The reaction of trans-RuCl(CCC6H4R1-4)(dppe)2 (2: R1 = Me (a), C5H11 (b), OMe (c), CO2Me (d), NO2 (e), CCSiMe3 (f), CCBut (g), NH2 (h)), prepared in situ from reactions of RuCl(dppe)2OTf ...(1OTf) with terminal alkynes in CH2Cl2 solutions containing 1,8-diazabicycloundec-7-ene (DBU) and TlBF4, provides a convenient and rapid route to bis(acetylide) complexes trans-Ru(CCC6H4R1-4)2(dppe)2 (3a–h) and trans-Ru(CCC6H4R1-4)(CCC6H4R2-4)(dppe)2 (4, R1 = CCSiMe3, R2 = NH2; 5, R1 = CO2Me, R2 = NH2; 6, R1 = CO2Me, R2 = OMe). However, even in the absence of the chloride abstracting reagent, more strongly electron donating substituents (e.g., R1 = OMe (2c), NH2 (2h)) promote sufficient ionization of the Ru–Cl bond in trans-RuCl(CCC6H4R1-4)(dppe)2 to lead to slow conversion to bis(alkynyl) complexes 3c,h in the presence of excess alkyne and DBU. Desilylation of 2f and 3f affords 2i and 3i (R1 = CCH), respectively. The molecular structures of 3a–d,f–i have been determined and are reported together with the structures of the monoalkynyl complexes 2f,g,i and compared with related compounds from the literature. Complexes 3a–i and 4–6 undergo one reversible electrochemical oxidation process, which can be attributed to depopulation of an orbital with significant alkynyl ligand character. The one-electron-oxidation products 3f•+, 3h•+, 4•+, and 5•+, chosen to serve as representative examples of this family of complexes, each exhibit a series of NIR absorptions between 15000 and 5000 cm–1 which on the basis of TDDFT calculations cannot be attributed to a single, static lowest energy molecular structure. Rather, the transitions that are responsible for the absorption band envelope have varying degrees of LMCT and inter-alkynyl ligand IVCT or MLCT character that depend not only on the nature of the Rn groups but also on the ensemble of thermally populated molecular conformers in solution with various relative orientations of the metal fragment and arylethynyl moieties.
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The influence of the number of molecule–metal interactions on the surface-enhanced Raman scattering (SERS) spectroscopy of 1,4-benzenedithiol (BDT) was investigated. For this purpose, a series of ...SERS-active samples were prepared featuring one or two molecule–metal interfaces. Molecules were adsorbed on the surface of a rough Au substrate, or sandwiched between Au nanoparticles (NPs) and a flat Au(111) substrate in a “sphere–plane” disposition. In the presence of the Au surface(s), vibrational energy and intensity of the SERS spectra differs significantly from the bulk. Molecule–metal charge transfer upon chemisorption weakens intramolecular bonds, resulting in the observed red shift of the breathing and CC stretching modes. This effect was found to be more pronounced for samples with multiple molecule–metal interfaces. In addition, the SERS spectra of BDT featured additional b 2 signals not present in the bulk spectra. Chemical enhancement of the b 2 modes takes place by means of photoinduced charge transfer from an occupied molecular orbital to an unoccupied metal orbital. Analysis of the normalized SERS intensity revealed a larger scattering enhancement for the samples with a sphere–plane disposition arising from the stronger electromagnetic enhancement effect via plasmonic localization of optical fields. Complementary studies on 4-aminobenzenethiol support these findings.
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Conductance across a metal|molecule|metal junction is strongly influenced by the molecule-substrate contacts, and for a given molecular structure, multiple conductance values are frequently observed ...and ascribed to distinct binding modes of the contact at each of the molecular termini. Conjugated molecules containing a trimethylsilylethynyl terminus, -C≡CSiMe(3) give exclusively a single conductance value in I(s) measurements on gold substrates, the value of which is similar to that observed for the same molecular backbone with thiol and amine based contacting groups when bound to under-coordinated surface sites.
