Abstract
Complexation of Eu(III) by two hydroxybenzoic acids, namely p-hydroxybenzoic acid (4-dihydroxybenzoic, HPhbH), and protocatechuic acid (3,4-dihydroxybenzoic, HProtoH2), is studied by time-resolved luminescence spectroscopy (TRLS) in mildly acidic solution. Comparable formation constants are determined at 0.1molL−1 NaCl for EuPhbH2+ – log10β°(EuPhbH2+)=2.18±0.09 (1σ) – and 0.01molL−1 NaCl for EuProtoH22+ – log10β°(EuProtoH22+)=2.72±0.07 (1σ). The stoichiometry and carboxylate complexation of the EuProtoH22+ complex is ascertained by varying both pH and ligand concentration. The luminescence decay time of EuPhbH2+ (τ=107±5μs) is comparable with that of Eu(H2O)n3+ (τ=110±3μs), suggesting that luminescence quenching processes compensate the expected increase in decay time due to the dehydration associated with complexation. For EuProtoH22+, the luminescence decay time is even shorter (τ=20±5μs), evidencing intricate quenching processes.
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