Rotation of Aromatic Solutes in Supercritical CO₂:  Are Rotation Times Anomalously Slow in the Near Critical Regime?

Abstract

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 CO 2 (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 CO 2 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 (F/F 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.

Dissolution of Trihexyltetradecylphosphonium Chloride in Supercritical CO2

“…Increased solvating power of scCO 2 is expected at high density and the dissolution of dimers back to monomers could be explained by improved solubility. Density-dependent behavior in supercritical fluids has been known for decades, and there are many reports that discuss density augmentation effects below the critical density (ρ r = 1), which are then diminished by a sufficient increase in fluid density [24,[81][82][83][84]. Limited solubility in supercritical fluids is expected below the critical density (ρ r = 1), where the fluid density is more gas-like, and therefore, any potential aggregation would be expected at these lowest densities.…”

1,3,6,8‐Tetraphenylpyrene Derivatives: Towards Fluorescent Liquid‐Crystalline Columns?

“…3). [30] A bathochromic shift of absorption and emission maxima are observed going from 1 (k max (Abs) = 383 nm and k max (Em) = 411 nm) to 9 (k max (Abs) = 388 nm and k max (Em) = 423 nm), from 9 to 6 (k max (Abs) = 393 nm and (k max (Em) = 433 nm), and from 6 to 8 (k max (Abs) = 395 nm and k max (Em) = 445 nm); this results from a fine balance between conformational effects and electronic effects linked to the substitution of the phenylene units by electroactive groups. From the shape of the absorption spectra, structureless for 6 and 8 and poorly resolved for 9, it can be deduced that these compounds display a pronounced conformational disorder associated to the rotation of the external rings.…”

“…In order to get a better insight into the structural organization of the tetraphenylpyrene cores and their solid-state optical properties we first performed molecular-mechanics calculations with the universal force field (UFF) [30] to investigate the possible arrangement of the molecules within isolated one-dimensional stacks. The optimized geometry of a single 1,3,6,8-tetraphenylpyrene molecule is characterized by torsion angles u 1±4 of 62 for the external rings (see Scheme 1); this value compares well with X-ray diffraction data indicating angles on the order of 50 (vide infra).…”

Vibrational Lifetimes and Spectral Shifts in Supercritical Fluids as a Function of Density: Experiments and Theory

“…[26][27][28] Some research shows that orientational motion slows down substantially, 27 though these results are debated by others. 28 Electron paramagnetic resonance experiments indicate that spin exchange and hyperfine coupling deviate from normal behavior near the critical point in some fluids. 29,30 Several excellent recent reviews have been written on these spectroscopic results.…”