New coplanar (ABA)_n-type donor-acceptor π-conjugated copolymers constituted of alkylthiophene (unit A) and pyridazine (unit B): Synthesis using hexamethylditin, self-organized solid structure, and optical and electrochemical properties of the copolymers

A new series of π-conjugated (ABA)_n-type polymers constructed of electron-donating thiophene or 3-alkylthiophene (as the unit A) and electron-accepting pyridazine (as the unit B) were synthesized via palladium-catalyzed dehalogenative polycondensation of Br-ABA-Br using hexamethylditin as the condensing reagent. The obtained polymers were characterized by NMR, IR, and elemental analysis. The polymers with alkyl side chains were soluble in organic solvents, and gave number-average molecular weights ranging from 3.2 × 10⁴ to 16 × 10⁴ in gel-permeation analysis. The polymers exhibited strong green photoluminescence (PL) with the peak in the range of 510-520 nm and quantum yields of 51-61% in chloroform. The PL intensity was sensitive toward acids. In films, the UV-vis and PL peaks shifted to a longer wavelength by about 30-40 nm. Powder X-ray diffraction data suggested that the polymer formed a highly ordered π-stacked assembly assisted by the side chain crystallization and dipole-dipole interaction. Cyclic voltammetry revealed that the polymer was susceptible to both electrochemical p- and n-doping, and the n-doping peak appeared in the range of -2.15 to -2.28 V vs Ag⁺/Ag. The polymer served as a good material for a thin-film field-effect transistor and gave a hole mobility of 3 × 10^-3 cm² V^-1 s^-1.