Synthesis and helicity induction of poly(phenylacetylene)s bearing crown ether pendants. reversible on-off switching of the induced helical chirality tunable with temperature

Cis-transoidal poly(phenylacetylene)s bearing 15-crown-5 (2a) and 2-crown-6 (2b) were prepared by stereoselective polymerization using Rh(nbd)BPh4. In the circular dichroism (CD) spectrum of 2a in the presence of the perchloric acid salt of L-phenylglycine (L-Pgly . HClO4), a characteristic induced CD (ICD) was observed in the absorption range corresponding to the polymer backbone. on the contrary, the 2b/L-Pgly . HClO4 system did not show such an ICD spectrum. The ICD intensity of the 2a/L-Pgly . HClO4 system drastically depended on temperature, i.e., the ICD disappeared at 30 degrees C or higher. Thus, the on-off switching of the ICD of 2a was controllable by either the temperature modulation or the doping/undoping process of the chiral guest.