A cyanine-derived near-infrared molecular rotor for ratiometric imaging of mitochondrial viscosity in cells

[Display omitted] •We incorporate the methine groups of heptamethine cyanines into the cyclic frame to construct a new near-infrared (NIR) molecular rotor Mu1.•Mu1 is superior to the traditional cyanines with high viscosity response, large stokes shift (˜100 nm), high photo-stability and selectivity.•Mu1 can ratiometrically track mitochondrial viscosity changes in live cells with high spatial and temporal resolution. We developed a new cyanine-derived near-infrared (NIR) molecular rotor Mu1, which is superior to the traditional cyanines with high viscosity response, large Stokes shift (˜100 nm), and high photo-stability to detect the microscopic viscosity ratiometrically. The time-dependent density functional theory calculations highlighted the structure-optical properties of Mu1 as molecular rotor. Due to the mitochondria-actived fluorescence characteristics, Mu1 was used to track mitochondrial viscosity changes in live cells with high spatial and temporal resolution. This new type of NIR molecular rotor presented herein may open up new opportunities of NIR sensors for biomedical diagnosis and imaging applications.