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The most probable single-molecule conductance of each member of a series of 12 conjugated molecular wires, 6 of which contain either a ruthenium or platinum center centrally placed within the backbone, has been determined. The measurement of a small, positive Seebeck coefficient has established that transmission through these molecules takes place by tunneling through the tail of the HOMO resonance near the middle of the HOMO-LUMO gap in each case. Despite the general similarities in the molecular lengths and frontier-orbital compositions, experimental and computationally determined trends in molecular conductance values across this series cannot be satisfactorily explained in terms of commonly discussed "single-parameter" models of junction conductance. Rather, the trends in molecular conductance are better rationalized from consideration of the complete molecular junction, with conductance values well described by transport calculations carried out at the DFT level of theory, on the basis of the Landauer-Büttiker model.

Stimuli-responsive molecular junctions, where the conductance can be altered by an external perturbation, are an important class of nanoelectronic devices. These have recently attracted interest as large effects can be introduced through exploitation of quantum phenomena. We show here that significant changes in conductance can be attained as a molecule is repeatedly compressed and relaxed, resulting in molecular folding along a flexible fragment and cycling between an anti and a syn conformation. Power spectral density analysis and DFT transport calculations show that through-space tunneling between two phenyl fragments is responsible for the conductance increase as the molecule is mechanically folded to the syn conformation. This phenomenon represents a novel class of mechanoresistive molecular devices, where the functional moiety is embedded in the conductive backbone and exploits intramolecular nonbonding interactions, in contrast to most studies where mechanoresistivity arises from changes in the molecule–electrode interface.

Stimuli-responsive molecular junctions, where the conductance can be altered by an external perturbation, are an important class of nanoelectronic devices. These have recently attracted interest as large effects can be introduced through exploitation of quantum phenomena. We show here that significant changes in conductance can be attained as a molecule is repeatedly compressed and relaxed, resulting in molecular folding along a flexible fragment and cycling between an anti and a syn conformation. Power spectral density analysis and DFT transport calculations show that through-space tunnelling between two phenyl fragments is responsible for the conductance increase as the molecule is mechanically folded to the syn conformation. This phenomenon represents a novel class of mechanoresistive molecular devices, where the functional moiety is embedded in the conductive backbone and exploits intramolecular nonbonding interactions, in contrast to most studies where mechanoresistivity arises from changes in the molecule-electrode interface.

We demonstrate here a new concept for a metal-molecule-semiconductor nanodevice employing Au and GaAs contacts that acts as a photodiode. Current-voltage traces for such junctions are recorded using a STM, and the "blinking" or "I(t)" method is used to record electrical behavior at the single-molecule level in the dark and under illumination, with both low and highly doped GaAs samples and with two different types of molecular bridge: nonconjugated pentanedithiol and the more conjugated 1,4-phenylene(dimethanethiol). Junctions with highly doped GaAs show poor rectification in the dark and a low photocurrent, while junctions with low doped GaAs show particularly high rectification ratios in the dark (>103 for a 1.5 V bias potential) and a high photocurrent in reverse bias. In low doped GaAs, the greater thickness of the depletion layer not only reduces the reverse bias leakage current, but also increases the volume that contributes to the photocurrent, an effect amplified by the point contact geometry of the junction. Furthermore, since photogenerated holes tunnel to the metal electrode assisted by the HOMO of the molecular bridge, the choice of the latter has a strong influence on both the steady state and transient metal-molecule-semiconductor photodiode response. The control of junction current via photogenerated charge carriers adds new functionality to single-molecule nanodevices.

As a small molecule possessing both strong H-bond donor and acceptor functions, 1H-imidazole can participate in extensive homo- or heteromolecular H-bonding networks. These properties are important in Nature, as imidazole moieties are incorporated in many biologically-relevant compounds. Imidazole also finds applications ranging from corrosion inhibition to fire retardants and photography. We have found a peculiar behaviour of imidazole during scanning tunnelling microscopy-break junction (STM-BJ) experiments, in which oligomeric chains connect the two electrodes and allow efficient charge transport. We attributed this behaviour to the formation of hydrogen-bonding networks, as no evidence of such behaviour was found in 1-methylimidazole (incapable of participating in intramolecular hydrogen bonding). The results are supported by DFT calculations, which confirmed our hypothesis. These findings pave the road to the use of hydrogen-bonding networks for the fabrication of dynamic junctions based on supramolecular interactions.