Monomolecular films of an oligo(phenylene)ethynylene, OPE, derivative 2-isocyano-1,3-dimethylbenzene4-(4′-aminophenylethynyl)phenylethynylgold, 1, containing a gold atom in the molecule backbone ...have been prepared by the Langmuir–Blodgett (LB) method in order to study how the electrical properties can be modulated in monolayers of OPEs by incorporation of a gold center in their structures. UV–vis reflection spectra of Langmuir monolayers of 1 at the air–water interface reveal strong aurophilic interactions between neighboring molecules that increase upon compression. Monolayer Langmuir–Blodgett (LB) films were readily fabricated by the transfer of Langmuir films of 1 onto solid substrates. Quartz crystal microbalance (QCM) experiments conclusively demonstrate formation of monolayer LB films with a high surface coverage. The morphology of these films was analyzed by atomic force microscopy (AFM), revealing formation of homogeneous layers with an optimum surface pressure of transference of 6 mN·m–1. Film homogeneity and integrity was confirmed by cyclic voltammetry, with efficient blocking of gold electrodes by these well-formed monolayers of 1. The electrical properties of LB films of 1 were investigated by scanning tunneling microscopy (STM) using a “tip-to-contact” method. Characteristic shape for a tunneling junction (Simmon’s type behavior) I−V curves were observed, with analysis of the pseudolinear (ohmic) region giving a conductance value G = 3.9 × 10–5 G 0, which is relatively high for an OPE derivative and may indicate a beneficial role of metal atom incorporation within the wire-like system.
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Well-ordered, tightly-packed (surface coverage 0.97 × 10
−9
mol cm
−2
) monolayer films of 1,4-bis((4-ethynylphenyl)ethynyl)benzene (
1
) on gold are prepared
via
a simple self-assembly process, ...taking advantage of the ready formation of alkynyl C-Au σ-bonds. Electrochemical measurements using Ru(NH
3
)
6
3+
, Fe(CN)
6
3−
, and ferrocenylmethanol Fe(η
5
-C
5
H
4
CH
2
OH)(η
5
-C
5
H
5
) redox probes indicate that the alkynyl C-Au contacted monolayer of
1
presents a relatively low barrier for electron transfer. This contrasts with monolayer films on gold of other oligo(phenylene ethynylene) derivatives of comparable length and surface coverage, but with different contacting groups. Additionally, a low voltage transition (
V
trans
= 0.51 V) from direct tunneling (rectangular barrier) to field emission (triangular barrier) is observed. This low transition voltage points to a low tunneling barrier, which is consistent with the facile electron transport observed through the C-Au contacted self-assembled monolayer of
1
.
Well-ordered, self-assembled monolayer films of 1,4-bis((4-ethynylphenyl)ethynyl)benzene on gold are shown to be nearly transparent to outer-sphere electron transfer processes.
Aromaticity is a fundamental concept in chemistry. It is described by Hückel's rule that states that a cyclic planar π-system is aromatic when it shares 4n+2 π-electrons and antiaromatic when it ...possesses 4n π-electrons. Antiaromatic compounds are predicted to exhibit remarkable charge transport properties and high redox activities. However, it has so far only been possible to measure compounds with reduced aromaticity but not antiaromatic species due to their energetic instability. Here, we address these issues by investigating the single-molecule charge transport properties of a genuinely antiaromatic compound, showing that antiaromaticity results in an order of magnitude increase in conductance compared with the aromatic counterpart. Single-molecule current-voltage measurements and ab initio transport calculations reveal that this results from a reduced energy gap and a frontier molecular resonance closer to the Fermi level in the antiaromatic species. The conductance of the antiaromatic complex is further modulated electrochemically, demonstrating its potential as a high-conductance transistor.
Langmuir films of 4‐{4‐({4‐(trimethylsilyl)ethynylphenyl}ethynyl)phenylethynyl} benzenaminium chloride (1 HCl) undergo anion metathesis when assembled on an aqueous auric acid (HAuCl4) subphase. ...Subsequent transfer to solid supports gives well‐formed Langmuir–Blodgett (LB) monolayers of 1 HAuCl4 in which the trimethylsilyl group serves as the surface contacting group. Photoreduction of the aurate on these monolayers leads to the formation of metallic gold nanoislands, which were distributed over the surface of the film. Electrical properties of these nascent devices were determined by recording current–voltage (I–V) curves with conductive atomic force microscopy (c‐AFM) using the PeakForce tunneling AFM (PF‐TUNA) mode. This gives consistent sigmoidal I–V curves that are indicative of well‐behaved junctions free of metallic filaments and short circuits. The photoreduction of a metal precursor incorporated onto monomolecular films is therefore proposed as an effective method for the fabrication of molecular junctions.
