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 NH(3). The behavior is intrinsic to the donor-(pi-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-C(10)H(21)//D-pi-A-C(10)H(20)-S-Au. Films contacted by gold tips exhibit rectification ratios of ca. 18 at +/-1 V, whereas those contacted by pentanethiolate (Au-S-C(5)H(11))- and decanethiolate (Au-S-C(10)H(21))-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-pi-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.