Bianthrone in a Single-Molecule Junction: Conductance Switching with a Bistable Molecule Facilitated by Image Charge Effects † Samuel Lara-Avila, ‡ Andrey Danilov, ‡ Victor Geskin, § Saı ¨d Bouzakraoui, § Sergey Kubatkin, ‡ Je ´ro ˆme Cornil,* ,§ and Thomas Bjørnholm | Department of Microtechnology and Nanoscience, Chalmers UniVersity of Technology, KemiVa ¨gen 9, S-41296 Go ¨teborg, Sweden, SerVice de Chimie des Mate ´riaux NouVeaux, UniVersite ´ de Mons, Place du Parc 20, B-7000 Mons, Belgium, and Nano-Science Center & Department of Chemistry, UniVersity of Copenhagen, UniVersitetsparken 5, DK-2100 Copenhagen, Denmark ReceiVed: July 1, 2010; ReVised Manuscript ReceiVed: September 23, 2010 Bianthrone is a sterically hindered compound that exists in the form of two nonplanar isomers. Our experimental study of single-molecule junctions with bianthrone reveals persistent switching of electric conductance at low temperatures, which can be reasonably associated with molecular isomerization events. Temperature dependence of the switching rate allows for an estimate of the activation energy of the process, on the order of 120 ( 50 meV. Quantum-chemical calculations of the potential energy relief of neutral bianthrone and its anion, including identification of transition states, yields the isolated molecule isomerization barriers too high vs the previous estimate, though compatible with previous experimental studies in solution. Nevertheless, we show that the attraction of the anion in the vicinity of the metal surface by its image charge can change the energetic landscape, in particular, by significantly reducing the barrier to values compatible with the observed switching behavior. Introduction Molecular electronics is a fast developing field which has a potential to dramatically extend the miniaturization limits of the integrated circuit technology. The ultimate ambition of molecular electronics is to implement the desired electronic functionality at a single-molecule level by a proper molecular design. 1 Spectacular recent advances in experimental techniques made it possible to bridge a single molecule to macroscopic electrodes and to study current transport through a molecule in two- and three-terminal devices. 2 Molecular resonance tunneling devices, 3 single electron transistors, 4,5 and switches 6 were demonstrated. In view of possible device applications, molecular switches appear to be the most promising candidates as CMOS succes- sors. First of all, switches are the basic elements for digital memory and field programming gate arrays (FPGA) - the two large groups of electronic devices that will benefit the most from radical miniaturization to be provided by molecular implementa- tion. Moreover, both memory chips and FPGAs are regular arrays of many identical units and the technology shall clearly benefit from the fact that chemical synthesis can provide moles of identical functional kernels. Three different design concepts for molecular switches were suggested so far: (I) switches operating by reversible isomer- ization without pronounced changes in molecular shape like tautomerization, 7 opening/closing of conjugation path, 8 or in- tramolecular charge redistribution; 9 (II) switches exploiting rotation or rolling of the whole molecule; 10 (III) conformational switches based on isomerization between two manifestly dif- ferent metastable shapes predetermined by molecular design (two states can differ, for example, by folding 11,12 or twisting 13,14 a part of the molecule around flexible link or by sliding 15 or circumrotation 16 of a molecular component in mechanically interlocked supramolecular structures). The main challenge on the way to a wafer-scale fabrication lies in a poor reproducibility of single-molecule devices. Being identical in solution, molecular kernels reveal big dispersion of operational parameters when placed in a solid-state environment. For example, it is currently a well-established fact that molecular electronic spectrum and addition energies can be dramatically affected by image charges generated in the substrate and electrodes. 4,17,18 Small variations in molecular placement/orienta- tion redistribute these image charges, which makes operational parameters of class I switches vulnerable to environmental effects and difficult to predict judging from the properties of functional molecules measured in solution. Class II switches suffer from high sensitivity of molecule-to-electrode coupling to intimate details of atomic arrangement within the contact area; a single atom displacement can, and often does dramatically affect the switching behavior. 10 Conceptually, the conformational switches have a potential to be less prone to environmental uncertainties. In this paper, we present a conformational switch based on the bianthrone (BA) molecule, which has two well-defined conformations, 19 as illustrated in Figure 1. The structural constraint that ensures the existence of two forms cannot be overruled by any environmental perturbations as long as the molecule retains its chemical identity. One can therefore expect that the switching behavior will be a robust feature of bianthrone single molecule junctions. Indeed, we observed the hysteretic switching between two states with different conductancies in all measured samples. However, the data analysis shows that the environment does play an important role: the barrier which separates two conformations is dramatically reduced. We argue that this effect † Part of the “Mark A. Ratner Festschrift”. * Corresponding author. E-mail: Jerome.Cornil@umons.ac.be. ‡ Chalmers University of Technology. § Universite ´ de Mons. | University of Copenhagen. J. Phys. Chem. C 2010, 114, 20686–20695 20686 10.1021/jp1060667  2010 American Chemical Society Published on Web 10/19/2010