Self-assembled monolayers of cationic donor-(pi-bridge)-acceptor dyes coupled with anionic donors exhibit asymmetric current-voltage (I-V) characteristics when contacted by Au or PtIr probes. Rectification ratios of 3000 at +/- 1 V are obtained from Au-S-C10H20-A+-pi-D|D-|Au structures in which the cationic moiety is 5-(4-dimethylaminobenzylidene)-5,6,7,8-tetrahydro-isoquinolinium and the counterion is copper phthalocyanine-3,4',4'',4'''-tetrasulfonate (SAM ). Similar behaviour, with a high rectification ratio of 700-900 at +/- 1 V, is also obtained for the CuPc(SO3-)4 salt of 4-[2-(4-dimethylaminonaphthalen-1-yl)-vinyl]-quinolinium (SAM ). The properties are dependent upon the D-pi-A+ moieties which, for these highly rectifying salts, have sterically locked non-planar structures causing the conjugation to be effectively broken. Its effect on the electrical asymmetry is less spectacular when the cationic species is sterically unhindered: the rectification ratio decreases to 15-70 at +/- 1 V for films of the 4-[2-(4-dimethylaminophenyl)-vinyl]-pyridinium salt (SAM ), which has single-ring substituents on opposite sides of the -CH=CH- bridge and an almost planar D-pi-A+ structure. Rectification ratios from the sterically hindered structures are on a par with electrical asymmetries from metal-insulator-metal (MIM) devices where oxide-induced Schottky barriers dominate the behaviour.

A self-assembled arylene-ethynylene molecular wire with a rigid 7 nm long backbone exhibits symmetrical current-voltage (I-V) characteristics and a single-molecule current of 0.35 +/- 0.05 nA at 0.3 V; these data are supported by theoretical calculations.

Self-assembled monolayers (SAMs) of arylene-ethynylene oligomers that incorporate electron-accepting 9-fluorenone and 9-[(4-pyridyl)methylene]fluorene units in the backbone exhibit symmetrical current-voltage (I-V) characteristics, but rectifying junctions with current ratios of 20-80 at +/-1 V have been obtained by protonating these wire-like molecules and ionically coupling with anionic donors.

This article describes arylene–ethynylene molecular wires with 7 nm long backbones and thiolated termini. Cyclic voltammetric studies in solution reveal that the reduction waves of the fluorene, 9-[(4-pyridyl)methylene]fluorene and 9-[di(4-pyridyl)methylene]fluorene units which are embedded in the conjugated π-systems endow these wires with n-doping characteristics. An x-ray crystal structure investigation of 2,7-diiodo-9-[bis(4-pyridinium)methylene]fluorene bis(tetrafluoroborate) 8 established that protonation occurs on both nitrogens of this unit. Self-assembled monolayers of the 7 nm wire 2 on gold substrates exhibit symmetrical current–voltage (I–V) characteristics when contacted by a gold scanning transmission microscope (STM) tip. The dipyridyl functionality of 2 served to obtain a rectifying junction in which the diprotonated cationic wire is the electron accepting component in combination with an adjacent anionic phthalocyanine as the electron-donating layer. This ionic Au–2H22+[CuPc(SO3−)4(Na+)n]2/(4−n) bilayer assembly exhibits rectification with current ratios of 15–50 at ± 1 V. This dramatic change in I–V characteristics upon simple chemical manipulation proves that the conductivity is a property of the wire molecules 2 in the junction. Ab initio calculations suggest that the molecular wires possess useful structural features which allow the conductance of the molecule to be altered by changing the properties of the side groups attached to the fluorene units.