Enantioseparation on helical poly(diphenylacetylene)s bearing optically-active pendants: Effects of differences in higher-order structures of kinetically-trapped and thermodynamically-stable states on chiral recognition ability.

An optically-active poly(diphenylacetylene) (PDPA) bearing carboxy pendant groups with left-handed helicity memory (M-h-poly-1), synthesized using the noncovalent helicity-induction-and-memory strategy, was converted into a PDPA bearing optically-active pendant groups through an amide bonding (M-hKT-poly-2S), while maintaining the left-handed helicity memory, by reaction with (S)-1-phenylethylamine ((S)-2) using a condensing reagent at room temperature. Its chiral recognition ability was investigated as a chiral stationary phase (CSP) for high-performance liquid chromatography (HPLC). M-hKT-poly-2S exhibited significantly different chiral recognition ability towards racemates compared to the previously reported corresponding helical PDPA bearing the same optically-active pendant groups (M-hTS-poly-2S) (prepared by the reaction of an optically-inactive PDPA bearing carboxy pendants with (S)-2, followed by thermal annealing, to induce a left-handed helical structure in the polymer main chain). Although the main chains of both M-hKT-poly-2S and M-hTS-poly-2S formed almost completely left-handed helical structures, their higher-order structures varied slightly, as confirmed by various spectroscopic methods (UV-Vis, circular dichroism (CD), IR, and vibrational CD). M-hKT-TS-poly-2S, the PDPA formed on the thermal annealing of M-hKT-poly-2S, exhibited the same higher-order structure and chiral discrimination ability as M-hTS-poly-2S. Therefore, slight differences in the higher-order structures of the kinetically-trapped metastable state (M-hKT-poly-2S) and the thermodynamically-stable state (M-hTS-poly-2S), due to differences in synthetic procedures, significantly impact their chiral recognition abilities as CSPs, even with identical primary structures and helix-sense of the polymer main chain.

Helical springs as a color indicator for determining chirality and enantiomeric excess.

Chirality plays a key role in the physiological system, because molecular functionalities may drastically alter due to a change in chirality. We report herein a unique color indicator with a static helicity memory, which exhibits visible color changes in response to the chirality of chiral amines. A difference of less than 2% in the enantiomeric excess (ee) values causes a change in the absorption that is visible to the naked eyes. This was further quantified by digital photography by converting to RGB values. This system relies on the change in the tunable helical pitch of the π-conjugated polymer backbone in specific solvents and allows rapid on-site monitoring of chirality of nonracemic amines, including drugs, and the simultaneous quantitative determination of their ee values.