Induced rectification from self-assembled monolayers of sterically hindered π-bridged chromophores

Abstract

Self-assembled monolayers (SAMs) obtained via the chemisorption of bis-[1-(10-decyl)-4-{2-(4-methoxynaphthalen-1-yl)vinyl}quinolinium]disulfide diiodide (1) on gold-coated highly oriented pyrolytic graphite (HOPG) substrates exhibit asymmetric current–voltage characteristics with rectification ratios of ca. 30 at ±1 V when contacted by PtIr or Au probes. The ratio decreases to ca. 10 at ±1 V for pentanethiolate-coated gold probes, which locate the donor–(π-bridge)–acceptor moiety approximately midway between the electrodes: Au–S–C₅H₁₁//D–π–A–C₁₀H₂₀–S–Au. The I–V characteristics are different but the current versus electric field dependence is indistinguishable for both types of electrode, the field being calculated for a monolayer thickness of 2.4 ± 0.1 nm and an assumed bilayer thickness of 3.2 nm (i.e. 0.8 nm for pentanethiolate). SAMs obtained from bis-[1-(10-decyl)-4-{2-(4-methoxyphenyl)vinyl}pyridinium]disulfide diiodide (2) exhibit symmetrical I–V characteristics and, unlike the bulkier rectifying analogue above, its D–π–A chromophore is almost planar. This structure–property relationship highlights the significance of steric hindrance for rectification from π-bridged donor–acceptor molecules and consistent I–V characteristics have been obtained from SAMs investigated by scanning tunnelling spectroscopy (STS) as well as electrometer-based techniques with an alkanethiolate-coated mercury droplet electrode.