Conductance Isotope Effect: A Chemical Tool to Explore the Microscopic Nature of Polarons in Pi-Conjugated Molecular Wires

在化学系大分子、超分子和纳米化学（MSN）项目的支持下，明尼苏达大学双城分校的C. Daniel Frisbie教授和他的学生将探索分子有机半导体的导电机制。对于有机半导体的导电机理，人们仍有许多未解之处，这阻碍了合成性能更高的新型半导体材料的努力。该研究计划利用了一项发现——分子内电导同位素效应——在弗里斯比教授之前的国家科学基金会的支持下完成，这为理解极化子输运机制提供了令人兴奋的机会，就像在物理有机化学中使用动力学同位素效应来理解化学反应的机制一样。该项目涉及有机合成、结构表征、电导测量和理论建模。目的是了解有机半导体的化学结构如何影响和调节它们的电导率。这些材料已经在有机发光器件（OLED tv）和手机显示屏等电子设备中实现了商用化。在实施研究计划的过程中，从事该项目的研究生将获得进入半导体行业工作所需的技能。本科生将获得暑期研究经验。这个项目的技术重点是理解极化子的本质，即分子半导体中事实上的电荷载流子，包括它们沿π共轭链的大小或离域长度，它们的热激活运动，它们的跳跃过渡态，以及所有这些因素对分子结构的敏感性。实验方法侧重于从金表面合成长度达10纳米的pi共轭分子线。整体合成策略的关键创新将是在导线的选定亚基中引入13C和15N同位素标签。长度大于4-5纳米的导线的电导率，其中电荷传输机制是热激活的极化子跳变，然后将使用既定的方法进行第二次电接触以产生金属-分子-金属结。实验设计的总体目标是将分子长度尺度上的电导测量与分子结构的精确单体控制相结合-通过选择性同位素标记增强-将电导与结构联系起来。第二个目标是更好地理解电导同位素效应现象本身；也就是说，它在不同分子线结构中的再现性和通用性。实验工作将通过与量子化学家的合作得到加强，量子化学家的计算将指导实验设计并帮助解释结果。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

With support from the Macromolecular, Supramolecular, and Nanochemistry (MSN) Program of the Division of Chemistry, Professor C. Daniel Frisbie of the University of Minnesota-Twin Cities and his students will explore the mechanisms of electrical conduction in molecular organic semiconductors. Much is still not understood about how organic semiconductors conduct electricity, and this stifles efforts to synthesize new semiconducting materials with enhanced performance. The research plan leverages a discovery – the intramolecular conductance isotope effect – made under Professor Frisbie’s prior NSF support, which offers exciting opportunities to understand polaron transport mechanisms in much the same way the kinetic isotope effect is used in physical organic chemistry to understand the mechanisms of chemical reactions. The project involves organic synthesis, structure characterization, conductance measurements and theoretical modeling. The objective is to understand how the chemical structure of organic semiconductors influences and modulates their electrical conductance. These materials are already commercialized in electronic devices such as organic light emitting devices (OLED TVs) and mobile phone displays. In carrying out the research plan, graduate students working on the project will acquire the skills needed to enter the workforce in the semiconductor industry, for example. Undergraduate students will be offered summer research experience. The particular technical focus of this project is on understanding the nature of polarons, the de facto charge carriers in molecular semiconductors, including their size or delocalization length along pi-conjugated chains, their thermally activated motion, their hopping transition states, and the sensitivity of all these factors to molecular structure. The experimental approach focuses on the synthesis of pi-conjugated molecular wires up to 10 nm in length from gold surfaces. The key innovation to the overall synthesis strategy will be the introduction of 13C and 15N isotopic labels to selected subunits of the wires. The conductance of wires of length greater than 4-5 nm, where the charge transport mechanism is thermally-activated polaron-hopping, will then be measured using an established approach for making a second electrical contact to produce metal-molecule-metal junctions. An overall objective of the experimental design is to combine conductance measurements on molecular length scales with precise monomer-by-monomer control of molecular structure – augmented by selective isotopic labeling – to relate conductance to structure. A second objective is to better understand the conductance isotope effect phenomenon itself; namely, its reproducibility and generality across different molecular wire architectures. The experimental work will be enhanced by collaborations with quantum chemists whose calculations will both guide experimental design and aid in the interpretation of results.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.