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 (C 1) or butylene (C 4)) 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, C 4, is found to efficiently enhance the coplanarity and conjugation of the D –A backbone, while the C 1 does not. The resulting C 4‐bridged TIPSBDT‐DPP‐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 C 1 system with d ₍₀₁₀₎ ≈3.93 Å. The C 4‐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 C 1 spacer system.