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...
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