Synthesis and properties of novel low band-gap polymers bearing squaraine units

Novel main-chain-conjugated poly(carbazol-alt-squaraine) and poly(dipyridyl-alt-squaraine) were successfully synthesized through direct polycondensation of 9-(2-ethylhexyl)carbazole-bridged or dipyridyl-bridged bispyrrole and squaric acid. The structures and properties of the polymers were characterized using 1H NMR, FT-IR, UV–vis and cyclic voltammetry. Both polymers exhibit excellent solubility in common organic solvents and good thermal stability. Their UV–vis absorption spectra indicated the polymers have broad and strong spectral responses from 200 nm to 900 nm, which reveals a low optical band gap around 1.38 eV, suggesting that they may be promising candidates for organic solar cells.