Eu(III) and Tb(III) Complexes of Octa- and Nonadentate Macrocyclic Ligands Carrying Azide, Alkyne, and Ester Reactive...

Azide- and alkyne-functionalized bioconjugable luminescent lanthanide complexes are reported. Reactive handles were introduced into the complexes by the late-stage modification of a methylenecarboxylic acid antenna pendent group. Tb and Eu quantum yields (11-13% and 3.4-3.6%, respectively) were not greatly affected by the presence of the azide or the alkyne compared to the parent complex (Φ_Tb = 10%, Φ_Eu = 2.8%). Two avenues were explored for improving the luminescence of the lanthanide (Ln) complexes: (1) attaching the antenna through a tertiary amide linker and (2) replacing a monodentate carboxylate ligand with a bidentate pyridylcarboxylate donor, which yielded a nonadentate ligand that could saturate the lanthanide coordination sphere and eliminate the quenching metal-bound water molecule that was present in the octadentate complexes. The combination of both approaches yielded Eu and Tb emitters with 5.8% and 46% quantum yields. For the Eu complex, this value was the same as Φ_Eu in the octadentate parent complex. We attribute this to increased photoinduced electron transfer quenching in the nonadentate species, which compensates for the reduced O-H quenching.