Optimization of the Coupling of Target Recognition and Signal Generation - Advanced Fluorescence Reporters in Chemistry and Biology - page 55

Fig. 8 Typical excimer probes utilizing two chelating sites and two fluorophores (20), a flexible
central composite receptor site and three fluorophores (21) and a single receptor site with two
pendant arms and two fluorophores (22). (a)Na + -induced excimer alignment in 20 and (b)
respective spectroscopic response; (c) selective probes for Fe 3+ (21) and Cu 2+ and Ni 2+ (22) that
show quenching of monomer and excimer emission upon binding. Color code: fluorophores in red
and atoms responsible for coordination in blue . (Reprinted in part with permission from [ 83 ].
Copyright 1995 American Chemical Society)
higher excimer yields and more pronounced quenching of the monomer emission,
which can be utilized for signaling (20, Fig. 8 )[ 83 ]. Many probes show residual
excimer/monomer emission in both the free and the complexed state (e.g., Fig. 8b ),
which is predominantly due to the interplay of static and dynamic excimers [ 84 ]. As
in the case of the exciplexes, the absorption spectrum is virtually identical to that of
the parent fluorophore and binding of the target species leads to a change in
monomer-to-excimer emission yield (Fig. 8 ). Since the possibilities of integrating
a receptor site (or more than one site as in 20, Fig. 8 ) and two (or more) fluorophores
into a supramolecule are manifold, a considerable number of structures have been
realized. However, one of the drawbacks of many excimer probe architectures is the
fact that on binding of potential quenchers, these species are too close to the
fluorophores and quench the emission nonspecifically, whether excimer or mono-
mer (e.g., 21,22, Fig. 8 )[ 85 , 86 ]. Attempts toward systems that are immune to
quenching have been realized but are much less frequent [ 87 ].
While research in the area started with probes for cations, the last years have
shown that the dual-fluorophore excimer concept seems especially promising for
the development of anion probes. The benefit here is often twofold. On one hand,
the observation of both emission bands provides a more stable differential or