Chemistry Reference
In-Depth Information
Absorption
Emission
ET
Chromophore
Ln
III
S
1
ISC
T
1
ET
E
Absorption
Emission
S
0
Ln
III
chromophore
sCHeMe 12.4
Top: Simplified model for sensitised lanthanide luminescence. Bottom: Typical energy level diagram for an emissive
chromophore-appended lanthanide complex sensitised via a ligand-centred triplet excited state.
O
O
O
O
OH
2
O
H
H
O
O
OH
N
N
N
N
OH
2
+2 H
2
O
O
O
Ln
Ln
N
N
N
N
O
O
O
O
O
O
sCHeMe 12.5
An example of reversible anion binding that induces a change in Ln
III
hydration.
the ligand core (i.e., aromatic/heterocyclic sensitiser). Anion binding to the metal ion typically occurs through a reversible
intermolecular process (Scheme 12.5), inducing reversible displacement of coordinated water molecules from the Ln
III
centre. Therefore, for a heptadentate ligand the resultant Ln
III
complex would typically expect to have 1 or 2 coordinated
water molecules; coordination of mono- or bi-dentate anions will likely liberate one or both of these water molecules, result-
ing in measurable changes in luminescent output (e.g., intensity and lifetime). An understanding of the anion-binding affin-
ities and the resultant perturbation of the Ln
III
luminescence are important in a biological context because various anionic
residues are available for binding [47]. In fact, this behaviour has allowed the development of responsive luminescent
probes, which can relay information on anion concentrations [48]. Finally, luminescing Ln
III
ions should be considered
'spherical' emitters and therefore avoid anisotropy; as a consequence, enantiopure Eu
III
/Tb
III
complexes can give intense
CPL signals in the visible region. The implications of this observation are important because binding to a bio-macromolecule
such as serum albumin can invert helicity, switching the sign of the CPL signal. Certain enantiopure complexes have been
shown, through competition studies, to demonstrate selective binding to specific drug sites on proteins; CPL may be a very
useful tool for tracking protein association through the use of chiral probes [49, 50].
