Molecular Rectification:  Self-Assembled Monolayers in Which Donor−(π-Bridge)−Acceptor Moieties Are Centrally Located and Symmetrically Coupled to Both Gold Electrodes

Self-assembled monolayers (SAMs) obtained from 1-(10-acetylsulfanyldecyl)-4-[2-(4-dimethylaminophenyl)vinyl]quinolinium iodide exhibit asymmetric current−voltage (I−V) characteristics. The rectification may be reversibly switched:  it is suppressed when the film is exposed to HCl vapor, the intramolecular charge-transfer axis being inhibited by protonation, but restored when exposed to NH3. The behavior is intrinsic to the donor−(π-bridge)−acceptor moiety, and ambiguity in the assignment has been excluded by matching the alkyl tails on the substrate and contacting STM tip to locate the chromophore midway between the electrodes:  Au−S−C10H21//D−π−A−C10H20−S−Au. Films contacted by gold tips exhibit rectification ratios of ca. 18 at ±1 V, whereas those contacted by pentanethiolate (Au−S−C5H11)- and decanethiolate (Au−S−C10H21)-coated tips have corresponding ratios of ca. 11 and 5, respectively. The I−V curves are different, but when adjusted for thickness the current versus electric field dependence is indistinguishable. Seven dyes are reported:  SAMs with sterically hindered D−π−A moieties, in which the donor and acceptor are twisted out of plane, exhibit rectification, whereas those that are planar or have a weak donor−acceptor combination do not.