Kinetically Stable Lanthanide Complexes Displaying Exceptionally High Quantum Yields upon Long-Wavelength Excitation: Synthesis, Photophysical Properties, and Solution Speciation

We demonstrate how
highly emissive, kinetically stable complexes can be prepared using
the macrocyclic scaffold of DO3A bearing coordinating aryl ketones
as highly effective sensitizing chromophores. In the europium complexes,
high quantum yields (up to 18% in water) can be combined with long-wavelength
excitation (370 nm). The behavior in solution upon variation of pH,
studied by means of UV–vis absorption, emission, and NMR spectroscopies,
reveals that the nature of the chromophore can give rise to pH-dependent
behavior as a consequence of deprotonation adjacent to the carbonyl
group. Knowledge of the molecular speciation in solution is therefore
critical when assessing the luminescence properties of such complexes.