The effect on the lanthanide luminescence of structurally simple Eu(iii) cyclen complexes upon deprotonation of metal bound water molecules and amide based pendant arms

Abstract

A series of substituted 1,4,7,10-tetraazacylcododecane ligands 1–4, possessing sensitizing nitrobenzene or naphthalene antennae, as one of the amide pendant arms, and their complexes with europium(III) were synthesised. The protonation constants and the metal ion stability constants of two of these ligands were determined by potentiometric titration. The pK_a of the water molecules coordinated to the complexed metal ion were determined by both luminescent and potentiometric measurements. The luminescence pH dependence of a further three Eu(III) complexes, 5–7, which lack any antennae, were also studied with the aim of gaining a better understanding of the role of the metal bound water molecules in the luminescence properties of such complexes upon direct excitation of the lanthanide ion. The results from these luminescent measurements demonstrate that the Eu(III) emission was significantly modulated as a function of pH for all the complexes, which we assigned to changes occurring in the coordination environment of the ion within the cyclen system, caused by deprotonation of metal bound water molecules and/or deprotonation of pendent amide arms.