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Dependence of Conductance in Cumulene Molecular Wire Angewandte Eine Zeitschrift der Gesellschaft Deutscher Chemiker Chemie www.angewandte.de Akzeptierter Artikel Titel: Unusual Length-Dependence of Conductance in Cumulene Molecular Wire Autoren: Wenjun Xu, Edmund Leary, Songjun Hou, Sara Sangtarash, Teresa González, Gabino Rubio-Bollinger, Qingqing Wu, Hatef Sadeghi, Lara Tejerina, Kirsten Christensen, Nicolás Agraït, Simon Higgins, Colin Lambert, Richard Nichols, and Harry Laurence Anderson Dieser Beitrag wurde nach Begutachtung und Überarbeitung sofort als "akzeptierter Artikel" (Accepted Article; AA) publiziert und kann unter Angabe der unten stehenden Digitalobjekt-Identifizierungsnummer (DOI) zitiert werden. Die deutsche Übersetzung wird gemeinsam mit der endgültigen englischen Fassung erscheinen. Die endgültige englische Fassung (Version of Record) wird ehestmöglich nach dem Redigieren und einem Korrekturgang als Early-View-Beitrag erscheinen und kann sich naturgemäß von der AA-Fassung unterscheiden. Leser sollten daher die endgültige Fassung, sobald sie veröffentlicht ist, verwenden. Für die AA-Fassung trägt der Autor die alleinige Verantwortung. Zitierweise: Angew. Chem. Int. Ed. 10.1002/anie.201901228 Angew. Chem. 10.1002/ange.201901228 Link zur VoR: http://dx.doi.org/10.1002/anie.201901228 http://dx.doi.org/10.1002/ange.201901228 Angewandte Chemie 10.1002/ange.201901228 COMMUNICATION Unusual Length-Dependence of Conductance in Cumulene Molecular Wires Wenjun Xu,+ Edmund Leary,+* Songjun Hou,+ Sara Sangtarash, M. Teresa González, Gabino Rubio- Bollinger, Qingqing Wu, Hatef Sadeghi, Lara Tejerina, Kirsten E. Christensen, Nicolás Agraït, Simon J. Higgins, Colin J. Lambert,* Richard J. Nichols,* and Harry L. Anderson* Abstract: Cumulenes are sometimes described as ‘metallic’ where β is the exponential attenuation factor, which is normally because an infinitely long cumulene would have the band structure in the range 0.2–0.5 Å–1 for a conjugated organic π-system.[2] It of a metal. Here we report the single-molecule conductances of a has been predicted that molecules with low bond length series of cumulenes and cumulene analogues, where the number of alternation (BLA) will give unusual attenuation factors, such as β consecutive C=C bonds in the core is n = 1, 2, 3 and 5. The ≈ 0 (i.e. conductance independent of length) or even β < 0 (i.e. [n]cumulenes with n = 3 and 5 have almost the same conductance, conductance increasing with length).[3,4] Cumulenes are the and they are both more conductive than the alkene (n = 1). This is simplest type of neutral π-system not to exhibit substantial remarkable because molecular conductance normally falls BLA.[5–7] Here we report an experimental and computational exponentially with length. The conductance of the allene (n = 2) is investigation of the length-dependence of charge transport much lower, due to its twisted geometry. Computational simulations through these linear carbon chains. Recently, near length- predict a similar trend to the experimental results and indicate that independent conductances were reported for a set of cyanine the low conductance of the allene is a general feature of dyes, which constitute a class of cationic π-system with BLA ≈ [n]cumulenes where n is even. The lack of length-dependence in the 0.[8] conductances of [3] and [5]cumulenes is attributed to the strong decrease in HOMO-LUMO gap with increasing length. R R • • R R R x R n Long molecules generally conduct electricity less well than short [n]cumulene (n = 2x + 1) polyyne ones, and this can be a problem when designing molecular wires for mediating efficient charge-transport over distances of Figure 1. Cumulenes and polyynes: two types of linear sp carbon chains. several nanometers. When a homologous series of oligomers Manuscript are connected between metal electrodes, and the transport mechanism is coherent tunneling, the conductance G of each Cumulenes and polyynes are the two types of linear chains oligomer typically decreases exponentially with its molecular of sp-hybridized carbon atoms: in cumulenes, the carbon atoms [1] length L according to equation (1), are linked by double bonds, whereas in polyynes there are � ∝ �–!" (1) alternating single and triple bonds (Figure 1). Cumulenes and polyynes have fascinated chemists for many years as models for carbyne, the infinite 1D form of carbon.