The ON/OFF switching by metal ions of the “Sergeants and Soldiers” chiral amplification effect on helical poly(phenylacetylene)s

Abstract

Here we report copolymers where the “Sergeants and Soldiers effect” can be switched ON and OFF by the presence of a metal ion. These copolymers have been prepared by a combination of achiral and chiral monomer units, where the chiral ones are unable to drive the chiral amplification unless a small amount of mono- or divalent metal ions is added. In this way, the ions act as promoters upgrading some of the chiral pendants, initially unable to induce a preferential helical sense, to a higher rank forcing the arrangement of the rest of the chain in a specific helical sense. In these copolymers, the classical “Sergeant” and “Soldier” roles are modified in such a way that the chiral units become “Sergeants” only by the effect of an achiral external stimulus, namely the metal ion. The structure of the metal complex determines its interaction in the helix with the surrounding chiral and achiral “Soldiers” and therefore also determines both the intensity of the amplification and the response of a copolymer to a certain metal. For instance, poly(1_r-co-7_(1−r)) shows chiral amplification (“Sergeants and Soldiers effect”) towards the right-handed helix only with divalent ions, while poly(1_r-co-8_(1−r)) amplifies the helix to the left-handed sense with mono- and to the right-handed sense with divalent ions. This behaviour allows, using a single copolymer, to selectively induce any of the two helical senses. The aggregation and encapsulation properties of these copolymers are also described.