Luminescence Studies of Ln III ParaCEST Agents

The CEST spectra of both Eu(STHP) and Yb(STHP) are turned on at near neutral pH in the presence of diethylphosphate (DEP) [64], yet the Fq excitation spectrum of 

Eu(STHP) at pH 6.5 is unchanged upon addition of excess DEP (Fig. 8.16). The luminescence lifetime of Eu(STHP) in the presence of DEP also does not change. Both of these experiments are consistent with the lack of formation of an innersphere complex by replacement of water with DEP. 

However, addition of DEP to Eu(STHP) at pH 7.5 leads to disappearance of the excitation spectrum corresponding to the ionised species and appearance of the excitation spectrum of the non-ionised species (Fig. 8.17). The interaction of DEP with Eu(STHP)(OH2) essentially inhibits the ionisation of the Eu(III) complex (Fig. 8.18). 

Phantom images of Yb(STHP) at pH 4,5, and 7 exhibit contrast only at the most acidic pH. However, addition of two equivalents of DEP (D, E, F) shows that contrast is now brightest for the tube at neutral pH [64]. 

Curiously, the outersphere interaction of phosphate diester with Eu(STHP) is selective for the type of phosphate diester. Only symmetrically substituted esters such as BNNP or DEP 

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