Interaction-induced negative differential resistance in asymmetric molecular junctions

Combining insights from quantum chemistry calculations with master equations, we discuss a mechanism for negative differential resistance (NDR) in molecular junctions, operated in the regime of weak tunnel coupling. The NDR originates from an interplay of orbital spatial asymmetry and strong electron-electron interaction, which causes the molecule to become trapped in a nonconducting state above a voltage threshold. We show how the desired asymmetry can be selectively introduced in individual orbitals in, e.g., oligo(phenyleneethynylene)-type molecules by functionalization with a suitable side group, which is in linear conjugation to one end of the molecule and cross-conjugated to the other end.     

Nonbenzenoid aromatic isocyanides: New coordination building blocks for organometallic and surface chemistry

This review is focused on the emerging chemistry of nonbenzenoid aromatic isocyanides, a relatively new family of aryl isocyanide molecules. Two types of systems are discussed: (1) isocyanoazulenes, for which five archetypal isomeric structures may be envisioned, and (2) η⁵-stabilized isocyanocyclopentadienides. So far, the latter are represented by isocyanoferrocene, 1,1′-diisocyanoferrocene, and isocyanocymantrene. In addition, the synthesis and chemistry of the linear 2,6-diisocyanoazulene motif, including regioselective installation and complexation of its –NC termini with controlled orientation of the azulenic dipole, are described. Self-assembly of nonbenzenoid aryl isocyanides and diisocyanides on gold(1 1 1) surfaces is reviewed as well.     

Length-dependent transport in molecular junctions based on SAMs of alkanethiols and alkanedithiols: effect of metal work function and applied bias on tunneling efficiency and contact resistance

Nanoscopic tunnel junctions were formed by contacting Au-, Pt-, or Ag-coated atomic force microscopy (AFM) tips to self-assembled monolayers (SAMs) of alkanethiol or alkanedithiol molecules on polycrystalline Au, Pt, or Ag substrates. Current-voltage traces exhibited sigmoidal behavior and an exponential attenuation with molecular length, characteristic of nonresonant tunneling. The length-dependent decay parameter, beta, was found to be approximately 1.1 per carbon atom (C(-1)) or 0.88 A(-)(1) and was independent of applied bias (over a voltage range of +/-1.5 V) and electrode work function. In contrast, the contact resistance, R(0), extrapolated from resistance versus molecular length plots showed a notable decrease with both applied bias and increasing electrode work function. The doubly bound alkanedithiol junctions were observed to have a contact resistance approximately 1 to 2 orders of magnitude lower than the singly bound alkanethiol junctions. However, both alkanethiol and dithiol junctions exhibited the same length dependence (beta value). The resistance versus length data were also used to calculate transmission values for each type of contact (e.g., Au-S-C, Au/CH(3), etc.) and the transmission per C-C bond (T(C)(-)()(C)).     

Ab initio electron propagator calculations in molecular transport junctions: predictions of negative differential resistance

In this work we study current-voltage characteristics in transport molecular junctions with a 1,4-benzene dithiol molecule as a bridge by using different ab initio electron propagator methods such as OVGF and P3 which are both programs in a Gaussian software package. The current-voltage characteristics are calculated for different values of Fermi energy in various basis sets such as 6-311++G(p,d) and cc-pVDZ and are compared with the experimental data. A good agreement is found in almost the entire voltage range. In addition, the results of our calculations indicate that the accuracy of ab initio electron propagator methods is in the range of 0.2-0.3 eV. Since the computational methods are truly ab initio, implying no adjustable parameters, functions, or functionals, the theoretical predictions can be improved only by changing the model of a transport device. The current-voltage characteristics predict peaks, i.e., negative differential resistances, for the various values of Fermi energy. As shown, the origin of the negative differential resistances is related to the voltage dependences of overlap integrals for the active terminal orbitals, expansion coefficients of partial atomic wavefunctions in Dyson orbitals, and the voltage dependences of Dyson poles (ionization potentials). We find that two peak behavior in the current-voltage characteristics can be explained by the anharmonicity of potential energy surfaces. As a result of our studies, we predict that negative differential resistances can be experimentally found by changing a position of Fermi level, i.e., by using different metal electrodes coated by a gold atomic monolayer.     

