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，C。明尼苏达大学双城分校的丹尼尔弗里斯比和他的学生将探索分子有机半导体中的导电机制。 关于有机半导体如何导电，人们仍然不太了解，这阻碍了合成具有增强性能的新半导体材料的努力。 该研究计划利用了一项发现-分子内电导同位素效应-在Frisbie教授先前的NSF支持下进行，这为理解极化子传输机制提供了令人兴奋的机会，就像动力学同位素效应在物理有机化学中用于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其电导。 这些材料已经在电子器件中商业化，例如有机发光器件（OLED TV）和移动的电话显示器。 例如，在执行研究计划的过程中，从事该项目的研究生将获得进入半导体行业劳动力所需的技能。 本科生将获得夏季研究经验。这个项目的技术重点是了解极化子的性质，分子半导体中事实上的电荷载流子，包括它们的大小或沿沿着π共轭链的离域长度，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。实验方法的重点是从金表面的π共轭分子线的长度可达10 nm的合成。 整体合成策略的关键创新将是将13 C和15 N同位素标记引入到所选的丝亚基中。 长度大于4-5 nm的导线的电导，其中电荷传输机制是热激活极化子跳跃，然后将使用建立的方法进行第二电接触以产生金属-分子-金属结来测量。实验设计的总体目标是将分子长度尺度上的联合收割机电导测量与分子结构的精确单体控制（通过选择性同位素标记增强）相结合，以将电导与结构相关联。第二个目标是更好地理解电导同位素效应现象本身，即其在不同分子线架构中的可重复性和一般性。 通过与量子化学家的合作，实验工作将得到加强，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NSF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.