Clickable Pyclen-Based Luminescent Lanthanide Complexes: Application to Two-Photon Microscopy

Lanthanide luminescent bioprobes incorporating a reactive group for the functionalization of biomolecules via click chemistry are of high interest for the straightforward preparation of smart luminescent tools for biological imaging and sensing applications. This article describes the synthesis and the photophysical properties of two lanthanide complexes based on the pyclen macrocycle bearing a terminal alkyne group on its pyridine moiety and a 4-(4-alkylthiophenyl)picolinate charge transfer chromophore to sensitize the lanthanide luminescence. The Eu³⁺ and Sm³⁺ complexes show excellent two-photon absorption and emission properties. These complexes were conjugated to a cell penetrating peptide bearing an azide via copper-catalyzed azide-alkyne cycloaddition. The resulting conjugates exhibit luminescence properties comparable to those of the parent alkyne-complex. Two-photon microscopy imaging of live HeLa cells was performed with both the Eu³⁺ and the Sm³⁺ conjugates. These bioprobes are efficiently delivered to the cytosol of the living cells. The Eu³⁺ luminescence was detected in cells even at very low incubation concentration (50 nM) and the Sm³⁺ complex gave excellent quality images despite a low emission quantum yield (< 1 %).