Molecular motion and mobility in an organic single crystal: Raman study and model

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.