Sensitizing Tb(III) and Eu(III) Emission with Triarylboron Functionalized 1,3-Diketonato Ligands

Four BMes2Ar (Mes = mesityl,
Ar = phenyl or duryl) functionalized
1,3-diketonato ligands have been investigated for use in selective
sensitization of Tb­(III) and Eu­(III) emission. These ligands have
the general formula of [R1C­(O)­CR2C­(O)­R3]− (R1 = Ph, R2 = H, R3 = p-Ph-BMes2, L1; R1 = R3 = p-Ph-BMes2, R2 = H, L2; R1 = R3 = Me, R2 = p-Ph-BMes2, L3; R1 = R3 = Me, R2 = p-duryl-BMes2, L4) and
belong to class I (L1 and L2) and class
II (L3 and L4), respectively. In class I,
the boron unit is conjugated with the phenyl linker and the diketone
backbone, while in class II, the boron unit, the linker unit, and
the diketone unit are nonconjugated with a mutually orthogonal arrangement.
To understand the impact of the location of the BMes2Ar
unit on the electronic properties of the 1,3-diketone molecules and
their ability in activating lanthanide emission, the difluoroboron
chelate compounds (1-BF2 to 4-BF2) of ligands L1–L4 were synthesized
and examined. The class I ligands were effective in activating Eu­(III)
emission, while the class II ligands were effective in activating
Tb­(III) emission. Four Ln­(III) complexes, 1Eu, 2Eu, 3Tb, and 4Tb, based on the L1–L4 ligands, respectively, were prepared
and examined. The emission quantum efficiency of 1Eu and 2Eu is low (ΦEu ≤ 0.01 in THF, 0.07–0.13
in the solid state), but can be greatly enhanced by the addition of
fluoride ions. In contrast, the complex 4Tb has a moderate
emission efficiency (ΦTb = 0.14 in THF, 0.47 in the
solid state) and experiences a distinct emission quenching upon the
addition of fluoride. The selective sensitization of Eu­(III) and Tb­(III)
by L1–L4 and the distinct luminescent
response of their Ln­(III) complexes toward fluoride ions are caused
by the distinct intraligand charge transfer transitions of the two
different classes of ligands involving the BMes2 unit.