Enhanced translational diffusion of rubrene in sucrose benzoate

The translational diffusion of rubrene in the fragile molecular glass former, sucrose benzoate (SB) (fragility index m≈94), has been studied from T _g+6 K to T _g+71 K(T _g=337 K) by using the technique of holographic fluorescence recovery after photobleaching. In the temperature range of the measurements, the translational relaxation functions were observed to decay exponentially, indicating that Fick's law of diffusion governs the translational motion of rubrene in sucrose benzoate. The value of the translational diffusion coefficient D _T obtained from the 1/e time of the translational relaxation function varied from 5.3×10 ^-15 cm ² s ^-1 at 343 K to 5.0×10 ^-9 cm ² s ^-1 at 408 K. The temperature dependence of D _T for diffusion of rubrene in SB is compared with that of the viscosity and the dielectric relaxation time τ _D of SB. The temperature dependence of D _T is weaker than that of T/η for T<1.2T _g but tracks the reciprocal of the dielectric relaxation time 1/τ _D for 1.05T _g<T<1.21T _g. The translational diffusion coefficient at T _g is enhanced by a factor of ≈ 2.5×10 ² over the value predicted by the Stokes-Einstein equation. The decoupling of probe diffusion from the viscosity is characterized by a scaling law, D _T∼η ^-ξ, with ξ=0.729.