Short chains of porphyrin molecules can mediate electron transport over distances as long as 5-10 nm with low attenuation. This means that porphyrin-based molecular wires could be useful in nanoelectronic and photovoltaic devices, but the mechanisms responsible for charge transport in single oligo-porphyrin wires have not yet been established. Here, based on electrical measurements of single-molecule junctions, we show that the conductance of the oligo-porphyrin wires has a strong dependence on temperature, and a weak dependence on the length of the wire. Although it is widely accepted that such behaviour is a signature of a thermally assisted incoherent (hopping) mechanism, density functional theory calculations and an accompanying analytical model strongly suggest that the observed temperature and length dependence is consistent with phase-coherent tunnelling through the whole molecular junction.

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 report the electrical conductance at the single molecule level of the oligoyne molecular wires Py-(C[triple bond]C)(n)-Py (n = 1, 2 and 4; Py = 4-pyridyl) using STM-molecular break junction techniques in Au|molecule|Au configurations. The conductance histograms reveal multiple series of peaks attributed to differing contact geometries between the pyridyl head groups and the gold electrodes. Both experimental and theoretical evidence point to the higher conduction groups being related to adsorption of the pyridyl group at more highly coordinated sites such as step edges or alongside gold adatoms. All three conduction groups in the oligoyne series show a remarkably low beta value of (0.06 +/- 0.03) A(-1), that is, the conductance is almost independent of molecular length. 4,4'-Bipyridyl studied under the same conditions does not follow this exponential decay series. Theoretical calculations using a combination of density functional theory and nonequilibrium Green's function formalism support the experimental results. We conclude that oligoynes and polyynes are a very promising class of molecular wires for integration into electronic circuitry.

The measurement of the electrical properties of molecules, down to the single molecule level, has become an experimental reality in recent years. A number of methods are now available for experimentally achieving this feat. The common aim of these methods is to entrap a single or small numbers of molecules between a pair of metallic contacts. This topical review focuses on describing and comparing experimental methods for entrapping and measuring the electrical properties of single molecules in metallic contact gaps. After describing the methods, reasons are tendered for apparent discrepancies in the literature between measured single molecule conductance values, with a focus on the most widely studied alkanedithiol system. Illustrative examples are then presented of the determination of the electrical properties of a range of single molecular systems, in order to highlight the progress which has been made in recent years.

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.

Electrochemical gating at the single molecule level of viologen molecular bridges in ionic liquids is examined. Contrary to previous data recorded in aqueous electrolytes, a clear and sharp peak in the single molecule conductance versus electrochemical potential data is obtained in ionic liquids. These data are rationalized in terms of a two-step electrochemical model for charge transport across the redox bridge. In this model the gate coupling in the ionic liquid is found to be fully effective with a modeled gate coupling parameter, ξ, of unity. This compares to a much lower gate coupling parameter of 0.2 for the equivalent aqueous gating system. This study shows that ionic liquids are far more effective media for gating the conductance of single molecules than either solid-state three-terminal platforms created using nanolithography, or aqueous media.

The length dependence of charge transport is evaluated in three families of porphyrin-based wires. Planar edge-fused tapes and alkyne-linked oligomers mediate efficient charge transport with exceptionally shallow distance dependence, whereas the conductances of the twisted singly linked chains decrease steeply with increasing oligomer length. The planar tapes are more conjugated than the alkyne-linked oligomers, but these two types of wires have similar conductance attenuation factors.

Citation for published item:wil nD h vid gF nd elEyw ediD yd y eF nd yerthelD w rieEghristine nd w rqu¡ esEqonz¡ lezD nti go nd frookeD i h rd tF nd fry eD w rtin F nd ge D il r nd perrerD t ime nd rigginsD imon tF nd v m ertD golin tF nd vowD ul tF nd solt w nriqueD h vid nd w rtinD nti go nd xi holsD i h rd tF nd hw rz herD lther nd q r ¡ % E u¡ rezD ¡ % tor wF @PHITA 9 olvent dependen e of the single mole ule ondu t n e of oligoyneE sed mole ul r wiresF9D tourn l of physi l hemistry gFD IPH @PWAF ppF ISTTTEISTURF Further information on publisher's website:This document is the Accepted Manuscript version of a Published Work that appeared in nal form in The Journal of Physical Chemistry C, copyright c American Chemical Society after peer review and technical editing by the publisher. To access the nal edited and published work see http://dx.doi.org/10.1021/acs.jpcc.5b08877. Additional information:Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-pro t purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details.