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Fig. 9 (a) Two examples of excimer probes for anions and (b) response of 23 toward Cl
in
MeCN:CHCl
3
(95:5 v/v; excess of anion indicated). For color code, see Fig.
8
. (Reprinted in part
with permission from [
88
]. Copyright 2006 American Chemical Society)
ratiometric signal similar to the ratiometric ICT probes introduced above. On the
other however, the excimer mechanism in cleverly designed systems especially
allows obtaining FEF that are otherwise difficult to obtain for anions and probes
that rely on a direct electronic perturbation of an intramolecular process by the
analyte such as ICT or PET. Representative examples of excimer anion probes are
shown in Fig.
9
[
88
,
89
]. Here, for instance, chloride binding leads to the disruption
of the perfectly aligned pyrene moieties in 23, reducing excimer and enhancing
monomer emission and H
2
PO
4
complexation entails a “switching on” of the
excimer emission of 24 because of a reduction in PET interaction between the
pyridinium and anthracene groups and alignment of the latter. Current research
activities explore the use of other excimer forming fluorophores such as 1,4,5,8-
naphthalenetetracarboxdiimide [
90
], target more complex anionic molecules such
as ATP [
91
], or aim at enantioselective recognition [
92
].
3 Strategies of Signal Optimization in Fluorescent Probes
The optimization of a fluorescence signal upon target recognition can principally be
accomplished along four different paths. Path #1 is a synthetic approach and utilizes
combinatorial techniques to synthesize a larger library of functional fluorophores of
a rather simple type with the aim of screening their performance. Path #2 follows
mechanistic considerations and tries to rationally optimize the different photophy-
sical process partly by taking into account as many data available on a certain class
of fluorophores and receptor units as possible, including redox and spectroscopic
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