Fluorine-free blue phosphorescent emitters for efficient phosphorescent organic light emitting diodes

Abstract

Two blue phosphorescent Ir(III) compounds with a main fluorine-free bipyridine ligand have been synthesized. These molecules have the general formula of Ir(C⁁N)₂(L⁁X), where C⁁N = 2′,6′-dimethoxy-4-methyl-2,3′-bipyridine [(MeO)₂pypy] and L⁁X = ancillary ligand, such as acetylacetonate (acac) (1) or 2-picolinate (pic) (2). Thermal, photophysical, and electrochemical properties of 1 and 2 have also been investigated systematically. The two compounds show high thermal stability with a decomposition temperature of ∼350 °C due to strong and varied intermolecular interactions. A sky-blue emission with moderate phosphorescence quantum efficiency (Φ_PL = 0.49–0.54) is observed in the spectral region of 460–470 nm for all compounds, which is attributed to both metal-to-ligand charge transfer (MLCT) and ligand-centered (LC) transition. Compounds 1 and 2 show only ∼10 nm red-shifted emission in the fluid state, compared to similar fluorinated iridium counterparts. Multi-layered electroluminescent devices using compounds 1 and 2 as the dopant and 9-(3-(9H-carbazol-9-yl)phenyl)-9H-carbazol-3-yl)diphenylphosphine oxide as a host material have been fabricated. The electroluminescent device of 2 at a doping level of 5 wt% shows the best performance with an external quantum efficiency of 15.3% and the color coordinate of (0.16, 0.28) at a brightness of 100 cd m⁻².