Molecular Construction Kit for Tuning Solubility, Stability and Luminescence Properties: Heteroleptic MePyrPHOS-Copper Iodide- Complexes and their Application in Organic Light-Emitting Diodes Daniel Volz, †,‡ Daniel M. Zink, †,‡ Tobias Bocksrocker, § Jana Friedrichs, ‡ Martin Nieger, ∥ Thomas Baumann,* ,‡ Uli Lemmer, § and Stefan Bra ̈ se* ,†,# † Institute of Organic Chemistry, Karlsruhe Institute of Technology (KIT), Fritz-Haber-Weg 6, 76131 Karlsruhe, Germany ‡ cynora GmbH, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany § Light Technology Institute, Karlsruhe Institute of Technology (KIT), Engesserstraße 13, 76131 Karlsruhe, Germany ∥ Laboratory of Inorganic Chemistry, University of Helsinki, P.O. Box 55, A.I. Virtasen aukio 1, FIN-00014 University of Helsinki, Finland # Institute of Toxicology and Genetics, Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, D-76344 Eggenstein-Leopoldshafen, Germany * S Supporting Information ABSTRACT: Organic light-emitting diodes (OLEDs) are currently being commercialized for lighting and display applications, but more work has to be done. In addition to the ongoing optimization of materials and devices in terms of efficiency and lifetime, the substitution of processing steps involving vacuum deposition for solution processing techniques is favorable. To reach this aim, good soluble materials are required. A modular family of highly emissive PyrPHOS-copper iodide complexes featuring various ancillary phosphine ligands has been synthesized. Photoluminescence spectroscopy, TCSPC (time-correlated single photon counting), cyclic voltammetry, X-ray diffraction, and DFT calculations were performed to gain a broad understanding of the complexes. While the photophysical properties are consistent within the family, thermal stability and solubility depend on the ligands. The materials showed very high photoluminescence quantum efficiencies up to 99% in powders and 85% in thin films. Selected examples were tested in devices, confirming the suitability of heteroleptic PyrPHOS-complexes for OLEDs. KEYWORDS: copper complexes, organic light-emitting diode, structure−property relationships, photoluminescence E ven after the commercial launch of organic light-emitting diodes (OLED) as a technology 1 for lighting and display applications some years ago, essential questions remain unsolved: 14 years after the introduction of transition-metal compounds as efficient emitting materials by Forrest et al., 2,3 the fabrication of solution-processed OLEDs with metal complexes as emitting materials still has to be developed further into a truly reliable, industrial process. The three main problems preventing the use of solution processing are (i) poor solubility of many OLED-materials in common solvents, 4,5 (ii) morphological inhomogeneity due to aggregation and crystal- lization of small molecules, 6 and (iii) blending of the functional layers during or after deposition. 5,7 The latter is also relevant even for vacuum-processed OLEDs, but is often neglected. 7 Comparing solution- to vacuum-processing, in most cases both efficiency and device-lifetime are lower, while the turn-on- voltage is higher for the former technique, even when identical materials are used. This is expected to be a result of defects at the interfaces. 8 Grave problems arise from the insolubility of many functional materials known from vacuum-deposited OLEDs in common organic solvents. Common solvents used for the processing of molecular, polymer host systems are chloro- benzene and toluene. Ir(ppy) 3 , one of the standard emitters used in high-performance OLEDs, 9,10 has a solubility of 1 mg mL −1 in chlorobenzene, 4 even less in toluene. The preparation of emitting layers (usually a mixture of a host and the emitting compound as dopant) with a suitable thickness for OLEDs, typically in the order of 50 nm, is hindered by this low solubility. Morphological defects like crystalline grains in functional layers act as charge traps, 11 while aggregation, especially for triplet-harvesting emitters, causes emission quenching. 12 Such morphology defects can be avoided by using materials with a low crystallization tendency, which corresponds to a low lattice energy, a good solubility, or by immobilizing the relevant molecules, for example, by attaching them to a polymeric backbone. 13−19 To address this issue, common materials may be substituted with solubility-enhancing groups. Because of the steric demand of these substituents, the Received: April 3, 2013 Revised: July 2, 2013 Published: July 16, 2013 Article pubs.acs.org/cm © 2013 American Chemical Society 3414 dx.doi.org/10.1021/cm4010807 | Chem. Mater. 2013, 25, 3414−3426