Optimization of the Coupling of Target Recognition and Signal Generation - Advanced Fluorescence Reporters in Chemistry and Biology - page 46

Fig. 2 Chemical structures of fluorescent ligands (anion coordination sites are indicated in blue )
and tentative reaction schemes on interaction of 6 with chloride and fluoride in dichloromethane
according to the color patterns observed in [ 41 ]. The wavelengths of absorption given in brackets
have been calculated for the geometry optimized species 6, 6-Cl and 6 in the gas phase by
semiempirical AM1 calculations (Ampac V6.55, Semichem)

individual design of binding and signaling site and the tailoring of the communica-
tion between them are essential for broader success. 6

2.1

p -Conjugated Binding Site and Fluorophore

When individual units are combined in a modular fashion, perhaps the most
obvious approach again relies on a direct integration of (part of) the receptor to
the chromophore with only the electronic transitions in the resulting probe being of
a more refined nature than in the ligands discussed above. In contrast to the latter
where HOMO and LUMO are delocalized over the same parts of the molecule, i.e.,
usually over the entire ligand molecule, the reporters with -conjugated binding site

and fluorophore generally show different charge distributions in the molecular
orbitals involved in the optical transitions. Absorption and emission processes
thus have a (certain) charge transfer (CT) character, leading to a reversal of the
charge densities upon photoexcitation. This feature is also responsible for the term
commonly used for this class of reporters - intramolecular charge-transfer or ICT
probes. 7 In a simplified picture, the molecular entity that is electron-rich in the
ground state becomes electron poor in the excited state and vice versa. Accordingly,
integration of a binding site to the electron richest/poorest atom/molecular fragment
should lead to the strongest modulation of its electron donating/accepting character

p

6

A certain control of selectivity for simple ligands without altering their fluorescence features is
possible when integrating them into more developed, often macrocyclic receptor units [ 45 , 46 ].
7 The term PCT probe for photoinduced charge transfer probe is also frequently used.