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-4L), 3 (Eu-3L), 2 (1,4-disubstituted Eu-o-2L and 1,7-disubstituted Eu-p-2L) and 1 (Eu-1L). It was found that the definitive distinction between Eu-o-2L and Eu-p-2L could be interpreted from the emission spectra of Eu³⁺ 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-4L with an overall +3 charge demonstrated subcellular localization towards lysosomes and was a potential compound for a selective time-resolved bio-labelling probe. This journal is