Large Room-Temperature Magnetoresistance in a High-Spin Donor-Acceptor Conjugated Polymer.

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.