Single Molecular Conductance of Tolanes: Experimental and Theoretical Study on the Junction Evolution Dependent on the Anchoring Group Wenjing Hong, † David Zsolt Manrique, ‡ Pavel Moreno-García, †,∥ Murat Gulcur, § Artem Mishchenko, † Colin J. Lambert,* ,‡ Martin R. Bryce,* ,§ and Thomas Wandlowski* ,† † Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, CH-3012 Bern, Switzerland ‡ Department of Physics, Lancaster University, Lancaster LA1 4YB, England § Department of Chemistry, Durham University, Durham DH1 3LE, United Kingdom ∥ Instituto de Física, Beneme ́ rita Universidad Autó noma de Puebla, Apartado Postal J-48, Puebla 72570, Me ́ xico * S Supporting Information ABSTRACT: Employing a scanning tunneling microscopy based beak junction technique and mechanically controlled break junction experiments, we investigated tolane (diphenylacetylene)-type single molecular junctions having four different anchoring groups (SH, pyridyl (PY), NH 2 , and CN) at a solid/liquid interface. The combination of current−distance and current−voltage measurements and their quantitative statistical analysis revealed the following sequence for junction formation probability and stability: PY > SH > NH 2 > CN. For all single molecular junctions investigated, we observed the evolution through multiple junction configurations, with a particularly well-defined binding geometry for PY. The comparison of density functional theory type model calculations and molecular dynamics simulations with the experimental results revealed structure and mechanistic details of the evolution of the different types of (single) molecular junctions upon stretching quantitatively. ■ INTRODUCTION The formation of molecular junctions is a prerequisite for addressing charge transport in molecular components and devices. 1−3 Several approaches have been developed to monitor and to characterize charge transport in nanoscale junctions. They include techniques for formation of molecular junctions 4 such as scanning tunneling microscopy (STM), 5−7 conductive- probe atomic force microscopy (CP-AFM), 8−10 scanning tunneling microscopy break junctions (STM-BJs), 3,11−15 crossed wires, 16 nanoparticle assemblies, 17 mechanically con- trolled break junctions (MCBJs), 2,18−22 electromigration break junctions (E-BJs), 23,24 nanopores, 25 and liquid metal junctions employing mercury 26,27 or gallium−indium eutectic alloys (EGaIn). 28 A critical issue in all these experimental techniques is the electrical contact between single and/or small ensembles of molecular wires and the macroscopic leads. The ideal molecular anchoring group should form reproducible and mechanically stable contacts with well-defined binding sites. To optimize charge transport, a second essential property is the strong electronic coupling between the ends of the molecule and the macroscopic (metal) electrodes. 29,30 Chemical syn- thesis offers unique possibilities to tailor anchoring groups to specific contact sites. This strategy is the main topic of the present paper. However, there are also promising alternative approaches which are based on surface grafting via covalent bonds, such as carbon−carbon, 31 metal−carbon, 32 silicon− carbon. 33 Amino (NH 2 ), pyridyl (PY), and thiol (SH) groups are the most frequently used “chemical” anchoring groups in charge transport studies of single-molecule junctions because of their rather stable binding to metals (often gold electrodes) as well as their reasonable electrical coupling in nanoscale hetero- junctions with contact to macroscopic metal leads. Thiol was the first and still is the most widespread anchoring group in fundamental charge transport studies with single molecular junctions because of its strong binding to many metals, such as gold, copper, and silver. 2,3,36 Charge transport through SH- bound molecular junctions is dominated by hole transport through the highest occupied molecular orbital (HOMO) because this is the closest level to the metal Fermi level. 34,35 Several authors have demonstrated that SH linkers bind to different sites on metal surfaces, such as gold, which often leads to a wide spread in experimentally measured conductan- ces. 12,36,37 Furthermore, the strong covalent bond between SH and in particular gold surfaces leads to distinct changes in the surface crystallography, such as a weakening of the Au−Au spacing between the first and second metal layers, 38 which may Received: October 27, 2011 Published: December 16, 2011 Article pubs.acs.org/JACS © 2011 American Chemical Society 2292 dx.doi.org/10.1021/ja209844r | J. Am. Chem.Soc. 2012, 134, 2292−2304