Charge Transport in Conjugated Polymers with Pendent Stable Radical Groups

Synthesizing a stable radical polymer with a conjugated backbone seems like a natural way to introduce conductivity to radical polymers, which are traditionally synthesized with insulating, nonconjugated backbones. For charge storage applications that take advantage of the redox-active nature of stable radical polymers, enhanced conductivity would improve performance. To explore the interplay between stable radicals and a conjugated backbone, we prepared and studied soluble polythiophene with high regioregularity and various concentrations of pendent radical groups to systematically examine any change in conductivity with radical incorporation. Using electron paramagnetic resonance and electrical conductivity measurements, we show that there is an exponential decrease in conductivity as we increase the percentage of pendent groups attached to repeating units, which changes the conductivity by 6 orders of magnitude between the nonradical control polythiophene material and the material with the highest radical content (∼80%). These findings serve as an important guide to the future design of radical polymers on conjugated backbones with the goal of tuning conductivity as a function of stable radical content in redox-active energy storage applications.