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

Abstract

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 π-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.