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fluorophores play outstanding roles and are combined mainly with short alkyl
spacers (especially methylene) and the well-known neutral and charged anion
recognition moieties such as (thio)ureas or imidazolium salts. In contrast to their
cation analogs, fluorescence quenching is the predominant signaling mechanism for
PET anion probes. Moreover, for those probes which show amplified fluorescence
upon binding, the enhancement is often less dramatic than in the cation case and is
often ascribed to a rigidification of the probe on anion coordination. PET probes
that show comparatively strong responses (i.e., FEF
5) such as 15-17 (Fig.
7
)
commonly involve the suppression of an oxidative PET process (e.g., 15,16) or the
inhibition of a reductive PET through coprotonation (e.g., 17)[
74
-
76
]. As for
cation reporters, issues of selectivity also play a significant role in anion probe
design nowadays [
77
].
Besides inorganic anions, a lot of research effort is focussed on the development
of indicator molecules for small organic molecules that possess an anionic and a
cationic function such as, e.g., a carboxylate and an ammonium group as in the
neurotransmitter
>
g
-aminobutyric acid (18, Fig.
7
)[
78
] or an anionic and a neutral
Fig. 7 PET probes for anions (15-17) and charged small molecules (18,19) showing enhanced
fluorescence upon target binding. Typical FEF are ca. 4 for 15 and acetate in MeCN, ca. 5 for 16
and HPO
4
2
in H
2
O:MeCN (6:94, v/v), 2.5 for 17/PPi in 50 mM aq. HEPES, 3.5 for 18/GABA in
H
2
O:MeOH (2:3, v/v) and ca. 5 for 19/
D
-glucarate in MeOH:0.1 M aq. HEPES (1:1, v/v). For color
code, see Fig.
6
; target analytes for 18,19 are shown as well; GABA
ΒΌ g
-aminobutyric acid. For
19 upon
D
-glucarate binding, the 9,10 substituents are presumably rearranged in such a way that
the boron atom can interact strongly with the PET-active nitrogen atom (
orange
), hampering PET
quenching [
73
]
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