ion, though peak splittings and relative intensities can vary signifi-
cantly in some cases. Due to the various EnT steps that occur in
lanthanide luminescence (IC, ISC, etc.),thereis usually a very large
difference between the absorption and emission maxima in these
complexes (the Stokes shift) (39,43). A larger Stokes shift reduces
overlap between absorbance and emission bands which minimizes
energy lost to reabsorption. Further, as the major transitions in
these complexes are electric dipole forbidden, the excited state
lifetimes tend to be very long, often micro- to milliseconds (21,44).
Though thepropertyof longluminescence lifetime isnotnecessarily
unique, the fact that it occurs under ambient conditions is unusual.
Most organic species that exhibit phosphorescence only do so at low
temperatures and/or in the absence of oxygen ( 19 ).
Owing to these unique properties, lanthanides have several
advantages over traditional organic fluorophores, quantum dots,
or other fluorescent species commonly used as sensors. These
FIG. 4. Energy level diagram depicting the triplet excited states of
various aromatic ligands (left) along with the excited and ground states
of the four luminescent lanthanides (right). Ground states that do not
contribute to luminescence are shown in gray. The area highlighted
in the box illustrates the region where the energy gap between ligand
triplet state and lanthanide excited state is optimal for efficient energy
transfer (4000500 cm–^1 less than the ligand triplet state). Aromatic
ligands: dipicolinic acid (DPA); salicylic acid (SA); 1,10-phenanthroline
(phen); benzoate (ben); 2,2^0 -bipyridine (bpy).
8 MORGAN L. CABLEet al.