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.

Figure 1

Packing of two nearest-neighbor molecules in the rubrene crystal, with the hydrogen atoms hidden; inset showing torsion angle $\varphi$, with the backbone perpendicular to the paper.

Figure 2

(a) Comparison of Raman spectra at low temperature (30 K) and room temperature (300 K); (b) evolution of the peak positions with temperature (note the different temperature scales in the upper and lower parts of the graph); (c) evolution of the peak linewidths with temperature.

Figure 3

(Color online) (a) Rubrene molecules in the low-energy state; (b) two phenyl groups (1 and 3) are cofacial where the maximum of the energy barrier occurs; (c) the phenyl groups have exchanged sides; [(d)–(f)] with one phenyl group pair having exchanged sides, another pair becomes cofacial simultaneously.

Figure 4

Evolution of energy differences between different phenyl group positions with temperature.