Helicity induction on a poly(phenylacetylene) derivative bearing a sulfonic acid pendant with chiral amines and memory of the macromolecular helicity in dimethyl sulfoxide

A stereoregular poly(phenylacetylene) derivative bearing a sulfonic acid residue (poly-1) as the pendant was found to form a predominantly one-handed helix upon complexation with various chiral amines through noncovalent acid-base interactions in dimethyl sulfoxide (DMSO). In sharp contrast to analogous poly(phenylacetylene)s bearing relatively weak carboxy and phosphonic acid residues as the pendants, the poly-1-amine complexes exhibited a weak induced circular dichroism (ICD) in the UV-visible region of the polymer backbone. However, in the presence of a strong acid, such as p-toluenesulfonic acid (TosOH), the ICD intensity significantly increased, resulting from a favorable ion pair formation between the sulfonic acid residues of the poly-1 and chiral amines, leading to an increase in the helical sense excess of poly-1. Moreover, the macromolecular helicity of poly-1 induced by chiral amines in the presence of TosOH was "memorized" after the chiral amines and their salts were completely removed and replaced with an achiral diamine, i.e., ethylene diamine, in DMSO, while no memory effect was observed for a helical poly-1 induced by the same chiral amines in the absence of TosOH.