Molecular Conductance Measurements Research Articles

Boosting the Photoresponse of Azobenzene Single-Molecule Junctions via Mechanical Interlock and Dynamic Anchor.

As the most classic photoisomerization system, azobenzene has been widely utilized as a building unit in various photoswitching applications. However, attempts to build azobenzene-based single-molecule photoswitches have met with limited success, giving low on/off ratios. Herein, we demonstrate two designs of azobenzene-based photoresponsive single-molecule junctions, based on mechanically interlocked diazocine and azobenzene-based dynamic anchors, respectively. Molecular conductance measurements using the scanning tunneling microscope breaking junction (STMBJ) technique revealed dramatic conductance changes upon photoillumination, achieving a high on/off ratio of ∼3.7. Using density functional theory (DFT), we revealed peculiar quantum interference (QI) effects in the diazocine molecular switch, indicating that diazocine is an excellent candidate for molecular photoswitches. The asymmetric azobenzene devices with a dynamic anchor exhibit switching behavior between a fully off state and a highly conductive state associated with the trans/cis conformation transition. The findings of this work not only present the design and development of functional molecular devices based on azobenzene units but also provide insight into the fundamental properties of light-induced quantum interference in azobenzene-based molecular devices.

Design, synthesis, characterization, in vitro cytotoxic, antimicrobial, antioxidant studies, DFT, thermal and molecular docking evaluation of biocompatible Co(II) complexes of N4O4-macrocyclic ligands

Bioactive cobalt (II) macrocyclic complexes [Co(N4O4ML1)Cl2]-[Co(N4O4ML3)Cl2] have been synthesized by using the macrocyclic ligands [N4O4ML1], [N4O4ML2], and [N4O4ML3] that have an N4O4 core. These three macrocyclic ligands were all isolated in pure form, together with their complexes. Microanalytical investigations, FT-IR NMR, Mass, magnetic moments, electronic, PXRD, TGA, and EPR spectrum studies were used to analyse their structures. For these complexes, an octahedral geometry is proposed for the metal ion. By using molecular weights and conductivity measurements the monomeric and non-electrolytic nature has been confirmed. The Coats-Redfern and FWO methods are used to determine the thermodynamic characteristics of the ligands and their Co(II) complexes. The molecular modelling using the DFT technique displays the bond angle, bond lengths and quantum chemical properties. To determine their ability to prevent the growth of harmful fungus and bacteria, the ligands [N4O4ML1]- [N4O4ML3] and their complexes were tested in vitro against A. Niger, C. albicans and B. subtilis, S. aureus, E. coli and S. typhi fungal and bacterial organisms, respectively. By using DPPH free radical scavenger assays, the in vitro antioxidant capabilities of each compound were evaluated. The [Co(N4O4ML3)Cl2] antioxidative capabilities revealed significant radical scavenging power. The MTT assay was used to assess the toxicity of all the synthesised compounds under inquiry on MCF-7, HeLa, and A549 cancer cells. The findings revealed that the ligand and the compounds gave outstanding IC50 values in the range of 9.07–36.25 (uM) at a concentration of 25 ppm. Among all the substances evaluated, [Co(N4O4ML3)Cl2] complex was discovered to be the most active and least cytotoxic. Additionally, docking investigations of the produced compounds were carried out in order to validate the biological outcomes.

Synthesis, spectroscopic elucidation, in vitro antimicrobial, cytotoxic and CT-DNA binding evaluation of heterobimetallic complexes of Ni(II) with main group/transition metal dichlorides

Heterobimetallic complexes of Nickel (II) have been synthesized and characterized of the type [Ni(L)2(M)2R4Cl2], where L = 2,6-diaminopyridine (C5H7N3), M = Sn, Si, Ti and Zr and R = Ph, Me, C5H5 etc. The monometallic complexes of nickel have been synthesized by using 2,6-diaminopyridine in the presence of NiCl2·6H2O dissolved in methanol to form complexes of the type [Ni(L)2]Cl2. To synthesize heterobimetallic complexes, these synthesized complexes were further processed with main group metal or transition metal dichlorides in a 1:2 molar ratio in MeOH. Elemental analyses, molecular weight calculations, electronic, infrared, magnetic susceptibility, conductivity measurements, mass spectra, 1HNMR , 13CNMR , and powder X-ray diffraction studies have been used to evaluate these synthesized compounds. Using a monometallic complex, the heterobimetallic complexes were synthesized, and an octahedral geometry was perceived for these heterobimetallic complexes. The antimicrobial potency of the synthesized complexes has been evaluated against four different bacterial and three fungal strain at different concentrations. The cytotoxicity activity of these resulting complexes against MCF-7, HeLa and HaCaT cell lines using MTT assay has been scrutinized. Also, the interaction of the reported complexes with calf thymus DNA (CT-DNA) by different techniques revealed that the complexes could bind to CT-DNA by intercalative mode. The binding of the reported complexes to CT-DNA has been investigated using UV–vis and fluorescence spectra. The results indicated a much lower binding affinity of monometallic complex than that of the heterobimetallic complexes.

