Coplanar Donor-Acceptor Semiconducting Copolymers to Achieve Better Conjugated Structures: Side-Chain Engineering.

It is reported on the allocation effects of branched alkyl chains, when used as solubility and ordering enhancers of the conjugated donor-accepter (D-A) copolymer backbones, on the ordering and π-π overlapping of the copolymers, that drastically affect the electrical properties of organic field-effect transistors (OFETs). Triisopropylsilylethynyl-benzo[1,2-b:4,5-b′]dithiophene (TIPSBDT) and diketopyrrolopyrrole (DPP)-based copolymers, which have two linear alkyl spacers (methylene (C1) or butylene (C4)) between the DPP and side-substituent (C₁₀H₂₁) CH(C₈H₁₇)-, are synthesized by Suzuki cross-coupling. These copolymer films are spun cast onto a polymer-treated SiO₂ dielectric surface, and some are further thermally annealed. The longer spacer, C4, is found to efficiently enhance the coplanarity and conjugation of the D-A backbone, while the C1 does not. The resulting C4-bridged TIPSBDTDPP-based copolymer readily develops a superior π-extended layer on the dielectric surface; the edge-on chains with randomly oriented side chains can be closely packed with a short π-planar distance (d₍₀₁₀₎) of 3.57 Å. Its properties are superior to those of the short spacer C1 system with d(₀₁₀) ≈3.93 Å. The C4-bridged TIPSBDT-DPP copolymer films yield a field-effect mobility up to 1.2 cm² V^-1 s^-1 in OFETs, 12 times as higher than that of the C1 spacer system. [ABSTRACT FROM AUTHOR]