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

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, 1H‐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.