Rotation of aromatic solutes in supercritical CO₂: Are rotation times anomalously slow in the near critical regime?

Picosecond fluorescence anisotropy decay measurements are used to examine the rotational dynamics of three solutes, 1,3,6,8-tetraphenylpyrene ("TPP"), 9,10-bis(phenylethynyl)anthracene ("PEA"), and N.N′-bis(2,5-di-tert-butylphenyl)-3,4,9,10-perylenedicarboximide ("BTBP") in supercritical CO₂ (35 °C = T_c+ 4 °C) and in a variety common liquid solvents. In liquids the rotation times of all three probes show an approximate proportionality to solvent viscosity, in rough agreement with simple hydrodynamic theories. In supercritical CO₂ two of the probes, TPP and BTBP, are found to exhibit rotation times consistent with the extrapolation of the hydrodynamic trends found in liquid solvents. In the case of BTBP, these results disagree with recently published reports of very long rotation times near the critical point [Heitz and Bright J. Phys. Chem. 1996, 100, 6889]. However, the rotation times of PEA deviate significantly from hydrodynamic predictions based on the viscosity of the supercritical fluid for near critical densities. In this case, it appears that local density augmentation leads to increased rotational friction on the solute compared to what would be expected on the basis of the bulk solvent properties. Using the observed rotation times, an effective density that is 50-100% greater than the bulk density is estimated for reduced densities (ρ/ρ_c) of 0.8-1. Similar estimates of the extent of local density augmentation are also obtained from the behavior of the electronic frequency shifts of this solute.