Enhancing quantum efficiency in Pt-based emitters using a pendant closo-monocarborane cluster to enforce charge neutrality

The use of organometallic triplet emitters in OLED applications may be complicated when the luminescent complex bears a formal charge. By designing ligands containing pendant closo-monocarborane clusters [CB₁₁H₁₁]⁻, it is possible to neutralize the otherwise cationic cores of cyclometalated Pt(II) complexes bearing bis-carbenes, resulting in significantly enhanced photophysical properties. Herein, we report four Pt(II) complexes decorated with a monoanionic closo-monocarborane by coupling the cluster to the classic cyclometallating moiety, phenylpyridine (ppy). All four complexes were fully characterized by NMR, HR-MS, UV–Vis, and cyclic voltammetry. Complexes 1b, 1c and 2a were characterized by single-crystal X-ray diffraction. Moreover, all four complexes show interesting photophysical behaviors, with complex 1a exhibiting a photoluminescent quantum yield of 97% in a 2 wt% PMMA film. This high efficiency prompted the incorporation of 1a and 1c into an OLED device using solution processing, resulting in green emission with CIE coordinates of (0.28, 0.55) for 1a and yellow emission with CIE coordinates of (0.44, 0.50) for 1c. The measured maximum external quantum efficiency (EQE_max) was 18.51% for 1a, an EQE_max that is among the highest for solution-processed OLED devices containing Pt(II) complexes with simple cyclometalated ppy ligands.