Electronic Characteristics and Charge Transport Mechanisms for Large Area Aromatic Molecular Junctions

This paper reports the electron transport characteristics of carbon/molecule/Cu molecular junctions, where aromatic molecules (azobenzene or AB and nitroazobenzene or NAB) are employed as the molecular component. It is shown that these devices can be made with high yield (>90%), display excellent reproducibility, and can withstand at least 1.5 × 10 9 potential cycles and temperatures of at least 180°C. Transport mechanisms are investigated by analysis of current density/voltage (J-V) curves as a function of the molecular layer thickness and temperature. Results show that J decreases exponentially with thickness, giving a measured value for the low-bias attenuation factor ( ) of 2.5 ( 0.1 nm -1 for AB and NAB. In addition, it is shown that transport is not thermally activated over a wide range of temperatures (5-450 K) and that the appearance of a thermally "activated" region at higher temperatures can be accounted for by the effect of temperature on the distribution of electrons around the Fermi level of the contact(s). These results indicate that quantum mechanical tunneling is likely the mechanism for charge transport in these junctions. Although application of the Simmons tunneling model leads to transport parameters consistent with nonresonant tunneling, the parameters obtained from fitting experimental data indicate that the barrier height and/or shape, effective mass, and dielectric constant (ε) can all change with thickness. Experimental measurements of ε and density functional theory (DFT) calculations of molecular energy levels and polarizability support these conclusions. Finally, the implications of the transport mechanisms are discussed from the viewpoint of designing functional molecular electronic devices

HOMO–LUMO energy gap control in platinum(ii) biphenyl complexes containing 2,2′-bipyridine ligands

A series of platinum(II) biphenyl 2,2’-bipyridine complexes containing electron-donating and electron-withdrawing moieties on the 4 and 4’ positions of the bipyridine ligand exhibit emission from excited states in the 600 nm region of the spectrum upon excitation in the metal-to-ligand charge transfer transition located near 450 nm. These complexes are distorted from planarity based on both single crystal structure determinations and density functional theory (DFT) calculations of isolated molecules in acetonitrile. The DFT also reveals the geometry of the lowest-lying triplet state (LLTS) of each complex that is important for emission behavior. The LLTS are assigned based on the electron spin density distributions and correlated with the singlet excited states to understand the mechanism of electronic excitation and relaxation. Timedependent DFT calculations are performed to compute the singlet excited state energies of these complexes so as to help interpret their UV-Vis absorption spectra. Computational and experimental results, including absorption and emission energy maxima, electrochemical reduction potentials, LLTS, singlet excited states, and LUMO and HOMO energies, exhibit linear correlations with the Hammett constants for para-substituents σp. These correlations are employed to screen complexes that have not yet been synthesized. The correlation analysis indicates that electronic structure and the HOMO-LUMO energy gap in Pt(II) complexes can be effectively controlled using electron-donating and electron-withdrawing moieties covalently bonded to the ligands. The information presented in this paper provides analysis and better understanding of the fundamental electronic and thermodynamic behavior of these complexes and could be used to design systems with specific applications

UV Stimulated Manganese Dioxide for the Persulfate Catalytic Degradation of Bisphenol A

One of the most commonly produced industrial chemicals worldwide, bisphenol A (BPA), is used as a precursor in plastics, resins, paints, and many other materials. It has been proved that BPA can cause long-term adverse effects on ecosystems and human health due to its toxicity as an endocrine disruptor. In this study, we developed an integrated MnO2/UV/persulfate (PS) process for use in BPA photocatalytic degradation from water and examined the reaction mechanisms, degradation pathways, and toxicity reduction. Comparative tests using MnO2, PS, UV, UV/MnO2, MnO2/PS, and UV/PS processes were conducted under the same conditions to investigate the mechanism of BPA catalytic degradation by the proposed MnO2/UV/PS process. The best performance was observed in the MnO2/UV/PS process in which BPA was completely removed in 30 min with a reduction rate of over 90% for total organic carbon after 2 h. This process also showed a stable removal efficiency with a large variation of pH levels (3.6 to 10.0). Kinetic analysis suggested that 1O2 and SO4

DFT, molecular docking, and ADME/Tox screening investigations of market‑available drugs against SARS‑CoV‑2

A series of drugs was investigated to determine structural, electronic and pharmacological properties, as well as the molecular affinity for the main protease of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). The drugs were submitted to density functional theory calculations to optimize structures and predict binding preferences. The optimized geometries were used in molecular docking simulations. In the docking study, the receiver was considered rigid and the drugs flexible. The Lamarckian genetic algorithm with global search and Pseudo-Solis and Wets with local search were adopted for docking. Absorption, distribution, metabolism, excretion and toxicological properties were obtained from the Pre-ADMET online server. In this series, the antiviral atazanavir showed the potential to inhibit the main protease of SARS-CoV-2, based on the free binding energy, inhibition constant, binding interactions and its favorable pharmacological properties. Therefore, we recommend carrying out further studies with in vitro tests and subsequent clinical tests to analyze its effectiveness in the treatment of SARS-CoV-2

Modeling of bitumen fragment adsorption on Cu+ and Ag+ exchanged zeolite nanoparticles

