Encoding molecular-wire formation within nanoscale sockets.

We detail here a method to integrate chemical synthesis with the formation of nanoscale electrical “sockets” to allow the in situ construction of three-component molecular wires. Reaction chemistry offers molecular materials an overwhelming amount of diversity and functionality and could provide a method for nanoscale electronics to be synthesized rather than fabricated. Though presently underutilized, this approach can be used to produce multifaceted electrical components on the molecular scale by a combination of selfassembly and programmed reactivity. This approach contrasts with previous work in molecular electronics, which has typically relied on ex situ synthesis of wire components followed by their subsequent insertion into devices. The wires typically used in this context are dithiolated aromatics or other bifunctionalized molecules, which are intended to bridge from one electrode surface to another. 3,5, 21–24] One complication associated with dithiols is their tendency to undergo oxidative oligomerization, which allows them to span gaps much longer than an individual molecule. In addition, these molecules can orient both of their surface-active groups toward the same electrode. 26,27] The present study circumvents these problems through the implementation of a two-step reaction sequence between molecular-scale electrodes: first, we assemble a bifunctional molecule into a monolayer on the electrode surface, such that only one end of the molecule reacts with the electrode; then, we use a second molecule to bridge the gap between the termini of the films (Figure 1a). The two reaction sequences explored in this study are shown in Figure 1b, c. In both cases, a metal surface is reacted with a monothiolated aromatic compound to form a monolayer. We chose platinum as the electrode material because the film is metallic despite being only a few atoms thick on the surface of zirconium oxide. We chose thiols because they are known to readily form strong bonds with platinum surfaces. 30,33] For the monolayers that are terminated with terpyridyl units, we chose transition-metal ions to connect the two ends through well-known coordination chemistry (Figure 1b). Cobalt ions, in particular, are attractive because their complexes are readily formed at room temperature with a variety of terpyridyl ligands. We also studied aldehyde-terminated bisoxazole monolayers and their reactions with aromatic amines (Figure 1c). This reaction is advantageous because it provides a system that is conjugated across the entire length of the molecular bridge. We performed model studies on large-area, planar metal surfaces to assess the feasibility of these linking reactions by first preparing monolayers and then reacting them with the bridging subunits (shown in Figure 2a,b). The reaction between the aldehyde-terminated monolayers 1 and the amine-functionalized aromatics to give 2 (Figure 2a) readily occur and are described elsewhere. The other reaction we investigated is that between the terpyridyl-terminated monolayer 3 and a solution of cobalt(II) diacetate. Using X-ray photoelectron spectroscopy (XPS), we found that that the coverage of both monolayers, 1 and 3, is quantitative and that the layer heights ( 2.3 nm and 1.4 nm, respectively) are consistent with an upright orientation of the molecule. Figure 2c shows the XPS spectra from the cobalt and carbon regions from monolayer 3 as it is reacted with Co(OAc)2. We observed transitions for the Co 2p3/2 (781 eV) and Co 2p1/2 (797 eV) energy levels, which are indicative of a cobalt(II) species bound to the surface. In addition, we observed the smaller satellite peaks (786 and 803 eV) indicative of the formation of cobalt hydroxide or oxidized cobalt species. In the carbon (C1s) region of the XPS spectrum after reaction, a new peak characteristic of the [*] Dr. S. J. Wind Department of Applied Physics/Applied Mathematics and The Center for Electron Transport in Molecular Nanostructures Columbia University New York, NY 10027 (USA) Fax: (+1) 212-854-1909 E-mail: [email protected] Homepage: http://nuckolls.chem.columbia.edu