Helicity Induction in a Poly(phenylacetylene) Bearing Aza-18-crown-6 Ether Pendants with Optically Active Bis(amino acid)s and Its Chiral Stimuli-Responsive Gelation

Abstract

A stereoregular poly(phenylacetylene) bearing the aza-18-crown-6 ether as the pendant (poly-1) formed a predominantly one-handed helical conformation upon complexation with optically active bis(amino acid)s such as l-homocystine perchlorate (l-2) in acetonitrile, and the complex solution gradually gelled with time. The gel exhibited an induced circular dichroism (ICD) in the polymer backbone region, but the absolute value of the ICD intensity decreased with an increase in temperature and the gel gradually shrunk before melting at 55 °C. This gel−sol transition was irreversible, but once the solution was annealed at 60 °C and then rapidly cooled to 0 °C, the formation of the gel occurred again at ambient temperature. The poly-1−l-2 gel shrunk in response to external stimuli such as the addition of a poor solvent. The effect of the enantiomeric excess (ee) of 2 on the helicity induction in the poly-1 and its gelation was also investigated. In the presence of 60% ee of 2, poly-1 showed an intense CD like that of 100% ee. The gelation took place when the ee of 2 was greater than 60%. The helicity induction in poly-1 and its gelation behavior were also investigated with other optically active bis(amino acid)s and achiral diamines.