Highly coplanar polythiophenes with-C≡CR side chains: Self-assembly, linear and nonlinear optical properties, and piezochromism

Abstract

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 2,2́-bithiophenes with C≡ CR side chains were synthesized. The new polymers include alternating copolymers between bithiophenes with C≡ Calkyl side chains and thiophene and those between bithiophenes with C≡ Calkyl side chains and 2,2́-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-2,2́-bithiophenes with C≡ CR side chains were coplanar. The polythiophenes with C≡ CR side chains showed a strong tendency to selfassemble, and assumed edge-on alignment and side-on alignment on the surface of substrates. Self-assembly caused a decrease in the transition energy the polymers by 0.30.4 eV. A copolymer of thiophene and dialkoxy-pphenylene 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≡CDec) Th with a C≡ Cdecyl side chain gave a larger optical third-order nonlinear susceptibility (3) than regio-regular poly(3-hexylthiophene)s. © 2009 The Chemical Society of Japan.