EuIII Complexes of Octadentate 1-Hydroxy-2-pyridinones: Stability and Improved Photophysical Performance

Abstract

The luminescence properties of lanthanoid ions can be dramatically enhanced by coupling them to antenna ligands that absorb light in the UV-visible and then efficiently transfer the energy to the lanthanoid centre. The synthesis and the complexation of LnIII cations (Ln = Eu, Gd) for a ligand based on four 1-hydroxy-2-pyridinone (1,2-HOPO) chelators appended to a ligand backbone derived by linking two l-lysine units (3LI-bis-LYS) is described. This octadentate EuIII complex ([Eu(3LI-bis-LYS-1,2-HOPO)]–) has been evaluated in terms of its thermodynamic stability, UV-visible absorption and luminescence properties. For this complex, the conditional stability constant (pM) is 19.9, which is an order of magnitude higher than diethylenetriaminepentacetic acid at pH = 7.4. This EuIII complex also shows an almost two-fold increase in its luminescence quantum yield in aqueous solution (pH = 7.4) when compared with other octadentate ligands. Hence, despite a slight decrease of the molar absorption coefficient, a much higher brightness is obtained for [Eu(3LI-bis-LYS-1,2-HOPO)]–. This overall improvement was achieved by saturating the coordination sphere of the EuIII cation, yielding an increased metal-centred efficiency by excluding solvent water molecules from the metal’s inner sphere.