Modulation of Photoluminescence of BODIHY Dye Using Water-Soluble Coordination Cages With Different Shapes.

The confinement of guest molecules within supramolecular hosts can alter their photophysical properties. However, the shapes of the hosts in regulating the guest's emission remains underexplored. Herein, we investigate how the shape of the host alters the emission behavior of boron difluoride hydrazone (BODIHY) (G) dye encapsulated within two iso-stoichiometric water-soluble coordination cages: MC1 (double-square cage) and MC2 (octahedral cage). Encapsulation of G within MC1 results in a highly emissive solution, whereas similar confinement in MC2 leads to a non-emissive host-guest solution. A similar trend was observed with different sets of iso-stoichiometric cages MC3 (double-square cage) and MC4 (octahedral cage). Using a combination of femtosecond transient absorption and time-resolved fluorescence spectroscopy, we observed that the disparity in fluorescence behavior of BODIHY is attributed to charge transfer interactions between the guest and ligand panels of cages. The shape of the coordination cage dictates the preorganization of the guest within the cavity, thereby suppressing or promoting this charge transfer interactions. Moreover, we demonstrate a turn-on emission of BODIHY dye due to its preferential binding to a double-square cage. These findings provide fundamental insights into host-mediated modulation of the guest's photophysics and offer a blueprint for designing supramolecular systems with tunable emissive behavior.