Probing Charge Transport in π-Conjugated Molecular Wires

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Probing Charge Transport in π-Conjugated Molecular Wires

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2016-10

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This thesis is centered on studying the charge transport in molecular wire films with a variety of architectures and investigating the structure-property relationships in these systems. In particular, a comprehensive surface and electrical characterization of self-assembled monolayers (SAMs) of π-conjugated molecular wires is demonstrated. We extensively used conducting probe atomic force microscopy (CP-AFM) as the primary tool for making electrical measurements on molecular wires and elucidating the charge transport mechanisms in these ultrathin organic films. Nanoscopic metal-molecule-metal junctions (molecular junctions) were formed by bringing an Au-coated AFM probe into contact with monolayers on Au substrates and current-voltage (I-V) characteristics of the junction were measured. The contact areas are in the order of 50 nm2 at a compressive load of 1-2 nN and the junctions typically consist of ~50 molecules. We also used cyclic voltammetry (CV) as a complementary method to investigate the kinetics of electron-transfer (ET) in a series of molecular wires terminated with the redox active ferrocene unit. The molecular wires studied in this thesis were all derivatives of the previously reported oligophenyleneimine (OPI) wires and were synthesized on Au surfaces using a stepwise approach based on the imine condensation reaction. The formation of the wires on the surface were monitored by variety of techniques including infrared spectroscopy, ellipsometry, and x-ray photoelectron spectroscopy (XPS). Optical band gaps (Eg) of the wires were determined from UV-Vis spectroscopy measurements and surface coverages were obtained from cyclic voltammetry (CV) data. In this work, we studied the charge transport in a series of molecular wires for which the π-conjugation was intentionally interrupted by saturated cyclohexyl spacers at certain locations within the molecular backbone. The current-voltage (I-V) characteristics of the corresponding molecular junctions were measured by CP-AFM and complementary density functional theory (DFT) calculations were performed on radical cation species to further understand the charge transport mechanism in these wires. The combination of experimental and computational data confirmed the polaronic nature of the charge carriers and suggested that the transport takes place via a thermally-assisted polaron tunneling mechanism. Using the CP-AFM technique, we also explored the electrical properties of three light-switchable π-conjugated molecular wires. These wires consisted of a photochromic dithienylethene linker (the “photoswitch”) embedded in a conjugated OPI backbone. We 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. Finally, the kinetics of electron-transfer (ET) to ferrocene was investigated for a series of ferrocene-capped OPI molecular wire in two different electrolytes. It was found that the variations in standard rate constants (k0) of ET in these wires strongly depends on the choice of the electrolyte. Overall, these experiments offer promising opportunities for further understanding the details of microscopic charge transport in π-conjugated molecular wires.

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University of Minnesota Ph.D. dissertation. October 2016. Major: Chemistry. Advisor: Carl Frisbie. 1 computer file (PDF); x, 160 pages.

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Taherinia, Davood. (2016). Probing Charge Transport in π-Conjugated Molecular Wires. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/183327.

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