Asymmetries were observed across a monolayer of dimethylanilinoaza[C 60 ]fullerene, (DMA-NC 60 , 1) sandwiched between gold electrodes of relatively large size (0.265 mm 2 ). Two modes of behavior are observed: (1) a sigmoidal and slightly asymmetric behavior, bespeaking of a moderate unimolecular rectifier (rectification ratio of about 2), and (2) above a threshold voltage V 1 (≈0.6 to 1.0 V), a dramatic increase of current to 0.3 to 1 A (as high as 1.36 × 10 7 electrons molecule -1 s -1 at 1.5 V), followed by ohmic behavior from V 1 to a relatively smaller negative bias V 2 (≈ -0.5 V to -0.6 V). At more negative potentials (e.g., at -1.5 V) the current is very small (a few µA). This high asymmetry in current persists for between 10 and 20 cycles of voltage scan. This increased, but ohmic conductivity is probably due to defects that grow at domain boundaries, since this behavior is not seen when very small electrodes (1 µm 2 area) are used. The defects could be stalagmitic filaments of gold which grow from the bottom electrode above V 1 but are broken at the negative bias V 2 , or else they could be due to some unknown electrochemical couple. This device is vaguely reminiscent of Zener diodes or varistors: if operated between, say, + 2 V and -2 V, it is a super-rectifier, with a rectification ratio of up to 20 000 at 1.5 V.

Rectification was observed across a monolayer of 2,6-di[dibutylamino-phenylvinyl]-1-butylpyridinium iodide, (Bu2NφV)2BuPy+I-, 1 sandwiched between gold electrodes. The current is enhanced at a forward bias above 0.5 to 1.0 V, as electrons flow preferentially from the iodide ions to the pyridinium ring, and is as high as 100 000 electrons molecule-1 s-1, with a rectification ratio of up to 90.

We report spectroscopic characterization and unimolecular rectification (asymmetric electrical conduction) measurements of three donor-sigma-acceptor (D-sigma-A) compounds N-(10-nonadecyl)-N-(1-pyrenylmethyl)perylene-3,4,9,10-bis(dicarboximide) (1), N-(10-nonadecyl)-N-(4-[1-pyrenyl]butyl)perylene-3,4,9,10-bis(dicarboximide) (2), and N-(10-nonadecyl)-N-(2-ferrocenylethyl)perylene-3,4,9,10-bis(dicarboximide) (3). These molecules were arranged as one-molecule thick Langmuir-Blodgett monolayers between Au electrodes. In such an "Au | D-sigma-A | Au" sandwich, molecule 1 is a unimolecular rectifier, with rather small rectification ratios (between 2 and 3 at +/-1 V) that decrease upon cycling. Molecule 2 does not rectify. Molecule 3 rectifies, with a rectification ratio of between 14 and 28 at +/-1 V; the through-film rectification and currents persist, even with scans of +/-2 V, for up to 40 cycles of measurement. Qualitative arguments, based on a two-level rectification mechanism, are consistent with the current asymmetries observed in the monolayers of 1 and 3.