Influence of temperature on rotational diffusion of dipolar laser dyes in glycerol

Temperature dependent rotational diffusion of three medium sized dipolar laser dyes viz., Fluorescein 27 (F27), Fluorescein Na (FNa) and Sulforhodamine B (SRB) has been studied in glycerol using both steady-state and time-resolved fluorescence depolarization techniques. The rotational reorientation times of these probes are observed to vary linearly as a function of viscosity over the range of temperature studied. The results have been discussed and analyzed in the light of Stokes–Einstein–Debye (SED) hydrodynamic theory. The reorientation times of the probes follow a trend that is well within hydrodynamic slip and stick limits. Finally, the applicability of quasihydrodynamic models of Geirer–Wirtz (GW) and Dote–Kivelson–Schwartz (DKS) is discussed with a view to understand the nature of solute–solvent interactions. ► τ r of three structurally similar dipolar probes are studied in glycerol as function of temperature. ► τ r of probes vary linearly with η over the temperature range studied even at high viscosities. ► Rotational dynamics follows a trend that lies in between slip and stick hydrodynamic limits. ► Formation of weak hydrogen bonds between solute and solvent molecules responsible for the results. ► A coincidence of slip hydrodynamic and DKS predictions has been noted for both F27 and FNa. ► Fair agreement between DKS theory and experiment is noted for the first time, for the largest probe SRB.