It is now possible to reliably measure single molecule conductance in a wide variety of environments including organic liquids, ultrahigh vacuum, water, ionic liquids, and electrolytes. The most commonly used methods deploy scanning probe microscopes, mechanically formed break junctions, or lithographically formed nanogap contacts. Molecules are generally captured between a pair of facing electrodes, and the junction current response is measured as a function of bias voltage. Gating electrodes can also be added so that the electrostatic potential at the molecular bridge can be independently controlled by this third noncontacting electrode. This can also be achieved in an electrolytic environment using a four-electrode bipotentiostatic configuration, which allows independent electrode potential control of the two contacting electrodes. This is commonly realized using an electrochemical STM and enables single molecule electrical characterization as a function of electrode potential and redox state of the molecular bridge. This has emerged as a powerful tool in modern interfacial electrochemistry and nanoelectrochemistry for studying charge transport across single molecules as a function of electrode potential and the electrolytic environments. Such measurements are possible in electrolytes ranging from aqueous buffers to nonaqueous ionic liquids. In this Account, we illustrate a number of examples of single molecule electrical measurements under electrode potential control use a scanning tunneling microscope (STM) and demonstrate how these can help in the understanding of charge transport in single molecule junctions. Examples showing charge transport following phase coherent tunneling to incoherent charge hopping across redox active molecular bridges are shown. In the case of bipyridinium (or viologen) molecular wires, it is shown how electrochemical reduction leads to an increase of the single molecule conductance, which is controlled by the liquid electrochemical gating. This has been referred to as to a "single molecule transistor configuration" with the gate voltage being provided by the controllable potential achieved through the electrochemical double layer. It is shown how the electrolyte medium can control such gating, with ionic liquids providing more efficient gate coupling than aqueous electrolytes. Control of the conductance of viologen molecular wires can also be achieved by encapsulating the viologen redox moiety within a molecular cage, thereby controlling its immediate environment. Molecular conductance can also be gated through multiple redox states. This has been shown for the redox moiety pyrrolo-tetrathiafulvalene, which undergoes single molecule electrochemical transistor gating through three redox states in molecular junctions. Charge transport through this junction follows a two-step hopping mechanism, demonstrating the role of the redox center in electron transfer across the molecular bridge. Recent electrolyte gating studies of rigid, conjugated redox-active metal complexes with tailored terpyridine coordinating ligands and anchors are also presented. These aforementioned studies have all been performed with gold electrode contacts. The Account concludes with recent data showing that it is now possible to study single molecule electrochemical gating with nickel electrodes. This opens up new perspectives for studying interfacial charge transfer with a wide variety of other electrode materials including semiconductor electrodes and also points toward future opportunities for coupling molecular spintronics and nanoelectrochemistry.

Control of quantum interference features: molecular junctions incorporating pyrrolodipyridine-based molecular wires were fabricated by scanning probe methods. Quantum interference effects were introduced by employing meta-connected molecules, and modulated in magnitude by changing the substituent on the pyrrolic N. Dramatic changes in molecular conductance and DFT transport calculations demonstrate the storng effect that small changes in electronic density can have on the overall conductance of a molecular wire.

We use a scanning tunneling microscope to form and electrically interrogate metal - molecule - metal junctions. To form such junctions, molecules must be functionalised with suitable contact groups (e.g. thiols, thioethers, 4-pyridyls, amines) at each extremity. We show here that 2-(methylthio)thiophene units not only act as contact groups, but can reversibly switch between a monodentate configuration (MeS-only) and a bidentate configuration (MeS- and thienyl S) upon junction compression; as the junction is compressed the electrical conductance increases greatly with the increased molecule-contact interaction. This means that such molecules show a large-amplitude mechanical switching behavior; we also show that this is reversible and that switching can occur at a rate of at least 10 kHz. Control molecules with MeSC6H5 contact groups do not show this behavior. This, together with detailed theoretical and transport calculations on the compressed and extended molecular junctions, supports our contention that it is the thienyl S that is involved in the switching mechanism.

