trans-N‑(Heterocyclic Carbene) Platinum(II) Acetylide Chromophores as Phosphors for OLED Applications

A family of complexes of the type trans-(NHC)₂Pt­(CC–Ar)₂ (NHC = N-heterocyclic carbene and Ar = substituted phenyl or 4-pyridyl) exhibits blue or blue-green phosphorescence. The photoluminescence is 10–50-fold more efficient when the materials are in dispersed in a solid poly­(methyl methacrylate) glass compared to in THF solution. The phosphorescence quantum efficiencies in PMMA glass range from 0.20–0.65, varying with the structure of the aryl acetylide ligands. Organic light-emitting diodes (OLEDs) were fabricated by thermal evaporation, using bis­[2-(diphenylphosphino)­phenyl] ether oxide (DPEPO) as the host, and 15% of the trans-(NHC)₂Pt­(CC–Ar)₂ complexes as the dopant/emitters. Most of the OLEDs display good performance, with EL spectra that closely match the PL in the PMMA glass. The best performing materials have a peak EQE ranging from 9.6–14.1%, with deep blue spectral profiles (minimum CIE (0.16, 0.13)). Several of the complexes have deep HOMO levels, and they display poor EL performance attributed to inefficient host to emitter hole transfer.