Silsesquioxane cages as fluoride sensors

Pyrene functionalized silsesquioxane cages (PySQ) not only provide significant fluorescence from pyrene–pyrene excimers with a very large Stokes shift (Δλ = 143 nm, 69 930 cm−1) in DMSO but also exhibit fluoride capture results coincidentally with a π–π* fluorescence enhancement. On the other hand, PySQ-F− in THF significantly exhibits π–π* fluorescence quenching and a color change can be observed with the naked eye from light yellow to deep orange by forming a charge-transfer (CT) complex among the pyrenyl rings. Moreover, PySQ selectively captures F− with a response time of <2 min and with a very low detection limit (1.61 ppb), while 19F NMR is used to confirm encapsulation of F− with Δδ = 19 ppm.