基于电化学方法研究以铜和银为电极的对苯二甲酸单分子结电导

利用基于电化学跳跃接触的扫描隧道显微镜裂结法(ECSTM-BJ), 通过现场形成金属电极, 对以Cu和Ag为电极的对苯二甲酸单分子结电导进行了测量. 研究结果表明: 利用该方法对所有数据直接线性统计即可得到很好结果; 两种电极下都存在两套高和低电导值, 其中以Cu为电极的单分子结电导高低值分别为11.5和4.0 nS, 而以Ag为电极的单分子结电导分别为10.3和3.8 nS, 高值都约为低值的3倍, 且以Cu为电极的单分子结电导要略大于以Ag为电极的电导, 可归结于电极和分子的耦合不同造成的. 与同样条件下测量得到的烷基链羧酸单分子结电导只存在一套值相比,对苯二甲酸表现出两套电导值, 反应了分子内主链对分子结电导的影响.     

Asymmetry induction by cooperative intermolecular hydrogen bonds in surface-anchored layers of achiral molecules.

The mesoscale induction of two-dimensional supramolecular chirality (formation of 2D organic domains with a single handedness) was achieved by self-assembly of 1,2,4-benzenetricarboxylic (trimellitic) acid on a Cu(100) surface at elevated temperatures. The combination of spectroscopic [X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS)], real-space-probe [scanning tunneling microscopy (STM)], and computational [density functional theory (DFT)] methods allows a comprehensive characterization of the obtained organic adlayers, where details of molecular adsorption geometry, intermolecular coupling, and surface chemical bonding are elucidated. The trimellitic acid species, comprising three functional carboxylic groups, form distinct stable mirror-symmetric hydrogen-bonded domains. The chiral ordering is associated with conformational restriction in the domains: molecules anchor to the substrate with an ortho carboxylate group, providing two para carboxylic acid moieties for collective lateral interweaving through H bonding, which induces a specific tilt of the molecular plane. The ease of molecular symmetry switching in domain formation makes homochiral-signature propagation solely limited by the terrace width. The molecular layer modifies the morphology of the underlying copper substrate and induces mum-sized strictly homochiral terraces.     

对巯基苯甲酸的表面增强拉曼光谱

应用表面增强拉曼光谱研究了吸附于粗糙银电极表面的对巯基苯甲酸。对巯基苯甲酸以去质子的形式通过巯基端进行吸附,表面Ag-S键的形成及羧基的结构改变直接影响苯环的电子结构。羧基的振动谱峰均对其质子化较为敏感,其峰强度随pH值的变化表明吸附态对巯基苯甲酸的pKa约为5．9。铜离子可与吸附对巯基苯甲酸形成表面络合物,配位反应与羧基的质子化反应密切相关。     

Extraction of copper(II) carboxylates with chloroform under substoichiometric conditions

The features are considered of binding carboxylic acids (HR) as dimeric solvated copper(II) complexes at the copper(II) extraction with chloroform under substoichiometric conditions, with excess of the metal ion in the aqueous phase. The fundamental difference in the optimization of the quantitative extraction of copper(II) in the form of any carboxylate with a maximum ratio of Cu:R and the total binding with carboxylic acids to form the copper complex of the minimum stoichiometry was noted. It was found that quantitative binding of HR occurred at the extraction of mixed-ligand acetate-carboxylate copper(II) complexes without control over the aqueous phase acidity when acetic acid medium or copper(II) acetate was used, but not copper chloride, nitrate, perchlorate, or sulfate. The possibility of determination by extraction-photometric method by the color of the Cu(II) complex was shown that was suitable only to those carboxylic acids, whose logarithm of the partition coefficient in the water-chloroform mixture was higher than three.     

