Synthesis and characterization of 4‐dodecyloxyphenyl and (4′‐dodecyloxy‐4‐biphenyl)methylene‐substituted bispyrrolylvinylthiophene‐based polysquaraines having good solubility and very low bandgap for light absorption

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