Rotational relaxation of biphenyl and p-terphenyl in n-alkanes: the breakdown of the hydrodynamic description

Rotational reorientation rates of biphenyl and p-terphenyl in n-alkanes at ambient pressure have been investigated using ultrafast pump-probe polarization spectroscopy. The results show that the Stokes-Einstein-Debye theory can describe the experimental results at low viscosities whereas at higher viscosities the experimental results deviate significantly from the hydrodynamic prediction. The breakdown of the hydrodynamic model occurs in solvents whose size is comparable to the size of the rotating molecule.