Modulation of Water Exchange in Europium(III) DOTA–Tetraamide Complexes via Electronic Substituent Effects

The chemical exchange saturation transfer (CEST) efficiency for a series Eu3+-based tetraamide complexes bearing p-substituents on a single coordinating pendant arm is highly sensitive to water exchange rates. The CEST effect increases in the order Me \u3c MeO \u3c F ∼ CO2tBu \u3c CN ≪ H. These results show that CEST contrast can be modulated by changes in electron density at a single ligating atom, and this forms the basis of creating imaging agents that respond to chemical oxidation and reduction