Mechanism of Helix Induction on a Stereoregular Poly((4-carboxyphenyl)acetylene) with Chiral Amines and Memory of the Macromolecular Helicity Assisted by Interaction with Achiral Amines

Cis-transoidal poly((4-carboxyphenyl)acetylene) (poly-1) is an optically inactive polymer but forms an induced one-handed helical structure upon complexation with optically active amines such as (R)-(1-(1-naphthyl)ethyl)amine ((R)-2) in DMSO. The complexes show a characteristic induced circular dichroism (ICD) in the UV-visible region of the polymer backbone. Moreover, the macromolecular helicity of poly-1 induced by (R)-2 can be "memorized" even after complete replacement of (R)-2 by various achiral amines. We now report fully detailed studies on the mechanism of the helicity induction and memory of the helical chirality of poly-1 by means of UV-visible, CD, and infrared spectroscopies. We have found that a one-handed helix is cooperatively induced on poly-1 upon the ion pair formation of the carboxy groups of poly-1 with optically active amines and that the bulkiness of the chiral amines plays a crucial role for inducing an excess of a single-handed helix. On the other hand, the free ion formation was found to be essential for the macromolecular helicity memory of poly-1 after the replacement of the chiral amine by achiral amines, since the intramolecular electrostatic repulsion between the neighboring carboxylate ions of poly-1 significantly contributes to reduce the atropisomerization process of poly-1. On the basis of the mechanism of helicity induction and the memory of the helical chirality drawn from the present studies, we succeeded in creating an almost perfect memory of the induced macromolecular helicity of poly-1 with (R)-2 by using 2-aminoethanol as an achiral chaperoning molecule to assist in maintaining the memory of helical chirality.