Structure, dynamics and conductivities of ionic liquid-alcohol mixtures

We study the microscopic structure and transport properties of ions in mixtures of 1-butyl-3-methylimidazolium and 1-butyl-3-ethylimidazolium iodide with ethanol using atomistic molecular dynamics simulations and conductivity measurements. Compared with the same ionic liquids in water, we reveal essential differences in ionic structure that are closely related to the differences in the solubility mechanisms of both types of solvents. In particular, unlike for aqueous solutions, we find a homogeneous distribution of solvent molecules in the system, i.e., we observe no cluster formation, which agrees with the nano-structured solvation paradigm. In addition, we calculate the conductivities of these systems in the whole concentration range and compare them with experimental data. Although the simulated values slightly underestimate the experimental ones, they reproduce the shape of the experimental conductivity dome reasonably well. We also show that the pseudo-lattice random-alloy model, which is based on microscopic ion jumping frequencies, describes the conductivity data accurately. We compute the average jumping frequencies directly from simulations and find that they agree well with those obtained by fitting the simulation conductivity data. These results show that the pseudo-lattice random-alloy model provides a valuable tool to describe the conductivities of ionic liquid–solvent mixtures and particularly their concentration dependence. It shall also apply to other systems, e.g., inorganic electrolytes and dispersed ionic conductors.

Palladium(II) Complexes with S-Benzyl Dithiocarbazate: Synthesis, Characterization in vitro Antimicrobial and Anticancer Activities

Square planar complexes of palladium(II) were prepared by the reaction of PdCl2 with four different thio Schiff bases, namely 2-acetyl-pyridine S-benzyl dithiocarbazate (L1H), 2-acetylthiophene-S-benzyl dithiocarbazate (L2H), 2-acetylfuran-S-benzyl dithiocarbazate (L3H) and 2- acetylnaphthalene-S-benzyl dithiocarbazate (L4H) in a 1:2 molar ratio. Elemental analysis, molecular weight determinations, conductance measurements and spectral data, including electronic, IR, 1H and 13C NMR and X-ray powder diffraction studies, confirmed the formation of palladium(II) complexes. The electrochemical behaviour of one of the palladium(II) complex has also been determined by cyclic voltammetry. The antimicrobial activities of the Schiff base ligands and their corresponding palladium(II) complexes have been tested in vitro against various pathogenic bacterial and fungal strains. The cytotoxicity of [Pd(L1)2] complex shows the promising results when analyzed using MTT cell proliferation assay.

Synthetic, structural and biochemical approach of the macro cyclic complexes of manganese (II) and tin (II)

This study presents a brief account of the synthesis, spectroscopic and biochemical aspects of tetraazamacrocyclic complexes of managenese (II) and tin (II). The complexes of manganese (II) and tin (II) were prepared by the template condensation of metal salts with phthalic acid and diamines (1,3-diaminobutane and 1,4- diaminobutane) in 1:2:2 molar ratio. All the complexes are soluble in polar solvent. The structures were investigated using elemental analyses, molecular weight determinations, conductivity measurements, electronic, infrared and X-Ray diffraction spectral studies. The elemental analyses are consistent with the formation of complexes of the type [M(Macn)Cl2] (where, n=1-2, M=Mn(II), Sn (II)). An octahedral geometry around the metal ion is suggested for these complexes. All the complexes were screened against several fungi and bacteria to assess its biological properties and results are discussed.The importance of this method includes shorter reaction time and high yield.

