Predicting Finite-Bias Tunneling Current Properties from Zero-Bias Features: The Frontier Orbital Bias Dependence at an Exemplar Case of DNA Nucleotides in a Nanogap

The electrical current properties of single-molecule sensing devices based on electronic (tunneling) transport strongly depend on molecule frontier orbital energy, spatial distribution, and position with respect to the electrodes. Here, we present an analysis of the bias dependence of molecule frontier orbital properties at an exemplar case of DNA nucleotides in the gap between H-terminated (3, 3) carbon nanotube (CNT) electrodes and its relation to transversal current rectification. The electronic transport properties of this simple single-molecule device, whose characteristic is the absence of covalent bonding between electrodes and a molecule between them, were obtained using density functional theory and non-equilibrium Green’s functions. As in our previous studies, we could observe two distinct bias dependences of frontier orbital energies: the so-called strong and the weak pinning regimes. We established a procedure, from zero-bias and empty-gap characteristics, to estimate finite-bias electronic tunneling transport properties, i.e., whether the molecular junction would operate in the weak or strong pinning regime. We also discuss the use of the zero-bias approximation to calculate electric current properties at finite bias. The results from this work could have an impact on the design of new single-molecule applications that use tunneling current or rectification applicable in high-sensitivity sensors, protein, or DNA sequencing.

Tuning the Acceptor Unit of Push–Pull Porphyrazines for Dye-Sensitized Solar Cells

A family of four push–pull porphyrazines of A3B type, where each unit A contains two peripheral propyl chains and the unit B is endowed with a carboxylic acid, were prepared. The carboxylic acid was attached to the b-position of the pyrrolic unit, either directly (Pz 10), or through cyanovinyl (Pz 11) and phenyl (Pz 7) groups. The fourth Pz (14) consisted in a pyrazinoporphyrazine wherein the dinitrogenated heterocycle provided intrinsic donor–acceptor character to the macrocycle and contained a carboxyphenyl substituent. The direct attachment of the carboxylic acid functions and their linkers to the porphyrazine core produces stronger perturbation on the electronic properties of the macrocycle, with respect to their connection through fused benzene or pyrazine rings in TT112 and 14, respectively. The HOMO and LUMO energies of the Pzs, which were estimated with DFT calculations, show little variation within the series, except upon introduction of the cyanovinyl spacer, which produces a decrease in both frontier orbital energetic levels. This effective interaction of cyanovinyl substitution with the macrocycle is also evidenced in UV/Vis spectroscopy, where a large splitting of the Q-band indicates strong desymmetrization of the Pz. The performance of the four Pzs as photosensitizers in DSSCs were also investigated.

Biodegradation Mechanism of Anionic Polyacrylamide in Coal Preparation Using Molecular Simulation

Biodegradation of anionic polyacrylamide (HPAM) and polyacrylate (PAA) by key enzymes, such as amidase and bacterial laccase, have been reported. However, the interaction mechanism between HPAM or PAA and enzymes is still poorly unclear. Here, docking study was undertook to demonstrate the binding modes and interaction details for degradation of HPAM or PAA. Then, bioactivities between PAA and HPAM were compared with frontier orbital theory. The docking results showed that HPAM completely buried in pocket of Rhodococcus sp. N-771 amidase (Rh Amidase), while most of PAA molecule exposed outside pocket of Bacillus subtilis laccase ( B. subtilis laccase ), further suggesting PAA was much more difficult to degrade than HPAM. Hydrophobic interactions and hydrogen bonds were necessary for stabilizing HPAM-Rh Amidase or PAA- B. subtilis laccase complex. The frontier orbital analysis indicated that bioactivity of PAA was higher than that of PAA. These findings provide an insight into enzyme-catalyzed degradation of HPAM. It is helpful in designing highly efficient enzymes against HPAM or PAA to protect environment.

Group VIII coordination complexes of bidentate P^N ligands bearing π-extended quinoline or phenanthridine N-heterocycles

Group 8 complexes of π-extended N-heterocyclic ligands present some unexpected results. Benzannulation shifts the lowest energy excitation to the blue by impacting the character of the transition rather than affecting frontier orbital energies.