Induction of Supramolecular Chirality by Chiral Solvation in Achiral Azo Polymers with Different Spacer Lengths and Push-Pull Electronical Substituents: Where will Chiral Induction Appear?
Lu Yin, Yin Zhao, Meng Liu, Nianchen Zhou, Wei Zhang, Xiulin Zhu
Construction of supramolecular chirality from achiral compounds has been exploited in small molecule, oligomer and polymer systems. However, the relationship between the structure of building blocks and induction of supramolecular chirality is often poorly understood. Herein, we designed and synthesized a series of azobenzene (Azo) side-chain polymers with different spacer lengths (n is the number of methylene units in the spacer, n = 0, 2, 4, 6, 11) and push-pull electronical substituents (R is the terminal groups of Azo unit, R = OCH3, CH3, CN, N(CH3)2, H) by reversible addition-fragmentation chain transfer polymerization. The structure of achiral side-chain Azo polymer with different push-pull groups and spacer lengths had strong effects on supramolecular chirality induced by chiral limonene. No apparent chiral amplification phenomena were observed for all Azo polymer systems. Those aggregates of several Azo polymers in dichloroethane/limonene mixed solvent were characterized by circular dichroism (CD) and UV-vis spectroscopy, demonstrating the successful chirality transfer from limonene molecules to PM4Az, PM6Az, PM11Az, PM6CN and PM6N. However, when spacer lengths were shorter (PM0Az and PM2Az), the supramolecular chirality arising from wellassembled achiral side-chain Azo units was not observed and the polymer aggregates had no CD signal. Meanwhile, the CD signal was absent for Azo polymers containing CH3 or H terminal groups. The polymers with longer spacer lengths (PM11Az) showed better reversibility of chiral-achiral switch by trans-cis photoisomerization and cis-trans thermalisomerization process compared to PM4Az, PM6Az and PM6CN. Intriguingly, the chiral aggregate of PM6N showed good stability of supramolecular chirality under 405 nm light exposure over 40 min. These findings will not only provide significant insight into the structure-function relationship for constructing supramolecular chirality from achiral polymers, but also give cues for design of stabilized chiroptical materials.
Circular dichroism, Polymers, Aggregation, Chemical stability