OFF-ON Fluorescence Sensing of Fluoride by Donor-Antimony(V) Lewis Acids

A series of triarylmethylstibonium Lewis acids of general formula [Ph2MeSb-(p-(C6H4))-FLUO]+ bearing a peripheral electron-rich fluorophore (FLUO = 10H-phenoxazine ([3a]+), diphenylamine ([3b]+), and 9H-carbazole ([3c]+)) have been synthesized and investigated for the fluorescence turn-on sensing of fluoride anions. Treatment of the stibonium cations with fluoride anions leads to the corresponding fluorostiboranes (3a-F-3c-F). While the stibonium cations are almost nonemissive, the fluorostiboranes display fluorophore-centered emissions arising from the corresponding π-π∗ excited state. The carbazole-containing derivative [3c]+ exhibits the most intense fluorescence turn-on response. It also displays a high binding constant (K > 107 M-1) in MeCN and shows compatibility with protic media such as MeOH (K = 950(±50) M-1). Computational studies aimed at identifying the origin of the turn-on response show that the excited state of the stibonium cations is best described as charge transfer in nature with the π system of the fluorophore acting as the donor and the π-σ system of the stibonium unit acting as the acceptor. This π(FLUO)-π ∗/σ ∗ (Ph2MeSb-(p-(C6H4))) excited state is nonemissive, making these cations dark in the absence of fluoride anions. Conversion to the fluorostiboranes occurs via donation of a fluoride lone pair into the antimony-centered σ∗. Formation of this Sb-F bond modifies the electronic structure of the platform and restores the emissive π-π∗ excited state of the fluorophore, thus accounting for the observed OFF-ON fluorescence response.