Single-Molecule Conductance in a Series of Extended Viologen Molecules Viliam Kolivos ̌ ka, † Michal Vala ́ s ̌ ek, ‡,∥ Miroslav Ga ́ l, †,⊥ Romana Sokolova ́ , † Jana Bulíč kova ́ , † Lubomír Pospís ̌ il, †,‡ Ga ́ bor Me ́ sza ́ ros, § and Magdale ́ na Hromadova ́ * ,† † J. Heyrovsky ́ Institute of Physical Chemistry of ASCR, v.v.i., Dolejs ̌ kova 3, 18223 Prague, Czech Republic ‡ Institute of Organic Chemistry and Biochemistry of ASCR, v.v.i., Flemingovo n. 2, 16610 Prague, Czech Republic § Institute of Materials and Environmental Chemistry, Research Centre for Natural Sciences, Hungarian Academy of Sciences, Pusztaszeri strasse 59−67, H-1025 Budapest, Hungary * S Supporting Information ABSTRACT: Single-molecule conductance in a series of extended viologen molecules was measured at room temperature using a gold−molecule−gold scanning tunneling microscopy break junction arrangement. Conductance values for individual molecules change from 4.8 ± 1.2 nS for the shortest compound to 2.9 ± 1.0 nS for the compound with six repeating units and length of 11 nm. The latter value is almost 3 orders of magnitude higher than that reported for all-carbon-based aromatic molecular wires of comparable length. On the basis of the length of the molecules, an attenuation factor of only 0.06 ± 0.004 nm −1 (0.006 ± 0.0004 Å −1 ) was obtained. To the best of our knowledge, this is the smallest value reported for the conductance attenuation in a series of molecular wires. SECTION: Physical Processes in Nanomaterials and Nanostructures S earch for a robust molecular wire, which could transport charge repeatedly over long distances, became one of the major important goals of the molecular electronics. 1−8 Several groups have studied series of molecules with repeating motifs allowing the effective manipulation of the molecular length in order to achieve the long distance electron transfer through molecular wires. 9−18 Extended viologens belong to a group of rigid molecules of variable molecular length, which are easily dopable with electrons. 19−23 They were synthesized with the purpose to be used as molecular wires in the molecular electronics devices. 24,25 Structurally simpler 4,4′-bipyridine and dialkyl viologens represent today one of the most frequently studied models of the conducting organic molecular wires. 26−41 The extended viologen molecules used in this communica- tion are labeled in the text according to the number of repeating units n as 1−6, whereas the number of extended viologen units in each molecule of the series equals to n − 1. Redox and adsorption properties of these extended viologen molecules have been studied previously. 23,42 It was confirmed that these molecules can withstand switching between multiply oxidized and reduced states without compromising their chemical integrity, and each extended viologen unit behaves as a spatially localized π-conjugated electron acceptor. 23 Chart 1 shows the schematic representation of the gold− molecule−gold STM break junction, which was formed repeatedly by driving the gold STM tip in and out of the gold substrate as described previously by Tao et al. 27,43 The inset shows the chemical formula of the extended viologen molecules 1 to 6 used in this work. The current−distance withdrawal curves were obtained both in the absence and presence of the extended viologen Received: December 12, 2012 Accepted: January 29, 2013 Published: January 29, 2013 Chart 1. Schematic Representation of the Gold−Molecule− Gold Contact Junction with Extended Viologen Molecule Containing n = 2 Repeating Units a a Generalized chemical formula is shown in the inset. Letter pubs.acs.org/JPCL © 2013 American Chemical Society 589 dx.doi.org/10.1021/jz302057m | J. Phys. Chem. Lett. 2013, 4, 589−595