Optimization of the Coupling of Target Recognition and Signal Generation - Advanced Fluorescence Reporters in Chemistry and Biology - page 58

possess intrinsically different properties as detailed above, the following sections
are divided into probes for cations and probes for anions and small molecules.

3.2.1 For Metal Cations

As we have discussed in Sect. 2 , ICT and PET probe architectures are in principle
well-suited concepts for the development of reporters that show pronounced spec-
troscopic responses upon metal ion binding. For ICT probes, the major challenge
perhaps is how to design probe molecules which predictably show an analyte-
induced increase in emission in a rational way. A key to success here is the
installation of a second ICT process in the chemosensor molecule. Depending on
the aspired architecture, the two ICT processes can be identical or different with
intrinsically different emissivities. One such example of the latter type is asymmet-
ric 25 in Fig. 10 [ 106 ]. For 25, ICT 1 involving D 1 and A is highly emissive whereas
ICT 2 involving D 2 and A is weakly emissive. The net effect is an intermediate
fluorescence of the free probe (

0.03). Accordingly, when a metal ion is bound
at D 2 , this donor is turned into an acceptor ( A 2 ) and the ICTs in the molecule are
reconfigured: ICT 1 is accelerated whereas ICT 2 is blocked. The net effect is a red-
shift of the fluorescence spectrum and a strong 32-fold increase in quantum yield.
Symmetric D - A - D probes operate in a related way, only that here both processes
are convergent and the net effect is the amplification of a single ICT. Of course,
both types are possible, i.e., reporters where cation binding takes place in the
acceptor (e.g., 26 in Fig. 10 ) or in the two donor parts of the molecule (e.g., 27 in
Fig. 10 )[ 107 , 108 ]. Although there are not too many reports on such type of dyes in
the literature, other approaches relying on cruciform fluorophores [ 109 ] or utilizing
the concept of colorimetric [ 110 ] and two-photon absorption [ 111 ] metal ion probes
have also been published.
Before discussing a very powerful approach to signal enhancement that relies on
the virtual decoupling of donor and acceptor moieties in fluorescent reporters, we
will briefly highlight a strategy already introduced with 27 - the integration of more
than one receptor site into such double ICT probes. Besides achieving an enhanced
response as in 27 with its two identical binding sites, the integration of two different
receptor units can be employed for the indication of more than one species or for the
generation of cooperative signals. For instance, 28 expresses the same pattern as 25,
only the ICT from the tetraoxa monoaza crown can selectively be addressed with
alkaline earth metal ions such as Ca 2+ and the ICT from the tetrathia monoaza
crown with thiophilic cations such as Ag + [ 112 ]. 29 on the other hand utilizes a
tandem donor configuration with H + serving the nitrogen addressee and Mg 2+ or
Ca 2+ the pentaoxa benzocrown moiety [ 113 ]. Accordingly, binding of one, the
other, or both target species with 28 and 29 yields different spectral and intensity
responses, allowing for various detection schemes.
Leaving the classic ICT architecture and moving toward electronically
decoupled systems, a strategy for obtaining extraordinarily high fluorescence
enhancement factors has been developed at the end of the 1990s - the concept of

F f ¼