Photoswitchable Hopping Transport in Molecular Wires 4 nm in Length

(Chemical Equation Presented). We report the synthesis and electrical characterization of photoswitchable π-conjugated molecular wires. The wires were designed based on the previously reported oligophenyleneimine (OPI) wires [Frisbie et al. Science 2008, 320, 1482] with a slight modification to incorporate the dithienylethene linker (the "photoswitch") into the wire backbone (e.g., PS3-OPI 5; PS stands for the photoswitch, and the number following "PS" indicates its position within the OPI chain). Stepwise arylimine condensation reaction between 1,4-diaminobenzene and terephthalaldehyde (1,4-benzenedicarbaldehyde) was employed to grow these wires from Au surfaces. To insert the "photoswitch" into the wire, 1,4-diaminobenzene was replaced with perfluoro-1,2-bis(2-(4-aminophenyl)-5-methylthien-4-yl)cyclopentene (PS) at specific steps during the wire growth. A variety of surface characterization techniques were employed to investigate the structure of the wires including FT-IR spectroscopy, ellipsometry, cyclic voltammetry (CV), X-ray photoelectron spectroscopy (XPS), and UV-vis spectroscopy. The current-voltage (I-V) characteristics and resistances of the wires were acquired using conducting probe atomic force microscopy (CP-AFM). It was observed that all of the wires switch between high and low conductance modes ("ON" and "OFF" states corresponding to "closed" and "open" forms of the dithienylethene linker, respectively) when irradiated by UV and visible light, respectively. Measuring the temperature dependence of the resistance revealed that the charge transport mechanism in the PS3-OPI 3 wire is tunneling (temperature independent) whereas longer PS3-OPI 5 and PS5-OPI 5 showed Arrhenius temperature dependence which is characteristic of a hopping mechanism. These experiments demonstrate light-based control of transport in molecular wires in the hopping regime, which ultimately may be useful for switching applications in molecular electronics.