We report Raman spectra of the organic semiconductor 5,6,11,12- tetraphenyltetracene (rubrene) in the temperature range 30-300 K. The linewidths of certain low-frequency peaks increase significantly, especially in the range 150-200 K. These peaks correspond to the vibrations of the phenyl side groups of the rubrene molecules, and their couplings to intermolecular vibrational modes. We propose a model in which the strong increase in mobility observed with increasing temperature between 30 and 150 K results from disorder as the phenyl groups exchange sides of the backbone plane and break the symmetry. This model explains previous experimental observations of structural and calorimetric changes near 150 K