Efficient charge transport across long molecular wires enabled by charge-transfer complexation, through Fermi level pinning of interference features., Interference features in the transmission spectra can dominate charge transport in metal–molecule–metal junctions when they occur close to the contact Fermi energy (EF). Here, we show that by forming a charge-transfer complex with tetracyanoethylene (TCNE) we can introduce new constructive interference features in the transmission profile of electron-rich, thiophene-based molecular wires that almost coincide with EF. Complexation can result in a large enhancement of junction conductance, with very efficient charge transport even at relatively large molecular lengths. For instance, we report a conductance of 10–3G0 (∼78 nS) for the ∼2 nm long α-quaterthiophene:TCNE complex, almost two orders of magnitude higher than the conductance of the bare molecular wire. As the conductance of the complexes is remarkably independent of features such as the molecular backbone and the nature of the contacts to the electrodes, our results strongly suggest that the interference features are consistently pinned near to the Fermi energy of the metallic leads. Theoretical studies indicate that the semi-occupied nature of the charge-transfer orbital is not only important in giving rise to the latter effect, but also could result in spin-dependent transport for the charge-transfer complexes. These results therefore present a simple yet effective way to increase charge transport efficiency in long and poorly conductive molecular wires, with important repercussions in single-entity thermoelectronics and spintronics.

The single molecule conductances of a series of bis-2,2':6',2″-terpyridine complexes featuring Ru(II), Fe(II), and Co(II) metal ions and trimethylsilylethynyl (Me3SiC≡C-) or thiomethyl (MeS-) surface contact groups have been determined. In the absence of electrochemical gating, these complexes behave as tunneling barriers, with conductance properties determined more by the strength of the electrode-molecule contact and the structure of the "linker" than the nature of the metal-ion or redox properties of the complex.

The ligands 4'-(4-(methylthio)phenyl)-2,2':6',2″-terpyridine (L(1)), 4'-((4-(methylthio)phenyl)ethynyl)- 2,2':6',2″-terpyridine (L(2)), and bis(tridentate) bridging ligand 2,3,5,6-tetra(pyridine-2-yl)pyrazine (tpp) were used to prepare the complexes [Ru(L(1))2][PF6]2 ([1][PF6]2, [Ru(L(2))2][PF6]2 ([2][PF6]2), [{(L(1))Ru}(μ-tpp){Ru(L(1))}][PF6]4 ([3][PF6]4), and [{(L(2))Ru}(μ-tpp){Ru(L(2))}][PF6]4 ([4][PF6]4). Crystallographically determined structures give S···S distances of up to 32.0 Å in [4](4+). On the basis of electrochemical estimates, the highest occupied molecular orbitals of these complexes fall between -5.55 and -5.85 eV, close to the work function of clean gold (5.1-5.3 eV). The decay of conductance with molecular length across this series of molecules is approximately exponential, giving rise to a decay constant (pseudo β-value) of 1.5 nm(-1), falling between decay factors for oligoynes and oligophenylenes. The results are consistent with a tunnelling mechanism for the single-molecule conductance behavior.

An electrochemical method is presented for fabricating cobalt thin films for single-molecule electrical transport measurements. These films are electroplated in an aqueous electrolyte, but the crucial stages of electrochemical reduction to remove surface oxide and adsorption of alkane(di)thiol target molecules under electrochemical control to form self-assembled monolayers which protect the oxide-free cobalt surface are carried out in an ionic liquid. This approach yields monolayers on Co that are of comparable quality to those formed on Au by standard self-assembly protocols, as assessed by electrochemical methods and surface infrared spectroscopy. Using an adapted scanning tunneling microscopy (STM) method, we have determined the single-molecule conductance of cobalt/1,8-octanedithiol/cobalt junctions by employing a monolayer on cobalt and a cobalt STM tip in an ionic liquid environment and have compared the results with those of experiments using gold electrodes as a control. These cobalt substrates could therefore have future application in organic spintronic devices such as magnetic tunnel junctions.

A scanning tunnelling microscope (STM) has been used to measure the single-molecule conductance of a homologous series of α,ω-alkanedithiols in the room-temperature ionic liquid (RTIL) 1-butyl-3-methylimidazolium trifluoromethanesulfonate (BMIOTf). The alkanedithiol system is chosen because it is a well-known model system for single-molecule conductance evaluation, hence allowing us to evaluate ionic liquids as a medium for such determinations. The STM-based measurements have been made in the RTIL under environmentally controlled conditions with the exclusion of water and oxygen. Conductance values measured in the RTIL for the low (A) and medium (B) conductance groups compare very well with those previously made in different environments including air and organic solvents. Anomalous length dependence of the conductance has been observed for the shorter alkanedithiols in RTILs, and this is compared with similar previous observations for measurements in air. These measurements made on the model alkanedithio...

