Highly Coplanar Polythiophenes with -CCR 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 pi-pi* transition energy of the polymers by 0.3-0.4eV. 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 pi-pi* transition energy by about 0.2eV at 10GPa. Head-to-head-P3(C C-Dec)Th with a -C C-decyl side chain gave a larger optical third-order nonlinear susceptibility chi((3)) than regio-regular poly(3-hexylthiophene)s.