Hydrodynamics of molecular rotors in lipid membranes

Molecular rotors are molecules that form twisted conformations upon photoexcitation, with their fluorescence relaxation time serving as a measure of viscosity. They have been used to estimate the viscosity of biological and other membranes, but yield higher values than more common methods. We show that the rotor's relaxation time is influenced by a combination of membrane viscosity and interleaflet friction, and rationalize the existing discrepancy among methods.