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.