Luminescence of Eu3+ and Tb3+ Complexes of Two Macrobicyclic Ligands Derived from a Tetralactam Ring and a Chromophoric Antenna

Abstract

Two macrobicyclic ligands derived from an 18-membered tetralactam ring and 2,2′-bipyridine or 2,6-bis(pyrazol-1-yl)pyridine moieties, 1 and 2, respectively, form stable complexes with GdIII, EuIII, and TbIII ions in aqueous solution. The ligand-based luminescence is retained in the GdIII cryptates, whereas this radiative deactivation is quenched in the EuIII and TbIII cryptates by ligand-to-metal energy transfer, resulting in the usual metal-centered emission spectra. Singlet- and triplet-state energies, emission-decay lifetimes, and luminescence yields were measured. [Tb⊂1]3+ cryptate shows a long luminescence lifetime (τ=1.12 ms) and a very high metal luminescence quantum yield (Φ=0.25) in comparison with those reported in the literature for Tb3+ complexes sensitized by a bipyridine chromophore. By comparison to [Ln⊂1]3+, [Ln⊂2]3+ presents markedly lower luminescence properties, due to worse interaction between the 2,6-bis(pyrazol-1-yl)pyridine unit and the metal ion. Moreover, the luminescent metal and the triplet ligand energy levels of [Eu⊂2]3+ do not match. The effects of H2O molecules coordinated to the metal centre and of thermally activated decay processes on nonradiative deactivation to the ground-state are also reported.