Design and Fabrication of a MEMS-Based Break Junction Device for Mechanical Strain-Correlated Optical Characterization of a Single-Molecule

Here, we propose a robust lab-on-a-chip system for multi-dimensional (electrical, optical, and mechanical) characterization of a single-molecule utilizing a Si micromachined micro-electromechanical-system (MEMS) chip. Our MEMS-based break junction (MEMS-BJ) utilizes conventional MEMS chip fabrication techniques to create an electrostatic comb actuator capable of trapping a single-molecule between two nanoscale metal electrodes with sub-nanometer displacement resolution. In order to achieve this functionality, components of the chip (e.g flexural spring, comb drive fingers) are delicately designed based on an analytical model and basic performance parameters are verified using a finite element analysis simulation and capacitive sensor measurements. Using this system, we are able to perform combined single-molecule Raman spectroscopy and molecular conductance measurements in real-time. Finally, we present a representative case study of the multi-dimensional characterization of a single-molecule junction by analyzing correlated electrical, optical, and mechanical information while mechanically modulating the strain on the junction which allows for direct observation of the interplay between molecular structure and electron transport. We believe that our MEMS-BJ system will enable a deeper multi-dimensional study at a single-molecule level promoting the development of new methods for chemical and biological sensor applications.

PH-Mediated Single Molecule Conductance of Cucurbit[7]uril.

Recognition tunneling technique owns the capability for investigating and characterizing molecules at single molecule level. Here, we investigated the conductance value of cucurbit[7]uril (CB[7]) and melphalan@CB[7] (Mel@CB[7]) complex molecular junctions by using recognition tunneling technique. The conductances of CB[7] and Mel@CB[7] with different pH values were studied in aqueous media as well as organic solvent. Both pH value and guest molecule have an impact on the conductance of CB[7] molecular junction. The conductances of CB[7] and Mel@CB[7] both showed slightly difference on the conductance under different measurement systems. This work extends the molecular conductance measurement to aqueous media and provides new insights of pH-responsive host-guest system for single molecule detection through electrical measurements.

Quantum interference and heteroaromaticity of para- and meta-linked bridged biphenyl units in single molecular conductance measurements

Is there a correlation between the (hetero)aromaticity of the core of a molecule and its conductance in a single molecular junction? To address this question, which is of fundamental interest in molecular electronics, oligo(arylene-ethynylene) (OAE) molecular wires have been synthesized with core units comprising dibenzothiophene, carbazole, dibenzofuran and fluorene. The biphenyl core has been studied for comparison. Two isomeric series have been obtained with 4-ethynylpyridine units linked to the core either at para-para positions (para series 1–5) or meta-meta positions (meta series 6–10). A combined experimental and computational study, using mechanically controlled break junction measurements and density functional theory calculations, demonstrates consistently higher conductance in the para series compared to the meta series: this is in agreement with increased conjugation of the π–system in the para series. Within the para series conductance increases in the order of decreasing heteroaromaticity (dibenzothiophene < carbazole < dibenzofuran). However, the sequence is very different in the meta series, where dibenzothiophene ≈ dibenzofuran < carbazole. Excellent agreement between theoretical and experimental conductance values is obtained. Our study establishes that both quantum interference and heteroaromaticity in the molecular core units play important and inter-related roles in determining the conductance of single molecular junctions.

Electrochemically grafted single molecule junctions exploiting a chemical protection strategy

Diazonium grafted oligo(phenylene ethynylene) (OPE) monolayers suitable for single molecule conductance studies have been prepared. The surface-bound OPE is prepared from the amino tolan, 4-(2-(4-(2-(triisopropylsilyl)ethynyl)phenyl)ethynyl)benzamine, which is converted to the diazonium derivative in situ, and subsequently attached to a gold substrate using conventional cathodic electrochemical grafting. Multilayer formation is avoided by the presence of the bulky triisopropylsilyl (TIPS) protected ethynyl group. After removal of the TIPS group by treatment with fluoride, the film, now bearing exposed terminal acetylene moieties, is further functionalised by reaction with 1-iodo-4-acetylthiobenzene using Sonogashira coupling chemistry, to create sites bearing a strongly aurophilic top-contacting group. The success of these surface reactions is confirmed using Raman spectroscopy (gap-mode and SHINERS, employing gold nanoparticles and SiO2 coated gold nanoparticles, respectively). It is shown that such thiolate-modified, diazonium-grafted monolayers are suitable for STM-based molecular junction conductance measurements.

