Quantum Interference and Substantial Property Tuning in Conjugated Z-ortho-Regio-Resistive Organic (ZORRO) Junctions

Theoretical investigation of thermoelectric properties of methyl blue-based molecular junctions

Thermoelectric properties of a family of methyl blue-based molecular junctions were theoretically studied using a combination of density functional theory (DFT) methods, and quantum transport theory (QTT). Employing different numbers of amino groups not only proves itself as a powerful strategy for controlling the transport behaviour and lifting the transmission coefficient T(E) from 1.91 × 10-5 to 7.45 × 10-5 with increasing the amino groups from zero to four, but also it enhances the thermoelectric properties of these molecules, since it increases the Seebeck coefficient (S) from 106.8 to 202.4 μV K-1 and the electronic figure of merit (Z el T) has been raised from 0.15 to 0.35, making these molecules promising candidates for thermoelectric applications.

Carbon Nanohoops: Multiple Molecular Templates for Exploring Spectroscopic, Electronic, and Thermoelectric Properties

A combination of density functional theory (DFT) methods and quantum transport theory (QTT) has been used to investigate the spectroscopic, electronic, and thermoelectric properties of carbon nanohoop molecules with different molecular templates. The connectivity type, along with inherent strain, impacts the transport behavior and creates a destructive quantum interference (DQI), which proves itself to be a powerful strategy to enhance the thermoelectric properties of these molecules, making them promising candidates for thermoelectric applications.

Site and length dependent quantum interference and resonance in the electron transport of armchair carbon nanotube molecular junctions

The destructive quantum interference (DQI) effect in molecular devices, as characterized by a sharp valley in the transmission function and conductance suppression with several orders of magnitude, is of great interest for both fundamental reasons and technical applications. Planar π conjugated systems, such as benzene, graphene molecules and graphene nanoribbons, are typical examples showing DQI and have been studied most frequently. Carbon nanotubes (CNTs) can be considered as extended planar π conjugated systems, but with a different topology from graphene. In this work, using the Hückel analytical theory, we investigated the transport properties of molecular junctions constructed with armchair CNTs which are weakly coupled to the leads with single site connections. It is found that the transport properties demonstrate obvious oscillation with a period of 3 in nanotube length as defined by the number (n) of atomic planes along the transport direction, which is not observed in graphene nanoribbons. Specifically, when the length is n = 3p or 3p + 1, DQI will be observed at the Fermi level when both leads are connected to the same sublattice, but not observed when they are connected to different sublattices. In contrast, when the length is 3p + 2, the DQI sharp valley will never be observed at the Fermi level. Instead, a resonant peak will appear at the Fermi level when the two leads are connected to the same sublattice. Nevertheless, this resonant peak will not appear for connections with different sublattices. All these results are well explained in terms of the energy spectrum of an armchair graphene nanoribbon model and spatial distribution of the frontier molecular orbitals. The findings demonstrate the great difference between the DQI patterns of graphene nanoribbons and carbon nanotubes due to topology differences.

Ronald C.D. Breslow (1931-2017): A career in review

Reviewed herein are key research accomplishments of Professor Ronald Charles D. Breslow (1931-2017) throughout his more than 60 year research career. These accomplishments span a wide range of topics, most notably physical organic chemistry, medicinal chemistry, and bioorganic chemistry. These topics are reviewed, as are topics of molecular electronics and origin of chirality, which combine to make up the bulk of this review. Also reviewed briefly are Breslow's contributions to the broader chemistry profession, including his work for the American Chemical Society and his work promoting gender equity. Throughout the article, efforts are made to put Breslow's accomplishments in the context of other work being done at the time, as well as to include subsequent iterations and elaborations of the research.

Quantum interference enhances rectification behavior of molecular devices

A theoretical and computational study of the effect of quantum interference on the rectification behavior of unimolecular devices.

Toward a First-Principles Evaluation of Transport Mechanisms in Molecular Wires

Understanding charge transport through molecular wires is important for nanoscale electronics and biochemistry. Our goal is to establish a simple first-principles protocol for predicting the charge transport mechanism in such wires, in particular the crossover from coherent tunneling for short wires to incoherent hopping for longer wires. This protocol is based on a combination of density functional theory with a polarizable continuum model introduced by Kaupp et al. for mixed-valence molecules, which we had previously found to work well for length-dependent charge delocalization in such systems. We combine this protocol with a new charge delocalization measure tailored for molecular wires, and we show that it can predict the tunneling-to-hopping transition length with a maximum error of one subunit in five sets of molecular wires studied experimentally in molecular junctions at room temperature. This suggests that the protocol is also well suited for estimating the extent of hopping sites as relevant, for example, for the intermediate tunneling-hopping regime in DNA.

Simultaneous Suppression of π- and σ-Transmission in π-Conjugated Molecules

Molecular dielectric materials require ostensibly conflicting requirements of high polarizability and low conductivity. As previous efforts toward molecular insulators focused on saturated molecules, it remains an open question whether π- and σ-transport can be simultaneously suppressed in conjugated systems. Here, we demonstrate that there are conjugated molecules where the σ-transmission is suppressed by destructive σ-interference, while the π-transmission can be suppressed by a localized disruption of conjugation. Using density functional theory, we study the Landauer transmission and ballistic current density, which allow us to determine how the transmission is affected by various structural changes in the molecule. We find that in para-linked oligophenyl rings the σ-transmission can be suppressed by changing the remaining hydrogens to methyl groups due to the inherent gauche-like structure of the carbon backbone within a benzene ring, similar to what was previously seen in saturated systems. At the same time, the methyl groups fulfill a dual purpose as they modulate the twist angle between neighboring phenyl rings. When neighboring rings are orthogonal to each other, the transmission through both π- and σ-systems is effectively suppressed. Alternatively, breaking conjugation in a single phenyl ring by saturating two carbons atoms with two methyl substituents on each carbon, results in suppressed π- and σ-transport independent of dihedral angle. These two strategies demonstrate that methyl-substituted oligophenyls are promising candidates for the development of molecular dielectric materials.