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

University Of Minnesota-Twin Cities, Minneapolis MN

Investigators

Abstract

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.