Because electron transfer is often highly sensitive to bridge length and molecular conformation, changes in these can have a large impact on the conductance of single molecule electrical junctions. In this study, pH is used to control the conformation and effective bridge length of single molecule junctions containing the peptide sequence H(EL)5C (where H stands for histidine, E for glutamic acid, L for leucine, and C for cysteine). The ionizable glutamic acid residues in this peptide result in an oligo-peptide structure highly sensitive to pH. At low pH, the H(EL)5C bridge exists in its more compact α-helical state, while at high pH, deprotonation leads to electrostatic repulsion between the charged carboxylate groups of the glutamic acid residues, promoting more extended conformations. An scanning tunneling microscopy-based method is used to measure the single molecule conductance of Au|H(EL)5C|Au two-terminal junctions in buffered electrolyte solution at low pH (2) and higher pH (6.9). In its more comp...

Some of the possible roles that organometallic chemistry will play in the future are discussed and reflected upon against the backdrop of current global uncertainty. In particular, the manipulation of organometallic excited states, redox states, and magnetic properties for use in energy generation, photocatalysis, and molecular electronics is discussed.

Conjugated polymers derived from thiophene or 3,4-ethylenedioxythiophene derivatives have been widely investigated for use in organic thin-film transistors and organic photovoltaic devices. We describe here the synthesis of a series of conjugated microporous polymers formed by reaction of spiro-bis(2,5-dibromopropylenedioxythiophene) with one of three di- or triethynyl monomers. These polymers have surface areas up to 1631 m2/g and exhibit significant microporosity, suggesting possible applications in the fields of organic electronics or optoelectronics.

The realization of molecular-scale electronic devices will require the development of novel strategies for controlling electrical properties of metal/molecule/metal junctions, down to the single molecule level. Here, we show that it is possible to exert chemical control over the formation of metal/molecule...molecule/metal junctions in which the molecules interact by pi-stacking. The tip of an STM is used to form one contact, and the substrate the other; the molecules are conjugated oligophenyleneethynylenes (OPEs). Supramolecular pi-pi interactions allow current to flow through the junction, but not if bulky tert-butyl substituents on the phenyl rings prevent such interactions. For the first time, we find evidence that pi-stacked junctions can form even for OPEs with two thiol contacts. Furthermore, we find evidence for metal|molecule|metal junctions involving oligophenyleneethynylene monothiols, in which the second contact must be formed by the interaction of the pi-electrons of the terminal phenyl ring with the metal surface.

The temperature dependence of the single-molecule conductance of conformationally flexible alkanedithiol molecular bridges is compared to that of more rigid analogues which contain cyclohexane ring(s). Molecular conductance has been measured with a scanning tunneling microscope (STM) at fixed gap separation by observing the stochastic formation of molecule bridges between a gold STM tip and substrate (the so-called “I(t)” technique). Under these conditions, the junction can be populated by a wide distribution of conformers of alkanedithiol molecular bridges and a strong temperature dependence of the single-molecule conductance is observed. By contrast the rigid analogues that contain cyclohexane ring(s), which cannot form the thermally accessible gauche rich conformers open to the alkanedithiols, show no dependence of the single-molecule conductance on temperature. This comparison demonstrates that it is the conformational flexibility and access to thermally populated higher energy conformers of the linea...

In recent years, several experimental studies have shown that different values of single molecule conductance can be observed for the same type of molecule. Although this observation has been tentatively attributed either to differing molecular conformations or to differing contact geometries, the reason for the different conductance groups remains still unclear. To elucidate this issue, a comparison of four different experimental methods to measure single molecule conductance is presented here for the case of alkanedithiols between gold electrodes, which is considered to be a model system. Three different fundamental conductance groups exhibiting low, medium, and high conductance, respectively, were observed for each molecule. The comparison of measurements performed on surface areas with different step densities reveals that the medium (high) conductance group can be attributed to the adsorption of one (two) contacting S atoms at step sites, whereas the low conductance group can be attributed to molecules adsorbed between flat surface regions. This finding is corroborated by a gap separation analysis for the different conduction groups, by matrix isolation measurements, and by a comparison of the results presented here with conductance measurements performed on self-assembled monolayers. The results presented here help to resolve apparent discrepancies in single molecule conductance measurements and are of general significance for molecular electronics and electrochemistry, since they show how molecular conductance is influenced by the contact morphology and, thus, by the atomic structure of the substrate surface.

We present here a comprehensive study of the single molecule conductance of alkanes with carboxylic-acid terminal groups, investigated with the I(s) and I(t) scanning tunneling microscopy (STM) methods. These methods enable the formation of single molecular bridges between the gold substrate surface and gold STM tip without the need to first form a metallic break junction. The I(s) and I(t) methods have up until this study been used primarily with thiol head groups, and this current study shows that they can be extended to other contact groups. The single molecule conductance of a series of dicarboxylic acids, HOOC−(CH2)n−COOH, has been investigated as a function of length of the alkane chain (N = 4−12), temperature, pH, electrochemical potential, and contact gap separation. The latter parameter, which has been often neglected in single molecule conductance studies, is particularly noteworthy given the strong dependence of molecular conductance on the separation between the STM tip and the surface. In thi...

