Evidence for this“enhanced receptor effect”can be found for var-
ious other lanthanide/ligand/analyte systems by comparing
reported binding affinities with and without a helper ligand under
similar conditions (Table II) ( 103 – 107 ). In each case, employing an
ancillary ligand improves binding affinity of the oxyanion analyte
(picolinate, acetate, and lactate) by roughly an order of magnitude,
regardless of the type (cyclic or linear), denticity, or charge of the
helper ligand. We attribute this enhancement to a shift in electron
density of the lanthanide upon receptor ligand chelation,
generating a binding site with greater positive character due to
the electron-withdrawing oxygen and nitrogen moieties of the
helper ligand. Though the net charge of the complex may have
decreased, thelocalcharge in the binding site may be even greater
than the Ln^3 þaquo case, where the nine solvent molecules are
evenly distributed about the lanthanide coordination sphere and
the electron density is uniform.
Such a proposal is further supported by another interesting
trend in the dipicolinate system: the degree of ligand enhance-
ment is lanthanide dependent. We found a discrepancy in
10.009.509.008.508.007.507.00Tb(DO2A)+Dy(DO2A)+Eu(DO2A)+Sm(DO2A)+Sm3+0.90 0.91 0.92 0.93 0.94 0.95 0.96 0.97
Ionic radius (Å)log dipicolinate affinity
Dy3+
Tb3+ Eu3+FIG. 6. Plot of association constants for Ln^3 þand [Ln(DO2A)]þto
DPA^2 – against lanthanide ionic radius, 0.2 M NaOAc, pH 7.5. The addi-
tion of DO2A enhances dipicolinate binding affinity by an order of
magnitude for most lanthanides investigated and by nearly two orders
of magnitude for terbium (light gray).
LUMINESCENT LANTHANIDE SENSORS 17