Sequence modulation of tunneling barrier and charge transport across histidine doped oligo-alanine molecular junctions

Series tunneling across peptides composed of various amino acids is one of the main charge transport mechanisms for realizing the function of protein. Histidine, more frequently found in redox active proteins, has been proved to be efficient tunneling mediator. While how it exactly modulates charge transport in a long peptide sequence remains poorly explored. In this work, we studied charge transport of a model peptide junction, where oligo-alanine peptide was doped by histidine at different position, and the series of peptides were self-assembled into a monolayer on gold electrode with soft EGaIn as top electrode to form molecular junction. It was found that histidine increased the overall conductance of the peptide, meanwhile, its position modulated the conductance as well. Quantitative analysis by transport model and ultraviolet photoelectron spectroscopy (UPS) indicated a sequence dependent energy landscape of the tunneling barrier of the junction. Density-functional theory (DFT) calculation on the electronic structure of histidine doped oligo-alanine peptides revealed localized highest occupied molecular orbital (HOMO) on imidazole group of the histidine, which decreased charge transport barrier.     

Metal-molecule-metal junctions in Langmuir-Blodgett films using a new linker: trimethylsilane

Herein trimethylsilane (TMS) is demonstrated to be an efficient binding group suitable for construction of metal-molecule-metal (M-mol-M') junctions, in which one of the metal contacts is an atomically flat gold surface and the other a scanning tunnelling microscopy (STM) tip. The molecular component of the M-mol-M' devices is an oligomeric phenylene ethynylene (OPE) derivative Me(3)Si C≡C{C(6)H(4)C≡C}(2)C(6)H(4)NH(2), featuring both Me(3)SiC≡C and NH(2) metal contacting groups. This compound can be assembled into Langmuir-Blodgett (LB) films on Au--substrates by surface binding through the amine groups. Alternatively, low coverage (sub-monolayer) films are formed by adsorption from solution. In the case of condensed monolayers top electrical contacts are formed to STM tips through the TMS end group. In low coverage films, single molecular bridges can be formed between the gold surface and a gold STM tip. The similarity in the I-V response of a one-layer LB film and the single molecule conductance experiments reveals several points of critical importance to the design of molecular components for use in the construction of M-mol-M' junctions. Firstly, the presence of neighbouring π systems does not have a significant effect on the conductance of the M-mol-M' junction. Secondly, in the STM configuration, intermolecular electron hopping does not significantly enhance the junction transport characteristics. Thirdly, the symmetric behaviour of the I-V curves obtained, despite the different metal-molecule contacts, indicates that the molecule is simply an amphiphilic electron-donating wire and not a molecular diode with strong rectifying characteristics. Finally, the conductance values obtained from the amine/TMS-contacted OPE described here are of the same order of magnitude as thiol anchored OPEs, making them attractive alternatives to the more conventionally used thiol-contacting chemistry for OPE molecular wires.     

单分子电导测量技术及其影响因素

Molecular electronics is an important field for the application of nanotechnologies with an ultimate goal of building functional devices using single molecules or molecular arrays to realize the same functionality as macroscopic devices. To attain this goal, reliable techniques for measuring and manipulating electron transfer processes through single molecules are essential. There are various techniques and many environmental factors influencing single-molecule electronic conductance measurements. In this review, we first provide a detailed introduction and classification of the current well-accepted techniques in this field for measuring single-molecule conductance. All available techniques are summarized into two categories: the fixed junction technique and break junction technique. The break junction technique involves repeatedly forming and breaking molecular junctions by mechanically controlling a pair of electrodes moving into and out of contact in the presence of target molecules. Single-molecule conductance can be determined from the conductance plateaus that appear in typical conductance decay traces when molecules bind two electrodes during their separation process. In contrast, the fixed junction technique is to fix the distance between a pair of electrodes and measure the conductance fluctuations when a single molecule binds the two electrodes stochastically. Both techniques comprise different application methods and have been employed preferentially by different groups. Specific features of both techniques and their intrinsic advantages are compared and summarized in Section 4.     

