Complexation of europium(III) by hydroxybenzoic acids: A time-resolved luminescence spectroscopy study

Complexation of Eu(III) by two hydroxybenzoic acids, namely p-hydroxybenzoic acid (4-dihydroxybenzoic, HPhbH), and protocatechuic acid (3,4-dihydroxybenzoic, HProtoH(2)), is studied by time-resolved luminescence spectroscopy (TRLS) in mildly acidic solution. Comparable formation constants are determined at 0.1 mol L-1 NaCl for EuPhbH(2+) - log(10)beta degrees(EuPhbH(2+)) = 2.18 +/- 0.09 (1 sigma) - and 0.01 mol L-1 NaCl for EuProtoH(2)(2+) - log(10)beta degrees (EuProtoH(2)(2+)) = 2.72 +/- 0.07 (1 sigma). The stoichiometry and carboxylate complexation of the EuProtoH(2)(2+) complex is ascertained by varying both pH and ligand concentration. The luminescence decay time of EuPhbH(2+) (tau = 107 +/- 5 mu s) is comparable with that of Eu(H2O)(n)(3+) (tau = 110 +/- 3 mu s), suggesting that luminescence quenching processes compensate the expected increase in decay time due to the dehydration associated with complexation. For EuProtoH(2)(2+), the luminescence decay time is even shorter (tau = 20 +/- 5 mu s), evidencing intricate quenching processes. (C) 2015 Elsevier B.V. All rights reserved.