INSTITUTE OF PHYSICS PUBLISHING NANOTECHNOLOGY Nanotechnology 16 (2005) 695–702 doi:10.1088/0957-4484/16/6/012 Switching of a photochromic molecule on gold electrodes: single-molecule measurements Jin He 1 , Fan Chen 1 , Paul A Liddell 2 , Joakim Andr´ easson 2 , Stephen D Straight 2 , Devens Gust 2 , Thomas A Moore 2 , Ana L Moore 2 , Jun Li 1 , Otto F Sankey 1 and Stuart M Lindsay 1,2,3 1 Department of Physics and Astronomy, Arizona State University, Tempe, AZ 85287, USA 2 Department of Chemistry and Biochemistry, Arizona State University, Tempe, AZ 85287, USA 3 The Biodesign Institute, Arizona State University, Tempe, AZ 85287, USA E-mail: Gust@asu.edu and Stuart.Lindsay@asu.edu Received 10 November 2004, in final form 23 February 2005 Published 5 April 2005 Online at stacks.iop.org/Nano/16/695 Abstract We have studied the electronic changes caused by light-induced isomerization of a photochromic molecule between an open state (that absorbs in the UV to become closed) and a closed state (that absorbs in the visible to become open). Data obtained using a newly developed repetitive break junction method are interpreted in terms of single-molecule resistances of 526 ± 90 M in the open form and 4 ± 1M in the closed form when the molecule is bound between two gold contacts via dithiol linkages. The corresponding ratio of open to closed resistance is in close agreement with the results of ab initio calculations, though the measured resistances are about half of the calculated values. Optical spectroscopy indicates that the photoisomerization occurs in both directions on small gold particles, evaporated thin gold films, and in the break junction experiments. M A supplementary data file is available from stacks.iop.org/Nano/16/695 1. Introduction The utility of molecules as electronic building blocks is generally judged by what we know of their solution properties, or in the case of electrochemical data, their properties when in transient contact with a single electrode. Yet as devices shrink in size, and new functionalities are envisaged, the properties of the molecules when bonded to electrodes or sandwiched between electrodes become ever more important. The fundamental issue of how and why optoelectronic molecules function on metal surfaces at all has yet to be completely understood. It has long been known [1] that the lifetime of an excited state decays as (z /λ) −3 at short distances, z , from a metal surface whereas charge transfer decays as exp −β z where β −1 is of the order of a few ångstr¨ oms or less. So strong is this excited state quenching effect that it overwhelms field-enhancement owing to a sharp metal probe near a chromophore, so that fluorescence is, in fact, suppressed, not enhanced, near such a probe [2]. Yet optical function can be maintained on electrodes as evidenced by photoelectrochemical effects [3] and photoisomerization on an electrode [4]. Recently, Dulic et al [5] showed that a photochromic molecule could be photoisomerized in one direction while it was being studied in a metal–molecule– metal junction. We have been engaged in a parallel study of a related photochromic molecule, using a new technique that allows us to characterize large numbers of molecules. We find both similarities and differences with this earlier study [5] and our results are described here. One outcome of this work is statistically well-characterized values for the resistance of the two isomers of the molecule, based on a newly developed method for determination of single-molecule electrical properties. Two photochromic dithienylethene molecules were investigated. One of these (1) bears a thiol group at each end (for bonding to gold), whereas the other (2) has only a single 0957-4484/05/060695+08$30.00 © 2005 IOP Publishing Ltd Printed in the UK 695