Intramolecular deactivation processes in complexes of salicylic acid or glycolic acid with Eu(III)

Abstract

The complexation of Eu(III) by 2-hydroxy benzoic acid (2HB) or glycolic acid (GL) was investigated using steady-state and time-resolved laser spectroscopy. Experiments were carried out in H 2O as well as in D 2O in the temperature range of 80 K < T < 290 K . The Eu(III) luminescence spectra and luminescence decay times were evaluated with respect to the temperature dependence of (i) the luminescence decay time τ , (ii) the energy of the D 5 0 → F 7 0 transition, (iii) the width of the D 5 0 → F 7 0 transition, and (iv) the asymmetry ratio calculated from the luminescence intensities of the D 5 0 → F 7 2 and D 5 0 → F 7 1 transition, respectively. The differences in ligand-related luminescence quenching are discussed. Based on the temperature dependence of the luminescence decay times an activation energy for the ligand-specific non-radiative deactivation in Eu(III)–2HB or Eu(III)–GL complexes was determined. It is stressed that ligand-specific quenching processes (other than OH quenching induced by water molecules) need to be determined and considered in detail, in order to extract speciation-relevant information from luminescence data (e.g., estimation of the number of water molecules n H 2 O in the first coordination sphere of Eu(III)). In case of 2HB, conclusions drawn from the evaluation of the Eu(III) luminescence are compared with results of a X-ray structure analysis.