π-extension enhanced viscosity detection ability of indole derivative probes

Mitochondrial viscosity is a crucial microenvironmental parameter that significantly influences cellular metabolism and function, with its dysregulation being implicated in various pathological processes. Herein, we design and synthesize two molecular rotor-type mitochondrial-targeted fluorescent probes, Mito-indo1 and Mito-indo2, for monitoring mitochondrial viscosity. Through increasing the viscosity of solvents, Mito-indo2 with π-extension of the indole group is found to exhibit superior absolute brightness as compared to Mito-indo1. Both probes demonstrate excellent selectivity for viscosity over potential interferents such as polarity, outstanding mitochondrial targeting capability and low cytotoxicity. Theoretical calculations suggest that the inhibition of chemical bond rotation between the indole and the benzene ring leads to such a profound turn-on response. Their potential for visualizing drug-induced mitochondrial viscosity changes is demonstrated in living WiT49 cells.