Emissive and cell-permeable 3-pyridyl- and 3-pyrazolyl-4-azaxanthone lanthanide complexes and their behaviour in cellulo

A series of seven emissive europium(III) and terbium(III) complexes was prepared, incorporating a 3-pyridyl-4-azaxanthone or 3-pyrazolyl-4-azaxanthone sensitising moiety within a polydentate macrocyclic ligand. High overall emission quantum yields in aqueous media are attenuated in the presence of protein or certain oxy anions due to displacement of the N,N′-chelated sensitiser. Nevertheless, these complexes are taken into cells and tend to localise over the first few hours in mitochondria before being trafficked to endosomal compartments. Cell uptake studies, in the presence of competitive inhibitors or promoters of well-defined uptake pathways, reveal a common uptake mechanism involving macropinocytosis.