Synthesis and characterization of optically active poly(phenylene-ethynylene)s containing chiral oxazoline derivatives

Abstract

Novel optically active (S)-2-(3,5-diiodophenyl)oxazoline derivatives ((S)-DIPhROx)) (R=4-benzyl (1a), 4-phenyl (1b)) were synthesized and polymerized using the Sonogashira–Hagihara coupling reaction to obtain the poly(phenylene-ethynylene)s bearing oxazoline derivatives in the weight range from 1060 to 17 300 in 40–72% yields. The polymers were thermally stable up to 220 °C. All the polymers exhibited higher specific rotation values than the model compound did. Moreover, these polymers also showed intense circular dichroism (CD) signals in the transition region of the conjugated main chain, whereas almost no induced Cotton effect was observed for the model compound. The results of the specific rotation and CD and ultraviolet analyses indicate that the optical activities of the polymers arise not only from the configurational chirality of the optically active oxazoline pendant groups but also from elements of higher-order structure such as the helical conformation. Optically active 4-alkyl-2-(3,5-diiodophenyl)oxazoline was synthesized and polymerized by the Sonogashira–Hagihara coupling reaction to obtain the poly(phenylene-ethylnylene)s bearing oxazoline derivatives with number-average molecular weight ranging from 1060 to 17 300 in 40–72% yields. All the polymers exhibited higher values of specific rotation ([α]D=+41.6 to +83.7°) than the monomer ([α]D=−1.2 to −16.9°). On the basis of the results of specific rotation, circular dichroism and ultraviolet analyses, the optical activities of the polymers arise from not only the configurational chirality of the optically active oxazoline pendant groups but also the higher-order structure such as helical conformation.