Abstract A library of new fluorescent molecular rotors (FMRs) for viscosity sensing was synthesized. The sensitivity of the fluorescence emission toward solvent viscosity and polarity was investigated by using UV/Vis absorption, fluorescence emission spectra, and theoretical calculations. For the new FMRs, red-shifted emissions at 620 nm, Stokes shifts of 170 nm, and up to 40-fold fluorescence enhancement upon increasing the viscosity of ...