Electronic properties of phenanthrimidazoles as hole transport materials in organic light emitting devices and in photoelectron transfer to ZnO nanoparticles

Abstract

Phenanthrimidazoles as hole transport materials have been synthesized, characterized, and applied as nondoping emitters in organic light emitting devices. The synthesized molecules possess high fluorescent quantum yield and thermal properties and display film forming abilities. The highest occupied molecular orbital (HOMO) energies of these materials are shallower than the reported tris(8-hydroxyquinoline)aluminum (Alq3), which enables the hole transport ability of these phenanthrimidazoles. Taking advantage of the thermal stability and hole transporting ability, these compounds can be used as a functional layer between NPB [4,4-bis(N-(1-naphthyl)-N-phenylamino)biphenyl] and Alq3 layers and show that these phenanthrimidazoles can be alternatively used as novel hole transport materials and to improve the device performances. Geometrical, optical, electrical, and electroluminescent properties of these molecules have been probed. Further, natural bond orbital, nonlinear optical materials (NLO), molecular electrostatic potential, and HOMO–lowest unoccupied molecular orbital (LMO) energy analysis have been made by density functional theory (DFT) method to support the experimental results. Hyperpolarizability analysis reveals that the synthesized phenanthrimidazoles possess NLO behavior. The chemical potential, hardness, and electrophilicity index of phenanthrimidazoles have also been computed by DFT method. Photoinduced electron transfer explains the enhancement of fluorescence by nanoparticulate ZnO, and the apparent binding constant has been obtained. Adsorption of the fluorophore on ZnO nanoparticle lowers the HOMO and LUMO energy levels of the fluorophore. The strong adsorption of the phenanthrimidazoles on the surface of ZnO nanocrystals is likely due to the chemical affinity of the nitrogen atom of the organic molecule to Zn(II) on the surface of nanocrystal. Copyright © 2013 John Wiley & Sons, Ltd.