Photophysical Investigation of Energy Transfer Luminescence of Lanthanide Chelates with Aromatic Polyaminocarboxylate Ligands in Aqueous Solutions

Abstract

Some photophysical data including emission lifetimes (τ), total emission quantum yields (Φ), and ligand phosphorescence data are reported for the energy-transfer luminescence of the Eu(III) chelate of Quin 2 and the Tb(III) chelate of BAPTA: Quin 2 means 2-[(2-amino-5-methylphenoxy)methyl]-6-methoxy-8-aminoquinoline-N, N, N′, N′-tetraacetic acid; BAPTA means 1, 2-bis(2-aminophenoxy)ethane-N, N, N′, N′-tetraacetic acid. The energy diagrams for the ligand T₁ and the metal-center f-f levels are proposed. The τ values of Tb(III)-BAPTA chelates are 1.73ms in H₂O and 3.44ms in D₂O. The Eu(III)-Quin 2 chelate system shows a bi-exponential decay of emission; τ=0.048 and 0.20ms in H₂O and 0.066 and 1.44ms in D₂O. The Quin 2 chelate is kinetically inert, so that the interchange of these two conformer structures are very slow at room temperature. The number of water molecules in the primary coordination sphere is calculated from the lifetime data to be 1.9-2.4 for Eu-Quin 2 and 0.5 for Tb-BAPTA. The Φ values in aqueous solutions are rather small in these systems; 0.009 for Tb-BAPTA and 0.0023 for Eu-Quin 2, but these are enough counterbalanced by the large molar absorptivities giving the great sensitization factors for the ions; the sensitization factors against each aqua ion are 1380 for Eu-Quin 2 and 1600 for Tb-BAPTA.