Selected Current Research Projects
Our work has focused on the development of new plastics (polymers) that are designed for function. Some of this work has led to the invention of polymers that incorporate atoms like selenium (Se) and tellurium (Te). Other work has used Nature’s functional molecules to build polymers for energy harvesting and storage. While most of the projects have an application in mind (one has even led to the start-up company Pliant Power Devices), we are most proud of the impact we have hadin fundamental science and in particular of our ability to imagine new compositions of matter, create and study that matter in the laboratory, and learn a great deal along the way. The following vignettes describe the major ongoing projects in our group.
Conjugated Materials Synthesis. Living polymerizations play a central role in polymer chemistry, however, conjugated materials synthesis is currently dominated by non-controlled, step-growth polycondensation polymerizations. Synthesizing well-defined π-conjugated polymers is very limited in scope. Catalyst transfer polycondensation reactions are one of the most promising methods to prepare π-conjugated polymers in a controlled, chain-growth manner. Through theoretical investigations, mechanistic studies, monomer and catalyst design, our group has largely expanded the scope of this methodology. Lately, we described the synthesis of monodisperse polythiophenes by temperature cycling. This method adds one monomer to each chain for each cycle, and produces the first example of isolated living π-conjugated polymer chains and discrete oligomers. These advances create new knowledge in the polymer community and expand the scope of well-defined π-conjugated polymers as well as more complex structures, such as statistical and block copolymers. Moving beyond catalyst transfer polycondensation, we are also interested in developing new strategies to synthesize well-defined π-conjugated materials, which is exemplified by our recent report of a versatile template approach.
Group 16 Chemistry. Organic semiconductors have garnered widespread attention due to their unique properties. Nonlimiting examples incude their low mass, flexible form factors, solution and/or low temperature processability, as well as the concept that structure can be chemically tailored for function. Group 16 elements, also known as chalcogens, are quite commonly incorporated into π-conjugated systems to achieve distinct opto-electronic properties. Our group was among the first that incorporated 'heavy' chalcogens (Se and Te) into organic semiconductors. We have investigated the effects of heavy chalcogen substitution on the properties of both electron-rich and electron-deficient organic semiconductors. Through delicate single atom substitution, both the opto-electroonic properites and solid-state assembly behaviour can be finely tuned, and ultimately optimized. Synthetic advances have also enabled us to establish the relationship between chemical structure and property. We have subsequently several ongoing collaborations to prove the utility of the compounds in devices, such as organic light-emitting diodes, transistors, and thermoelectrics. We also have numerous patents on these novel and functional compositions of matter. Our research sets the stage for the continued development of new classes of Se- and Te- containing small molecules and polymers.
Energy Storage in Organic Materials. Electrochemical energy storage applications are growing at an unprecedented rate on multiple scales. These include smart card batteries, large-scale electric vehicle batteries, and warehouse-sized redox flow batteries. While much progress has been made, energy storage materials that are higher performing, more versatile, smaller, lighter, and most importantly, more environmentally viable, will be required in the future. With these goals in mind, our group is working on improving a variety of organic-based energy storage systems, such as lithium-ion, magnesium-ion, and lithium-sulfur batteries. By using synthetic strategies, we can incorporate novel organic small molecules or polymers into energy storage systems to improve their performance. We recently reported a series of microporous polymers that can undergo 'superlithiation,' a process where many redox groups are incorporated into the system. These groups can accept an extraordinary large number of Li+ ions during cycling, which leads to very high battery capacities. We plan to continue contributing to this field by incorporating abundant compounds and ones that are scalabe into energy storage systems.
Conjugated Polymers with Complex Architectures. Conjugated polymers, including polythiophenes, polyselenophenes, and polytellurophenes have been widely studied as optoelectronic materials. However, these polymers have largely been investigated as linear homopolymer or block copolymer systems, where self-assembly modifications are limited to adjusting molecular weights, side chain lengths, and block ratios. The introduction or branching points, grafts, and loops in the design of complex architectures offers an opportunity to tailor the physical and electronic properties of conjugated polymers through self-assembly, without changing their composition. To date, our group has synthesized and investigated an array of complex architectures, including stars, macrocycles, and bottlebrush polymers. Through the synthesis and study of polythiophene and polyselenophene macrocycles, we can elucidate the role that similar chain loop defects play in conjugated polymer materials. We have also found that unique hexagonal assemblies can be arranged from 4-arm star conjugated polymers, showing the extent to which self-assembly can be controlled. More recently, we reported the first synthesis of all-conjugated bottlebrush polymers, consisting of polyselenophene side chains extending from polythiophene cores. Addditionally, we have demonstrated that conjugated bottlebrush polymers can form fibrous end-on-end assemblies via thermal annealing, a property that has not been shown for unconjugated polymers. While much remains to be studied, the ability to control the formation and properties of conjugated polymer self-assemby paves the way toward more efficient optoelectronic devices.