A key area of activity in contemporary molecular electronics is the control of quantum interference (QI) in molecular junctions and devices. A range of strategies have been employed to understand and regulate QI, with features such as chemical substitution, connectivity, aromaticity and conjugation, electrochemical potential and non-covalent interactions being varied. Fused ring systems containing heteroatoms provide new opportunities for systematically exploring relationships between the structural, chemical and electronic states of a molecular wire and quantum interference, through measurement of its single-molecule electrical conductance. Here we study and modify a range of pyrrolodipyridines (carbazole-like) molecular wires. We are able to change the quantum interference patterns and hence the single-molecule conductance by chemically regulating the bridging nitrogen atom in the tricyclic ring system. A series of eight different N-substituted pyrrolodipyridines has been synthesized and subjected to single-molecule electrical characterization using an STM break junction. For meta connectivity to the electrodes a clear correlation is found between the charge residing on the pyrrolic nitrogen and the molecular conductance. This allows us to introduce a simple conceptual model which links the molecular conductance for each molecule in this series with pKa of the corresponding aniline used in the synthesis of the given compound. Correlations of these experimental data with theoretical calculations underline the importance of the pyrrolic nitrogen in facilitating conductance across the molecular bridge and controlling quantum interference. The large chemical gating for the meta connected series is not apparent for the para series, showing the competition between (i) meta connectivity quantum interference phenomena and (ii) the ability of the pyrrolic nitrogen to facilitate conductance, that can be modulated by chemical substitution. Our work demonstrates a purely synthetic method to harness and control quantum interference features, which complements existing electrochemical/electrostatic and (photo)chemical approaches.

When a linear aromatic molecule within a nanogap is bound only to a source electrode, and an adjacent molecule is bound only to a drain electrode, the two molecules can interact via pi–pi stacking, which allows electrons to flow from the source to the drain, via pi–pi bonds. Here we investigate the thermoelectric properties of such junctions, using mono-thiol oligo-phenylene ethynylene (OPE3)-based molecules as a model system. For molecules which are para-connected to the electrodes, we show that the Seebeck coefficient is an oscillatory function of the length L of the pi–pi overlap region and exhibits large positive and negative values. This bi-thermoelectric behavior is a result of quantum interference within the junction, which behaves like a molecular-scale Mach–Zehnder interferometer. For junctions formed from molecular monolayers sandwiched between planar electrodes, this allows both hole-like and electron-like Seebeck coefficients to be realized, by careful control of electrode separation On the other hand for meta-connected molecules, the Seebeck coefficient is insensitive to L, which may be helpful in designing resilient junctions with more stable and predictable thermoelectric properties.

DFT method was used to investigate the conductivities and thermoelectric properties for platinum metal complexes. The results were showed the type of ligands in the molecular complexes, which play a significant role in electrical and thermal properties calculations. The contact of the molecule with the electrodes along the plane of the molecule was given the molecule high ability to appear high conductance. Therefore, nitrogen atom in pyridyl rings that contacts with gold electrodes is more effective than the sulfur atoms. These molecules exhibit various conductance profiles, low conductance can be identified by molecule 1, and a higher conductance by molecule 4. We showed the molecule in the perpendicular to gold electrodes, and the presence of nitrogen atom in the position of contact with leads in place of sulfur gave the molecule high ability to an electron transfer.

The present work shows a theoretical results that have been used the functional Hybrid of three parameters Lee-Yang-Parr (B3LYP) of the quantum mechanical approach for density functional theory with (Spanish Initiative for Electronic Simulations with Thousands of Atoms) SIESTA code. All calculations were carried out employing the used method at the Gaussian 09 package of programs. It was reported the main point for research on dominance of the bandgap of elongated pi-conjugated molecules by using different chemical groups replacing hydrogen atom in the most molecules that used in this work. The side groups creates another factor that controls the value of the band gap. The dihedral angle between the two phenyl rings plays more important role in controlling the band gap in these molecules.