Preparation, characterization and optical properties of rare earth complexes with derivatives of N-phenylanthranilic acid

Highlights: • N-phenylanthranilic acid and various derivatives can be employed as ligands to Tb³⁺. • Efficient excitation of Tb³⁺ in the spectral range 350–400 nm is possible. • Flexibility of internal spatial arrangement of ligands on the base of DFT calculations. • High quantum efficiencies (31%) in this excitation range for Tb(meclofenamate)₃. N-phenylanthranilic acid and various derivatives (NPAs) can be employed to sensitize rare earth complexes, Gd³⁺ and Tb³⁺ in particular, for the excitation with UV-light in the range from 350 to 400 nm, where low cost, high power LEDs have become available during the last years. This range is of particular interest for applications in biophotonics or luminescent bioassays, which can exploit the unique features of the rare earth ions: large Stokes shifts, narrow line emissions and very long life times of the excited states. Among others, the 350–400 nm range allows the use of standard optics and closes the gap between numerous complexes known for wavelengths 3+, Tb³⁺ and Eu³⁺ were prepared and characterized with respect to chemical composition (elemental analyses, thermoanalysis) and spectroscopically (IR, phosphorescence, luminescence); quantum chemical calculations (DFT) accompanied the spectral interpretation. Several binary Tb³⁺ complexes were found to be excitable at wavelengths as high as 400 nm; quantum yields of 31% at decay times of 837 μs could be realized. Corresponding Eu³⁺ complexes were found to be essentially non-luminescent. The efficiencies can be enhanced substantially, if ancillary ligands are employed.