Switching of macromolecular helicity of optically active poly(phenylacetylene)s bearing cyclodextrin pendants induced by various external stimuli.

A series of novel phenylacetylenes bearing optically active cyclodextrin (CyD) residues such as alpha-, beta-, and gamma-CyD and permethylated beta-CyD residues as the pendant groups was synthesized and polymerized with a rhodium catalyst to give highly cis-transoidal poly(phenylacetylene)s, poly-1alpha, poly-2beta, poly-3gamma, and poly-2beta-Me, respectively. The polymers exhibited an induced circular dichroism (CD) in the UV-visible region of the polymer backbones, resulting from the prevailing one-handed helical conformations. The Cotton effect signs were inverted in response to external chiral and achiral stimuli, such as temperature, solvent, and interactions with chiral or achiral guest molecules. The inversion of the Cotton effect signs was accompanied by a color change due to a conformational change, such as inversion of the helicity of the polymer backbones with a different twist angle of the conjugated double bonds, that was readily visible with the naked eye and could be quantified by absorption and CD spectroscopies. The dynamic helical conformations of poly-2beta showing opposite Cotton effect signs in different solvents could be further fixed by intramolecular cross-linking between the hydroxy groups of the neighboring beta-CyD units in each solvent. The cross-link between the pendant CyD units suppressed the inversion of the helicity; therefore, the cross-linked poly-2betas showed no Cotton effect inversion, although the polymer backbones were still flexible enough to alter their helical pitch with the same handedness, resulting in a color change depending on the degree of intramolecular cross-linking.