The unimolecular rectifier: unimolecular electronic devices are coming ...

Unimolecular electronic devices use the energy levels, or conformations, of one molecule or a very few molecules, and are contacted electrically from the outside. When made practical, these devices, the 'advanced guard' of un imolecular electronics, should leapfrog below the 50 nm limit of convention al inorganic microelectronics. Aviram and Ratner proposed in 1974 rectifica tion of electrical current through a single molecule D-sigma-A, 1, where D= good one-electron donor, sigma=covalent, saturated 'sigma' bridge, A=good o ne-electron acceptor, because, from the undissociated ground state D-0-sigm a-A(0), the first zwitterionic excited state D+-sigma-A(-) is accessible un der electrical bias. Many such D-sigma-A molecules were prepared. We found unimolecular rectification in a molecule, gamma-hexadecylquinolinium tricya noquinomethanide, 2, in which the ground state is zwitterionic: D+-pi-A(-), while the first excited state is undissociated: D-0-pi-A(0).