Rotation of aromatic hydrocarbons in viscous alkanes. 2. Hindered rotation in squalane

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 degrees 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 alpha and beta processes observed in many relaxation studies of highly viscous liquids.