Helically π-Stacked Thiophene-Based Copolymers with Circularly Polarized Fluorescence: High Dissymmetry Factors Enhanced by Self-Ordering in Chiral Nematic Liquid Crystal Phase

Abstract

Novel derivatives of polythiophenes and their phenylene copolymers were synthesized by introducing chiral alkoxy carbonyl substituents into their side chains. Most of these polymers showed enantiotropic main-chain liquid crystallinity at high temperatures and across a wide range of elevated temperatures. They exhibited fluorescence that ranged in color from blue to orange in chloroform solutions and from blue to red in films. The bisignate Cotton effect was observed in the π–π* transition region of the circular dichroism (CD) spectra of the polymers consisting of three aromatic rings in the repeating unit, which was attributed to the formation of a polymer assembly with an interchain helically π-stacked structure. The dependence of UV–vis absorption and CD spectra on the concentration and temperature of the solution indicated that the polymer assembly exists, even in dilute concentration at room temperature, but it dissociated into single polymers at high temperatures. Additionally, the polymer films exhibited an enhanced Cotton effect caused by the strengthening of the helical π-stacking in the polymer assembly. Both the polymer solutions and films generated circularly polarized fluorescence with values for gem, the dissymmetry factor, on the order of 10–3 and 10–2, respectively. Values for gem as high as 10–1 were obtained by annealing the polymer films at temperatures corresponding to the liquid crystalline region and were due to the self-ordering of their chiral nematic phases. Furthermore, a mixture of red, green, and blue fluorescent polymers generated a unique, circularly polarized white luminescence not only in solution but also in the cast film prepared by dispersing the mixture in an excess of polystyrene.