Structural and vibrational characterization of the organic semiconductor tetracene as a function of pressure and temperature

Neutron powder diffraction and inelastic neutron scattering measurements were performed on crystalline tetracene, a molecular semiconductor of triclinic crystal structure that adopts a herringbone layered motif, as a function of pressure up to 358 MPa. In combination with theoretical and simulated computations, these measurements permit detailed characterization of the structural and vibrational changes of tetracene as a function of pressure. Powder diffraction at 295 K reveals anisotropic modification of the crystal structure with increasing pressure. Particularly, the unit cell parameters associated with the two-dimensional herringbone layers of the solid state structure displayed continuous change at all measured pressures, whereas perpendicular to the herringbone layers the structure remains relatively unchanged. The measured compressibilities along the [1 0 0], [0 1 0], and [0 0 1] crystal axes are -3.8 × 10^-4, -1.9 × 10^-4, and -3.4 × 10^-4 Å/MPa, respectively. Inelastic neutron scattering spectra were collected at several pressures in the 25-75 and 0-25 meV energy ranges using a filter analyzer and a Fermi chopper time-of-flight spectrometer, respectively. Assignment of the spectral peaks to specific intramolecular vibrational modes has been accomplished using ab initio density functional theory calculations and the low energy lattice phonon modes were interpreted from the results of molecular dynamics simulations at 1 atm and 358 MPa. Anisotropic behavior parallel to that observed in the structural measurements is also apparent in both the intramolecular and lattice phonon vibrational dynamics. Intramolecular vibrations having atomic displacements entirely within the plane of the molecule's aromatic ring remain unchanged with increasing pressure while vibrations with atomic displacements perpendicular to the molecular plane shift to higher energy. The lattice phonons display a similar anisotropy with increasing pressure. Phonon modes propagated within the herringbone layer are significantly shifted to higher energy with increasing pressure relative to the modes with displacements primarily perpendicular to the layers. Overall, both the planar internal geometry and the layered arrangement of the tetracene molecules significantly influence the observed structural and vibrational behavior with increasing pressure.