine essential properties of the electron transport in them. The derived rule allows us to predict single molecules’ essential transport properties, which significantly depend on the type of connection between a molecule and electrodes. Qualitative predictions based on frontier orbital analysis about the site-dependent electron transport in naphthalene, phenanthrene, and anthracene are compared with density functional theory calculations for the molecular junctions of their dithiolate derivatives, in which two gold electrodes have strong contact with a molecule through two Au−S bonds

Orbital Determining Spintronic Properties of a π‑Conjugated System

Spintronic properties of cyclobutadiene (CBD) systems are investigated based on a qualitative frontier orbital analysis. CBD undergoes a Jahn–Teller distortion from the square triplet state to the rectangular singlet state. According to the qualitative Hückel molecular orbital analysis, the electron transport through the square triplet state is symmetry allowed, whereas that through the rectangular singlet state is symmetry forbidden. The magnetic triplet state is a possible coexisting system of conductivity and magnetism. Sophisticated first-principles quantum chemical calculations are performed by using a realistic molecular junction model. Obtained results are in good agreement with the prediction based on the qualitative orbital analysis. Interesting spin filtering properties are found in the square-shaped CBD system. The high- and low-spin states of the square-shaped CBD system produce the spin-α and spin-β polarized conductance, respectively. The qualitative orbital analysis is useful as a guiding principle for designing molecular spintronics