Rotation of Aromatic Hydrocarbons in Viscous Alkanes. 2. Hindered Rotation in Squalane

Abstract

Time-resolved single-photon counting and the synchrotron radiation source at the CCLRC’s Daresbury Laboratory have been used to measure the decay of fluorescence anisotropy of solutions in squalane of the aromatic hydrocarbons triphenylene, coronene, benzoperylene, perylene, anthracene, and tetracene. For the last four, extra information was obtained by exciting both parallel and perpendicular to the fluorescence transition. The results differ markedly from those obtained in methylcyclohexane (part 1 of the study) probably because the solute molecules are smaller than those of the solvent. Contrary to the predictions of hydrodynamic theory, decay rates appeared to change with excitation wavelength, and three decays were observed in some cases. Temperature effects (+20 to −70 °C) provided evidence for bifurcation of the molecular rotations into two stages: slow overall motion and faster rotation over a restricted angular range. This resembles the split into α and β processes observed in many relaxation studies of highly viscous liquids.

Rotation of Aromatic Hydrocarbons in Viscous Alkanes. 1. Methylcyclohexane

“…Coronene has D 6 h symmetry; the others resemble anthracene. (Benzoperylene is included here and in Tables1 and 2 though it has been studied only in squalane …”

“…These results are reported here and compared to data for the same solutes in other solvents. In squalane, new phenomena are found that can be described in terms of limited range rotation and the α and β processes used to intepret relaxation near the glass transition; these are reported in the following paper . The earlier work has been extended in two ways: solutes have been excited at two different wavelengths and a wider range has been used.…”

“…The use of lasers has made possible measurements on the picosecond time scale and so of solutions in simple liquids such as n -alkanes. − , However, the liquid range, and therefore the viscosity range, in any one n -alkane is quite small; less symmetrical alkanes give a much wider range, and some form glasses rather than crystallize, on cooling. Our aim in this and the following paper has been to explore the opportunities afforded by two less symmetrical alkanes, methylcyclohexane (MCH) and squalane (2,6,10,15,19,23-hexamethyltetracosane). The melting point of MCH is −126.6 °C, and it is cf.…”

Model for the alpha and beta shear-mechanical properties of supercooled liquids and its comparison to squalane data

“…Squalane has been studied in molecular dynamics simulations of nonlinear flows [19]. Squalane has also been used as a solvent for studying the intriguing Debye dielectric relaxation of mono-hydroxy alcohols [20], the rotation of aromatic hydrocarbons in viscous alkanes [21], and the Stokes-Einstein relation for diffusion of organic solutes [22]. Due to its low vapor pressure squalane is used as a benchmark molecule for reaction-dynamics experiments performed under ultrahigh vacuum [23,24].…”

Is molecular rotation really influenced by subtle changes in molecular shape?

“…[3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] These investigations have provided a wealth of information on how the relative size of the solute to the solvent affects the reorientation time by altering the boundary condition parameter and also how the rotation of the solute is influenced by the free volume of the solvent. In such a scenario, Stokes-Einstein-Debye ͑SED͒ hydrodynamic theory 1,2 with slip boundary condition is more or less adequate to account for the rotational relaxation of a solute molecule.…”