Synthesis, structure and tuneable white-light emission of dinuclear Eu(III),Tb(III)-mixed Schiff base complexes

A new family of lanthanide-Schiff base complexes have been synthesized and fully characterized from the novel Schiff base ligand: H₂L¹ (acetic acid 2-hydroxy-benzylidene-hydrazide). The single crystal X-ray structure and mass spectroscopy reveal the isostructural dinuclear complexes: [Ln₂L¹(HL¹)₃(NO₃)] where Ln = Sm, Eu, Gd and Tb. Upon excitation of the Ln^III complexes in solution, their luminescence spectra display the broad ligand centred (LC) band at ∼470 nm as well as the narrow f–f transitions. In Eu^III complex, the LC emission is efficiently quenched, because of the better triplet state energy match to the Eu^III–⁵D₄ excited states. Doping methods of Eu^III with Tb^III into the complexes: [Eu_xTb_1-x(L¹)(HL¹)₃(NO₃)], x = 0.5 (A), 1.0 (B) and 1.5 (C); in MeOH/DCM (1:10), have revealed variable colours. By careful control of the amounts of Eu^III and Tb^III in the dinuclear mixed EuTb-complexes, white-light emission is obtained from blue LC band, green Tb^III luminescence and red Eu^III luminescence. Such behaviour is attributed to the efficient sensitization of Eu^III by the ligand and the metal to metal energy transfer from Tb^III to Eu^III excited 4f states.