Luminescence and non-radiative processes in lanthanide squarate hydrates

The luminescence properties of solid hydrated lanthanide squarates (Ln(2)(C4O4)(3)(H2O)(x); x = 8 or 13;Ln(3+) = Gd, La, Eu, To, Pr) are reported for temperatures down to 4.2 K. The luminescence of the squarate group is observed for the Gd3+ and La3+ compounds at low temperatures (below 150 K). The Pr3+ compound does not show any emission at all, not even at 4.2 K. This is ascribed to the quenching of the Pr3+ emission by multiphonon relaxation and/or concentration quenching. The quantum efficiencies of the D-5(0) emission of Eu3+ and the D-5(4) emission of Tb3+ in these squarate complexes are strikingly different. Whereas the Tb3+ emission shows a temperature independent quantum efficiency of 50% upon ligand excitation, the Eu3+ 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 charge-transfer state of Eu3+ in these compounds.