om7b00759_si_001.pdf (1.38 MB)
OFF–ON Fluorescence Sensing of Fluoride by Donor–Antimony(V) Lewis Acids
journal contribution
posted on 2017-12-14, 19:37 authored by Ajay Kumar, Mengxi Yang, Minji Kim, François P. Gabbaï, Min Hyung LeeA 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.