Unveiling photophysical properties of phenanthro[9,10-d]imidazole derivatives for organic light-emitting diodes

Recently, two phenanthro[9,10-d] imidazole derivatives exhibited excellent advantages in organic lightemitting devices (i.e. high luminous efficiency, high carrier mobility, and low turn-on voltage). However, the relationship between their photophysical properties and the structural characters or intermolecular interactions remain elusive, which is considerable importance to further performance improvement. Currently, density functional theory (DFT) and time-dependent DFT (TD-DFT) have become powerful tools to rationalize photophysical properties and to design new materials with improvement performance. The simulated electron absorption and emission wavelengths of compounds 1 and 2 are in good agreement with the experimental ones. For the studied compounds, the involvement of tert-butyl moiety has negligible effect on energy level and distribution of frontier molecular orbitals (FMOs), whereas greatly affects electron transition of deep energy level and charge transport property. Synergy of pi-pi and C-H... pi intermolecular interactions is responsible for the bipolar carrier transport, while C-H...pi for hole transport. The incorporation of NH2 on phenanthro[9,10-d] imidazole and NO2 on diphenylamino part is an effective way to tune FMOs energy level and intramolecular charge transfer, leading to the substantial enhancement of the second-order nonlinear optical (NLO) response. Our work is also important for understanding photophysical properties and designing photoelectric materials of phenanthro[9,10-d] imidazole derivatives. c 2017 Elsevier B. V. All rights reserved.