Polymorphism of crystalline α-quaterthiophene and α-sexithiophene:: Ab initio analysis and comparison with inelastic neutron scattering response

Phonons in the alpha-quaterthiophene (4T) and alpha-sexithiophene (6T) polymorph phases are investigated using the direct method combined with density functional theory (DFT)-based total energy calculations. The simulation of inelastic neutron scattering spectra (INS) on the LT and HT polymorph phases of 4T and 6T enable the corresponding spectral signatures of these materials to be identified. In particular, there are two fingerprints: (i) the low-frequency vibrational modes (frequencies lower than 200 cm(-1)) and (ii) the vibrational modes in the 600-900 cm(-1) frequency range. The good agreement with the INS experimental data allows us to assign unambiguously the origin of all features (first-order and high-order processes) of these spectra and to predict that the LT phase is the phase measured experimentally both on the 4T and 6T materials. Moreover, the broad background in the 600-1400 cm(-1) frequency range and the well-defined features which appear around 940 cm(-1) in the calculated INS spectra of 4T/HT and 6T/HT are assigned to multiphonon contributions. This multiphonon contribution at 940 cm(-1), which is absent in the 4T/LT and 6T/LT INS spectra, also constitutes a fingerprint of the HT phases. Finally, the calculated dispersion curves of the two polymorph phases of 4T and 6T are given.