Molecular diodes and ultra-thin organic rectifying junctions: Au-S-CnH2n-Q3CNQ and TCNQ derivatives.

Attempts to obtain derivatives of the molecular diode, 2-{4-[1-cyano-2-(1-(omega-acetylsulfanylalkyl)-1H-quinolin-4-ylidene)-ethylidene]-cyclohexa-2,5-dienylidene}-malonitrile [1, CH(3)CO-S-C(n)H(2n)-Q3CNQ], from either 2,3,5,6-tetrafluoro-7,7,8,8-tetracyano-p-quinodimethane (TCNQF(4)) or 2,3,5,6-tetramethyl-7,7,8,8-tetracyano-p-quinodimethane (TMTCNQ) result in ring closure via the cyano group of the pi-bridge and yield di-substituted analogues: 2-{2,3,5,6-tetrafluoro-4-[6-(10-acetylsulfanyldecyl)-3-(1-(10-acetylsulfanyldecyl)-1H-quinolin-4-ylidenemethyl)-6H-benzo[f][1,7]naphthyridin-2-ylidene]-cyclohexa-2,5-dienylidene}-malonitrile (2a) and the 2,3,5,6-tetramethyl derivative (2b). Self-assembled monolayers (SAMs) of these donor-(pi-bridge)-acceptor molecular diodes exhibit asymmetric current-voltage characteristics with electron flow at forward bias from the top contact to surface C(CN)(2) groups. Comparison is made with I-V curves from ultra-thin films of an organic rectifying junction in which TCNQ(-) is electron-donating and a donor-(sigma-bridge)-acceptor diode in which TCNQ degrees is electron-accepting.

Molecular rectification: stabilised alignment of chevron-shaped dyes in hybrid SAM/LB structures in which the self-assembled monolayer is anionic and the Langmuir-Blodgett layer is cationic.

The chevron-shaped dye, N-butyl-2,6-bis-[2-{4-(2-(4-dibutylaminophenyl)-vinyl)-phenyl}-vinyl]-pyridinium iodide, has a hydrophilic apex that is electron-accepting and hydrophobic it-bridged limbs that are electron-donating. It forms non-centrosymmetric monolayers at the air-water interface and when deposited, its LB films exhibit second-harmonic generation and asymmetric current-voltage (I-V) characteristics. However, the behaviour is short lived: molecular reorganisation causes the absorption maximum to shift from ca. 400 to 700 nm with suppression of the SHG and rectification. Long-term stability results when the cationic dye is LB deposited on an anionic surface, for example, a SAM formed via chemisorption of sodium 3-mercapto-1-propanesulfonate on a gold-coated substrate. The hybrid Au/SAM/LB device exhibits asymmetric I-V curves with rectification ratios of ca. 25 at +/- 1 V when investigated by scanning tunnelling spectroscopy and contacted by PtIr probes.