Luminescent Bimetallic Lanthanide Bioprobes for Cellular Imaging with Excitation in the Visible-Light Range

A series of homoditopic ligands H2LCX (X=4-6) has been designed to self-assemble with lanthanide ions (Ln(III)), resulting in neutral bimetallic helicates of overall composition [Ln2(L-CX)(3)] with the aim of testing the influence of substituents on the photo-physical properties, particularly the excitation wavelength. The complex species are thermodynamically stable in water (log beta(23) in the range 26-28 at pH 7.4) and display a metal-ion environment with pseudo-D-3 symmetry and devoid of coordinated water molecules. The emission of Eu-III, Tb-III, and Yb-III is sensitised to various extents, depending on the properties of the ligand donor levels. The best helicate is [EU2(L-C5)(3)] with excitation maxima at 350 and 365 nm and a quantum yield of 9%. The viability of cervix cancer HeLa cells is unaffected when incubated with up to 500 mu m of the chelate during 24 h. The helicate permeates into the cells by endocytosis and locates into lysosomes, which co-localise with the en-doplasmatic reticulum, as demonstrated by counterstaining experiments. The relatively long excitation wavelength allows easy recording of bright luminescent images on a confocal microscope (lambda(exc) =405 nm). The new lanthanide bioprobe remains undissociated in the cell medium, and is amenable to facile derivatisation. Examination of data for seven Eu-III and Tb-III bimetallic helicates point to shortcomings in the phenomenological rules Of thumb between the energy gap Delta E((3)pi pi*-D-5(J)) and the sensitisation efficiency of the ligands.