Stable and Efficient Light-Emitting Electrochemical Cells Rubén D. Costa * , Antonio Pertegás * , Daniel Tordera * , Martijn Lenes * , Enrique Ortí * , Henk J. Bolink * , Stefan Graber ** , Edwin Constable ** and Catherine E. Housecroft ** * Instituto de Ciencia Molecular, Universidad de Valencia, PO Box 22085, Valencia (Spain) henk.bolink@uv.es ** Department of Chemistry, University of Basel, Spitalstrasse 51, Basel, CH-4056 (Switzerland) ABSTRACT Three new heteroleptic iridium complexes that combine two approaches, one leading to a high stability and the other yielding a high luminescence efficiency, are presented. All complexes contain a phenyl group at the 6-position of the neutral bpy ligand, which holds an additional, increasingly bulky substituent on the 4-position. The phenyl group allows for intramolecular π–π stacking, which renders the complex more stable and yields long-living light-emitting electrochemical cells (LECs). The additional substituent increases the intersite distance between the cations in the film, reducing the quenching of the excitons, and should improve the efficiency of the LECs. Indeed, LEC devices employing these complexes as the primary active component show shorter turn-on times, higher efficiencies and luminances, and, surprisingly, also demonstrate longer device stabilities. Keywords light-emitting electrochemical cells, iridium complexes, ionic transition-metal complexes, high efficiency, bulky substituents. 1 INTRODUCTION Solid-state light-emitting electrochemical cells (LECs) have attracted considerable interest in the past few years.[1] LECs are single-component electroluminescent devices consisting of a charged luminescent material.[1, 2] The main characteristic of these devices is the insensitivity to the workfunction of the electrodes employed. This is due to the generation of a strong interfacial electric field caused by the displacement of the mobile ionic species towards the charged electrodes when an external electric field is applied over the device. Therefore, in contrast to organic light- emitting diodes (OLEDs), air-stable electrodes, such as gold, silver, or aluminium, can be used, which is an initial requirement for obtaining unencapsulated devices. In its simplest form, a LEC consists of a single active layer composed entirely of an ionic transition-metal complex (iTMC). iTMCs are triplet emitters similar to those used in OLEDs. iTMC-based LECs exhibiting low turn-on times and emitting blue, green, orange, red, and even white light have been reported.[1] Recently, we reported on a new approach to iTMCs that led to a significant increase in the lifetime of LECs that employed them as the main component.[3, 4] This was achieved with an iridium complex exhibiting intramolecular - stacking of two of its phenyl rings, resulting in a supramolecular cage formation. The simplest example is mentioned in this work for comparison and is referred to as complex 1 (see Fig. 1). This demonstrated that LECs can reach lifetimes suitable for first applications. Hence, all requirements seem to have been met to allow LECs to be applied in first products. However, the aforementioned achievements were obtained separately and never jointly in one device with a single complex. It is the object of this work to combine in one single complex two approaches, one leading to a high stability and the other yielding a high luminescence efficiency. High efficiencies can be reached by decreasing the quenching of the excitons by shielding the individual iTMCs from each other. This can be achieved by the introduction of bulky side groups to the periphery of the complex.[5] Bulky groups in the iTMC can also increase the stability of the LECs as they render the complex less susceptible for interaction with water.[6, 7] That interaction was identified as the primary reason for the short lifetimes of ruthenium based LECs.[8, 9] Three heteroleptic iridium complexes combining the above-mentioned features with cyclometallated 2-phenylpyridine ligands (Hppy = 2- phenylpyridine) were prepared, [Ir(ppy) 2 (Meppbpy)]PF 6 (2, Meppbpy = 4-(3,5-dimethoxyphenyl)-6-phenyl-2,2'- bipyridine), [Ir(ppy) 2 (C 10 ppbpy)]PF 6 (3, C 10 ppbpy = 4-(3,5- bis(decyloxy)phenyl)-6-phenyl-2,2'-bipyridine), and [Ir(ppy) 2 (G1ppbpy)]PF 6 (4, G1ppbpy = 4-(3,5-bis(3,5- bis(dodecyloxy)benzyloxy)phenyl)-6-phenyl-2,2'- bipyridine), and are presented in Figure 1. All complexes contain a phenyl group at the 6-position of the neutral 2,2'-bipyridine (bpy) ligand, which holds an additional, increasingly-bulky substituent on the 4-position. The phenyl group allows for the intramolecular - stacking, which renders the complex more stable and yields long-living LECs.[3, 4]The additional substituents increase the intersite distance between the cations in the film reducing the quenching of the excitons and should increase the efficiency of the LECs. Density functional theory (DFT) calculations indicate that all iTMCs have the desired - intramolecular interactions between the pendant phenyl ring of the bpy ligand and the phenyl ring of one of the ppy ligands. The photoluminescence quantum NSTI-Nanotech 2010, www.nsti.org, ISBN 978-1-4398-3402-2 Vol. 2, 2010 49