Anion identification using silsesquioxane cages

Anthracene-conjugated octameric silsesquioxane (AnSQ) cages, prepared via Heck coupling between octavinylsilsesquioxane (OVS) and 9-bromoanthracene, thermodynamically display intramolecular excimer emissions. More importantly, these hosts are sensitive to each anionic guest, thereby resulting in change of anthracene excimer formation, displaying the solvent-dependent fluorescence and allowing us to distinguish up to four ions such as F^-, OH^-, CN^- and PO₄ ^3- by fluorescence spectroscopy. Depending on the solvent polarity, for example, both F^- and CN^- quenched the fluorescence emission intensity in THF, but only F^- could enhance the fluorescence in all other solvents. The presence of PO₄ ^3- results in fluorescence enhancements in high polarity solvents such as DMSO, DMF, and acetone, while OH^- induces enhancements only in low polarity solvents (e.g. DCM and toluene). A picture of the anion recognizing ability of AnSQ was obtained through principal component analysis (PCA) with NMR and FTIR confirming the presence of host-guest interactions. Computational modeling studies demonstrate the conformation of host-guest complexation and also the change of excimer formation. Detection of F^-, CN^- and OH^- by AnSQ hosts in THF is noticeable with the naked eye, as indicated by strong color changes arising from charge transfer complex formation upon anion addition.