Evidence for chain length dependent local ordering surrounding perylene dissolved in poly(ethylene glycol)

Time-resolved fluorescence anisotropy measurements on dilute solutions of perylene dissolved in poly(ethylene glycol) (PEG) oligomers are carried out as a function of oligomer chain length and temperature. The perylene fluorescence anisotropy decay kinetics are best described by a double-exponential decay law. The temperature dependence of the perylene rotational reorientation dynamics in PEG is exponential activated; however, at the lower PEG chain lengths, the activation energies that describe the perylene rotational reorientation are significantly larger than the activation energy for PEG viscous flow. As the PEG molecular weight increases, the activation energies associated with the perylene rotational motions become equivalent to the activation energy for PEG viscous flow. The principal perylene rotational diffusion coefficients (D_∥ and D_⊥) are also a strong function of PEG chain length and temperature. The ratio of the principal diffusion coefficients (D_∥/D_⊥) shows that D_⊥ is hindered relative to D_∥ as the PEG chain length increases. These results are complementary to recent work from Goldie and Blanchard on the rotational reorientation dynamics of perylene dissolved in n-alkanols. On the basis of our data, we propose a chain length dependent ordering of the PEG oligomers around the perylene molecules that impedes D_⊥ to a greater extent compared to D_∥.