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

Abstract

<p>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 <em>closo</em>-monocarborane clusters [CB₁₁H₁₁]⁻, it is possible to neutralize the otherwise cationic cores of cyclometalated Pt(II) complexes bearing <em>bis</em>-carbenes, resulting in significantly enhanced photophysical properties. Herein, we report four Pt(II) complexes decorated with a monoanionic <em>closo</em>-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 <strong>1b</strong>, <strong>1c</strong> and <strong>2a</strong> were characterized by single-crystal X-ray diffraction. Moreover, all four complexes show interesting photophysical behaviors, with complex <strong>1a</strong> exhibiting a photoluminescent quantum yield of 97% in a 2 wt% PMMA film. This high efficiency prompted the incorporation of <strong>1a</strong> and <strong>1c</strong> into an OLED device using solution processing, resulting in green emission with CIE coordinates of (0.28, 0.55) for <strong>1a</strong> and yellow emission with CIE coordinates of (0.44, 0.50) for <strong>1c</strong>. The measured maximum external <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/quantum-efficiency" title="Learn more about quantum efficiency from ScienceDirect's AI-generated Topic Pages">quantum efficiency</a> (EQE_max) was 18.51% for <strong>1a</strong>, an EQE_max that is among the highest for solution-processed OLED devices containing Pt(II) complexes with simple cyclometalated ppy ligands.<br></p>