This journal is © The Royal Society of Chemistry 2016 J. Mater. Chem. C, 2016, 4, 10053--10060 | 10053 Cite this: J. Mater. Chem. C, 2016, 4, 10053 Tetrazole iridium(III) complexes as a class of phosphorescent emitters for high-efficiency OLEDs† B. Umamahesh, a N. S. Karthikeyan, b K. I. Sathiyanarayanan,* a J. M. Malicka* c and M. Cocchi* cd We show that iridium complexes incorporating a tetrazolate ligand can be used as new highly phosphorescent emitters in the fabrication of high-efficiency organic light emitting devices (OLEDs). The nature of the excited states of these Ir(III) complexes was probed by means of electrochemical analysis, absorption and photoluminescence spectroscopies and with the aid of TD-DFT calculations. To demonstrate the high stability of these classes of complexes we used TGA and DSC characterizations. These complexes are used as dopant emitters in the emissive layers. The as-fabricated OLEDs display high-efficiency and high-brightness with blue, green and yellow/orange emission. 1. Introduction Organic triplet-state emitting materials (organic phosphors) have been some of the most significant developments in organic (‘‘green’’) optoelectronics. Highly efficient (up to B10% exter- nal quantum efficiency) organic phosphorescent light-emitting diodes (PHOLEDs) based on cyclometallated iridium complexes have been developed during the first half of the past decade (see ref. 1 and references therein). The origin of their high performance is due to the efficient utilization of both singlet and triplet excited states in the radiative processes within the PHOLED’s emitting layers (EMLs). The 5d 6 complexes of Ir 3+ provide a flexible platform for triplet metal-to-ligand charge transfer ( 3 MLCT) room-temperature phosphorescence. The complexes utilized to date for this purpose possess a chemically stable octahedral symmetry, with three bidentate ligands com- plexed to the central Ir ion. For example, in tris-(2-phenylpyridine) iridium = Ir(ppy) 3 , the lowest triplet state emits 3 MLCT phospho- rescence at l = 510 nm, and thus the Ir(ppy) 3 -based PHOLEDs exhibit efficient green phosphorescence with a transient life- time of B1 ms. Ligands with lower triplet energies can be used to shift phosphorescence into the red {e.g. Ir(btp) 2 (acac) emits from the btp = bis[2-(2 0 -benzo[4,5-a]thienyl) pyridinato-N,C3 0 ] ligand at l = 610 nm}. 2 It has also been shown that, if electron withdrawing substituents are introduced into the cyclometal- lating rings, the 3 MLCT emission and hence the PHOLED’s color are blue-shifted, as is the case, for example, with the FIrpic = Ir(III)bis[4,6-di-fluorophenyl-pyridinato-N,C2 0 ] picolinate complex. 3,4 The triplet character of emission is not the only key to successful utilization of phosphorescent complexes in PHOLEDs. In addition to it and to appropriate device architec- tures, judicious matching of the orbital energies of the complex with those of the EML host compounds, another important requirement for achieving enhanced performance parameters as the phosphorescent complexes must not only serve as triplet emissive species, but also should act as effective traps for holes or/and electrons. Within the framework of our recent studies dealing with the use of Ir(III), Pt(II), and Re(I) tetrazolate complexes as highly efficient emitters for OLED-type devices, 5 we now describe other examples of very luminescent Ir(III)-tetrazolate phosphors, which allow the color change from blue to yellow/ orange. We demonstrate highly efficient OLEDs employing novel materials (dfmppy) 2 Ir(pytz) IrTz1, (ppy) 2 Ir(pytz) IrTz2 and (dpq) 2 - Ir(pytz) IrTz3 (Fig. 1), where (pytz) is (1H-tetrazol-5-yl)pyridine, (dfmppy) is 2-(2,4-difluorophenyl)-4-methylpyridine, (ppy) is phenylpyridine and (dpq) is 2,4-diphenylquinoline. The manipulation of the skeletal arrangement as well as the substituent groups (electron donating or electron withdrawing) of the cyclometallating ligand can be used as a platform to increase or decrease the electron density on the metal center a Department of Chemistry, School of Advanced Sciences, VIT University, Vellore-632014, India b Chemistry Department, Easwari Engineering College, Ramapuram, Chennai-600089, India c Consorzio MIST E-R, Via P. Gobetti 101, 40129 Bologna, Italy. E-mail: malicka@laboratoriomister.it d Istituto per la Sintesi Organica e la Fotoreattivita` (ISOF), Consiglio Nazionale delle Ricerche (CNR), Via P. Gobetti 101, 40129 Bologna, Italy. E-mail: massimo.cocchi@isof.cnr.it † Electronic supplementary information (ESI) available: TGA data, MO, electro- chemical properties, photophysics in the solid state, OLED configuration and energy diagrams, current–voltage–luminescence and EL efficiency–current den- sity curves, and NMR spectra. See DOI: 10.1039/c6tc03217f Received 28th July 2016, Accepted 27th September 2016 DOI: 10.1039/c6tc03217f www.rsc.org/MaterialsC Journal of Materials Chemistry C PAPER Published on 27 September 2016. Downloaded by VIT University on 31/10/2016 12:07:49. View Article Online View Journal | View Issue