Article Cite This: Organometallics XXXX, XXX, XXX−XXX
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Photoluminescence of Cyclometalated Iridium Complexes in Poly(methyl methacrylate) Films Hanah Na, Po−Ni Lai, Louise M. Cañada, and Thomas S. Teets* Department of Chemistry, University of Houston, 3585 Cullen Boulevard, Room 112, Houston, Texas 77204-5003, United States
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ABSTRACT: In this work, we describe the emission properties of 12 heteroleptic bis-cyclometalated iridium complexes immobilized in poly(methyl methacrylate) (PMMA) thin films. The compounds chosen for this study span a range of emission colors covering the visible spectrum and fall into three categories with electronically diverse ancillary ligandsfour cationic bis(aryl isocyanide) complexes, four with chelating bis(acyclic diaminocarbene) (bis-ADC) ancillary ligands which can exist in two protonation states, and four neutral compounds with monoanionic, strongly π donating β-ketoiminate (acNac) and β-diketiminate (NacNac) ancillary ligands. Included in this suite of compounds are five new compounds, one with two aryl isocyanide ancillary ligands, two with bis-ADC ancillary ligands, and one each with acNac and NacNac; their syntheses are described, and crystal structures of three are presented. The bulk of the work presented here describes the emission spectra of all 12 compounds, comparing spectral features in solution to those in thin PMMA films (2 wt % of the respective iridium complex). The emission wavelengths and spectral profiles do not change substantially in PMMA films, but in most cases there are dramatic (2-fold to more than 10-fold) increases in photoluminescence quantum yields, with many of the compounds approaching unity quantum yield in PMMA. The increases in quantum yield come from a combination of larger radiative rates and smaller nonradiative rates in the thin film samples, with significant differences between the members of this series of compounds.
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INTRODUCTION Cyclometalated iridium complexes are arguably the most successful class of organometallic compounds in optoelectronic applications,1 and especially in organic light-emitting diodes (OLEDs).2 The emission color of these compounds is easily tuned throughout the visible spectrum, and the radiative rates, quantum yields, and chemical stabilities are all generally high, because of the large ligand-field splitting and spin−orbit coupling engendered by the iridium center. These compounds exist as two major structural classes, with a great deal of the earliest work focusing on tris-cyclometalated Ir(C∧N)3 (C∧N = cyclometalating ligand) complexes, usually the facial (fac) isomer.3−5 There are also many categories of heteroleptic biscyclometalated iridium complexes, where the octahedral coordination sphere of a cationic [Ir(C∧N)2]+ fragment is completed with two monodentate ancillary ligands or one bidentate ancillary ligand, which can be charge neutral or anionic in nature; the most common structures include neutral Ir(C∧N)2(LX) complexes,6−10 where LX is a chelating, monoanionic ancillary ligand, cationic [Ir(C∧N)2(N∧N)]+ complexes with neutral diimine (N∧N) ancillary ligands,11,12 and anionic [Ir(C∧N)2(X)2]− complexes with two monodentate anionic ancillary ligands.13 In all classes of cyclometalated iridium complexes, the emission color is most often tuned by changing the structure of the cyclometalating ligand, whereas in heteroleptic complexes the ancillary ligand can sometimes influence the emission color but very often has a large effect on redox properties and excited-state dynamics.14 © XXXX American Chemical Society
The structural diversity of cyclometalated iridium complexes has allowed the synthesis of complexes and fabrication of efficient OLED devices with colors that span the entire visible range.1,2,6,15−17 When the potential of a compound for incorporation into OLEDs is evaluated, one of the important factors to consider is the photoluminescence quantum yield (ΦPL) of the compound. Most often, the quantum yields are evaluated in fluid solutions of organic solvents, even though in OLED devices the compound is immobilized as a thin film, generally dispersed in a host matrix with comparatively small (
