  Molecular rotors form twisted conformations upon photoexcitation, with their
fluorescent relaxation time serving as a measure of viscosity. They have been
used to assess membrane viscosities but yield higher values compared to other
methods. Here, we show that the rotor's relaxation time is influenced by a
combination of membrane viscosity and interleaflet friction. We present a
theory for the relaxation time and obtain a correction factor that accounts for
the discrepancy. If the membrane's viscosity is known, molecular rotors may
enable the extraction of the elusive interleaflet friction.