[5,6] Cumulenes are said [4-7,9-11] [*] W. Xu, Dr. L. Tejerina, Dr. K. E. Christensen, Prof. H. L. Anderson to have a ‘metallic’ electronic structure, because an Department of Chemistry, University of Oxford, Chemistry Research infinitely long cumulene would have a band structure Laboratory, Oxford OX1 3TA, UK characteristic of a metal, with a partially occupied band derived E-mail: [email protected] from the π and π* orbitals. In contrast polyynes have a π-π* gap Dr. E. Leary, Prof. S. J. Higgins, Prof. R. J. Nichols Department of Chemistry, Donnan and Robert Robinson that persists even in long chains, and an infinite polyyne is [10,12] Laboratories, University of Liverpool, Liverpool L69 7ZD, UK expected to be a semiconductor. This difference in Dr. E. Leary, Prof. R. J. Nichols electronic structure is a direct consequence of the different BLA. Surface Science Research Centre, University of Liverpool, Oxford Although there is some BLA in cumulenes,[7] it is much more Street, Liverpool L69 3BX, UK E-mail: [email protected], [email protected] subtle than the alternation between short C≡C and long C-C S. Hou, Dr. S. Sangtarash, Dr. Q. Wu, Dr. H. Sadeghi, Prof. C. J. bonds in polyynes. Accepted Lambert Here, we report an experimental investigation of the Department of Physics, Lancaster University, Lancaster LA1 4YW, conductances of the family of cumulenes 1, 2, 3 and 5 shown in UK E-mail: [email protected] Figure 2. The [4]cumulene 4 is included in our theoretical Dr. M. T. González, Prof. N. Agräit investigation, but has not yet been tested experimentally. All Instituto Madrileño de Estudios Avanzados (IMDEA), Calle Faraday these molecules have two terminal 4-thioanisole substituents for 9, Campus Universitario de Cantoblanco, 28049 Madrid, Spain. binding to gold electrodes. Single-molecule conductances were Prof. N. Agräit, Dr. G. Rubio-Bollinger Departamento de Física de la Materia Condensada, IFIMAC and measured using the scanning tunneling microscopy break- Instituto “Nicolás Cabrera”, Universidad Autónoma de Madrid, E- junction (STM-BJ) method, using a gold tip and a gold 28049, Madrid, Spain. surface,[13] as described in detail previously.[2d] [+] These authors contributed equally to this work. Supporting information for this article is given via a link at the end of the document. 1 This article is protected by copyright. All rights reserved. Angewandte Chemie 10.1002/ange.201901228 COMMUNICATION MeS MeS Ph • Ph Ph Ph SMe 1 (alkene, n = 1) 2 (allene, n = 2) SMe MeS MeS Ph • • • Ph Ph • • Ph SMe 3 ([3]cumulene, n = 3) 4 ([4]cumulene, n = 4) SMe MeS Ph • • • • Ph Figure 3. STM-BJ results on molecules 1–3 and 5 measured with a bias of 0.2 V. (a–d) 2D conductance histograms for the plateau-containing traces. The 5 ([5]cumulene, n = 5) number of traces in each (and percentage of the total) are as follows: n = 1: SMe 1309 (56%), n = 2: 2988 (58%), n = 3: 1139 (47%), n = 5: 3402 (34%). (e) 1D conductance histograms using all data points. (f) 1D conductance histogram Figure 2. Structures of compounds 1–5. (Note that only 1, 2, 3 and 5 have using only points from the end of the plateau distribution (g) Molecular been tested experimentally.) conductances from Gaussian fits to the data in (f); the experimental uncertainties in these values are smaller than the size of the black squares. Compounds 1, 2, 3 and 5 were synthesized as described in the Supporting Information (SI).[14] The alkene 1 was Table 1. Single-molecule conductances, lengths and HOMO-LUMO gaps for compounds 1–3 and 5. recrystallized to give the E isomer, as confirmed by single crystal [15] X-ray diffraction. Allene 2 is racemic. Cumulenes 3 and 5 are [a] L L E (UV) E (DFT) 1 compound log(G/G0) exp calc g g 1:1 mixtures of E and Z isomers (as confirmed by H NMR (nm)[b] (nm)[c] (eV)[d] (eV)[e] spectroscopy); we were unable to separate these stereoisomers. Manuscript We expect them to convert readily under ambient conditions,[16] 1 (n = 1) −4.09 0.94 1.45 3.7 2.21 and to have similar conductances (SI, Figure S7). (1.32) The experimental conductance results for compounds 1–3 2 (n = 2) –5.25 1.01 1.47 4.2 2.74 and 5 are summarized in Figure 3 and Table 1. For reference, (1.34) we have also measured bis-4,4’-(thiomethyl)biphenyl, which has no double bonds between the phenyl rings (SI, Section S3.7). 3 (n = 3) −3.74 1.07 1.63 2.7 1.54 (1.46) The 2D histograms (Figure 3a–d) show how the conductance 2 (G/G0, where G0 = 2e /h) of each junction varies as the STM tip 5 (n = 5) −3.64 1.09 1.80 2.4 1.24 is retracted from the surface (increasing distance, z) for a large (1.64) number of traces (see SI for details on the procedure and data from multiple experimental runs). All the compounds give well- [a] Experimental conductance peak positions from data in Figure 3f; Run to defined plateaus, which become longer as the length of the run variation in peak position is about 0.02, see SI Figure S19. [b] Lengths are calibrated by adding 0.4 nm to the peak position of a Gaussian fit to the total molecule increases, indicating that connection occurs at the distribution of plateau lengths.[2d,17] Values in brackets are derived from the SMe groups (see Table 1 for experimental and calculated 95th percentile.
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