THERMOCHROMIC PROPERTIES OF POLYTHIOPHENE DERIVATIVES - FORMATION OF LOCALIZED AND DELOCALIZED CONFORMATIONAL DEFECTS

Temperature-dependent UV-visible absorption measurements in the solid state and in solution have been performed on poly(3-butoxy-3'-decyl-2,2'-bithiophene) (PBDBT), poly(3-dodecyl-2, 2'-bithiophene) (PBT12), poly(3-(octyloxy)-4-methylthiophene) (POMT), and poly(3-dodecylthiophene) (PT12). These measurements revealed the existence of two different types of thermochromism which are correlated to the substitution pattern of the polymers. A ''two-phase'' thermochromic behavior was found in POMT and PT12 and was related to the formation of delocalized conformational defects (twistons) upon heating. The formation of such twistons is made possible by the presence of sterically demanding substituents between each consecutive repeat unit. In contrast, the partially substituted structure of PBDBT and PBT12 allows only the formation of localized conformational defects along the polymer backbone leading to a continuous and monotonic blue shift of the absorption maximum upon heating.