A Water-Soluble Pybox Derivative and Its Highly Luminescent Lanthanide Ion Complexes

A new water-soluble Pybox ligand, 1, has
been synthesized
and found to crystallize in the monoclinic P21/n space group with unit cell parameters a = 6.0936(1) Å, b = 20.5265(4) Å, c = 12.0548(2) Å, and β = 90.614(1)°. In the crystal, a water molecule is bound through hydrogen-bonding
interactions to the nitrogen atoms of the oxazoline rings. This ligand
was used to complex a variety of lanthanide ions, opening up new avenues
for luminescence and catalysis in aqueous environment. These complexes
are highly luminescent in aqueous solutions, in acetonitrile, and
in the solid state. Aqueous quantum yields are high at 30.4% for Eu­(III),
26.4% for Tb­(III), 0.32% for Yb­(III), and 0.11% for Nd­(III). Er­(III)
did not luminesce in water, but an emission efficiency of 0.20% could
be measured in D2O. Aqueous emission lifetimes were also
determined for the visible emitting lanthanide ions and are 1.61 ms
for Eu­(III) and 1.78 ms for Tb­(III). Comparing emission lifetimes
in deuterated and nondeuterated water indicates that no water molecules
are coordinated to the metal ion. Speciation studies show that three
species form successively in solution and the log β values are
5.3, 9.6, and 13.8 for Eu­(III) and 5.3, 9.2, and 12.7 for Tb­(III)
for 1:1, 2:1, and 3:1 ligand to metal ratios, respectively.