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) BPh4 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 center dot HClO4) at - 30 degrees C, a large Cotton effect with an intensity of + 2.96 x 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 center dot HClO4. However, the induced CD (ICD) intensity significantly decreased with the increasing temperature and almost completely disappeared at 30 degrees C or over. Furthermore, an alternate temperature modulation between - 30 and 30 degrees C brought about the reversible on-off switching of the ICD of 2.