Correlation between large polarons in molecular chains

We studied few extra electrons in a molecular chain with respect of electron-phonon coupling in the adiabatic approximation. It is shown that the lowest state of two extra electrons in a chain corresponds to the singlet bisoliton state with one deformational potential well. Two electrons with parallel spins form a localised triplet state, which corresponds to the two-hump charge distribution function. Three extra electrons form an almost independent nonlinear superposition of a soliton and bisoliton states. In the case of four electrons, the two almost independent bisolitons are formed. These two states tend to separate in the chain at the maximal distance due to the Fermi repulsion, accounted for in the zero-order adiabatic approximation. This repulsion is partly compensated by the attraction between the solitons due to their exchange with virtual phonons, described by the non-adiabatic part of the Hamiltonian. The formation of solitons is characterised by the appearance of the bound soliton and bisoliton levels in the forbidden energy band. This constitutes the qualitative difference of the large polaron (soliton) states from the almost free electron states and small polaron states.     

Switching in molecular transport junctions: polarization response

We discuss several proposed explanations for the switching and negative differential resistance (NDR) behavior seen in some molecular junctions. Several theoretical models are discussed, and we present results of electronic structure calculations on a series of substituted oligo(phenylene ethynylene) molecules. It is shown that a previously proposed polaron model is successful in predicting NDR behavior, and the model is elaborated with image charge effects and parameters from electronic structure calculations. This model now incorporates substituent effects and includes the effects of conformational change, charging, and image charge stabilization.     

Effect of length and contact chemistry on the electronic structure and thermoelectric properties of molecular junctions

We present a combined experimental and computational study that probes the thermoelectric and electrical transport properties of molecular junctions. Experiments were performed on junctions created by trapping aromatic molecules between gold electrodes. The end groups (-SH, -NC) of the aromatic molecules were systematically varied to study the effect of contact coupling strength and contact chemistry. When the coupling of the molecule with one of the electrodes was reduced by switching the terminal chemistry from -SH to -H, the electrical conductance of molecular junctions decreased by an order of magnitude, whereas the thermopower varied by only a few percent. This has been predicted computationally in the past and is experimentally demonstrated for the first time. Further, our experiments and computational modeling indicate the prospect of tuning thermoelectric properties at the molecular scale. In particular, the thiol-terminated aromatic molecular junctions revealed a positive thermopower that increased linearly with length. This positive thermopower is associated with charge transport primarily through the highest occupied molecular orbital, as shown by our computational results. In contrast, a negative thermopower was observed for a corresponding molecular junction terminated by an isocyanide group due to charge transport primarily through the lowest unoccupied molecular orbital.     

Relationship between retention behavior and molecular structure in HPLC. Correlation between solubility parameter and molecular properties

Chromatographia - The application and significance of the solubility parameter are detailed for chromatographic systems. A critical review of the general concept and several empirical and...     

Inelastic electron tunneling spectroscopy in molecular junctions: peaks and dips

We study inelastic electron tunneling through a molecular junction using the nonequilibrium Green's function formalism. The effect of the mutual influence between the phonon and the electron subsystems on the electron tunneling process is considered within a general self-consistent scheme. Results of this calculation are compared to those obtained from the simpler Born approximation and the simplest perturbation theory approaches, and some shortcomings of the latter are pointed out. The self-consistent calculation allows also for evaluating other related quantities such as the power loss during electron conduction. Regarding the inelastic spectrum, two types of inelastic contributions are discussed. Features associated with real and virtual energy transfer to phonons are usually observed in the second derivative of the current I with respect to the voltage Phi when plotted against Phi. Signatures of resonant tunneling driven by an intermediate molecular ion appear as peaks in the first derivative dI/dPhi and may show phonon sidebands. The dependence of the observed vibrationally induced lineshapes on the junction characteristics, and the linewidth associated with these features are also discussed.     

Relation between electrophoretic behavior and molecular shapes of aromatic anions.

