Synthesis of Double-Decker Cages by B(C ₆ F ₅ ) ₃ -Catalyzed Silylation of BINOL with Silsesquioxanes: Structural Analysis for Selective Organic-Aqueous Fluoride Sensing

The detection of fluoride ions (F – ) is critical for both environmental monitoring and human health. In this study, we report the design and synthesis of novel cage-like molecules ( L1 – L3 ) derived from double-decker silsesquioxane (DDSQ) hybridized with mono- or bis - R -BINOL fluorophores. These compounds were successfully synthesized through inter/intramolecular Piers–Rubinsztajn reactions, and their well-defined cage structures were confirmed by single-crystal X-ray diffraction. Photophysical investigation demonstrates that the hybrid molecules exhibit substantially enhanced fluorescence quantum yields in tetrahydrofuran (THF), showing up to an 8-fold improvement over the parent BINOL moiety. All three molecules display a highly selective fluorescence quenching response to fluoride ions, particularly in organic-aqueous systems, achieving detection limits of 2.87 μM, 2.86 μM, and 1.55 μM for L1, L2, and L3, respectively. A comprehensive structure–property analysis reveals that the enhanced sensitivity of molecule L3 is attributed to its larger internal cavity, which enables pre-enrichment with fluoride ions. This structural feature facilitates the disassembly of the hybrid cage on interaction with F –, leading to significant fluorescence quenching. This work provides an effective strategy for developing high-performance chemosensors for fluoride recognition through the strategic modification of the cavity environment within hybrid cage molecules.