New multifunctional phenanthroimidazole–phosphine oxide hybrids for high-performance red, green and blue electroluminescent devices

Abstract

In this work, two novel hybrids of an electron-accepting phosphine oxide moiety attached to a phenanthroimidazole have been designed and synthesized. The PO group is used as a point of saturation between the PPI moiety and the outer phenyl groups, so the high triplet energy of PPI is preserved to act as a host for red and green phosphorescent dopants. The strong intermolecular interactions and steric effect of the diphenylphosphine oxide (DPO) moiety endows the films with high quantum yields in the deep-blue emission region. Compared to PPI, the carrier (hole- and electron-)injection/transport properties were greatly promoted by the appended DPO group according to single-carrier device measurement. Besides, the morphological and thermal stabilities were also improved. The multiple functions enable adaptation of several simplified device configurations. The undoped deep-blue fluorescent device exhibits an external quantum efficiency of 2.24% with CIE (0.16, 0.08), very close to the NTSC blue standard CIE (0.14, 0.08). High performance for green (65.4 cd A⁻¹, 73.3 lm W⁻¹ and 18.0%) and red (19.0 cd A⁻¹, 21.3 lm W⁻¹ and 13.5%) phosphorescent devices used as hosts have been achieved. The experimental and theoretical relationships between the molecular structures and the optoelectronic properties are discussed.