Self-Assembly of Amino−Thiols via Gold−Nitrogen Links and Consequence for in situ Elongation of Molecular Wires on Surface-Modified Electrodes

The stepwise synthesis of molecular wires on goldnanoparticles and gold electrodes has been performed using amino-terminated and aldehyde-terminated thiols as anchoring groups to provide surface-active sites for imino coupling. X-ray photoelectron spectroscopy provides evidence that 4-mercaptoaniline (1) binds via either substituent, Au−S−C6H4−NH2 (N 1s, 400.1 eV) or Au−N(H2)−C6H4−SH (N 1s, 399.1 eV), therefore depleting the number of reactive amine sites at the surface. In contrast, 4-[(4-mercaptophenylimino)methyl]benzaldehyde (2) binds exclusively via a thiolate link (Au−S−wire−CHO) and, in relation to the former, highlights the significance of the second substituent. Amines compete with thiols for self-assembly on gold and may even bond via deprotonated nitrogen. For instance, 4-{(E)-1,3-dihydro-2H-benzimidazol-2-thione-5-yl)imino]methyl}benzaldehyde (3) binds via a nitrogen of the imidazole ring and the self-assembled monolayer (SAM) exhibits a 2.2 eV shift of the N 1s binding energy (SAM, 398.3 eV; solid sample, 400.5 eV) compared with a 1.0 eV shift for 1. Its in situ formed molecular wires with one to five bridged anthraquinone units exhibit symmetrical current−voltage characteristics, but the behavior alters to rectifying when the electron-accepting sequence is terminated by a 4-(dimethylamino)-1-naphthalene donor. Forward bias corresponds to electron flow from cathode to acceptor and from donor to anode, but the electrical asymmetry is dependent upon the number of bridging units. Molecules with two anthraquinones exhibit an optimum rectification ratio of 55 at ±1 V.