Structural and Photophysical Properties of Europium(III) Complexes Formed with an Optical Isomer of 4-Chloro-N,N'-bis(1-phenylethyl)-2,6,-Pyridinedicarboxamide

Luminescent lanthanide(III) complex-based molecular probes are commonly employed for analytical and biomedical applications, where they are used as diagnostic or  therapeutic tools. However, there is still a surge of interest to develop luminescent Ln(III) compounds possessing chiral properties along with their interesting spectral features.

Working along these lines, we reported on the interaction of a chiral ligand 4-chloro-N,N’-bis(1-phenylethyl)-2,6 pyridinedicarboxamide (Cl-(R,R)), with an emphasis on the photophysical and chiroptical properties of the resulting tris Eu(III) complex.

The photophysical properties of the Eu(III) complex have been examined in solution and the quantum yields of both the ligand- and the metal-centered emissions are reported, along with the lifetime of the excited ⁵D₀(Eu) ion. In addition, the luminescence efficiency, the influence of the chiral substituent on the helicity of the Eu(III) complex and the polarization of the emitted light have been investigated using circularly polarized luminescence, time-resolved and steady-state luminescence spectroscopic techniques.

In this study it was shown that the introduction of a chiral asymmetric carbon in each carboxamide moiety led to a diastereomeric resolution of the Ln(III) complex. Of special importance, the L metal stereochemistry was induced by the chiral nature of the ligand (Cl-(R,R)). It was also shown that the ligand triplet-excited-state energy was relatively well suited to sensitize the Eu(III) ion, although the sensitization of it was relatively weak due to poor energy transfer processes.

One can envision to use these stable Ln(III) complexes for their chiroptical properties to develop reliable empirical relationships between chiral structures and CPL sign patterns.