A computational protocol for the calculation of local (Holstein) and nonlocal (Peierls) carrier-phonon coupling in molecular organic semiconductors is presented and applied to orthorhombic rubrene (5,6,11,12-tetraphenyltetracene). In the phonon description, the rigid molecule approximation is removed, allowing mixing of low-frequency intramolecular modes with intermolecular (lattice) phonons. Notwithstanding, a rather clear distinction remains between the low-frequency phonons, which essentially modulate the transfer integral from a molecule to another (Peierls coupling), and the high-frequency, fully intramolecular phonons, which modulate the on-site energy (Holstein coupling). The implications for the current models of mobility are shortly discussed.

Crystals of tetracene have been studied by means of lattice phonon Raman spectroscopy as a function of temperature and pressure. Two different phases (polymorphs I and II) have been obtained, depending on sample preparation and history. Polymorph I is the most frequently grown phase, stable at ambient conditions. Application of pressure above $1\mathrm{GPa}$ yields polymorph II, which is also obtained by cooling the sample below $140\mathrm{K}$. However, the conditions for inducing the phase transitions depend on sample preparation and history, and polymorph II can also be maintained at ambient conditions. We have calculated the crystallographic structures and phonon frequencies as a function of temperature, starting from the configurations of the energy minima found by exploring the potential energy surface of crystalline tetracene. The spectra calculated for the first and second deepest minima match satisfactorily those measured for polymorphs I and II, respectively. The temperature dependence of the spectra is described correctly. All published x-ray structures, once assigned to the appropriate polymorph, are also reproduced.

Charge ordering in the (TMTTF)₂X salts with centrosymmetric anions (X = PF‾₆ , AsF‾₆ , SbF‾₆ ) leads to a ferroelectric state around 100 K. For the first time and in great completeness, the intra- and intermolecular vibrational modes of (TMTTF)₂X have been investigated by infrared and Raman spectroscopy as a function of temperature and pressure for different polarizations. In this original paper, we explore the development and amount of charge disproportionation and the coupling of the electronic degrees of freedom to the counterions and the underlying lattice. The methyl groups undergo changes with temperature that are crucial for the anion cage formed by them. We find that the coupling of the TMTTF molecules to the hexafluorine anions changes upon cooling and especially at the charge-order transition, indicating a distortion of the anion. Additional features are identified that are caused by the anharmonic potential. The spin-Peierls transition entails additional modifications in the charge distribution. To complete the discussion, we also add the vibrational frequencies and eigenvectors based on ab-initio quantum-chemical calculations.