CAREER:In Situ Measurement of Ultrafast Processes During Molecular Aggregation

University Of Oregon Eugene, Eugene OR

Investigators

Abstract

With funding from the Chemical Structure Dynamics and Mechanism (CSDM-A) program of the Chemistry Division, Professor Cathy Wong of the University of Oregon is developing an instrument to observe the interaction of light with organic molecules as these molecules assemble to form semiconducting materials. Organic molecule-based semiconductors have many potential applications, such as light-emitting diodes (LEDs) and light harvesting photovoltaics (solar panels). How the molecules come together to form materials is not well understood, in part because existing measurement tools are too slow to capture the fast formation events. Professor Wong overcomes this problem by using a specialized laser system that is capable of rapid measurement. The measurement of properties that directly impact the function of a material will provide feedback for rational materials design, and possibly more efficient and economical LEDs and photovoltaics. The graduate student researchers engaged in this project are gaining experience in a variety of disciplines, including physical chemistry, laser and optical instrument design, and computer simulations. The concepts of instrument design are also being shared with UO undergraduate students and local community college students via a virtual instrument simulation program. This project is focusing on the dynamics of exciton formation, singlet fission, and charge transfer processes in organic semiconducting materials. Of particular interest are the photo-physical processes that occur during molecular aggregate formation. An in situ transient absorption spectrometer is constructed by using a tilted pump pulse to spatially encode its arrival time at the sample. A second laser pulse probes these excited regions simultaneously, thus measuring absorption at a range pump-probe time delays in parallel. The spectrometer is incorporated into a solution deposition apparatus for optical interrogation during thin-film formation. A model of how molecular aggregation affects the rates of singlet fission and charge transfer processes is being developed. The production of molecular aggregates with designer properties is demonstrated by judiciously changing the environmental conditions during the aggregation process, informed by insights from the aggregation model. The scientific broader impacts of this work will be the extended application of transient absorption to non-equilibrium chemical systems. This is a valuable real-time diagnostic tool for understanding the physical phenomena that underpin chemical and materials synthesis, tightening the feedback loop and accelerating progress towards designer electronic materials. The measurements yield a deeper understanding of how molecular interactions impact singlet fission and charge transfer rates. 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.