Macroscopic Helical Assembly of One-Dimensional Coordination Polymers: Helicity Inversion Triggered by Solvent Isomerism

Abstract

Assembling macroscopic helices with controllable chirality and understanding their formation mechanism are highly desirable but challenging tasks for artificial systems, especially coordination polymers. Here, we utilize solvents as an effective tool to induce the formation of macroscopic helices of chiral coordination polymers (CPs) and manipulate their helical sense. We chose the Ni/R-,S-BrpempH₂ system with a one-dimensional tubular structure, where R-,S-BrpempH₂ stands for R-,S-(1-(4-bromophenyl)ethylaminomethylphosphonic acid). The morphology of the self-assemblies can be controlled by varying the cosolvent in water, resulting in the formation of twisted ribbons of R-,S-Ni(Brpemp)(H₂O)·H₂O (R-,S-2T) in pure H₂O; needle-like crystals of R-,S-Ni(Brpemp)(H₂O)₂·1/3CH₃CN (R-,S-1C) in 20 vol % CH₃CN/H₂O; nanofibers of R-,S-Ni(Brpemp)(H₂O)·H₂O (R-,S-3F) in 20–40 vol % methanol/H₂O or ethanol/H₂O; and superhelices of R-,S-Ni(Brpemp)(H₂O)·H₂O (R-,S-4H or 5H) in 40 vol % propanol/H₂O. Interestingly, the helicity of the superhelix can be controlled by using a propanol isomer in water. For the Ni/R-BrpempH₂ system, a left-handed superhelix of R-4H(M) was obtained in 40 vol % NPA/H₂O, while a right-handed superhelix of R-5H(P) was isolated in 40 vol % IPA/H₂O. These results were rationalized by theoretical calculations. Adsorption studies revealed the chiral recognition behavior of these compounds. This work may contribute to the development of chiral CPs with a macroscopic helical morphology and interesting functionalities.