Improved molecular rectification from self-assembled monolayers of a sterically hindered dye

Self-assembled monolayers (SAMS) formed from the reaction of 1-(10-acetylsulfanyldecyl)-4-{2-(4-dimethylaminonaphthalen-1-yl)-vinyl}-quinolinium iodide (1a) and gold-coated substrates exhibit asymmetric current-voltage (I-V) characteristics with a rectification ratio of 50-150 at 1 V. It is the highest to date for a molecular diode, and the improved behavior may be assigned in part to the controlled alignment of the donor-(pi-bridge)-acceptor moieties and in part to steric hindrance, which imposes a nonplanar structure and effectively isolates the molecular orbitals of the donor and acceptor end groups. The molecular origin of the rectification is verified by its suppression upon exposure to HCl vapor, which protonates the dimethylamino group and inhibits the electron-donating properties, with restoration upon exposure to NH3. It is also established by a reduced rectification ratio of ca. 2 at 1 V when the cationic D-pi-A(+) moieties adopt an antiparallel arrangement in self-assembled films of the derivative, bis-[1-(10-decyl)-4-{2-(4-dimethylaminonaphthalen-1-yl)-vinyl}-quinolinium]-disulfide diiodide (1b), which adsorbs via one of its terminal donors without rupture of the sulfur-sulfur bond: Au vertical bar D-pi-A(+)-C10H20-S-S-C10H20-(+)A-pi-D (l(-))(2).