Optimization of the Coupling of Target Recognition and Signal Generation - Advanced Fluorescence Reporters in Chemistry and Biology - page 45

reactions, or internal torsional motions around (a) flexible bond(s) [ 28 , 32 - 34 ].
Metal ion coordination can then shift the n p * energy levels with respect to those of

the ligand-centered * states, can shield the proton transfer sites or simply block

rotational motion, leading to strong fluorescence enhancement (Fig. 1 ).
However, because of their simple architecture, the metal ion selectivity of such
ligands is mostly rather poor and the binding affinity is basically dominated by the
Irving-Williams order [ 35 ]. In addition, the possibilities of discriminating between
different complexes of a certain ligand on the basis of absorption spectral shifts are
also very limited, the bands of the single complexes generally showing strong
overlap [ 36 ]. Moreover, although these compounds often undergo distinct changes
in their absorption spectra upon interaction with a metal ion (see Fig. 1 ), dramatic
fluorescence responses are restricted to “light” diamagnetic cations such as the
upper row group-II and group-III metal ions and Zn 2+ . Complexes with diamagnetic
“heavy” ions such as Pb 2+ and Hg 2+ usually show only weak fluorescence due to
enhanced spin-orbit coupling (heavy atom effect, [ 37 ]) 3 and complexes of para-
magnetic ions with an open d shell like Cu 2+ and Fe 3+ are often essentially
nonfluorescent due to efficient electronic energy transfer [ 39 ].
The main reason which is responsible for these restrictions is the concurrence of
chromophore and chelator, i.e., they are identical (like in 1 or 2) and any distinct
and deliberately manipulable communication channel between these functional
units simply does not exist. Any potentially quenching species interacting with
the chromophore/chelator can thus unfold its activity unhindered. Such a behavior
is not only found for metal ions, but also for anions , e.g., in the case of various
fluorometric chloride indicators [ 40 ]. In fact, for these analytes, the situation is
more complicated. Not only potential quenching interactions play a role (most of all
for Cl , Br, and I ) but also the intrinsic feature that utmost anions possess a much
lower charge density than metal cations, simply due to their polyatomic nature,
larger size for equal net charge, and higher atomic similarity to most organic
chromophores. Thus, already the spectral changes in absorption that have been
reported for anions of low basicity and fluorescent ligands such as 4-6 are consid-
erably weak and are only found in model environments like dichloromethane or
other organic solvents (Fig. 2 )[ 41 , 42 ]. 4 Obviously, this situation is not improved
for organic anions. Structurally, simple ligands that respond to anions with a
lighting up of their fluorescence are thus virtually unknown. 5 Taking the step
from charged to neutral analytes, i.e., small organic molecules which are partly
not even capable of forming electrostatic or hydrogen bonds with a receptor but
have to rely on weaker forces such as

pp

p stacking, dipole-dipole, or van der Waals

interactions, the picture remains unchanged. Apparently, the separation and

3

The heavy atom effect offers certain possibilities for fluorescence lifetime-based analytical
exploitation which will not be discussed here. The reader is referred to [ 38 ] for more details.
4

The dramatic bathochromic shifts found for ligands such as 6 and various more elaborated probes
in the presence of basic anions in organic solvents are mainly due to deprotonation effects [ 43 , 44 ].
5 Naturally, this holds for polyatomic (organic) cations as well. Simple fluorescent reporters for,
e.g., ammonium, alkylammonium or guanidinium species have also not been reported.