Optimization of the Coupling of Target Recognition and Signal Generation - Advanced Fluorescence Reporters in Chemistry and Biology - page 61

Fig. 12 Left : ICT-PET probe 34 with a small and rigid spacer ( thick black ), decoupling the PET
active donor -conjugated ICT fluorophore. Middle and right : probes 35

and 36 combining a conventional ICT and a virtually decoupled ICT process with two identical
receptor units (35, for Hg 2+ ) and two different receptor units (36, tetraoxa monoaza crown for Na +
and dithia monooxa monoaza crown for Ag + ). For color code, see Fig. 10

¼ receptor unit from the p

incorporation of a second binding site [ 120 ]. Potentially quenching species can be
deliberately detected with “light up” responses. In accordance with the doubly
receptor-substituted ICT dyes discussed before, the combination of a virtually
decoupled and a D - A - D configuration is also possible, giving rise to unique
signaling protocols with high fluorescence output for a single (35)[ 121 ] or given
combination of two analytes (36)[ 122 ].
The last approach for a designed cation signaling system with enhanced output
utilizes the differences in fluorescence quantum yield of two tautomeric forms, the
equilibrium of which is subject to analyte-induced changes (37, Fig. 13 )[ 123 ].

3.2.2 For Anions

One of the main forces that can be utilized for the recognition of most inorganic and
organic anions by fluorescent reporter molecules is hydrogen bonding. Whereas
electrostatic forces are usually divergent and do not allow to selectively address
a particular anion, hydrogen bonding is convergent and permits the specific
tailoring of receptor sites and interaction processes; electrostatic forces can then
support such binding interactions. Less strong and less hydrophilic forces such as
p

stacking, and van der Waals or hydrophobic interactions are usually much more
difficult to realize in indicator molecules that are functional derivatives of organic
dyes. Such effects can be utilized when the fluorescent reporter is incorporated
into composite sensory systems such as porous organic or inorganic materials like
molecularly imprinted polymers [ 124 ] or organic-inorganic hybrid materials [ 125 ]
which try to mimic antibodies or the binding pockets of proteins. Due to space
limitations, these composites will not be discussed here.
The use of hydrogen bonding assistance in fluorescent reporters is particularly
attractive for ICT probes. Because of the charge redistribution taking place upon
absorption of a photon and the ICT process, such anion-responsive probes can be
designed in a way that hydrogen bonds formed in the ground state are strengthened
in the excited state. These features can lead to enhanced rigidity, which would be
reflected in enhanced fluorescence signals. The role of hydrogen bonds can be