The effects of the increasing number of the same chromophore on photosensitization of water-soluble cyclen-based europium complexes with potential for biological applications

Five water-soluble cyclen-based europium complexes incorporated with the pendant chelating chromophore, 2-methyl-4-(2-(4-propoxyphenyl)ethynyl)pyridine ( L ), as the antenna in different ratios have been synthesized and full structural characterization, photophysical measurement and in vitro biological studies were carried out. The antenna-to-metal ratios were 4 (Eu-4 L ), 3 (Eu-3 L ), 2 (1,4-disubstituted Eu- o -2 L and 1,7-disubstituted Eu- p -2 L ) and 1 (Eu-1 L ). It was found that the definitive distinction between Eu- o -2 L and Eu- p -2 L could be interpreted from the emission spectra of Eu 3+ at room and low temperatures. With all the results from subcellular stability testing ( via titrations), in vitro imaging, cytotoxicity assays and cellular uptake profiles, Eu-4 L with an overall +3 charge demonstrated subcellular localization towards lysosomes and was a potential compound for a selective time-resolved bio-labelling probe.