Probing Hopping Conduction in Long, Pi-Conjugated Molecular Wires Assembled by Click Chemistry

In this award sponsored by the Macromolecular, Supramolecular and Nanochemistry Program of the NSF, Prof. Dan Frisbie of the University of Minnesota and his students aim to understand how pai-conjugated molecules conduct electricity. In particular, their goal is to connect pi-conjugated molecules between metal electrodes and to probe conduction in the 'hopping regime' in which electrons are injected into the molecular orbitals and hop from site-to-site along the molecular backbone, driven by the potential difference between the metal contacts. Building and characterizing the conduction of these structures will allow a better understanding of how molecular architecture impacts electrical conductivity in molecules. It will also allow useful electronic functions (e.g. current rectification) to be built into the molecular wires, which may enable new nanoelectronic applications.

The experimental approach involves building pi-conjugated molecular wires from metal electrodes using sequential 'click chemistry' reactions to connect monomers in a step-wise fashion. This chemistry allows the syntheses of molecular wires that are tens of nanometers in length with simultaneous control of the wire architecture on 1 nanometer length scales. Wires will be made with many different chemical functionalities, including redox-active sites, and with cascades of energy levels that can facilitate uni-directional electron transport (i.e., current rectification). After synthesis, the metal electrodes bearing oriented assemblies of wires will be inserted into an atomic force microscope (AFM), where the microscope probe tip will be brought into contact with the wire assembly with a controlled, nano-newton compressive load. In this manner, the AFM functions as an electrical probe station. Voltages applied between the metal AFM tip and the metal substrate drive current through the wires and the current-voltage (I-V) characteristics of the wires will be measured. The I-V characteristics will be recorded as a function of molecular length, chemical functionality and temperature. Analysis of this information will be used to create a more complete understanding of structure-conduction relationships in conducting molecules.

The broader impacts of this proposal will be in the training of graduate students and undergraduates in important emerging areas of chemistry and nanoscience. In particular students will develop skills in molecular synthesis, structure characterization, atomic force microscopy, and electrical measurements. They will also develop a more complete physical understanding of the physical principles involved in electrical conduction in molecules and nanoelectronics, more generally. In addition, this grant will offer summer research opportunities for three high school students from the Twin Cities metropolitan area, exposing them to scientific research at an early stage and positively impacting their understanding of chemical concepts.