Optimization of the Coupling of Target Recognition and Signal Generation - Advanced Fluorescence Reporters in Chemistry and Biology - page 86

was also shown. Interestingly, a strong amplification of the fluorescence quenching
signal was observed with increasing dendrimer generation, n . For the larger den-
drimers ( n

3, 4, and 5), 1:1 metal/dendrimer species were formed at very low
metal ion concentration, in which the Co 2+ guest quenches each of the dansyl units
that is excited after light absorption. In the case of n

¼

5, one Co 2+ is able to quench ¼

64 dansyl units of the dendrimer [ 202 ].
Optically active dendrimers have also been described for fluorescence detection
of chiral compounds, like those based on chiral 1,1 0 -bi-2-naphthol ( BINOL )[ 203 ].
The light harvesting antennas of the dendrimer funnel energy to the central BINOL
unit, whose hydroxyl groups lead to fluorescence quenching upon interaction with
a chiral amino alcohol. This mechanism renders the dendrimers' fluorescence
responses much more sensitive than those of the corresponding small-molecule
sensors. For instance, the enantioselective fluorescence recognition of mandelic
acid, a chiral -hydroxycarboxylic acid, was also realized in such a way [ 204 ]. This

last example is particularly interesting because, again in contrast to the majority of
systems, the light-harvesting effect in this case entails fluorescence enhancement
instead of quenching.
Mechanistically, exciton migration phenomena also seem to be responsible for
the enhanced fluorescence quenching in dendrimers. For instance, Guo et al. studied
the fluorescence quenching effects of an organic dendrimer in the presence of TNT
by two- and three-photon absorption techniques. Their investigations showed that
the quenching constant increased with the dendrimer generation number and that
excitons can migrate over the dendrimer surface to the quenching site. The ability
for exciton migration is thus the main contributor to the observed dynamic fluores-
cence quenching [ 205 ].

a

4.5 Resonance Energy Transfer

The last part of Sect. 4 deals with a concept that is in most cases not a true
amplification strategy in the sense that one analyte as input generates an output
from a larger number of fluorophores (for an exception, see the PPE/PA-FA
example in Sect. 4.4.1 ).The advantageous effect here, however, comes from the
unique signaling system, involving two chromophores and a distance-dependent
process, and from the fact that the analytically relevant parameters excitation
wavelength and emission spectrum are well separated, i.e., these systems show a
very pronounced virtual Stokes shift. The concept is termed Resonance Energy
Transfer (RET) and the mechanism can be basically described as follows. After
photoexcitation, energy absorbed by a molecule (the RET donor, D RET ) can be
transferred over a comparatively long distance to another molecule (the RET
acceptor, A RET ) by resonance energy transfer. The latter then can emit with its
characteristic spectrum. The pronounced virtual Stokes shift which is especially