Organic Light‐Emitting Diodes based on Charge‐Neutral Ru^II Phosphorescent Emitters

Abstract

Ever since the first discovery of organic electroluminescent (EL) devices, [1] an intensive research effort has been devoted to the development of new light-emitting materials. There is considerable optimism that organic light-emitting devices (OLEDs) may eventually offer an alternative to inorganic light-emitting diodes (LEDs) and liquid-crystal displays (LCDs), perhaps affording brighter, flexible displays at a lower cost. [2] In this regard, the fabrication of OLEDs with efficient, saturated red-light emission becomes essential. [3] This has been achieved, in part, by using third-row Os II , [4] Ir III , [5] and Pt II [6] phosphorescent dopant emitters, for which strong spin-orbit coupling effectively promotes singlet±triplet (S±T) intersystem crossing as well as enhancement of the subsequent T 1 ±S 0 transition. Theoretically, OLEDs with 100 % internal quantum efficiencies could be attained by harnessing both triplet and singlet excitons. [7] However, there are practical barriers to the commercialization of phosphorescent OLED technologies based on these third-row transition-metal complexes due to the prohibitive cost of the noble metals. Hence, from a manufacturing standpoint there is an urgent need to develop phosphorescent emitting materials from less expensive precursors. From a number of possible alternative materials evaluated, the cationic tris-substituted Ru II bipyridine complex Ru(ppy) 3 X 2 (where X is an anion such as ClO 4 ± or BF 4 ± ) and their functionalized derivatives have attracted much attention.[8] These Ru II complexes have been used to make solidstate light-emitting electrochemical cells (LECs), [9] in which the emissive layer contains an excess of mobile counter ions and the charge injection is relatively independent of the nature of the contacts. As a result, light emission occurs through electrochemical redox processes with high efficiency and low turn-on voltage.[10] Polymer light-emitting devices (PLEDs) containing these cationic Ru II dopant emitters have also been reported. [11] These PLED devices are characterized by good device performance and instantaneous light output when compared with LECs, which require longer response times to achieve steady and maximum emission. Unfortunately, these Ru II complexes are quite unsuitable for the fabrication of conventional, small-molecule OLEDs using the vacuum-deposition method. This can be mainly attributed to poor volatility due to their ionic nature, which results in severe thermal degradation during evaporation. In this communication, we report the synthesis and characterization of three charge-neutral Ru II complexes, (2), and [Ru(ifpz) 2 (PPh 2 Me) 2 ] (3) (ibpz: 3-tert-butyl-5-(1-isoquinolyl)-pyrazolate, ifpz: 3-trifluoromethyl-5-(1-isoquinolyl)pyrazolate) whose structures are depicted in Figure 1. In contrast to the PLEDs fabricated using the non-volatile, ionic Ru II tris-bipyridine-type of emitters, [11] remarkably high red-emission efficiencies have been achieved for OLED devices fabricated using codeposition techniques...

Homoleptic Tris(Pyridyl Pyrazolate) IrIII Complexes: En Route to Highly Efficient Phosphorescent OLEDs

“…Elemental analyses were carried out at the NSC Regional Instrumentation Center at National Chao Tung University, Hsinchu (Taiwan). The pyridyl pyrazole chelates, namely: 3-tert-butyl-5-(2-pyridyl) pyrazole (bppz)H, 3-trifluoromethyl-5-(2-pyridyl) pyrazole (fppz)H, [32] and their isoquinolyl derivatives, 3-tert-butyl-5-(1-isoquinolyl) pyrazole (bipz)H and 3-trifluoromethyl-5-(1-isoquinolyl) pyrazole (fipz)H [33] were prepared according to the literature procedures. All reactions were conducted under an N 2 atmosphere by using anhydrous solvents or solvents treated with an appropriate drying reagent.…”

Organic Semiconductor Electroluminescent Materials

“…The inadequate evaporation and serious thermal degradation resulting from the ionic nature makes cationic Ru(II) complexes unsuitable for wide application in PHOLEDs. However, a breakthrough has been achieved by Tung et al [179,184], who synthesized charge-neutral Ru(II) complexes which can be used in PHOLED by the vacuum-deposition method. An optimized device using (ifpz) 2 Ru (PPh2Me) 2 (ifpz = 3-trifluoromethyl-5-(1-isoquinolyl) pyrazolate) doped in CBP as EML reaches an EQE of 7.03 %, luminous efficiency of 8.02 cd/A, power efficiency of 2.74 lm/W at 20 mA/cm 2 , and CIE coordinates of (0.67, 0.33).…”

Synthesis, crystal structure, photoluminescence and theoretical studies of a series of copper(I) compounds based on imidazole derivatives

“…The phosphorescence compounds of the earlier studies focused on noble Ir(III), Pt-(II), and Ru(II)complexes [8][9][10][11][12]. In recent years, phosphorescent Cu(I) complexes receive much attention due to relatively abundant resource and nontoxic property, which make these complexes very applicable in solar energy conversion, biological probing, and organic lightemitting devices (OLEDs) [13][14][15][16][17][18].…”