We investigate bitumen desulfurisation on zeolite chabazite nanoparticles that contain Ag+ and Cu+ by using periodic density functional theory. The large bitumen molecules that contain thiophene derivative impurities and useful aromatic hydrocarbons cannot enter into zeolite pores but adsorb on the outer surface of the zeolite. The zeolite nanoparticle surface can be optimised for efficient impurities removal and bitumen upgrading, as we have shown recently. On chabazite nanoparticle surface, Ag+ that reside near the main channel enhance the bitumen fragment adsorption in the order benzene < thiophene < benzothiophene < dibenzothiophene. For Cu+, the bitumen fragment adsorption strength increases in the order benzene < dibenzothiophene < benzothiophene < thiophene. The different trends arise from the spatial constraint of the surface termination and the smaller ionic radius of Cu+ relative to Ag+. Our results show that zeolite surface modifications allow for stronger adsorption of thiophenes relative to hydrocarbons. Our results can be applied toward the rational design of zeolite nanoparticles for bitumen upgrading. We conclude that the preferred configurations of organic macromolecules adsorbed on zeolite outer surfaces can be safely predicted by using Fukui functions.Peer reviewed: YesNRC publication: Ye

Density functional theory calculations of selected Ru(II) two ring diimine complex dications

Full text of this article is not available in SOAR.Geometry optimization for a series of ten, two-ring diimine Ru(II) complexes was effected using the Gaussian 98 protocol at density functional theory (DFT) B3LYP level with basis sets 3-21G*and 3-21G**. HOMO-LUMO energy difference values compared favorably to the experimental data from electrochemistry [Delta E(1/2) = (E(1/2ox) - E(1/2red))] and the lowest energy absorption maxima, which for these complexes correspond to the metal-to-ligand charge transfer (MLCT) band. The HOMO and LUMO distributions from DFT support the idea that the lowest energy transitions are metal-to-ligand charge transfer and that the lowest energy LUMO for the mixed ligand complexes is located on 2,2'-bipyrazine (bpz), followed by 2,2'-bipyrimidine (bpm) and then 2,2'-bipyridine (bpy).peer reviewe

Spectroscopic Properties of [Pt2(μ-P2O5H2)4]:4- A Time-Dependent Density Functional Theory and Conductor-like Polarizable Continuum Model Investigation

Click on the DOI link to access the article (may not be free)The calculation of the singlet ground-state (SGS) and the lowest-lying triplet-state (LLTS) geometries of [Pt2(μ-P2O5H2)4]4- in the gas phase using density functional theory (DFT) produces 7% Pt−Pt bond shortening in the LLTS as compared to SGS. The transition from the Pt−Pt antibonding HOMO to the bonding LUMO+1 in the gas phase and to the bonding LUMO in water creates a metal−metal σ bond in both excited states. According to the molecular orbital population analysis in water performed using the conductor-like polarizable continuum model (CPCM) and the SGS geometry, the Pt−Pt bond arises from the overlap of the metal p orbitals. The singlet excited-state energy of 27 240 cm-1 in the gas phase is only 40 cm-1 higher than the experimental absorption energy. The first triplet excited-state energy of 22 730 cm-1 in the gas phase and 22 810 cm-1 in water correlates with the experimental phosphorescence excitation energy of 22 100 cm-1. The energy of the LLTS correlates with the experimental phosphorescence emission energy.We acknowledge the support of Kansas NSF Cooperative Agreement EPS 987432, the Wichita State University High Performance Computing Center, the Wichita State University Office of Research Administration, the Department of Energy, and Parker Fellowships (S.R.S. and J.M.V.). We also thank Dr. David M. Eichhorn for helping us to access the Inorganic Crystal Structure Database.Peer reviewe

Synthesis and photochemistry of Ru(II) complexes containing phenanthroline-based ligands with fused pyrrole rings

Full text of this article is not available in SOAR.Hydrolysis of 1,10-phenanthrolinopyrrole ethyl ester leads to the acid derivative which is unstable at room-temperature releasing CO(2) and forming 1,10-phenanthrolinopyrrole (php). The ligand reacts with ruthenium(II) to form a series of complexes of the general formula [Ru(php)(n)(bpy)(3-n)](2+), where bpy = 2,2'-bipyridine and n = 1-3. The photochemical properties reveal that the complexes have longer-lived excited states than the standard complex, [Ru(bpy)(3)](2+). Their emission lifetimes range from 9.04 micros (n = 1) to 35.5 micros (n = 3) at 77 K compared to 7.57 micros for the standard. Similarly, at room-temperature, emission lifetimes range from 1.20 micros (n = 1) to 1.70 micros (n = 3) relative to the standard (0.56 micros). The emission quantum yields also have higher values than the standard [Ru(bpy)(3)](2+) under similar conditions. The temperature-dependent studies for the complexes establish the distribution among the radiative, nonradiative, and (3)MLCT to (3)d-d decay channels and are in agreement with the energy gap law.peer reviewe

Time-Dependent Density Functional Theory Study of the Spectroscopic Properties Related to Aggregation in the Platinum(II) Biphenyl Dicarbonyl Complex

Click on the DOI link below to access the article (may not be free).Singlet ground-state geometry optimization of the monomer, four dimers, and the trimer of [Pt(bph)(CO)(2)], where bph = biphenyl dianion, was performed at the B3LYP level of density functional theory (DFT) with a mixed basis set (6-311G** on C, O, and H atoms; the Stuttgart/Dresden (SDD) effective core potential (ECP) on the Pt core; [6s5p3d] on the Pt valence shell). The aggregation was based on Pt[bond]Pt binding as well as on pi[bond]pi and electrostatic interactions. The lowest-lying triplet-state geometries of the monomer, one dimer, and the trimer of the complex were also optimized using the above theory. Significant shortening of the Pt[bond]Pt bond was recorded in the triplet state compared to the singlet one. A number of low-energy singlet and triplet allowed excited states were calculated using time-dependent density functional theory (TDDFT) and analyzed with respect to absorption, excitation, and emission spectra collected under various conditions. Simulated spectra of the monomer and dimer based on the singlet excited states were correlated with the absorption spectrum. The emission in concentrated solution was due to the triplet dimer, and the emitting states were (3)MLCT and Pt-centered states.peer reviewe