THz Raman spectroscopy to investigate structural order and polymorphism in charge transfer cocrystals

Vibrational spectroscopic techniques, as IR and Raman, are powerful tools to investigate crystal packing, polymorphism and phase transitions of molecular materials. Raman spectroscopy in the THz region probes the intermolecular vibrations (lattice phonons), that reflect the intermolecular interactions and unit cell symmetry, thus giving the fingerprint of a crystal phase. The use of polarized light on oriented single crystals also gives phonon symmetry species, associated with directional information on the molecular displacements involved. Thus, the interactions within and between chains or layers inside a crystal can be evaluated separately, eventually revealing the occurrence of low-dimensional disorder along specific directions [1],[2]. Following this approach, we investigate structural order and phase transitions in some charge transfer cocrystals we recently prepared [3]. The THz Raman measurements were integrated with the polarized IR and Raman spectra in the mid-IR range to assess the molecular orientation, the degree of charge transfer and the stack symmetry.