Template Synthesis, Characterization, Electrochemical Behavior, Antimicrobial, Nematicidal and Pesticidal Activities of Palladium(II) Macrocyclic Complexes

A new class of unsymmetrical palladium(II) macrocyclic complexes have been synthesized by template condensation reaction of the precursor, 2-[4-fluro-2-(2-oxo-1,2-diphenyl-ethylideneamino)-phenylimino]-1,2-diphenyl-ethanone (ML1) with appropriate diamines 1,2- diaminoethane, 1,3- diaminopropane, 1,4- diaminobutane, 1,5- diaminopentane and 1,6- diaminohexane, respectively in the presence of PdCl2.The reactions were carried out in 1:1:1 molar ratio. The precursor and its complexes have been characterized on the basis of elemental analyses, molecular weight determinations, conductance and magnetic susceptibility measurements and spectral studies including IR, 1H-NMR,electronic,mass, spectra and X-ray powder diffraction studies.Based on analytical and spectral data, a four coordinated square planar geometry has been assigned for Pd(II) macrocyclic complexes. The redox properties of the complexes were extensively investigated by electrochemical method using cyclic voltammetry (CV). The in vitro antimicrobial activity of Pd(II) macrocyclic complexes was studied and compared with that of free ligand. These complexes were also screened for their nematicidal and pesticidal activities.

Rectifying Electron-Transport Properties through Stacks of Aromatic Molecules Inserted into a Self-Assembled Cage.

Aromatic stacks formed through self-assembly are promising building blocks for the construction of molecular electronic devices with adjustable electronic functions, in which noncovalently bound π-stacks act as replaceable modular components. Here we describe the electron-transport properties of single-molecule aromatic stacks aligned in a self-assembled cage, using scanning probe microscopic and break junction methods. Same and different modular aromatic pairs are noncovalently bound and stacked within the molecular cage holder, which leads to diverse electronic functions. The insertion of same pairs induces high electronic conductivity (10(-3)-10(-2) G0, G0 = 2e(2)/h), while different pairs develop additional electronic rectification properties. The rectification ratio was, respectively, estimated to be 1.4-2 and >10 in current-voltage characteristics and molecular orientation-dependent conductance measurements at a fixed bias voltage. Theoretical calculations demonstrate that this rectification behavior originates from the distinct stacking order of the internal aromatic components against the electron-transport direction and the corresponding lowest unoccupied molecular orbital conduction channels localized on one side of the molecular junctions.

分子コンダクタンス計測に基づく生体分子の単分子検出

分子コンダクタンス計測に基づく生体分子の単分子検出

Kinetic Monte Carlo Simulations and Molecular Conductance Measurements of the Bacterial Decaheme Cytochrome MtrF

AbstractMicroorganisms overcome the considerable hurdle of respiring extracellular solid substrates by deploying large multiheme cytochrome complexes that form 20 nanometer conduits to traffic electrons through the periplasm and across the cellular outer membrane. Here we report the first kinetic Monte Carlo simulations and single‐molecule scanning tunneling microscopy (STM) measurements of the Shewanella oneidensis MR‐1 outer membrane decaheme cytochrome MtrF, which can perform the final electron transfer step from cells to minerals and microbial fuel cell anodes. We find that the calculated electron transport rate through MtrF is consistent with previously reported in vitro measurements of the Shewanella Mtr complex, as well as in vivo respiration rates on electrode surfaces assuming a reasonable (experimentally verified) coverage of cytochromes on the cell surface. The simulations also reveal a rich phase diagram in the overall electron occupation density of the hemes as a function of electron injection and ejection rates. Single‐molecule tunneling spectroscopy confirms MtrF’s ability to mediate electron transport between an STM tip and an underlying Au(111) surface, but at rates higher than expected from previously calculated heme‐to‐heme electron transfer rates for solvated molecules.

Rapid Time Domain Molecular Conductance Measurement of Alkanedithiols Using STM

ABSTRACTMolecules such as dithiols are of significant interest for potential molecular electronics applications. To investigate their properties, an efficient method for measuring their electrical conductance is crucial. This research focuses on the time domain measurement, a novel technique capable of measuring hundreds of molecules within a matter of seconds. Measurements were conducted using STM with the tip positioned within tunneling distance over a SAM of 1,8-octanedithiol on Au(111)-mica substrate submerged in toluene. Bonding/debonding events between the tops of molecules and the tip were observed through jumps in the time domain current waveform. A new time-spent histogram data analysis technique was developed to extract conductance values from complex waveforms. Conductance of 2.6 nS was obtained for a single 1,8-octanedithiol molecule, consistent with results obtained from wellestablished but time consuming break junction technique, validating the new STM based time domain technique for fast measurement of molecular conductance.

