Temperature-driven switching of helical chirality of poly [(4-carboxyphenyl)acetylene] induced by a single amidine enantiomer and memory of the diastereomeric macromolecular helicity

Two novel optically active amidines, (R,R)-N,N'-bis(1-phenylethyl)-2,4,6-triphenylbenzamidine [(R,R)-2] and (R,R)-N,N'-bis(1-phenylethyl)benzamidine [(R,R)-3] were synthesized, and their helicity induction abilities for poly[(4-carboxyphenyl)acetylene] (poly-1) in dimethyl sulfoxide (DMSO) were investigated by UV-visible and circular dichroic spectroscopies. Poly-1 exhibited a split-type induced circular dichroism (ICD) in the polymer backbone region in the presence of the optically active amidines due to a predominantly one-handed helical conformation. The ICD pattern of poly-1 complexed with the bulky (R,R)-2 ([(R,R)-2]/[poly-1] = 1) dramatically changed in DMSO and the Cotton effect signs reversibly inverted by changing the temperature, whereas the complex of poly-1 with the less bulky (R,R)-3 complex did not show such a temperature-driven change in the ICD. These results indicate that the helix-sense of poly-1 induced by (R,R)-2 through noncovalent acid-base interactions undergoes a transition from one helix to another in response to temperature. Furthermore, the diastereomeric right- and left-handed helices of poly-1 induced by (R,R)-2 at different temperatures could be separately memorized by the replacement of (R,R)-2 with achiral amines, thus generating enantiomeric helices of the mirror images of each other.