The capillary electrophoretic behavior of 44 aromatic organic ions was investigated. The observed ionic radii (r(obs0)) for the aromatic organic ions were obtained from the electrophoretic mobilities of sodium tetraborate (pH 9.2), potassium tetraborate (pH 9.2), ammonium borate (pH 9.2), and trisodium phosphate (pH 11.7) buffers with zero ionic strength. The linear relationships between the r(obs0)) values and the ionic radii (r(calc)), calculated by either the AM1 or PM3 method, were determined for benzyltrialkylammonium and aromatic sulfonate ions. However, the r(obs0)) values were constant for the aromatic carboxylate ions in buffers, in spite of the different r(calc) values. This indicates that aromatic carboxylate ions, such as benzenecarboxylate, pyridinecarboxylate, naphthalenecarboxylate, and anthracenecarboxylate ions, migrate as planar ions in buffers, whereas aromatic sulfonate ions could migrate as approximately spherical ions.     

Single molecule junctions formed via Au-thiol contact: stability and breakdown mechanism

The stability and breakdown mechanism of a single molecule covalently bound to two Au electrodes via Au-S bonds were studied at room temperature. The distance over which a molecular junction can be stretched before breakdown was measured using a scanning tunneling microscopy break junction approach as a function of stretching rate. At low stretching rates, the stretching distance is small and independent of stretching rate. Above a certain stretching rate, it increases linearly with the logarithm of stretching rate. At very high stretching rates, the stretching distance reaches another plateau and becomes insensitive to the stretching rate again. The three regimes are well described by a thermodynamic bond-breaking model. A comparative study of Au-Au atomic point contacts indicates that the breakdown of the molecular junctions takes place at Au-Au bonds near the molecule-electrode contact. By fitting the experimental data with the model, the lifetime and binding energy were extracted. Both quantities are found to have broad distributions, owing to large variations in the molecule-electrode contact geometry. Although the molecular junctions are short-lived on average, certain contact geometries are considerably more stable. Several types of stochastic fluctuations were observed in the conductance of the molecule junctions, which are attributed to the atomic level rearrangement of the contact geometry, and bond breakdown and reformation processes. The possibility of bond reformation increases the apparent lifetime of the molecular junctions.     

Cyanides and isocyanides of first-row transition metals: molecular structure, bonding, and isomerization barriers

Cyanides and isocyanides of first-row transition metal M(CN) (M=Sc-Zn) are investigated with quantum chemistry techniques, providing predictions for their molecular properties. A careful analysis of the competition between cyanide and isocyanide isomers along the transition series has been carried out. In agreement with the experimental observations, late transition metals (Co-Zn) clearly prefer a cyanide arrangement. On the other hand, early transition metals (Sc-Fe), with the only exception of the Cr(CN) system, favor the isocyanide isomer. The theoretical calculations predict the following unknown isocyanides, ScNC(3Delta), TiNC(4Phi), VNC(5Delta), and MnNC(7Sigma+), and agree with the experimental observation of FeNC(6Delta) and the CrCN(6Sigma+) cyanide. First-row transition metal cyanides and isocyanides are predicted to have relatively large dissociation energies with values within the range 80-101 kcal mol(-1), except Zn(CN), which has a dissociation energy around 50-55 kcal mol(-1), and low isomerization barriers. A detailed analysis of the bonding has been carried out employing the topological analysis of the charge density and an energy decomposition analysis. The role of the covalent and electrostatic contributions to the metal-ligand bonding, as well as the importance of pi bonding, are discussed.     

Correlation between retention data of polycyclic aromatic hydrocarbons and several descriptors in supercritical-fluid chromatography

Chromatographia - The correlation between retention data of polycyclic aromatic hydrocarbons (PAHs) obtained in various supercritical fluid-chromatographic systems (SFC systems) and several...     

