Electrostatic Polarization Energies of Charge Carriers in Organic Molecular Crystals: A Comparative Study with Explicit State-Specific Atomic Polarizability Based AMOEBA Force Field and Implicit Solvent Method.

Abstract

The electrostatic polarization plays an important role in determining the energy levels of charge carriers in organic solids, which is controlled by the atomic polarizability in AMOEBA polarizable force field. QTAIM-based space partitioning of molecular polarizability is utilized to uniformly parametrize the state-specific atomic polarizability (SSAP) of π-conjugated organic small molecules to avoid fitting molecular polarizability of some artificial training set. Herein, the SSAPs are applied to explicitly extrapolate the electrostatic polarization energy ( E pol) of the charge carriers of nine π-conjugated organic crystals including six p-type transfer materials, oligoacenes and TIPS-substituted oligoacenes, and three n-type transfer materials, F-substituted oligoacenes and TCNQ. Our results demonstrate that the electrostatic polarization energies of the hole carrier ( E+pol) are smaller than that of the electron carrier ( E-pol) for p-type molecules while E+pol are larger than E-pol for n-type molecules. SSAP-based E pol values of oligoacenes behave as a nearly unvaried feature with the increase of conjugation length which is similar to implicit polarizable continuum model (PCM) results, while E pol obtained from the default atomic polarizability behaves with a notable decrease. Implicit PCM can correctly capture most of electrostatic polarization of ions in bulk system although it slightly underestimates the gap between the electrostatic polarization of hole and electron carriers in oligoacene crystals. Our results demonstrate that this unified parametrized SSAP provides a reliable and cheap tool to estimate the energy landscape of charge carriers in condensed-phase organic solids.