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

Time-resolved single-photon counting and the synchrotron radiation source a t the CCLRC's Daresbury Laboratory have been used to measure the decay of f luorescence anisotropy of solutions in squalane of the aromatic hydrocarbon s triphenylene, coronene, benzoperylene, perylene, anthracene, and tetracen e. For the last four, extra information was obtained by exciting both paral lel and perpendicular to the fluorescence transition. The results differ ma rkedly from those obtained in methylcyclohexane (part 1 of the study) proba bly because the solute molecules are smaller than those of the solvent. Con trary to the predictions of hydrodynamic theory, decay rates appeared to ch ange with excitation wavelength, and three decays were observed in some cas es. Temperature effects (+20 to -70 degrees C) provided evidence for bifurc ation of the molecular rotations into two stages: slow overall motion and f aster rotation over a restricted angular range. This resembles the split in to alpha and beta processes observed in many relaxation studies of highly v iscous liquids.