Helically\nChiral Hybrid Cyclodextrin Metal–Organic\nFramework Exhibiting Circularly Polarized Luminescence

Abstract

Three\nachiral polycyclic aromatic fluorophoresnamely, 1-pyrenecarboxylic\nacid, 9-anthracenecarboxylic acid, and perylene-3,9-dicarboxylic acidwere\nchosen based on their desired properties before being incorporated\ninto the construction of a K⁺-carrying gamma-cyclodextrin\n(γ-CD)-based metal–organic framework (CD-MOF-1) and γ-CD-containing\nhybrid frameworks (CD-HFs). Among these fluorophores, only the pyrene-carrying\none shows significant noncovalent bonding interactions with γ-CD\nin solution. This fluorophore is encapsulated in a CD-HF with a trigonal\nsuperstructure instead of the common cubic CD-MOF-1 found in the case\nof the other two fluorophores. Single-crystal X-ray diffraction analysis\nof the trigonal CD-HF reveals a π-stacked chiral positioning\nof the pyrene-carrying fluorophore inside the (γ-CD)₂ tunnels and held uniformly around an enantiomorphous 3₂ screw axis along the <i>c</i> direction in the solid-state\nstructure. This helix-like structure demonstrates an additional level\nof chirality over and above the point-chiral stereogenic centers of\nγ-CD and the axial chirality associated with the self-assembled\nπ-stacked fluorophores. These arrangements result in specifically\ngenerated photophysical and chiroptical properties, such as the controlled\nemergence of circularly polarized luminescence (CPL) emission. In\nthis manner, a complete understanding of the mechanism of chirality\ntransfer from a chiral host (CD-HF) to an encapsulated achiral fluorophore\nhas been achieved, an attribute which is often missing in the development\nof materials with CPL.