Understanding Carrier Delocalization and Transport in Micelle Forming Amphiphilic Conjugated Polymers

University Of California-Los Angeles, Los Angeles CA

Investigators

Abstract

With the support of the Macromolecular, Supramolecular and Nanochemistry program in the Division of Chemistry, Professors Sarah H Tolbert, Benjamin J Schwartz and Yves Rubin of the University of California at Los Angeles are systematically studying assemblies of semiconducting polymers in aqueous solutions in order to seek more favorable geometries for electrical conductivity. Semiconducting polymers are an exciting class of optoelectronic materials because of their solution processability, low cost, and structural tunability. These characteristics make them useful in a range of organic electronic devices, including photovoltaics, thermoelectrics, light-emitting diodes, and transistors. However, the conformational freedom of conjugated polymers leads to intrinsic disorder that can result in poor electrical conductivity and hence limited commercial applicability. This research will address these issues and use organic synthesis, structural studies, and modern spectroscopy to explore water-soluble amphiphilic semiconducting polymers that self-assemble into cylindrical micelles as a way to straighten polymer chains and reduce defects without the need for a crystalline network. The efforts toward controlling polymer self-assembly while interrogating chain conformation with respect to carrier mobility have the potential for broad impact in the field of organic electronics and could lead to a development of new and low-cost polymeric systems for a variety of applications in which temperature and/or light are converted to electricity and vice versa. The project will provide opportunities for undergraduate and graduate students to be involved in cutting-edge interdisciplinary research. In an effort to bring the ideas of nanostructured materials, organic electronics, and self-assembly to a broader audience, experiments related to this work will be brought to secondary school classrooms throughout the greater Los Angeles area via a series of graduate-student run workshops for teachers. This research will focus on the synthesis of micelle-forming amphiphilic conjugated polymers based on poly(cyclopentadithiophene)-alt-thiophene (PCT) backbones, and will investigate of how their assembled structure controls charge mobility upon doping. In the first objective, organic synthesis and amphiphilic assembly will be used to precisely control the position of charge-balancing counterions in chemically-doped PCT polymers. PCT-based polymers with cationic, anionic, non-ionic, and zwitterionic size chains will be prepared and solution-phase small-angle X-ray scattering (SAXS) will be used to characterize micelle formation. Doping will be achieved with iron(III) salts in water and the number and nature of charge carriers will be probed using steady-state and transient IR/visible absorption spectroscopy. Based on theoretical calculations and modeling, doubly charged side chains and/or divalent solution-phase counterions will be employed to control polaron pairing into bipolarons at high doping densities. In order to create systems where the anionic side chains serve as counterions for the polarons, copolymers of anionic and either zwitterionic or non-ionic polymers will be applied. Such an approach will eliminate the need for additional ions in solution. The second objective will target new polymer backbones that are easier to chemically dope. The final goal will seek to develop methods to transition optimized assemblies from aqueous solutions into the solid state to create new materials with improved conductivity. The comprehensive approach for controlling polymer conformation and charge localization associated with this research has the potential to provide important strategies to further understand fundamental charge carrier dynamics in conjugated polymers. 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.