Switching a Helical Polymer between Mirror Images Using Circularly Polarized Light

Abstract

A system has been created in which chiral information inherent in circularly polarized light can be used to influence the helical sense characteristics of a polymer. A racemic mixture of a photoresolvable ketone-containing group was appended through different linkage patterns to a polymer having dynamically interconverting equal populations of left- and right-handed helical backbone conformations. Irradiation with circularly polarized light in the ketone's chromophore gave rise to easily measurable circular dichroism signals in the polymer backbone helix, which changed sign with a change in the sense of the circularly polarized light. This demonstrates that the small enantiomeric excess produced by the irradiation, even diluted with large proportions of achiral pendants, is capable of enforcing a disproportionate excess of one helical sense in the polymer. The results, expressed as optical activity as a function of the degree of polymerization, could be analyzed using an approximate solution of a one-dimensional quenched random-field Ising model. The experimental data were fit to the theory and demonstrated a crossover between statistical and thermal randomness in the response of the helical sense to the chiral information generated by the light.