Relaxometric and luminescence behaviour of triaquahexaazamacrocyclic complexes, the gadolinium complex displaying a high relaxivity with a pronounced pH dependence

Abstract

The relaxivity of [GdL¹]³⁺ in aqueous solution has been studied as a function of temperature and magnetic field strength. The complex possesses a high relaxivity at pH 6 (12.8 mmol^-1 s^-1) that diminishes as successive water molecules are deprotonated, leading to a complex species whose relaxivity is typical of a purely outer-sphere complex. Measurements of the luminescence lifetime of a very similar complex, [YbL²]³⁺, in H₂O and D₂O were used to confirm that the complex possesses three bound waters at pH 5.5, but above pH 11 there are no bound OH groups in the putative di-oxo-bridged dimer.