Correlation of electrochemical properties of expanded pyridinium compounds with their single molecule conductance  St  ep  anka Nov  akov  a Lachmanov  a a, b , Jakub  Sebera a , Viliam Kolivo  ska a , Jind  rich Gasior a , G  abor M  esz  aros c , Gr  egory Dupeyre d , Philippe P. Lain  e d, ** , Magdal  ena Hromadov  a a, * a J. Heyrovský Institute of Physical Chemistry of the Czech Academy of Sciences, Dolej  skova 3, 18223, Prague, Czech Republic b University of Chemistry and Technology, Prague, Technick a 5, 166 28, Prague 6, Czech Republic c Research Centre for Natural Sciences, HAS, Magyar Tud osok krt. 2, H-1117, Budapest, Hungary d Univ Paris Diderot, Sorbonne Paris Cit e, ITODYS, UMR CNRS 7086,15 rue J-A de Baïf, 75013, Paris, France article info Article history: Received 23 October 2017 Received in revised form 10 January 2018 Accepted 13 January 2018 Keywords: Expanded pyridinium Electron transfer and electron transport Single molecule conductance Scanning tunneling microscopy break junction Density functional theory abstract A series of four expanded pyridinium molecules were used to investigate the correlation between a single molecule conductance (electron transport) and redox (electron transfer) properties at the elec- trodejelectrolyte interface. Quantum chemical calculations of the transmission functions using DFT and non-equilibrium Green's function approach confirmed LUMO‒mediated electron transport in the break junction experiment. Single molecule conductance data can be rationalized within the framework of the non‒resonant tunneling mechanism. More interestingly, a linear correlation was found between the conductance values and the apparent electron transfer rate constants for three molecules of this series. © 2018 Elsevier Ltd. All rights reserved. 1. Introduction Expanded pyridinium compounds (possessing multiple aro- matic substituents) show versatile electrophoric activity. Depend- ing on their chemical structure (branched or fused) two electrons per pyridinium electrophore can be transferred either stepwise or in a single step allowing for a potential compression or even inversion [1 ,2]. Potential inversion in the reduction of pyridinium electrophore means that the standard redox potential of the second electron transfer has less negative value compared to the standard redox potential of the first electron transfer, i.e. it is energetically more favorable. In this work we present the electrochemical char- acterization of a series of expanded pyridinium molecules (formal redox potentials and heterogeneous electron transfer rate con- stants) and correlate these electrochemical properties with their single molecule conductance values obtained by a scanning tunneling microscopy break junction (STMBJ) technique. This approach will contribute to understanding of the relationship be- tween the electron transfer (ET) and electron transport properties of molecules aiming at their possible use in molecular electronics. Experimental work leading to better comprehension of such a relationship is rare [3e10] in spite of the fact that theoretical studies suggest such correlation [11e20]. Common to all of the theoretical works dealing with redox systems is to consider charge transport in the metaljredox-active moleculejmetal junction as a sequence of individual charge transfer steps (hopping mechanism) with their corresponding charge transfer rate constants being treated within the framework of Marcus ET theory [11e 13,17,20]. A sequential two-step electron transport formulated by Kuznetsov and Ulstrup falls into this category [17]. Alternatively, description of charge transport through a resonant coherent one-step tunneling has been proposed [21‒23]. Since most of the STMBJ experiments are done under the non-resonant conditions the conductance in the junctions of redox-active molecules is also being treated within the framework of a non-resonant direct tunneling (superexchange) model [3, 10]. Experiments to examine the relationship between single molecule conductance and ET rate constant from the electrode to * Corresponding author. ** Corresponding author. E-mail address: magdalena.hromadova@jh-inst.cas.cz (M. Hromadov a). Contents lists available at ScienceDirect Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta https://doi.org/10.1016/j.electacta.2018.01.094 0013-4686/© 2018 Elsevier Ltd. All rights reserved. Electrochimica Acta 264 (2018) 301e311