Studies on Molecular Recongnition of (a)-Cyclodextrin with Diphenyl compounds

The cyclodextrins(CDs) are a class of cyclic oligosaccharides made up of six(a), seven(a)or more [a-(1,4)-linked] D-glucopyranose units, and shaped like truncated cones with primary and secondary hydroxyl groups crowning the narrower rim and wider rim respectively. As they have a hydrophobic cavity of appropriate dimension, they can bind with various guest moleculars, such as hydrocarbon, cyclohexane, aromatic compounds, to form inclusion complexes. The cyclodextins inclusion complexation has been considered an ideal model mimicking the enzyme-substrate interaction and a lot of effect has been devoted to it. In this paper, we report our investigation on the inclusion complexation behavior of a-cyclodextrin(a-CD) with diphenyl compounds in order to further explore the molecular recongnition mechanism of 2:1 inclusion complexation of a-CD with aromatic compounds.Figure 1: Possible structures of the inclusion compounds.The inclusion complexation behavior of a-CD with sym-diphenyl-urea, sym-diphenyl-thiourea and diphenyl kotone as respective guest moleclars was studied by ultraviolet spectrometric titrations.The absorption spectral changes observed for the compounds in the absence and presence of a-CD are used to draw the corresponding Benesi-Hildebrand plots and caculate the complex stability constant value (Ks) for the inclusion compounds.The 2:1 inclusion complexations show higher binding constants by cooperative binding of one guest molecular in the closely two hydrophobic cyclodextrin cavites as compared with 1:1 inclusion complexations.The highest value observed for sym-diphenyl-urea could be due to the formation of a hydrogen bond between the carbonyl group and the hydrogen in the hydroxyl group of CD and this is not possible with sym-tiphenyl-thiourea. The lowest value observed for diphenyl kotone indicate the hydrophobic interaction is one of the binding force of cyclodextrin inclusion complex.     

Synthese de benzodioxines-1,4 disubstitutes sur l'heterocycle

The dianion of benzodioxin 2- carboxylic acid can be prepared and reacts with carbonyl compounds to give 2,3- disubstituted 1,4- benzodioxines.     

Fabrication of asymmetric molecular junctions by the oriented assembly of dithiocarbamate rectifiers

The oriented assembly of molecules on metals is a requirement for rectification in planar metal-molecule-metal junctions. Here, we demonstrate how the difference in adsorption kinetics between dithiocarbamate and thioacetate anchor groups can be utilized to form oriented assemblies of asymmetric molecules that are bound to Au through the dithiocarbamate moiety. The free thioactate group is then used as a ligand to bind Au nanoparticles and to form the desired metal-molecule-metal junction. Besides allowing an asymmetric coupling to the electrodes, the molecules exhibit an asymmetric molecular backbone where the length of the alkyl chains separating the electrodes from a central, para-substituted phenyl ring differs by two methylene units. Throughout the junction fabrication, the layers were characterized by photoelectron spectroscopy, infrared spectroscopy, and scanning tunneling microscopy. Large area junctions using a conducting polymer interlayer between a mercury-drop electrode and the self-assembled monolayer prove the relationship between electrical data and molecular structure.     

The Interaction of Co(II), Ni(II), Cu(II), Zn(II), and Cd(II) with Mixed Donor Macrocycles Containing Pendant Carboxylic Acid Functions

The syntheses of mixed oxygen-nitrogen donor macrocycles incorporating two or three pendant carboxylic acid groups are described. Potentiometric titrations in water (I = 0.1; KNO₃) at 25°C have been used to determine the stability constants for the 1: 1 (metal:ligand) complexes of Co(II). Ni(II), Cu(II), Zn(II), and Cd(II). The constants obtained are compared with the previously determined values for the corresponding complexes of the unsubstituted macrocyclic precursors. The results of these studies indicate that each carboxylate function participates in binding to the central metal. For some metal-ion/ligand systems there is evidence that ring size effects influence the overall stability patterns and that, in such cases, both the ether oxygens as well as the tertiary amines of the macrocyclic rings appear to bind to the metal.     

