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

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 II there are no bound OH groups in the putative di-oxo-bridged dimer.