Solvent-dependent helix inversion in optically active poly(diphenylacetylene)s and their chiral recognition abilities as chiral stationary phases for high-performance liquid chromatography.

We report the first example of solvent-dependent helix inversion in poly(diphenylacetylene) (PDPA) derivatives. Asymmetrically substituted PDPAs bearing optically active substituents linked through amide bonds formed preferred-handed helical conformations because of the optically active substituents in the pendants, whose helix-senses were inverted upon thermal annealing in polar solvents such as N,N-dimethylformamide and dimethylsulfoxide and in nonpolar solvents such as tetrachloroethane. Unlike the solvent-dependent helix inversion reported for other dynamic helical polymers, the macromolecular helicity induced in the polymer backbone of these PDPAs upon thermal annealing was stably maintained at room temperature, independent of the solvent polarity. These diastereomeric PDPAs with opposite helix-senses generated almost mirror-imaged left- and right-handed circularly polarized light in the same solvent at room temperature. Taking advantage of this unique solvent-dependent helix inversion property, the diastereomeric PDPAs with opposite helix-senses were coated on macroporous silica gel and applied to chiral stationary phases for high-performance liquid chromatography. Despite having the same optically active substituents on the pendant phenyl rings, they showed completely different chiral recognition abilities toward many racemates depending on the helix-sense of the polymer backbone, and the elution order of the enantiomers was reversed for some racemates. The combination of the helix-sense of the polymer backbone and the chirality of the pendants, which afforded a higher chiral recognition ability, differed depending on the racemates.

Synthesis of Optically Active Poly(diphenylacetylene)s Using Polymer Reactions and an Evaluation of Their Chiral Recognition Abilities as Chiral Stationary Phases for HPLC.

A series of optically active poly(diphenylacetylene) derivatives bearing a chiral substituent (poly-2S) or chiral and achiral substituents (poly-(2Sx-co-31-x)) on all of their pendant phenyl rings were synthesized by the reaction of poly(bis(4-carboxyphenyl)acetylene) with (S)-1-phenylethylamine ((S)-2) or benzylamine (3) in the presence of a condensing reagent. Their chiroptical properties and chiral recognition abilities as chiral stationary phases (CSPs) for high-performance liquid chromatography (HPLC) were investigated. Poly-2S and poly-(2Sx-co-31-x) (0.06 < x < 0.71) formed a preferred-handed helical conformation with opposite helical senses after thermal annealing despite possessing the same chiral pendant (h-poly-2S and h-poly-(2Sx-co-31-x)). Furthermore, h-poly-2S and h-poly-(2S0.36-co-30.64) emitted circularly polarized luminescence with opposite signs. h-Poly-2S showed higher chiral recognition abilities toward a larger number of racemates than poly-2S without a preferred-handed helicity and the previously reported preferred-handed poly(diphenylacetylene) derivative bearing the same chiral substituent on half of its pendant phenyl rings. h-Poly-(2S0.36-co-30.64) also exhibited good chiral recognition abilities toward several racemates, though the elution order of some enantiomers was reversed compared with h-poly-2S.