A Versatile Ditopic Ligand System for Sensitizing the Luminescence of Bimetallic Lanthanide Bio-Imaging Probes

The homoditopic ligand 6,6’- [methylenebis(1-methyl-1H-benzimid- azole-5,2-diyl)]bis(4-{2-[2-(2-methoxy- ethoxy)ethoxy]ethoxy}pyridine-2-carboxylic acid) (H2LC2) has been tailored to self-assemble with lanthanide ions (LnIII), which results in the formation of neutral bimetallic helicates with the overall composition [Ln2(LC2)3] and also provides a versatile platform for further derivatization. The grafting of poly(oxyethylene) groups onto the pyridine units ensures water solubility, while maintaining sizeable thermodynamic stability and adequate antenna effects for the excitation of both visible- and NIR-emitting LnIII ions. The conditional stability constants (logb23) are close to 25 at physiological pH and under stoichiometric conditions. The ligand triplet state features adequate energy (0-phonon transition at ~21 900 cm-1) to sensitize the luminescence of EuIII (Q=21%) and TbIII (11%) in aerated water at pH 7.4. The emission of several other VIS- and NIR-emitting ions, such as SmIII (Q=0.38%) or YbIII (0.15%), for which in cellulo luminescence is evidenced for the first time, is also sensitized. The EuIII emission spectrum arises from a main species with pseudo-D3 symmetry and without coordinated water. The cell viability of several cancerous cell lines (MCF-7, HeLa, Jurkat and 5D10) is unaffected if incubated with up to 500 mm [Eu2(LC2)3] during 24 h. Bright EuIII emission is seen for incubation concentrations above 10 mm and after a 15-minute loading time; similar images are obtained with TbIII and SmIII. The helicates probably permeate into the cytoplasm of HeLa cells by endocytosis. The described luminescent helical stains are robust chemical species which remain undissociated in the cell medium and in presence of other complexing agents, such as edta, dtpa, citrate or l-ascorbate. Their derivatization, which would open the way to the sensing of targeted in cellulo phenomena, is currently under investigation.