Thermoresponsive On−Off Switching of Chiroptical Property Induced in Poly(4‘-ethynylbenzo-15-crown-5)/α-Amino Acid System

Cis-transoidal poly(4‘-ethynylbenzo-15-crown-5) ( 2 ) prepared by the polymerization of 4‘-ethynylbenzo-15-crown-5 with Rh(nbd)BPh 4 has been used as a thermoresponsive material with a chiral on−off switching property. In the circular dichroism (CD) spectrum of 2 in the presence of the perchloric acid salt of l -phenylglycine ( l -Pgly·HClO 4 ) at −30 °C, a large Cotton effect with an intensity of +2.96 × 10 4 deg cm 2 dmol -1 was observed in the range from 300 to 550 nm corresponding to the absorption of the conjugated polymer backbone, indicating that 2 formed the one-handed helical structure triggered by the host−guest complexation with l -Pgly·HClO 4 . However, the induced CD (ICD) intensity significantly decreased with the increasing temperature and almost completely disappeared at 30 °C or over. Furthermore, an alternate temperature modulation between −30 and 30 °C brought about the reversible on−off switching of the ICD of 2 .