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

Abstract

Abstract Recently, two phenanthro[9,10-d]imidazole derivatives exhibited excellent advantages in organic light-emitting 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 π-π and C H···π intermolecular interactions is responsible for the bipolar carrier transport, while C H···π 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.