Anisotropic Conductivity at the Single‐Molecule Scale

In most junctions built by wiring a single molecule between two electrodes, the electrons flow along only one axis: between the two anchoring groups. However, molecules can be anisotropic, and an orientation‐dependent conductance is expected. Here, we fabricated single‐molecule junctions by using the electrode potential to control the molecular orientation and access individual elements of the conductivity tensor. We measured the conductance in two directions, along the molecular plane as the benzene ring bridges two electrodes using anchoring groups (upright) and orthogonal to the molecular plane with the molecule lying flat on the substrate (planar). The perpendicular (planar) conductance is about 400 times higher than that along the molecular plane (upright). This offers a new method for designing a reversible room‐temperature single‐molecule electromechanical switch that controllably employs the electrode potential to orient the molecule in the junction in either “ON” or “OFF” conductance states.     

Linker dependent bond rupture force measurements in single-molecule junctions

We use a modified conducting atomic force microscope to simultaneously probe the conductance of a single-molecule junction and the force required to rupture the junction formed by alkanes terminated with four different chemical link groups which vary in binding strength and mechanism to the gold electrodes. Molecular junctions with amine, methylsulfide, and diphenylphosphine terminated molecules show clear conductance signatures and rupture at a force that is significantly smaller than the measured 1.4 nN force required to rupture the single-atomic gold contact. In contrast, measurements with a thiol terminated alkane which can bind covalently to the gold electrode show conductance and force features unlike those of the other molecules studied. Specifically, the strong Au-S bond can cause structural rearrangements in the electrodes, which are accompanied by substantial conductance changes. Despite the strong Au-S bond and the evidence for disruption of the Au structure, the experiments show that on average these junctions also rupture at a smaller force than that measured for pristine single-atom gold contacts.     

A series of tripodal cysteine derivatives as water-soluble chelators that are highly selective for copper(I)

A series of tripodal ligands derived from nitrilotriacetic acid and extended by three converging, metal-binding, cysteine chains was synthesised. Their ability to bind soft metal ions thanks to their three thiolate functions was investigated by means of complementary analytical and spectroscopic methods. Three ligands that differ by the nature of the carbonyl group next to the coordinating thiolate functions were studied: L(1) (ester), L(2) (amide) and L(3) (carboxylate). The negatively charged derivative L(3), which bears three carboxylate functions close to the metal binding site, gives polynuclear copper(I) complexes of low stability. In contrast, the ester and amide derivatives L(1) and L(2) are efficient Cu(I) chelators with very high affinities, close to that reported for the metal-sequestering metallothioneins (log K≈19). Interestingly, these two ligands form mononuclear copper complexes with a unique MS(3) coordination in water solution. An intramolecular hydrogen-bond network involving the amide functions in the upper cavity of the tripodal ligands stabilises these mononuclear complexes and was evidenced by the very low chemical-shift temperature coefficient of the secondary amide protons. Moreover, L(1) and L(2) display large selectivities for the targeted metal ion that is, Cu(I), with respect to bioavailable Zn(II). Therefore the two sulfur-based tripods L(1) and L(2) are of potential interest for intracellular copper detoxication in vivo, without altering the homeostasis of the essential metal ion Zn(II).     

Influence of positional isomers on the macroscale and nanoscale architectures of aggregates of racemic hydroxyoctadecanoic acids in their molecular gel, dispersion, and solid states

Inter/intramolecular hydrogen bonding of a series of hydroxystearic acids (HSAs) are investigated. Self-assembly of molecular gels obtained from these fatty acids with isomeric hydroxyl groups is influenced by the position of the secondary hydroxyl group. 2-Hydroxystearic acid (2HSA) does not form a molecular dimer, as indicated by FT-IR, and growth along the secondary axis is inhibited because the secondary hydroxyl group is unable to form intermolecular H-bonds. As well, the XRD long spacing is shorter than the dimer length of hydroxystearic acid. 3-Hydroxystearic acid (3HSA) forms an acyclic dimer, and the hydroxyl groups are unable to hydrogen bond, preventing the crystal structure from growing along the secondary axis. Finally, isomers 6HSA, 8HSA, 10HSA, 12HSA, and 14HSA have similar XRD and FT-IR patterns, suggesting that these molecules all self-assemble in a similar fashion. The monomers form a carboxylic cyclic dimer, and the secondary hydroxyl group promotes growth along the secondary axis.     

