(603i) Elucidating the Charge Transfer Mechanism in Conjugated Radical Polymers

Conjugated polymers bearing pendant nitroxide radicals have been an intriguing research topic in energy storage due to their dual electroactive nature with different redox potentials within the same polymer backbone. Recently, we demonstrated polythiophenes bearing pendant nitroxide radical groups such as the TEMPO radical (2, 2, 6, 6-tetramethyl-1-piperidinyloxy). Polythiophenes possess a high oxidation potential with tunable electronic conductivity, and the organic radicals possess a very high electron transfer rate. However these polymers exhibited worse battery performance than that of either PTMA (the polymeric form of TEMPO radical) or polythiophenes alone. One reason for this phenomenon is that conjugated polymers and nitroxide radicals are both redox active and that intra and inter-charge transfer is possible. As such, it is relevant to investigate the charge transfer pathways in such a system. In this study, we report the systematic study of polythiophenes carrying pendant TEMPO radicals. By engineering the alkyl chain spacer length between the thiophene ring and TEMPO radical, we tune the electronic and electrochemical properties of these electroactive polymers. In addition, the charge storage mechanism during the galvanostatic charge/discharge process is also interrogated and the respective contributions from each redox moiety are also discussed. Evidence of internal charge transfer is observed using a potentiostatic pulse technique. These results provide a path forward in the design of conjugated radical polymers for energy storage.