Correlation between isoniazid resistance and superoxide reactivity in mycobacterium tuberculosis KatG

Isoniazid is an antituberculosis prodrug that requires activation by the catalase-peroxidase (KatG) of Mycobacterium tuberculosis. The activated species, presumed to be an isonicotinoyl radical, couples to NADH forming an isoniazid-NADH adduct that ultimately confers antitubercular activity. We have compared the catalytic properties of three KatGs associated with isoniazid resistance (resistance mutation KatGs, (RM)KatGs: R104L, H108Q, S315T) to wild-type enzyme and two additional lab mutations (wild-type phenotype KatGs, (WTP)KatGs: WT KatG, Y229F, R418L). Neither catalase nor peroxidase activities, nor the presence/absence of the Met-Tyr-Trp cross-link (as probed by LC/MS on tryptic digests of the protein), exhibited any correlation with isoniazid resistance. The yields of isoniazid-NADH adduct formed were determined to be 1-5, 4-12, and 20-70-fold greater for the (WTP)KatGs than the (RM)KatGs for the compound I, II, and III pathways, respectively, strongly suggesting a role for oxyferrous KatG (supported by superoxide consumption measurements) that correlates with drug resistance. Stopped-flow UV-visible spectroscopic studies revealed that all KatGs were capable of forming both compound II and III intermediates. Rates of compound II decay were accelerated 4-12-fold in the presence of isoniazid (vs absence) for the (WTP)KatGs but were unaffected by the drug for the (RM)KatGs. A mechanism for isoniazid resistance which accounts for the observed reactivity for each of the compound I, II, and III intermediates is proposed and suggests that the compound III pathway may be the primary factor in determining overall isoniazid resistance by specific KatG mutants, with secondary contributions arising from the compound I and II pathways.     

Electron delocalization in various triply linked zinc(II) porphyrin arrays: role of antiaromatic junctions between aromatic porphyrins

A series of meso-meso, β-β, β-β triply linked linear, radial and square-type zinc(II) porphyrin arrays consist of the constituent porphyrin units and naphthalene junctions. To understand the unique nature of triply linked porphyrin arrays, numerous research activities have been focused on the electronic structures of the constituent porphyrin units. In this study, however, we have paid attention to the role of the naphthalene junction in the electronic delocalization of various triply linked porphyrin arrays. On the basis of our study, we have unveiled that unique π-conjugation behaviors in triply linked porphyrin arrays are induced by their intrinsic molecular orbital interactions and subsequently by antiaromatic junctions. Furthermore, the structural deformation by triple linkages gives rise to a deteriorative effect on the electronic delocalization between inner and outer porphyrin units. Finally, we propose a different type of electron delocalization in linear multichromophoric systems by alternating aromatic and antiaromatic units.     

Correlation between magnetic properties and molecular structure of some metallo-mesogens

The behaviour of a large number of paramagnetic metallo-mesogenic molecules with Cu and VO in different mesophases in a magnetic field was investigated by EPR techniques and magnetic susceptibility measurements. The investigation of the angular dependence of the EPR spectra enabled conclusions to be reached concerning the molecular orientation in the external magnetic field. Temperature dependence magnetic susceptibility measurements were carried out in order to obtain information about the overall susceptibility anisotropy. The good agreement between experimental results and calculated data based on the known increment scheme is obvious. It is shown that the direction of orientation of the molecules in a magnetic field is predetermined by the sum of the anisotropy of the phenyl ring and the chelate core in the molecular structure.     

Dithiocarbamate anchoring in molecular wire junctions: a first principles study

Recent experimental realization [J. Am. Chem. Soc., 127 (2005) 7328] of various dithiocarbamate self-assembly on gold surface opens the possibility for use of dithiocarbamate linkers to anchor molecular wires to gold electrodes. In this paper, we explore this hypothesis computationally. We computed the electron transport properties of 4,4'-bipyridine (BP), 4,4'-bipyridinium-1,1'-bis(carbodithioate) (BPBC), 4-(4'-pyridyl)-peridium-1-carbodithioate (BPC) molecule junctions based on the density functional theory and nonequilibrium Green's functions. We demonstrated that the stronger molecule-electrode coupling associated with the conjugated dithiocarbamate linker broadens transmission resonances near the Fermi energy. The broadening effect along with the extension of the pi conjugation from the molecule to the gold electrodes lead to enhanced electrical conductance for BPBC molecule. The conductance enhancement factor is as large as 25 at applied voltage bias 1.0 V. Rectification behavior is predicted for BPC molecular wire junction, which has the asymmetric anchoring groups.