The Molecular Conductance Problem: Exploring the Effects of Isotopes and Chemical Doping on Hopping Conduction in Pi-Conjugated Molecular Wires Over 4 nm in Length

In this project, Professor Daniel Frisbie of the University of Minnesota-Twin Cities and his students will determine mechanisms of electrical conduction in organic semiconductors. Organic semiconductors are widely commercialized in OLED TVs and mobile phone displays, yet poor understanding of microscopic conduction mechanisms hinders efforts to expand the applications of these materials to other types of nanoelectronic devices. Using a combination of molecular synthesis, structure characterization, conductance measurements, and theory, Frisbie and his collaborators will determine how molecular structure and externally applied fields determine the rate of charge transport along individual molecules on nanometer length scales. This will provide a foundation for the development of new organic semiconductor technology advancing both materials and manufacturing readiness. A significant broader impact of the project will be the workforce training of two PhD students in organic and physical chemistry. Technical aims of the project are (1) to synthesize new pi-conjugated molecular wires, including wires with 13C isotopic labels and precisely positioned redox units; (2) to determine the mechanism of the recently discovered Conductance Isotope Effect (CIE) for polaron hopping in molecular wires; and (3) to control carrier doping levels in molecular wires as a means to estimate intramolecular polaron mobility and polaron-polaron interactions. The synthesis efforts will focus on Sonogashira coupling and classic click chemistries to build conjugated molecular wires up to 10 nm in length. For long wires above ~4 nm in length the general mechanism of charge transport is (positive) polaron hopping as determined using conducting probe atomic force microscopy (CP-AFM). Under prior NSF support, the principal investigator discovered that 13C isotopic substitution in the backbone of long wires (> 4 nm) results in a very large and unprecedented CIE, which may be a useful chemical tool for understanding charge/polaron delocalization. In this project, 13C labeling will be introduced into new molecular wire architectures to establish the generality of the effect. Further, external magnetic fields will be applied during CP-AFM conductance measurements on the 13C labeled and unlabeled wires to assess whether there is significant magnetoresistance behavior. The presence of magnetoresistance for labeled wires, but not for unlabeled wires, would be strong evidence that the underlying mechanism for the CIE is polaron-nuclear spin coupling (a hyperfine interaction). In parallel, controlled doping experiments by Frisbie and colleagues will examine wire conductance as a function of the number of polarons per molecular wire. This experiment, enabled by the new wire synthesis chemistries, will allow extraction of the single molecule polaron mobility as a function of doping level and wire architecture, answering long-standing questions regarding the ultimate limits on conduction in pi-conjugated molecules. In general, 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.