Film buffs: Langmuir films undergo anion metathesis when assembled on an aqueous HAuCl4 subphase. Subsequent transfer to solid supports gives well‐formed Langmuir–Blodgett monolayers in which the trimethylsilyl group serves as the surface contacting group. Photoreduction of the monolayers leads to the formation of metallic gold nanoislands on the surface of the film (see scheme). The electrical properties of these devices have been determined.
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Adsorption sites of molecules critically determine the electric/photonic properties and the stability of heterogeneous molecule–metal interfaces. Then, selectivity of adsorption site is essential for ...development of the fields including organic electronics, catalysis, and biology. However, due to current technical limitations, site-selectivity, i.e., precise determination of the molecular adsorption site, remains a major challenge because of difficulty in precise selection of meaningful one among the sites. We have succeeded the single site-selection at a single-molecule junction by performing newly developed hybrid technique: simultaneous characterization of surface enhanced Raman scattering (SERS) and current–voltage (I–V) measurements. The I–V response of 1,4-benzenedithiol junctions reveals the existence of three metastable states arising from different adsorption sites. Notably, correlated SERS measurements show selectivity toward one of the adsorption sites: “bridge sites”. This site-selectivity represents an essential step toward the reliable integration of individual molecules on metallic surfaces. Furthermore, the hybrid spectro-electric technique reveals the dependence of the SERS intensity on the strength of the molecule–metal interaction, showing the interdependence between the optical and electronic properties in single-molecule junctions.
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The preparation, characterization and electrical properties of Langmuir–Blodgett (LB) films composed of a symmetrically substituted oligomeric phenylene ethynylene derivative, namely, ...4,4′‐1,4‐phenylenebis(ethyne‐2,1‐diyl)dibenzoic acid (OPE2A), are described. Analysis of the surface pressure versus area per molecule isotherms and Brewster angle microscopy reveal that good‐quality Langmuir (L) films can be formed both on pure water and a basic subphase. Monolayer L films were transferred onto solid substrates with a transfer ratio of unity to obtain LB films. Both L and LB films prepared on or from a pure water subphase show a red shift in the UV/Vis spectrum of about 14 nm, in contrast to L and LB films prepared from a basic subphase, which show a hypsochromic shift of 15 nm. This result, together with X‐ray photoelectron spectroscopic and quartz crystal microbalance experiments, conclusively demonstrate formation of one‐layer LB films in which OPE2A molecules are chemisorbed onto gold substrates and consequently COOAu junctions are formed. In LB films prepared on a basic subphase the other terminal acid group is also deprotonated and associates with an Na+ counterion. In contrast, LB films prepared from a pure water subphase preserve the protonated acid group, and lateral H‐bonds with neighbouring molecules give rise to a supramolecular structure. STM‐based conductance studies revealed that films prepared from a basic subphase are more conductive than the analogous films prepared from pure water, and the electrical conductance of the deprotonated films also coincides more closely with single‐molecule conductance measurements. This result was interpreted not only in terms of better electron transmission in COOAu molecular junctions, but also in terms of the presence of lateral H‐bonds in the films formed from pure water, which lead to reduced conductance of the molecular junctions.
Modulation of conductance in Langmuir–Blodgett (LB) films of a symmetrically acid‐terminated oligomeric phenylene ethynylene derivative was achieved by means of the pH value of the aqueous subphase from which the corresponding Langmuir films were transferred. Current–voltage curves of the LB films on a gold substrate (see figure), recorded with an STM tip positioned just above the monolayer, and good fits to the Simmons model indicate that charge flow through the metal |molecule|metal junction occurs by a non‐resonant tunnelling mechanism.
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