Lattice Dynamics of Quinacridone Polymorphs: A Combined Raman and Computational Approach

Polarized low-frequency Raman microscopy and a posteriori dispersion-corrected density functional simulations are combined to investigate the lattice vibrations of the α^I, β, and γ polymorphs of the model organic semiconductor quinacridone, which are known to display different optical and electronic properties. The comparison between experiments and calculations allows for unambiguous mode assignment and identification of the scattering crystal faces. Conversely, the agreement between simulations and experiments validates the adopted computational methods, which correctly describe the intermolecular interaction of the molecular material. The acquired knowledge of quinacridone lattice dynamics is used to describe the α^I to β thermal transition and, most consequentially, to reliably characterize the electron-lattice phonon coupling strength of the three polymorphs, obtaining hints about the electrical transport mechanism of the material.