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 boolean AND N)(2)(N boolean AND N)]+PF6- (C boolean AND 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 boolean AND N ligand. By changing the C boolean AND 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.