Parameter describing the time dependence of the tumbling of a molecular entity in a medium of viscosity. The rotational correlation time can be obtained from the decay of the fluorescence or phosphorescenceanisotropy and is related to the average molecular rotational diffusion coefficient, , in turn related to the hydrodynamic molecular volume of the fluorophore, , and to (see Note 3).
- Mathematical definition: with the emission anisotropy at time and the fundamental emission anisotropy.
- In the case of a spherical emitting species reorienting itself in a homogeneous fluid, .
- Often, the Stokes–Einstein relationship is used for the calculation of , i.e., with the gas constant, the absolute temperature and the hydrodynamic molecular volume. However, the use of this relationship at a molecular level is questionable, and should be independently determined by time-resolved fluorescencepolarization methods. Compare with rotational relaxation time.
PAC, 2007, 79, 293 (Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)) on page 416