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Keywords:

  • polysquaraines;
  • substituted PVTVPSs;
  • low bandgap;
  • solubility;
  • thermochromism;
  • conjugated polymers;
  • UV–vis spectroscopy;
  • thermal properties

Abstract

In this work, two very low band-gap (∼1.0 eV) alternate conjugated copolymers bearing bispyrrolylvinylthiophene-based polysquaraine backbone (PVTVPS) have been synthesized. In comparison with their analogous polymer with 2-ethylhexyl side chain on the pyrrole segment, which possesses poor solubility after long-time storage, the two target polymers with 4-dodecyloxyphenyl (PVTVPS-Ph) or (4′-dodecyloxy-4-biphenyl)methylene (PVTVPS-Ph2) side chains exhibit dramatically improved solubility. Furthermore, PVTVPS-Ph shows unexpected thermochromism in the Vis–near-infrared (NIR) region of 600–1100 nm at 80–160°C in solution and thin film. This may be attributed to the presence of relatively rigid phenyl substituent restricting the free rotation between the D (pyrrole) and A (squaraine) segments of the main chain. To our knowledge, this is the first report on NIR thermochromic polysquaraines. Nevertheless, in the case of PVTVPS-Ph2, no thermochromism could be observed because of the existence of free-rotating methylene linkage bridge between biphenyl unit and the conjugated polymer skeleton. When compared with PVTVPS-Ph, PVTVPS-Ph2 has much improved thermostability and broader absorption property. Hence, PVTVPS-Ph2 is a more prospective candidate as photovoltaic materials. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013