Effects of Side Chain Branch Point on Self Assembly, Structure, and Electronic Properties of High Mobility Semiconducting Polymers

Semiconducting polymers exhibiting lyotropic liquid-crystalline (LC) mesophases allow for precise control over crystallinity and crystallite orientation using scalable equilibrium processing techniques. The ability to control morphologies from molecular to macroscopic scales is desirable for optimizing polymer electronic devices such as field-effect transistors. LC mesophases can form in semiconducting polymers with branched side chains, but steric repulsion from the side chain structure increases π-stacking distances. This change in unit cell negatively impacts electronic transport, negating any improvements gained by achieving more favorable mesostructures. Herein, we show that the π-stacking distance in cyclopenta­dithiophene-benzo­thiadiazole donor–acceptor copolymers is successively shortened by moving the branch point further away from the conjugated backbone. These materials retain their favorable LC properties, and the change in unit cell is accompanied by a monotonic increase in charge carrier mobility in field effect transistors. Mobility values span from ∼10^–4 to 0.41 cm² V^–1 s^–1, reaching their highest value when the branch point is furthest from the polymer backbone. In addition, these LC materials can be aligned across large length scales on rubbed polyimide, resulting in an improvement of charge carrier mobility by a factor of 56 when comparing transistors fabricated parallel and perpendicular to the alignment direction.