Synthesis and properties of novel low-band-gap thienopyrazine-based poly(heteroarylenevinylene)s

Thieno[3,4-b]pyrazine-based poly(heteroarylenevinylene)s (PHAVs) (P-1-P-6) by Horner polycondensation and cyano-PHAVs (P-7-P-9) by Knoevenagel polycondensation have been synthesized in quantitative yields. The copolymers are characterized by NMR, IR, UV, GPC, and elemental analysis. High molecular weight (M̄_w up to 279 000 g/mol), thermostable, soluble, and film-forming materials were obtained. The absorption spectra of the polymers show two peaks located in the UV and visible region from ∼384 to 650 nm and are promising for the application in polymer solar cells. The absorption and emission in solution and in solid state of the polymers revealed that the polymers generate a π-stacked structure in the solid state, and the polymer molecules in the film were ordered. Thin films of the polymers P-1-P-9 exhibit an optical band gap of ∼ 1.56-2.08 eV. Cyclic voltammograms display that the p-doping/dedoping and n-doping/dedoping processes of the polymers (P-1, P-2, P-3, P-5, and P-6) are reversible. The HOMO and LUMO levels of the polymers were determined from electrochemical measurements. From the absorption spectra of the polymers (P-1, P-2, and P-8) in chloroform/ methanol mixtures, all the three polymers have revealed solvatochromic effects, which have been related to the formation of aggregates.