Almost as important as the choice of lanthanide is the choice of
ancillary ligand, which transforms the lanthanide into a recep-
tor. This helper ligand must bind to the lanthanide with high
affinity to prevent solvent coordination, which can severely
quench luminescence via nonradiative decay pathways. However,
the ligand must not interfere with binding of the target analyte,
or the lanthanide loses all functionality as a sensor. Thus, the
denticity and stability of the receptor ligand, as well as the
size and geometry of the binding pocket it generates, are key
factors. Further, as most coordination to lanthanides is based
on electrostatic interactions, the charge of the ancillary
ligand—and ultimately the entire receptor complex—should
influence the binding affinity of the analyte. We will show that
there are exceptions to this“rule,”as other factors can affect
analyte binding in very significant ways.
The goal of our program is to build receptors with lanthanide ions
containing ancillary ligands that confer stability, solubility, and high
analyte specificity for target molecules. Binding of an aromatic
analyte to such a binary complex should trigger intense lumines-
cence upon UV excitation due to an AETE mechanism, producing a
signal that is orders of magnitude greater in intensity than emission
of the lanthanide alone. We have investigated various binary
complexes containing a luminescent lanthanide (Sm^3 þ, Eu^3 þ, Tb^3 þ,
and Dy^3 þ) and a helper ligand to optimize detection of a particular
analyte of interest. These investigations, which involved analysis of
structure, photophysics, stability, and resistance to interferents,
turned up some interesting results that forced us to question several
basic assumptions regarding lanthanide–ligand interactions.
The common properties attributed to helper ligands in improv-
ing lanthanide-based detection techniques include enhanced
stability, sensitivity, and selectivity. We will start with the
effects of ancillary ligands on lanthanide photophysics and then
explore the impacts of these ligands on sensor properties (Section
II). Other factors influencing stability, such as sterics and
oxophilicity, will be summarized in Section III. A brief discussion
of future applications will follow (Section IV).
II. Effects of Ancillary LigandsA. PHOTOPHYSICS
Ancillary ligands were first applied in lanthanide chemistry
due to their ability to encapsulate the metal ion and protect it
from solvent (47,48). The greatest vulnerability of sensitized
10 MORGAN L. CABLEet al.