Water-soluble phosphorescent iridium(iii) complexes as multicolor probes for imaging of homocysteine and cysteine in living cells

With the emergence of phosphorescent heavy-metal complexes as a class of attractive probes for bioimaging, there is a parallel need to develop new phosphorescent probes with complete solubility in pure water for phosphorescent staining in living cells. Herein, a convenient and general design strategy for realizing phosphorescent heavy-metal complexes with complete water-solubility is provided and a series of cationic iridium(iii) complexes [Ir(C^N) ₂(N^N)]⁺PF₆^- (C^N = 2-(2,4-difluorophenyl)pyridine (dfppy), 2-(4-(tert-butyl)phenyl)pyridine(t- buppy), 2-(thiophen-2-yl)quinoline) (thq), 4-(pyridin-2-yl)benzaldehyde (pba)) are prepared. The water-solubility of the complexes was successfully realized through the quaternization of the tertiary amino group in the N^N ligand. By changing the C^N ligands, the luminescent emission colors of these complexes can be tuned from green to red. These cationic iridium(iii) complexes are membrane-permeable and can be applied as phosphorescent dyes for cell imaging in phosphate buffer solution (PBS). Complexes Ir1-Ir3 displayed specific staining of the cytoplasm and complex Ir4 containing two aldehyde groups could detect the changes of cysteine/homocysteine concentration in living cells. These results demonstrated that our design strategy offers an effective way to develop excellent phosphorescent cellular probes for real applications.