Crystal Structure and Spectroscopic Characterization of a Luminescent Europium Chelate

We report the crystal structure at 1 Å resolution of europium bound to diethylenetriaminepentaacetate covalently coupled to an organic chromophore, carbostyril 124 (DTPA-cs124; carbostyril 124 ) 7-amino-4-methyl-2(1H)quinolinone). The complex is highly luminescent in solution and in the crystal form. The Eu-DTPA-cs124 crystallizes in the space group C2/c with two molecules in the asymmetric unit. The two molecules form a dimer which is stabilized by π-stacking of two carbostyrils and by the ligation of the carbonyl oxygen of one cs124 to the europium of the second half of the dimer. This ligation removes the last water bound to the primary coordination sphere of europium, resulting in no waters bound to the lanthanide. The crystal structure also shows the amide oxygen between the cs124 and the DTPA is ligated to the metal. Spectroscopic measurements of the europium luminescence in the crystal show that 66% of the luminescence is centered around 617 nm with a full width at half-maximum of 2.5 nm. In solution, 48% of the europium luminescence intensity is in the 617 nm peak, with a fwhm of 3.5 nm. The europium lifetime in the crystal structure is 0.90 ms; the lifetime of the monomeric Eu-DTPA-cs124 is 0.62 ms in H2O solution and 2.4 ms in D2O solution. The differing lifetimes can be explained by the well-known effects of H2O on nonradiative europium emission rates and by postulating an electric (crystal) field surrounding the Eu which is more anisotropic in the crystal than in the solution, creating a faster radiative rate of Eu excited state deactivation. This greater anisotropy is expected on the basis of the crystal structure. The crystal structure also suggests improvements in the linkage of cs124 to DTPA which may improve the energy transfer between the organic chromophore and the europium and sharpen the Eu emission spectrum in solution.