Highly Coplanar Polythiophenes with –C≡CR Side Chains: Self-Assembly, Linear and Nonlinear Optical Properties, and Piezochromism

Self-assembly of polythiophenes with –C≡CR (R = alkyl, phenyl, etc.) side chains has been investigated. Seven new polymers consisting of head-to-head and tail-to-tail 2,2′-bithiophenes with –C≡CR side chains were synthesized. The new polymers include alternating copolymers between bithiophenes with –C≡C–alkyl side chains and thiophene and those between bithiophenes with –C≡C–alkyl side chains and 2,2′-bithiophene. The polythiophene main chain is considered to be coplanar because of the absence of steric repulsion between the main chain and the –C≡CR side chain. Single-crystal X-ray crystallography and DFT calculations indicated that the head-to-head- and tail-to-tail-2,2′-bithiophenes with –C≡CR side chains were coplanar. The polythiophenes with –C≡CR side chains showed a strong tendency to self-assemble, and assumed edge-on alignment and side-on alignment on the surface of substrates. Self-assembly caused a decrease in the π–π* transition energy of the polymers by 0.3–0.4 eV. A copolymer of thiophene and dialkoxy-p-phenylene showed analogous self-assembly. The polythiophenes with –C≡CR side chains showed piezochromism, with a decrease in the π–π* transition energy by about 0.2 eV at 10 GPa. Head-to-head-P3(C≡C–Dec)Th with a –C≡C–decyl side chain gave a larger optical third-order nonlinear susceptibility χ(3) than regio-regular poly(3-hexylthiophene)s.