Unimolecular rectifier

Single organic molecules can function as rectifiers (one-way conductors) of electrical current: this was first proposed in 1974 by Arieh (later Ari) Aviram, then at IBM, and Mark Ratner, then at New York University: their publication was the first serious and concrete theoretical proposal in the new field of molecular electronics (UE).

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Their proposed rectifying molecule was designed so that electrical conduction within it would be favored from the electron-rich subunit or moiety (electron donor) to an electron-poor moiety (electron acceptor), but disfavored (by several electron Volts) in the reverse direction.

Many potential rectifying molecules were studied by the groups of Robert Melville Metzger, Charles A. Panetta, and Daniell L. Mattern (University of Mississippi) between 1981 and 1991, but were not tested successfully for conductivity.

This proposal was verified in two papers in 1990 and 1993 by the groups of John Roy Sambles (University of Exeter, UK) and Geoffrey Joseph Ashwell (Cranfield University, UK), using a monolayer of hexadecylquinolinium tricyanoquinodimethanide sandwiched between dissimilar metal electrodes (magnesium and platinum) [2,3] and then confirmed in three papers in 1997 and 2001 by Metzger (now at the University of Alabama) and coworkers, who used identical metals (first aluminum, then gold) [4,5,6]. These papers use Langmuir-Blodgett monolayers (one molecule thick), with maybe 10^14 to 10^15 molecules measured in parallel. About nine similar rectifiers of vastly different structure have been found by Metzger's group between 1997 and 2006 [7]. Single molecules bonded covalently to gold have been studied by scanning tunneling spectroscopy, and some of them are unimolecular rectifiers, studied as single molecules, as shown by the groups of Luping Yu (University of Chicago) and Ashwell (now at the University of Bangor, Wales).

The driving idea in UE (also called molecular-scale electronics) is that properly designed “electroactive” molecules, of the size between 1 and 3 nm in length, can supplant silicon-based devices in the ultimate reductio ad absurdum of circuit component sizes, providing the ultimate concomitant increase in integrated circuit speeds. However, amplification has not yet been realized, and the chemical interaction between metal electrodes and molecules are complex.

References

1. Aviram, A and Ratner, M.A.; “Molecular Rectifier” Chemical Physics Letters 29: 277 (1974)).
2. Ashwell, G.J., Sambles, J.R., Martin, A.S., Parker, W.G. and Szablewski, M.; J. Chem. Soc. Chem. Commun. 1374 (1990).
3. Martin, A.S., Sambles, J.R. and Ashwell, G.J.; Phys. Rev. Lett. 70: 218 (1993).
4. Xu, T., Peterson, I.R., Lakshmikantham, M.V. and Metzger, R.M.; Angew. Chem. Intl. Ed. 40: 1749 (2001).
5. Metzger, R.M.; Chen, B., Höpfner, U., Lakshmikantham, M.V., Guillaume, D, Kawai, T., Wu, X., Tachibana, H, Hughes, T.V., Sakurai, T.V., Baldwin, J.W., Hosch, C., Cava, M.P., Brehmer, L. and Ashwell, G.J.; J. Am. Chem. Soc. 119: 10455 (1997).
6. Metzger, R.M., Xu, T. and Peterson, I.R.; J. Phys. Chem. B105: 7280 (2001).
7. Metzger, R.M.; Chem. Physics 326, 176-187 (2006).