Molecular dynamics simulation and electrical conductivity measurement of Na2O•3SiO2 melt under high pressure; relationship between its structure and properties

The relationship between structure and properties of the Na2O•3SiO2 melt was investigated using molecular dynamics (MD) simulations and electrical conductivity measurements under high pressure. The inter-atomic potential was newly developed using energy surfaces derived by quantum chemical calculations. In MD simulations, changes of the structure and properties were observed as a function of pressure. An increase in self-diffusion coefficients of network forming atoms is closely related to the distortion of the –Si–O– network and the changes in distributions of poly-membered rings which forms the –Si–O– network. In the electrical conductivity measurements, we found a negative pressure dependence at least up to 3GPa. This suggests that electric conductivity in Na2O•3SiO2 melt is controlled by ionic conduction of sodium atoms.

Fine tuning of pentachromium(ii) metal string complexes through elaborate design of ligand

New pentachromoium metal string complexes [Cr5(μ5-L)4X2] (X = Cl−, L = dppzda2− (1), dpzpda2− (2); X = NCS−, L = dppzda2− (3), dpzpda2− (4)) were designed and synthesized through pyrazine-modulation of tripyridyldiamine ligand. X-Ray crystallographic studies revealed a linear metal chain structure consisting of two quadruple Cr–Cr bonds and a separated high spin Cr(II) at an end in crystallized form. A quintet ground state was observed for all pentachromium(II) molecules by magnetic study with g values of 2.04–2.18. While the electronic structure remained unchanged after the modification of ligands, electrochemistry showed a significant change in the molecular orbital energy levels of metal string molecules. Observation of the first oxidation peak of 1 at +0.57 V and of 2 at +0.73 V revealed that these complexes are quite resistant to oxidation. Single molecular conductance measurements showed that the complex exhibited good electronic conductance.

Direct electrode-electrode tunneling in break-junction measurements of molecular conductance

In break-junction experiments, the electrical contact between two metal electrodes is broken. When opening the junction, one or more molecules can be trapped between the electrodes and thereby contacted which allows their transport properties to be investigated. Statistical analysis of data plotted in histograms is generally used to determine the values of molecular conductance and the variations therefrom. In this paper, the effect of direct electron tunneling between the metal electrodes on the molecular conductance measurement is discussed. It is shown that this tunneling forms a parallel conductance channel that can influence both the position and the distribution of the apparent molecular conductance extracted from histogram peaks. Criteria to recognize the effect of this form of parallel tunneling path in comparison to the molecular transport path in the experimental data are established. With the help of analytic models and simulations, it will be shown how to disentangle the contributions of several physical variation sources on the resulting position, height, and width of the histogram peaks. Finally, the plotting of 2D histograms is described as a practical method to help distinguish the effects of molecular conductance from those of direct electrode-electrode tunneling.

Template synthesis, spectroscopic studies, antimicrobial, nematicidal and pesticidal activities of chromium(III) macrocyclic complexes

A new series of Cr(III) macrocyclic complexes have been synthesized by template condensation of ligands 2-[4-chloro-2-(2-oxo-1,2-diphenyl-ethylideneamino)-phenylimino]-1,2-diphenyl-ethanone (ML1) and 2-[4-fluro-2-(2-oxo-1,2-diphenyl-ethylideneamino)-phenylimino]-1,2-diphenyl-ethanone (ML2) respectively, with appropriate diamines i.e. 1,2-phenylenediamine, 4- chloro 1,2-phenylenediamine and 4-fluro- 1,2-phenylenediamine in the presence of CrCl3.6H2O. The ligands and their complexes have been characterized on the basis of elemental analyses, molecular weight determinations, conductance and magnetic susceptibility measurements and spectral studies including IR, ESR, electronic spectra and X-ray powder diffraction studies. On the basis of these studies, a six-coordinated octahedral geometry has been proposed for all these complexes. The newly synthesized ligands and their complexes have been screened for their antimicrobial, nematicidal and pesticidal activities. The results are indeed positive.

The gauge including magnetically induced current method

An overview of applications of the recently developed gauge including magnetically induced current method (GIMIC) is presented. The GIMIC method is used to obtain magnetically induced current densities in molecules. It provides detailed information about electron delocalization, aromatic character, and current pathways in molecules. The method has been employed in aromaticity studies on hydrocarbons, complex multi-ring organic nanorings, Möbius twisted molecules, inorganic and all-metal molecular rings and open-shell species. Recent studies on hydrogen-bonded molecules indicate that GIMIC can also be used to estimate hydrogen-bond strengths without fragmentation of the system. Preliminary results are presented on the applicability of GIMIC for investigating current transport in molecules attached to clusters simulating molecular conductivity measurements. Advantages and limitations of the GIMIC method are reviewed and discussed.