Theoretical Study of the Spectral and Charge-Transport Parameters of an Electron-Transporting Material Bis(10-hydroxybenzo[h]qinolinato)beryllium (Bebq2) Alexandra Ya. Freidzon,* ,†,‡ Andrei A. Safonov, † and Alexander A. Bagaturyants †,‡ † Photochemistry Center, Russian Academy of Sciences, Moscow 119421, Russia ‡ National Research Nuclear University MEPhI (Moscow Engineering Physics Institute), Kashirskoye shosse 31, Moscow 115409, Russia * S Supporting Information ABSTRACT: The multireference XMCQDPT2/CASSCF method is used to get insight into the charge transport mechanism of bis(10- hydroxybenzo[h]quinolinato)beryllium (Bebq2) and to explain some features of its light absorption and emission in monomeric and dimeric forms. Energy profiles corresponding to electron and hole hopping in Bebq2 monomer and in three close-packed dimers that can occur in the solid phase are calculated. Our calculation revealed that charges and excitons could be either localized on individual ligands or delo- calized over a pair of stacking ligands in dimers. Delocalized hole states serve as deep charge traps hindering hole transport. On the other hand, the electron states are localized, and hopping electron transport can take place with low barriers. The excited states of dimers exhibit exciton splitting. In some dimers, the transition dipole moment arrangement is unfavorable for luminescence. Therefore, our calculations explain why Bebq2 is an electron transporter (hole blocker) and why regular packing with ligand stacking in Bebq2 layers favors electron transport along the stacks but decreases luminescence. ■ INTRODUCTION Charge transport in small-molecule disordered organic semi- conductors proceeds via the hopping mechanism. 1 In the hopping process, the charge is transferred from an initial state localized on a certain molecule (A) to a final state localized on a neighboring one (B). *+ → + * A B A B Charge localization is accompanied by molecular reorganiza- tion (electron-phonon coupling). Therefore, the potential energy surface (PES) of the ionic supermolecule (AB)* is characterized by distinct minima separated by a barrier. Such charge localization accompanied by structure deformation can be considered as a manifestation of the pseudo-Jahn-Teller effect. 2 The same also relates to excitation transfer; that is, the star may designate either a charged or excited state. Therefore, charge localization directly affects the kinetics of charge and exciton transport in organic semiconductors. Another issue in organic electronics is chemical stability of the materials during device operation. 3 The molecular structure changes when charges or excitons travel through the material, causing weakening of some bonds (which can further break) or electron density accumulation (or depletion) on some atoms, resulting in the formation of highly reactive species. These structural changes are also directly related with charge or exciton localization in the molecule. It follows from the above that an accurate simulation of structural changes upon ionization and excitation of organic semiconductors is of crucial importance for predictive modeling of organic electronic devices. Hopping charge transport in disordered small-molecule semiconductors is usually described by the Marcus model. 4-9 This process can be considered as electron exchange between two molecules, a neutral one and its radical anion (for electron transport) or radical cation (for hole transport). The rate constant of this process is π πλ λ λ = ℏ | | − +Δ ° ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ k H kT G kT 2 1 4 exp ( ) 4 et AB 2 b 2 b where k et is the rate constant for electron transfer, k b is the Boltzmann constant, T is the absolute temperature, ΔG 0 is the Gibbs free energy of the electron transfer reaction (or site energy disorder), λ is the reorganization energy of the molecule, and |H AB | is the so-called hopping integral. For hopping between molecules of same sort (A = B), ΔG 0 ≈ 0. All these parameters, ΔG 0 , λ, and |H AB | depends on the layer material and can be calculated ab initio. Received: August 24, 2015 Revised: November 3, 2015 Published: November 9, 2015 Article pubs.acs.org/JPCC © 2015 American Chemical Society 26817 DOI: 10.1021/acs.jpcc.5b08239 J. Phys. Chem. C 2015, 119, 26817-26827