The rotational diffusion of three probes: coumarin 522B (C522B), coumarin 307 (C307) and coumarin 138 (C138) with nearly identical
size was studied at room temperature employing steady-state and time-resolved fluorescence anisotropy techniques in series
of alcohols and alkanes. Experimental observations indicate faster rotation of C138 compared to the other two dyes in alcohols
and a faster rotation of C522B than C307 in alkanes. The dielectric friction theories of Nee-Zwanzig (NZ) and van der Zwan-Hynes
(ZH) were employed to estimate the friction experienced by the probes in alcohols, in addition to the mechanical friction
calculated using Stokes-Einstein-Debye (SED) hydrodynamic with slip boundary condition and Dote-Kivelson-Schwartz (DKS) quasihydrodynamic
theories. The observed reorientation times for the three probes do not follow the trend predicted by dielectric friction theories
of NZ and ZH. The dipole moments determined from solvatochromic techniques were found to be different for the three probes.
Keywords Rotational dynamics - SED hydrodynamic theory - Dielectric friction theories - Electrostriction effect