Phenanthroimidazole derivatives showing mild intramolecular charge transfer and high quantum yields and their applications in OLEDs

Abstract

Phenanthroimidzole (Phen-I) has attracted the attention of researchers for applications in materials science including organic electronics and biological applications. Herein, we designed small Phen-I derivatives showing mild bipolar characters and strong emissions in the blue region and discussed their applications in OLEDs. Designing of materials is based on the presence of electron-deficient nitrogen (sp²) (acting as the mild electron-withdrawing core) and the availability of other nitrogen (N1) and carbon (C2) atoms for substitution at the imidazole ring. Earlier studies on the structural properties of Phen-I and our theoretical studies show that substitution at N1 is orthogonal to the plane of the imidazole ring, thus limiting the effect of extended conjugation. The aryl groups at the C2-carbon of the imidazole moiety caused considerable alteration in the electronic properties which is evident from the absorption and emission spectra. Absorption and emission in different solvents show slight alterations in the peak maxima suggesting mild intramolecular charge transfer in these molecules. These materials were also studied in thin films when doped in blends with polyvinylcarbazole (PVK). Furthermore, one of these Phen-I derivatives is used for OLED applications. A high luminance of ∼ 2.5 × 10⁴ Cd m⁻² at a current density of 35 mA cm⁻² was found for Phen-I based OLEDs. This gave a high current efficiency of ∼ 60 Cd A⁻¹ (@ 15 V) and an external quantum efficiency of ∼3.2%.