Structural and spectroscopic investigations of redox active seven coordinate luminescent lanthanide complexes

The ligand H3L and its corresponding tris(phenolato) lanthanide complexes L-Tb, L-Gd, L-Yb and L-Lu were synthesized. The X-Ray crystal structures demonstrated a monocapped octahedron environment of the lanthanide ion, with exclusion of all solvent molecules from the coordination sphere due to the protection by the tertbutyl groups. The coordination bond distances followed the lanthanide contraction, with Ln-O bond lengths in the range 2.148-2.217 angstrom and Ln-N-imine bond lengths in the range 2.405-2.535 angstrom. Each of the complexes showed three one-electron oxidation processes. These potentials changed as a function of the lanthanide due to the difference in size of the metal ions. Both chemical and electrochemical oxidations of these complexes permitted the one-electron oxidised species to be generated. They all show a sharp absorption band at around 420 nm in their UV-Vis spectra, which demonstrated phenoxyl radical formation. Consistently, the radical species L-Lu+ displays a resonance at g = 2.001 in its EPR spectrum. The other cations are EPR-silent or difficult to detect due to magnetic interactions, confirming the proximity of the phenoxyl radical to the metal centre. For both L-Lu+ and L-Yb+ DFT calculations predict the radical to be delocalized over the three arms. The L-Tb and L-Yb complexes experienced a quenching of the luminescence upon oxidation to the radical species whereby the quenching of up to 83% in the visible region and 93% in the NIR was observed.