Carbazole-Based Eu Complexes for Two-Photon Microscopy Imaging of Live Cells.

Lanthanide(III) complexes with two-photon absorbing antennas are attractive for microscopy imaging of live cells because they can be excited in the NIR. We describe the synthesis and luminescence and imaging properties of two Eu complexes, and , with (-carbazolyl)-aryl-alkynyl-picolinamide and (-carbazolyl)-aryl-picolinamide antennas, respectively, conjugated to the TAT cell-penetrating peptides. Contrary to what was previously observed with related Eu complexes with carbazole-based antennas in a mixture of water and organic solvents, these two complexes show very low emission quantum yield (Φ < 0.002) in purely aqueous buffers. A detailed spectroscopic study on reveals that the quantum yield of emission is strongly polarity dependent─the less polar the medium, the higher the quantum yield─and that the emission quenching in water is likely due to a photoinduced electron transfer between the excited carbazole-based antenna and Eu that efficiently competes with the energy transfer process. Nevertheless, shows a significant two-photon cross-section of 100 GM at 750 nm, which is interesting for two-photon microscopy. The live cell imaging properties of and the two other conjugates were investigated. Cytosolic delivery was clearly evidenced in the case of when cells are coincubated with this compound and a nonluminescent dimeric TAT derivative, dFFLIPTAT.