Polymers are long chains of hundreds or thousands of molecules, called monomers. When many polymer chains are aggregated together they form plastics, whose properties depend on the monomer's chemical structure. While some polymers are made to be stiff or flexible, others are designed to absorb light, transport excited state energy (called excitons), and conduct electricity, giving rise to electronic devices that are thin, light, and flexible. While such polymers have been known for some time, designing materials that have the same performance as traditional electronics materials (e.g. silicon) is difficult. The movement of excitons through a polymer involves both motion along a single chain, as well as transfer between chains, and studying this interchain hopping is a challenge. With funding from the Macromolecular, Supramolecular, and Nanochemistry Program in the Division of Chemistry, Professor Laura Kaufman at Columbia University is using sophisticated microscopies to study the flow of excitons through polymer aggregates. The insights gained from the project could help pave the way to inexpensive solar cells and flexible electronics, which would have significant societal benefits. The project is also providing training opportunities for graduate and undergraduate students, who will enter the Nation's science and technology workforce, and Professor Kaufman and her students are engaging with local elementary school students to promote interest in scientific exploration and discovery.
The project is exploring the excited state properties of individual polymer chains, as well as polymers that are organized together as aggregates. The focus is on polyfluorenes, perylene diimides, and polyacenes, each of which are promising for optoelectronic applications. Professor Kaufman and her students use approaches recently developed in her laboratory to control the assembly of single polymer chains into aggregates of particular size and structure through variation in solvent mixture and swelling. Preparation of aggregates of different packing densities is accomplished by careful selection of molecular properties and by tuning the aggregation to be driven by Ostwald ripening or coalescence. Spectroscopic properties (e.g. absorption, emission, anisotropy) are characterized on chromophore-by-chromophore basis using polarization modulation and super-resolution microscopies, which provide a detailed picture of the correlation between polymer conformation and excited state photophysical properties. Through the combination of controlled aggregate formation and detailed spectroscopic measurement, the research team is attempting to ascertain whether morphological ordering on mesoscopic length scales can be used to tune exciton diffusion length and other photophysical characteristics of aggregates.
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.
