Abstract
If the factors controlling the decay in single-molecule electrical conductance G with molecular length L could be understood and controlled, then this would be a significant step forward in the design of high-conductance molecular wires. For a wide variety of molecules conducting by phase coherent tunnelling, conductance G decays with length following the relationship G = Ae-βL. It is widely accepted that the attenuation coefficient β is determined by the position of the Fermi energy of the electrodes relative to the energy of frontier orbitals of the molecular bridge, whereas the terminal anchor groups which bind to the molecule to the electrodes contribute to the pre-exponential factor A. We examine this premise for several series of molecules which contain a central conjugated moiety (phenyl, viologen or α-terthiophene) connected on either side to alkane chains of varying length, with each end terminated by thiol or thiomethyl anchor groups. In contrast with this expectation, we demonstrate both experimentally and theoretically that additional electronic states located on thiol anchor groups can significantly decrease the value of β, by giving rise to resonances close to EF through coupling to the bridge moiety. This interplay between the gateway states and their coupling to a central conjugated moiety in the molecular bridges creates a new design strategy for realising higher-transmission molecular wires by taking advantage of the electrode-molecule interface properties.
Highlights
Understanding electron transport in metal–molecule–metal (MMM) junctions and identifying molecular wires whose conductance decays only slowly with length is important for the advancement of molecular electronics
While a wide variety of molecular backbones can be synthesised, the nature of the anchor groups that act as connectors to the metallic leads is limited by the strength of their interaction with the metal
Used electrode material in molecular electronics, the choice of anchor can be made from moieties that can form X–Au covalent bonds, such as thiols[2,3] and carbodithioates,[4] moieties that react to give a C–Au bond, such as organostannanes[5] or diazonium salts,[6] and moieties that interact with gold with a coordination bond, such thiomethyls,[7,8,9] amines,[7,10] pyridines,[11,12,13] and phosphines.[10]
Summary
Understanding electron transport in metal–molecule–metal (MMM) junctions and identifying molecular wires whose conductance decays only slowly with length is important for the advancement of molecular electronics. The critical factors which determine conductance in a MMM junctions are the metal–molecule contacts and the structure of the molecular backbone.[1] While a wide variety of molecular backbones can be synthesised, the nature of the anchor groups that act as connectors to the metallic leads is limited by the strength of their interaction with the metal. The nature of the molecular wire bridging the two metallic leads has a strong effect on the exponential attenuation factor β, as demonstrated by Wold et al in 2002.14 Conjugated molecular wires such as oligophenylene exhibit conductance values that decay with increasing number of phenyl units to the extent of β = 0.41 Å−1, and other conjugated systems such as oligophenyleneimine[15] and oligonaphthalenefluoreneimine[16] showed lower attenuation factors of 0.3 Å−1 and 0.25 Å−1, respectively. Low values of β were found in systems such as meso-to-meso bridged oligoporphyrins[13,17,18] (0.040 ± 0.006 Å−1), axially-bridged oligoporphyrins[19] (0.015 ± 0.006 Å−1), oligoynes[20] (0.06 ± 0.03 Å−1), carbodithioate-capped oligophenylene-ethynylene[4] (0.05 ± 0.01 Å−1), 3060 | Nanoscale, 2018, 10, 3060–3067
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