Vibrational Davydov Splittings and Collective Mode Polarizations in Oriented Organic Semiconductor Crystals

Vibrational properties of highly ordered crystalline perfluoropentacene (PFP) films epitaxially grown on KCl(100) and NaF(100) substrates have been studied by means of transmission infrared spectroscopy and density functional theory. The different molecular orientations adopted by PFP on both substrates (standing vs lying) and their epitaxial ordering enable precise polarization-resolved measurements along individual crystallographic directions and thus allow an unambiguous experimental determination of the polarization of the IR modes. Computations of the vibrational spectra beyond the single-molecule approximation were employed at the periodic dispersion-corrected density functional level (PBE-D2_PBC) and compared with nonperiodic calculations (PBE-D2/def2-TZVPP). Thereby, a detailed mode assignment based on vibrational energies and polarization information was attained. A microscopic explanation for the experimentally observed Davydov splitting of some modes and the IR inactivity of others was derived based on the mutual coupling of the dynamical dipole moments of the two molecules within the unit cell. Experimentally observed modes not covered by our theoretical analysis have been identified as combination bands of IR-active modes coupled to totally symmetric modes of similar displacement patterns. These findings have important implications for future studies on structure and charge transport in organic semiconductors and the validation of theoretical approaches for the modeling of vibrational spectra.