Effect of Ionic Liquid on Structure and Properties of Polysquaraines

Polycondensation of 4-bis­[2-(1-methylpyrrol-2-yl)­vinyl]-2,5-didodecyloxybenzene and squaric acid yielded the polysquaraine <b>SQ</b> with two isomeric subunits1,3-addition (zwitterionic) and 1,2-addition (diketonic) moietiesin the main chain structure. The former featured a C–O/CO infrared (IR) absorption frequency at 1622 cm^–1; the latter, a CO signal at 1716 cm^–1. Traditional synthesis yielded <b>SQ</b> as a powder with metallic luster that could not be cast as a polymer film from solution. When the ionic liquid [Oct₃NMe]­[TfO] (<b>IL</b>) was present in the cosolvent of BuOH and benzene (1:3), however, the resulting <b>SQI</b>_{<i><b>x</b></i>} polymers (<i>x</i> = 0.01–5 wt %) did not precipitate from solution, making it possible to cast continuous free-standing films with a large area (>1 × 1 cm²). A greater content of <b>IL</b> in the solution favored the formation of the 1,3-addition zwitterionic subunits in the <b>SQI</b>_{<i><b>x</b></i>} polymer main chains, thereby changing the physical and optical properties of the polysquaraine, as evidenced in IR and optical absorption spectra. The features in the UV–Vis–NIR absorption spectra of <b>SQ</b> and <b>SQI</b>_{<i><b>x</b></i>} were dependent on the <b>IL</b> concentration and the nature of the solvent. Among our synthesized <b>SQI</b>_{<i><b>x</b></i>} polymers, <b>SQI</b>_<b>0.01</b> and <b>SQI</b>_<b>0.1</b> formed flexible free-standing films with metallic luster, smooth surfaces, and good semiconductivities (2.27 × 10^–5 and 4.74 × 10^–5 S/cm, respectively). X-ray diffraction patterns revealed that the presence of <b>IL</b> in the polymerization medium increased the <b>SQI</b>_{<i><b>x</b></i>} interchain packing distance. <b>SQI</b>_<b>0.01</b> and <b>SQI</b>_<b>0.1</b> possessed thermal stabilities comparable with that of <b>SQ</b>. Our successful use of <b>IL</b> in the preparation of <b>SQI</b>_{<i><b>x</b></i>} polymers appears to have great potential for application in ionic liquid–related organic or polymeric preparation and processing.