LUMINESCENT EU-3+, TB-3+, AND GD-3+ COMPLEXES OF A BRANCHED-TRIAZACYCLONONANE LIGAND CONTAINING 3 2,2'-BIPYRIDINE UNITS

The Eu3+, Tb3+, and Gd3+ complexes of the branched-triazacyclononane ligand 1, which contains three 2,2'-bipyridine coordinating units, have been prepared and their absorption spectra, luminescence spectra, luminescence decays, and transient absorption spectra and decays have been investigated. The three complexes show intense absorption bands at 243 nm (epsilon congruent-to 14 000 M-1 cm-1) and 311 nm (epsilon congruent-to 22 000 M-1 cm-1), which are considerably red-shifted and weaker compared with the bands of the free ligand. The free ligand and the Gd3+ complex show ligand-centered (LC) fluorescence and phosphorescence, as well as triplet-triplet transient absorption. The transient absorption can also be observed for the Tb3+ complex but not for the Eu3+ complex. For the Tb3+ and the Eu3+ complexes excitation in the LC bands causes the characteristic luminescence of the lanthanide ion. The excitation spectra match closely the absorption spectra in the region of LC absorption. Measurements carried out in H2O and D2O at 77 and 300 K show that, when coordinated to 1, the metal ion is almost completely shielded from interaction with solvent molecules, in agreement with expectations based on molecular models. The emission lifetime at 77 K in D2O for [Tb(1)]3+ is considerably shorter than that of analogous complexes. This may be due either to a high radiative rate constant or, more likely, to radiationless deactivation via coupling with the very close lying CH2 groups of the triazacyclononane ring. For [Tb(1)]3+ no temperature effect is observed on the luminescence decay, because of the relatively high energy gap between the luminescent 5D4 level and the upper lying 3-pi-pi* LC level. For [Eu(1)]3+, the lifetime in D2O at 77 K is again rather short, and furthermore, it decreases with increasing temperature. The latter effect is presumably caused by an activated radiationless decay via a low-lying CT excited state, related to the presence of the aliphatic amine groups (intramolecular electron-transfer quenching). Because of its high luminescence quantum yield (0.37 in H2O at 300 K), [Tb(1)]3+ can be an excellent luminescence label.