Copolymers with pendent phenoxazine/arylbenzimidazole moieties as bipolar polymeric host materials for green PhOLEDs

A group of vinyl copolymers containing electronically isolated phenoxazine as an electron-donating unit and either 2-phenylbenzimidazole or 2-(2-pyridyl)benzimidazole as electron-accepting units was synthesized via a multistep synthetic route. The corresponding vinyl monomers were polymerized by cationic polymerization in solution to obtain amorphous bipolar polymeric materials. The chemical structures and molecular characteristics of the synthesized copolymers were fully characterized by NMR spectroscopy, elemental analysis, and gel permeation chromatography. Thermal and morphological properties were investigated using thermogravimetric analysis and differential scanning calorimetry. All materials exhibited high thermal stability, with initial thermal degradation temperatures in the range of 300–320 °C. Furthermore, the polymers demonstrated very high glass transition temperatures between 151 and 160 °C, indicating the excellent morphological stability of amorphous films, with only a minor dependence on the nature of the aromatic acceptor chromophore. The synthesized copolymers were evaluated as host materials in phosphorescent organic light-emitting diodes (PhOLEDs), using bis(2-phenylpyridine)(acetylacetonato)iridium(III), [Ir(ppy)₂(acac)] as the emissive dopant. Devices based on the copolymer PPxPy1, incorporating phenoxazine and 2-(2-pyridyl)benzimidazole units, exhibited promising electroluminescent performance, characterized by a low turn-on voltage of 3.0 V, a maximum luminance exceeding 2000 cd/m², and a maximum current efficiency of approximately 13.4 cd/A. A similar device employing the copolymer PPxP3, containing phenoxazine and 2-phenylbenzimidazole chromophores, showed the best overall performance, achieving a low turn-on voltage of 3.0 V, a maximum brightness above 2800 cd/m², and a maximum current efficiency of about 13.0 cd/A. These results demonstrate the potential of phenoxazine–benzimidazole-based vinyl copolymers as thermally robust and efficient host materials for phosphorescent OLED applications.