Gelation-Induced Visible Supramolecular Chiral Recognition by Fluorescent Metal Complexes of Quinolinol–Glutamide

Three metal complexes consisting of Li⁺, Zn²⁺, and Al³⁺ and quinolinol-functionalized l-glutamides (HQLG), (abbreviated as LiHQLG, Zn­(HQLG)₂, and Al­(HQLG)₃) were found to form fluorescent metallogels in several organic solvents. In solution, these chiral complexes showed neither any CD signal in the chromophore region nor chiral recognition of the chiral species. However, upon gel formation, the supramolecular chirality emerged because of the self-assembled nanostructures, which provided an opportunity for the chiral recognition of enantiomeric ligands. The metallogels showed different fluorescence changes when they met with enantiomeric (R,R)- or (S,S)-1,2-diaminocyclohexane. Among them, the Al­(HQLG)₃ metallogels did not show any change whereas the LiHQLG gels exhibited the same decrease in fluorescence. Interestingly, the Zn­(HQLG)₂ gels showed obviously different fluorescent color with respect to (R,R)- and (S,S)-1,2-diaminocyclohexane, thus providing visible chiral recognition via the naked eye. Such different recognition ability was discussed on the basis of the assembled chiral nanostructures and the primary molecular structures of the metal complexes. It was shown that both of them played important roles in chiral recognition.