Influence of ω-Bromo Substitution on Structure and Optoelectronic Properties of Homopolymers and Gradient Copolymers of 3-Hexylthiophene

The nature of side chains and substituents influence the resulting electronic and structural properties of conjugated polymers. Here, we report fundamental studies on the influence of a ω-bromohexyl side chain in polythiophene (resulting in P3BrHT) as well as in a series of gradient copolymers using 3-hexylthiophene (3HT) comonomer. The rate of polymerization of 3BrHT is slower compared to that for 3HT. Temperature dependent absorption and emission show that, with increasing content of 3BrHT, aggregation, critical transition temperature, and the fraction of aggregates decrease. Wide angle X-ray scattering shows that P3BrHT has a triclinic crystal lattice; the crystallinity is almost half of that of P3HT and an increase in the crystallinity and a gradual monotonic decrease of the a and b crystal lattice parameters with decreasing 3BrHT content in copolymers is observed. Finally, the consequences of change in aggregation and crystallinity on charge transport are elucidated using OFET, giving a general explanation for the influence of a ω-bromo substituent in polythiophene copolymers.