Novel 2,4‐divinyl‐3‐alkylthiophene/1,3,4‐oxadiazole alternating conjugated copolymer synthesized by the Heck coupling method: Synthesis, characterization, and electronic and optical properties

Abstract

A novel alternating copolymer with 3-alkylthiophene and oxadiazole (or pyridine) units in ordered arrangement was synthesized with vinyl as a bridge for the first time. The synthesis process included four steps: bromomethylation, preparation of the ylide monomer, the formation of 2,4-divinyl-3-alkylthiophene, and Heck alternating copolymerization. The Fourier transform infrared spectroscopy, 1 H-NMR, and gel permeation chromatography measurements showed that all of the copolymers had the required structures. The weight-average molecular weights of the copolymers were in the range 5500-15,000 with a relatively low polydispersity index of 1.4-1.7. The solubility of the copolymers in common solvents (e.g., methylene chloride, chloroform, tetrahydrofuran) was excellent. The optical properties and bandgap of the copolymers was compared with corresponding poly(3-alkylthiophene) homopolymers. The photoluminescence quantum efficiency (QE) of the copolymers improved markedly in chloroform. The QEs of poly(2,4-divinyl-3-hexylthiophene-alt-2,5-diphenyl-1,3,4-oxadiazole) and poly(2,4-divinyl-3-octythiophene-alt-2,5-diphenyl-1,3,4-oxadiazole) were 43.2 and 34.2%, respectively, which were about 20 and 21 times higher than those of the homopolymers, respectively. The ionization potential of the copolymers between 5.53 and 6.13 eV was appropriated to poly(3-alkylthiophene)s. The high electron affinity of the copolymers (2.71-2.95 eV) made the electrons inject from the cathode more easily. With excellent solubility, low bandgap energy, high QE, and both electron-transporting and hole-transporting abilities, the proposed copolymers might be excellent polymeric materials for applications in polymer light-emitting diodes, light-emitting electrochemical cells, and polymer solar cells.