Self-Assembly of Oxidatively Doped Conjugated Bottlebrush Polymers into Donor-Acceptor Nanostructures

Redox doping is a process which enables the practical application of conjugated polymer materials in optoelectronic devices. However, it induces conjugated polymer precipitation through polaron formation, so it is often limited to the solid state. Herein, we report core-shell bottlebrush polymers, comprising a polynorbornene backbone with either polythiophene or polyselenophene core blocks and poly(ethylene glycol) shell blocks. Bottlebrush polymers are architectures with densely grafted polymeric side chains with low chain entanglements that are of interest due to their elastomeric properties. We find that conjugated core blocks of the bottlebrush polymers are well doped by approximately 6 mol equiv of F₄TCNQ, a common oxidative dopant, while remaining colloidally stable due to PEG shell blocks. Furthermore, spectroelectrochemistry reveals that these bottlebrush architectures efficiently stabilize polarons upon oxidative doping relative to diblock copolymer analogues, as evidenced by the emergence of bipolaron bands 1.5-2× larger than the unoxidized absorption transitions. Conjugated polymer crystallization is known to improve parameters such as conductivity and exciton diffusion. Inducing crystallization of conjugated polymer core blocks in a selective solvent system while doping results in larger polaron signals by EPR and UV-visible-NIR spectroscopy, while retaining the colloidal stability of the core-shell bottlebrush polymers. Interestingly, the polythiophene-based bottlebrush polymers stack end-on-end into nanofibers with dopant concentration-dependent lengths, up to number-average (L_n) lengths of 51 nm for 1 weight equivalent of F₄TCNQ relative to the core-shell bottlebrush. The longest observed nanofiber was 192 nm. Overall, we report a new class of donor-acceptor materials that combine colloidally stable redox doping of conjugated polymer core blocks with bottlebrush self-assembly promoted by the generation of charge transfer sites.