Syntheses and crystal structures of gadolinium and europium complexes of AAZTA analogues

Modified 6-amino-6-methylperhydro-1,4-diazepinetetraacetic acid (AAZTA) ligands with a hydroxyl arm have been synthesized and characterized. The crystal structures of its lanthanide complexes, determined by single crystal X-ray crystallography, show centrosymmetric nonacoordinated dimeric M₂L₂ forms where one carboxylate group from each ligand forms a bridge between two metal centers. Each metal center is coordinated with one inner sphere water molecule. The hydroxyl arm does not interfere in the coordination environment of the metal and therefore acts as a good anchor for attaching other functional motifs.     

Binding of metal ions to melanin and their effects on the aerobic reactivity

The binding of Mg(II), Ca(II), Zn(II), Cu(II) and Fe(III) to ETDA-washed Sepia melanin is quantified by inductively coupled plasma mass spectrometry. By monitoring the solution pH change associated with metal uptake, it is concluded that Mg(II), Ca(II) and Zn(II) bind to carboxylic acid groups in melanin, Cu(II) binds to hydroxyl (OH) groups and Fe(III) binds to OH or amine groups. The aerobic reactivity of melanins with different metal contents is analyzed by examining their ability to cause strand breaks in supercoiled pUC18 DNA. Cu(II)- and Fe(III)-enriched melanins induce the most damage. Hydroxyl radical, *OH, is proposed to be one of the reactive oxygen species responsible.     

Synthese von neuen bifunktionellen Maleinimidverbindungen zur Herstellung von Chemoimmunokonjugaten

Summary Bifunctional maleimide compounds are suitable for binding small molecules to carrier proteins in that they bind to the sulfhydryl group of proteins through the double bond of the maleimide group and to molecules of low molecular weight (e.g. anticancer drugs) through a functional groupX. 18 maleimide compounds of the general formula Maleimid-R-X (R=phenylene, benzyl-, methylene-, ethylene, or am-benzoylethylamide group andX=hydroxy-, amino-, hydrazino-, carboxylic acid-, carboxylic anhydride-, carboxylic acid chloride-, carboxylic acid hydrazide-, oxycarbonylchloride-, aldehyde, keto-, orp-toluenesulfonate-group) were synthesized and characterized through¹H- and¹³C-NMR-spectroscopy, elemental analysis, and mass spectrometry.

     

In situ infrared spectroscopic analysis of the adsorption of aromatic carboxylic acids to TiO2, ZrO2, Al2O3, and Ta2O5 from aqueous solutions

In situ infrared spectroscopy has been used to investigate the adsorption of a range of simple aromatic carboxylic acids from aqueous solution to metal oxides. Thin films of TiO2, ZrO2, Al2O3 and Ta2O5 were prepared by evaporation of aqueous sols on single reflection ZnSe prisms. Benzoic acid adsorbed very strongly to ZrO2, in a bridging bidentate fashion, but showed only weak adsorption to TiO2 and Ta2O5. Substituted aromatic carboxylic acids; salicylic, phthalic and thiosalicylic, were found to adsorb to each metal oxide. Salicylic and phthalic acids adsorbed to the metal oxides via bidentate interactions, involving coordination through both carboxylate and substituent groups. Thiosalicylic acid adsorbed to the metal oxides as a bridging bidentate carboxylate with no coordination through the thiol substituent group.     

The Effect of Acid Strength on the Mitsunobu Esterification Reaction: Carboxyl vs Hydroxyl Reactivity

In the presence of carboxylic acids, the adduct formed between triphenylphosphine and diisopropyl azodicarboxylate reacts to form mono- and bis-acylated hydrazides and the carboxylic acid anhydrides. These products are formed via attack of the carboxylate on the triphenylphosphonium group of the adduct, with weaker acids reacting much faster than stronger acids. This provides an explanation for the observation in the literature that acids stronger than acetic acid, such as 4-nitrobenzoic acid and chloroacetic acid, provide better yields in esterification reactions, since reaction of the alcohol with the phosphonium group of the adduct is more rapid than the competing reaction of the carboxylate for the phosphonium group.