The ligand Ph2PC(CH3)2PPh2 (2,2-dppp) reacts with Ni(II) salts to give robust square-planar chelate complexes [NiX2(2,2-dppp-P,P‘)] (X = Cl, 1; X = Br, 2) and the salt [Ni(2,2-dppp)2](ClO4)2 (3). The Pd(II) analogues [PdCl2(2,2-dppp)] (4), [PdI2(2,2-dppp)] (5), and [Pd(2,2-dppp)2](BF4)2 (6) have also been made. The crystal structure of 5 (obtained as 5·0.8CH2Cl2) has been determined by X-ray diffraction. The geminal C(CH3)2 group, when compared with related Ph2PCH2PPh2 (dppm) complexes, causes a small compression of the P−C−P chelate ring angle (to 91.3(3)° for 5). With [PtCl2(PhCN)2], 2,2-dppp affords [PtCl2(2,2-dppp)] (7), which undergoes metathesis with NaI to give [PtI2(2,2-dppp)] (8), and [Pt(2,2-dppp)2]Cl2 (9) was obtained by reaction of [PtCl2(PhCN)2] with 2 equiv of 2,2-dppp. Reactions were then attempted with 7 and 9, which are known to result in ring-opening reactions with the corresponding complexes of dppm, but in all cases, only 2,2-dppp chelate complexes could be isolated. Thus, treatment of...

The ligand (Ph2P)2CCH2 (dppen) reacts with [RuCl2(PPh3)3] in CH2Cl2 to give trans-[RuCl2(dppen)2], 1. Complex 1 has reversible Ru(II)/Ru(III) electrochemistry, and on treatment with NOBF4, 1 affords [RuCl2(dppen)2]BF4, 1+. Refluxing solutions of 1 in chlorobenzene affords cis-[RuCl2(dppen)2], 2. With excess RNH2 in chlorobenzene or toluene, 1 reacts to give functionalized diphosphine complexes of general formula trans-[RuCl2({Ph2P}2CHCH2NHR)2] (R: PhCH2, 3a; [CH2]3NH2, 3b; n-octyl, 3c; R-α-CH(Me)Ph, 3d; [CH2]3Si(OEt)3, 3e) characterized principally by a small upfield shift in their 31P{1H} NMR spectra compared to that of 1 and by their distinctive 1H NMR spectra. Complex 3b·4MeOH crystallizes in the space group P1 with a = 11.448(6) A, b = 13.10(1) A, c = 11.178(7) A, α = 93.16(6)°, β = 99.51(5)°, γ = 92.19(5)°, V = 1648(2) A3, and Dcalcd = 1.250 g cm-3 for Z = 1. Complex 3e reacts with the surface hydroxyl groups of indium-doped tin oxide (ITO) electrodes, to give monolayers of anchored, redox-active R...

A series of thioacetate-terminated butadiyne-linked porphyrin oligomers have been synthesized with one to three porphyrin repeat units. Single molecule electrical scanning tunneling microscopy measurements using the I(s) and I(t) methods were used to determine the molecule conductances for this series of oligomers. The molecular conductance shows an exponential falloff with sulfur-sulfur distance with a remarkably low attenuation factor of beta = (0.04 +/- 0.006) A-1.

A guided miniproject involving aspects of Ru(II)-diphosphine complex synthesis and characterization, suitable for advanced undergraduate students, has been devised. It combines teaching of new synthetic and characterization techniques with the opportunity for a taste of research work in the latter part of the project. Students work in teams to prepare dppm (Ph2PCH2PPh2) and the labile starting material, [RuCl2(PPh3)3]. They then individually prepare [RuCl2(dppm)2], characterize it as trans by 1H and 31P NMR spectroscopy, and investigate its thermal isomerization to cis. This part of the project is based on a literature report (Sullivan, B. P.; Meyer, T. J. Inorg. Chem. 1982, 21, 1037). Each student then undertakes a different extension to the project; this can involve a study of Ru(II) complexes of the analogous distibine Ph2SbCH2SbPPh2, of a different diphosphine (e.g. dppe, Ph2PCH2CH2PPh2), or of dppm with a different halide ligand. Assessment is on the basis 50% for laboratory work and results (partly ...