Recti®cation by a single molecule Robert M. Metzger * Department of Chemistry, Laboratory for Molecular Electronics, University of Alabama, Tuscaloosa, AL 35487-0336, USA Abstract Unimolecular recti®cation of electrical current was con®rmed between 105 and 370 K in a single molecule, g-hexadecylquinolinium tricyanoquinomethanide, 1, in which the ground state is zwitterionic D  ±p±A  ) with a large dipole moment 43 Debyes) while the ®rst excited state is undissociated: D 0 ±p±A 0 ) with a smaller estimated moment 3±9 Debyes). The intervalence optical absorption band connecting these two states is strongly hypsochromic, and other spectroscopic measurements all con®rm this assignment. This step to unimolecular electronics, i.e. a recti®er only 2.3 nm thick, is a realization of the 1974 proposal by Aviram and Ratner. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Hexadecylquinolinium tricyanoquinomethanide; Aviram±Ratner mechanism; Unimolecular electronics; Unimolecular recti®er 1. Introduction Unimolecular electronics started with the 1974 proposal by Aviram and Ratner [1], that an appropriately designed single molecule may rectify electrical current. The scienti®c conferences organized by the late F.L. Carter stirred further interest in ``molecular electronics'', the paradigm that elec- tronic devices consisting solely of molecules are possible and feasible [2±4]. By now some important milestones towards that goal have been reached. 1. Tunneling currents through aliphatic chains are larger than through aromatic chains [5]. 2. The resistance of a single 1,4-benzenedithiol molecule bonded to two Au electrodes was several MO [6]. 3. The quantum of electrical resistance 12 kO) was measured at room temperature between a single carbon nanotube, glued to a conducting AFM tip, and liquid Hg [7]. 4. The Aviram±Ratner mechanism [1] was confirmed in conductivity measurements through a monolayer of Z-b-N-hexadecyl-4-quinolinium)-a-cyano-4-styryldi- cyano-methanide or g-n-hexadecyl)quinolinum tricya- noquinodimethanide, C 16 H 33 -Q3CNQ, 1: this is the first proven two-terminal unimolecular electronic device [8]. We summarize here our results [8±22]. Aviram and Ratner [1] proposed that a single D±s±A organic molecule 2 could be a recti®er of electrical current Fig. 1). The D end tetrathiafulvalene) is a good organic one-electron donor but poor acceptor), s is a covalent saturated ``sigma'') bridge, and A tetracyanoquinodi- methane) is a good organic one-electron acceptor but poor donor). This D±s±A molecule, placed between two metal electrodes M 1 and M 2 , should form the recti®er M 1 |D±s± A|M 2 , with a working thickness of 2±3 nm, and easy electron transfer from M 2 to M 1 , facilitated by the ``down-hill'' through-molecule tunneling from the electronically excited state D  ±s±A  to the ground state D 0 ±s±A 0 . The practical hurdles were: 1) the HOMOS and LUMOs of even potent organic one-electron donors and one-electron acceptors do not easily match the Fermi levels of inorganic metals; 2) it is not easy to assemble defect-free monolayers by Langmuir± Blodgett LB) or by covalent ``self-assembly'' SA) tech- niques); 3) one must make reliable electrical contacts with macroscopic metal electrodes. This challenge of making a unimolecular rectifying device took about 20 years. The efforts by the groups of Metzger and Panetta, Metzger and Cava, Sambles and Sandman, and Sambles and Ashwell have also been reviewed [10,11,16±30]. Synthetic Metals 124 2001) 107±112 * Tel.: 1-205-348-5952; fax: 1-205-348-9104. E-mail address: rmetzger@bama.ua.edu R.M. Metzger). 0379-6779/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII:S0379-677901)00440-4