Interaction of charge carriers with lattice vibrations in organic molecular semiconductors: Naphthalene as a case study

Recent theoretical studies suggest that the modulation of the electronic couplings (transfer integrals) between adjacent molecules by lattice vibrations, i.e., the so-called nonlocal electron-phonon coupling, plays a key role in the charge-transport properties of molecular organic semiconductors. However, a detailed understanding of this mechanism is still missing. Here, we combine density functional theory calculations and molecular mechanics simulations and use a chemistry-based insight to derive the nonlocal electron-phonon coupling constants due to the interaction of charge carriers with the optical lattice vibrations in the naphthalene crystal. The results point to a very strong coupling to both translational and librational intermolecular vibrational modes as well as to intramolecular modes. Along some crystal directions, the nonlocal interactions are found to be dominated by nontotally symmetric vibrational modes which lead to an alternation (Peierls-type dimerization) pattern. Importantly, we introduce two parameters that can be used: (i) to quantify the total strength of the nonlocal electron-vibration mechanism in the form of a reorganization energy term; and (ii) to define the extent of the thermal fluctuations of the electronic couplings. Interestingly, zero-point fluctuations are seen to be very significant.