Polymorphism of Crystalline α-Quaterthiophene and α-Sexithiophene:  Ab Initio Analysis and Comparison with Inelastic Neutron Scattering Response

Abstract

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.

Lattice Dynamics of Oligo(phenylenethienylene)s: A Far-Infrared and Inelastic Neutron Scattering Study

“…The second frequency range is mainly dominated by thiophene phonon modes. These proposed assignments are consistent with those reported in the literature on oligothiophenes 16,20,40,39,41 and oligophenylenes. 21,[26][27][28] …”

“…In this frequency range, the phonon modes in π-conjugated molecular crystals are mainly intramolecular with a small frequency dispersion. 20,38 Thus, although the crystallographic structure of the different oligo(phenylenethienylene)s are unknown, the isolated molecule approximation should be reliable for the calculations of high-frequency phonon modes. Table 1 reports the main TBT-Me experimental INS frequencies, together with their corresponding calculated frequencies and assignments in this frequency range.…”

“…12,13 Thus, the assignment of the low-frequency phonon modes, affecting the instantaneous electronic structure of the conjugated systems via the π-π* overlap, is of particular interest in these materials. 14 The investigation of intramolecular phonon modes in crystalline oligothiophenes [15][16][17][18][19][20] and oligophenylenes [21][22][23][24][25][26][27][28] has been extensively studied in the literature by using several spectroscopic techniques such as mid-infrared, Raman, and inelastic neutron scattering. By contrast, a few papers report the lattice modes and ring librations of these materials.…”

Polymorphism and Phonon Dynamics of α‐Quaterthiophene

“…[18][19][20] To properly interpret the lattice phonon spectra, it is useful to consider the selection rules based on the RMA, and to compare the spectral predictions for the HT and LT phases.…”

Intermolecular interactions in solid benzene

“…Increasingly, these calculations are being used to simulate not only the coherent INS spectra from single crystals but also the incoherent INS spectra of powdered samples. [12][13][14] The latter is a far more straightforward experimental technique and alleviates the need for large single crystals of deuterated materials in the study of lattice dynamics.…”