Abstract: This talk will describe fundamental measurements aimed at understanding the conductance of pi-conjugated molecules connected between metal electrodes. We explore conductance in two regimes: 

• The tunneling regime, applicable to short molecules. 
• The polaron hopping regime, which pertains to longer molecules. 

In the tunneling regime, quantitative analysis of current-voltage (I-V) characteristics is aided greatly by application of an analytical single level model, which allows extraction of the HOMO or LUMO offset from the electrode Fermi level e and the electrode-orbital coupling G. We show that the single level model applies extremely well to common molecular junctions and we are able to relate the junction parameters e and G to molecular structure and the nature of the metal-molecule contacts. 

Our experiments in the polaron hopping regime rely on high yield click-like chemistry to build pi-conjugated molecular wires up to 10 nm in length from metal substrates. We probe the conductance and I-V behavior as a function of wire length and we observe a clear crossover from tunneling to hopping near 4 nm. Transport for long wires > 4 nm is thermally activated and we have recently observed a very strong conductance isotope effect (CIE), which may allow us to understand transition states and polaron localization effects for intramolecular conductance along pi-conjugated chains. In general, there are many opportunities to understand charge transport kinetics in molecules in much the same way that reaction kinetics are explored in classical physical organic chemistry. 

Bio: C. Daniel Frisbie is Distinguished McKnight University Professor of Chemical Engineering and Materials Science (CEMS) at the University of Minnesota. He joined the faculty in 1994 and served as Head of CEMS from 2014-2024. A physical chemist by training, he obtained a Ph.D. from MIT in 1993 and was an NSF postdoctoral fellow in chemistry at Harvard. His research focuses on materials for printed electronics, including organic semiconductors and their applications in devices such as transistors and sensors. He also has a longstanding program in molecular electronics. Research themes include the characterization of novel organic semiconductors, structure-property relationships, device physics and the application of scanning probe techniques. Recent efforts also include manufacturing approaches for large area flexible electronics and strategies for electrocatalysis.