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 αI, 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 αI 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.