Switching and inversion of the handedness and circularly polarized luminescence (CPL) in chiral supramolecular system has attracted great interests, not only for their potential applications in the construction of chiroptical materials, but also for their unique roles in understanding the chirality-switching and circularly polarized bioluminescence mechanism observed in nature.

However, a major challenge in this field is to use tunable supramolecular system based on a single enantiomer of assembly unit, to selectively obtain left-handed or right-handed CPL depending on the type and sequence of external trigger signals, which is appealing to fabricate smart chiral supramolecular materials.

In a study published in Angew. Chem. Int. Ed., Prof. LIU Minghua from Institute of Chemistry of the Chinese Academy of Sciences reported that a novel strategy by using coordination-driven pyrene-Histidine supramolecular system can achieve the switchable reversal of supramolecular chirality and circularly polarized fluorescent signals in different assembly pathways.

Researchers used Zn(II) coordinated with imidazole groups and changed the nanofiber structures into nanospheres. The nanofiber structures gave reversed chirality as well as CPL. The coordination interaction changed the chromophore packing model from T-shaped stacking into parallel п-п stacking.

Noticeably, the reversible inversion of supramolecular chirality could be repeated for several cycles by adding complementary external stimuli in turns. Collective characterization with extensive spectra data coupled with X-ray crystallographic analysis and DFT calculations revealed that the change of pi stacking modes between T-shaped and п-п stacking was responsible for these switching phenomena.

This study provided a deep insight into how coordination and pi-stacking modes could affect supramolecular chirality as well as their excited states chirality in chiral assemblies. It also showed a new way for developing switchable chiroptical supramolecular materials.

Illustration of inversion and switching of CPL regulated by coordination and π-stacking. (Image by Prof. LIU Minghua)