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

在这个由美国国家科学基金会大分子、超分子和纳米化学项目赞助的奖项中，明尼苏达大学的Dan Frisbie教授和他的学生旨在了解偶联分子如何导电。特别是，他们的目标是在金属电极之间连接π共轭分子，并探测“跳跃状态”中的传导，在这种状态下，电子被注入分子轨道，并在金属接触之间的电位差的驱动下，沿着分子骨架从一个位点跳到另一个位点。建立和表征这些结构的传导将使我们更好地理解分子结构如何影响分子中的导电性。它还允许将有用的电子功能（例如电流整流）内置到分子线中，这可能会实现新的纳米电子应用。实验方法包括用连续的“点击化学”反应从金属电极上构建pi共轭分子线，以循序渐进的方式连接单体。这种化学反应允许合成几十纳米长度的分子线，同时控制1纳米长度尺度上的线结构。电线将具有许多不同的化学功能，包括氧化还原活性位点，以及可以促进单向电子传递的能级级联（即电流整流）。合成后，金属电极将被插入原子力显微镜（AFM）中，显微镜探针尖端将在受控的纳牛顿压缩载荷下与导线组件接触。以这种方式，原子力显微镜起着电探针站的作用。施加在金属AFM尖端和金属基板之间的电压驱动通过导线的电流，导线的电流-电压（I-V）特性将被测量。I-V特性将被记录为分子长度、化学功能和温度的函数。对这些信息的分析将用于对导电分子的结构-传导关系有更全面的了解。这项提议的更广泛的影响将是在化学和纳米科学的重要新兴领域培养研究生和本科生。特别是学生将发展分子合成、结构表征、原子力显微镜和电测量方面的技能。他们还将对分子和纳米电子学中导电的物理原理有更全面的物理理解。此外，这项资助将为来自双子城大都会地区的三名高中生提供暑期研究机会，让他们在早期阶段接触科学研究，并积极影响他们对化学概念的理解。

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.