RESUMEN
Open-shell conjugated polymers (CPs) offer new opportunities for the development of emerging technologies that utilize the spin degree of freedom. Their light-element composition, weak spin-orbit coupling, synthetic modularity, high chemical stability, and solution-processability offer attributes that are unavailable from other semiconducting materials. However, developing an understanding of how electronic structure correlates with emerging transport phenomena remains central to their application. Here, the first connections between molecular, electronic, and solid-state transport in a high-spin donor-acceptor CP, poly(4-(4-(3,5-didodecylbenzylidene)-4H-cyclopenta[2,1-b:3,4-b']dithiophen-2-yl)-6,7-dimethyl-[1,2,5]-thiadiazolo[3,4-g]quinoxaline), are provided. At low temperatures (T < 180 K), a giant negative magnetoresistance (MR) is achieved in a thin-film device with a value of -98% at 10 K, which surpasses the performance of all other organic materials. The thermal depopulation of the high-spin manifold and negative MR decrease as temperature increases and at T > 180 K, the MR becomes positive with a relatively large MR of 13.5% at room temperature. Variable temperature electron paramagnetic resonance spectroscopy and magnetic susceptibility measurements demonstrate that modulation of both the sign and magnitude of the MR correlates with the electronic and spin structure of the CP. These results indicate that donor-acceptor CPs with open-shell and high-spin ground states offer new opportunities for emerging spin-based applications.
RESUMEN
Open-shell macromolecules (i.e., polymers containing radical sites either along their backbones or at the pendant sites of repeat units) have attracted significant attention owing to their intriguing chemical and physical (e.g., redox, optoelectronic, and magnetic) properties, and they have been proposed and/or implemented in a wide range of potential applications (e.g., energy storage devices, electronic systems, and spintronic modules). These successes span multiple disciplines that range from advanced macromolecular chemistry through nanoscale structural characterization and on to next-generation solid-state physics and the associated devices. In turn, this has allowed different scientific communities to expand the palette of radical-containing polymers relatively quickly. However, critical gaps remain on many fronts, especially regarding the elucidation of key structure-property-function relationships that govern the underlying electrochemical, optoelectronic, and spin phenomena in these materials systems. Here, we highlight vital developments in the history of open-shell macromolecules to explain the current state of the art in the field. Moreover, we provide a critical review of the successes and bring forward open opportunities that, if solved, could propel this class of materials in a meaningful manner. Finally, we provide an outlook to address where it seems most likely that open-shell macromolecules will go in the coming years. Our considered view is that the future of radical-containing polymers is extremely bright and the addition of talented researchers with diverse skills to the field will allow these materials and their end-use devices to have a positive impact on the global science and technology enterprise in a relatively rapid manner.
RESUMEN
Nonconjugated radical polymers (i.e., macromolecules with aliphatic backbones that have stable open-shell sites along their pendant groups) have arisen as an intriguing complement to π-conjugated polymers in organic electronic devices and may prove to have superior properties in magneto-responsive applications. To date, however, the design of nonconjugated radical polymers has primarily focused on linear homopolymer, copolymer, and block polymer motifs even though conjugated dendritic macromolecules (i.e., polyradicals) have shown significant promise in terms of their response under applied magnetic fields. Here, we address this gap in creating a nonconjugated, three-arm radical macromolecule with nitroxide open-shell sites using a straightforward, single-step reaction, and we evaluated the electronic and magnetic properties of this material using a combined computational and experimental approach. The synthetic approach employed resulted in a high-purity macromolecule with a well-defined molecular weight and narrow molecular weight distribution. Moreover, epoxide-based units were implemented in the three-arm radical macromolecule design, and this resulted in a nonlinear radical macromolecule with a low (i.e., below room temperature) glass transition temperature and one that was an amorphous material in the solid state. These properties allowed thin films of the three-arm radical macromolecule to have electrical conductivity values on par with many linear radical polymers previously reported, and our computational efforts suggest the potential of higher generation open-shell dendrimers to achieve advanced electronic and magnetic properties. Importantly, the three-arm radical macromolecule also demonstrated antiferromagnetic exchange coupling between spins at temperatures < 10 K. In this way, this effort puts forward key structure-property relationships in nonlinear radical macromolecules and presents a clear path for the creation of next-generation macromolecules of this type.