Luminescence and non-radiative processes in lanthanide squarate hydrates

The luminescence properties of solid hydrated lanthanide squarates (Ln2(C4O4)3(H2O)x; x = 8or13;Ln 3+ = Gd, La, Eu, Tb, Pr) are reported for temperatures down to 4.2 K. The luminescence of the squarate group is observed for the Gd 3+ and La 3+ compounds at low temperatures (below 150 K). The Pr 3+ compound does not show any emission at all, not even at 4.2 K. This is ascribed to the quenching of the Pr 3+ emission by multiphonon relaxation and/or concentration quenching. The quantum efficiencies of the 5D0 emission of Eu 3+ and of the 5D4 emission of Tb 3+ in these squarate complexes are strikingly different. Whereas the Tb 3+ emission shows a temperature independent quantum efficiency of 50% upon ligand excitation, the Eu 3+ emission is strongly quenched, showing a temperature dependent quantum efficiency of 0.8% at 4.2 K upon ligand excitation. This quenching is ascribed to the low energy position of the chargetransfer state of Eu 3+ in these compounds.