Cyclometalated iridium(III) complexes containing bithiazole ligands for preferable viscosity detection

Detecting viscosity is vital for pathological investigation since subtle changes in cellular microviscosity are related to pathologies and diseases. In this work, five cationic Ir(III) complexes Ir1–Ir5 with the formulas as [Ir(ppy)₂L]PF₆ have been synthesized, where L1-L5 are a family of coplanar bithiazole derivatives having versatile donor/acceptor tails (proton, bromo, tetraphenylethylene, benzothiophene, ethyl benzoate). Fluorescent experiment shows that L1-L5 have a fluorescent activity with high quantum yields (Ф > 33%). However, the quantum yields of Ir1–Ir5 are weak in the range from 2.7% to 7.7%. There is a linear relationship between Log(I_{620 nm}) and Log(viscosity), which can quantitatively measure viscosity in the range of 1.36–669 cP. Furthermore, a cell colocalization experiment has been performed using commercial nuclear dye (DAPI), lysosomal dye (LysoGreen), and mitochondrial dye (MitoGreen) to explore the subcellular localization ability of Ir1 in living cells, indicating that the probe Ir1 can easily pass through the cell membrane, and can accurately target lysosomes in living cells. In addition, complex Ir1 can quantitatively detect viscosity and target lysosomes, possessing the potential for practical application in intracellular viscosity detection.