Efficient Host Material Containing 1,3,5-Triazine Moiety for Red Phosphorescent Organic light-emitting diodes (OLEDs)

Abstract

Organic light-emitting diodes (OLEDs) using organic fluorescent dyes as emitting materials have multilayered structure of the following order in general: cathode/hole injection layer/hole transporting layer/emissive layer/electron-transporting layer/electron-injection layer/anode.1 The internal quantum efficiency of fluorescent dye, however, is limited up to 25% maximum because of the singlet exciton it utilizes.2 Phosphorescent organic light-emitting diodes have also received a great deal of attention because of their high internal efficiency (nearly 100%) from the utilization of both singlet and triplet excitons.3 In spite of a number of advantages in phosphorescence organic light-emitting diodes (PHOLEDs), several drawbacks still remain, most notably is concentration quenching of the emitter.4 Host-embeded phosphorescent emitters are generally used to avoid this defect. Additionly, energy transfer between host and dopant molecule is used in PHOLEDs to achieve high efficiency. 5Therefore, the development of suitable host materials in accordance with a dopant is considered major issue because the emission in the emissive layer depends on the energy matching of the host and guest molecule.Recently, a few report have demonstrated that triazine based heterocycles with bipolar character showed high efficiency in OLEDs performance.7 However, the number of triazine derivatives with bipolar character is very limited, 4,7 therefore, there is still a strong need for the development of triazine based compounds. Our interest focuses on the development of efficient host materials based on triazine because of their unique thermal and optical properties. During our ongoing efforts on the development of luminescent materials and host for PHOLEDs, 8 we have found that triazine based molecules, such as 2,4,6-tris(3-methyl-4-(thiophen-2-yl)phenyl)-1,3,5-triazine (TTPT) and 2,4,6-tris(3-methyl-4-(trimethylsilyl)phenyl)-1,3,5-triazine (TSPT) exhibit good glass-forming nature, low-lying HOMO energy and unique optical/electrochemical behaviors as compared to those of other triazine derivatives.4 Based on these facts, we envisioned that these molecules would potentially be good hosts for red guest in PHOLEDs. The facts have also prompted us to investigate electroluminescent (EL) characteristics of these materials. We herein report the synthesis, characterization and OLED performance of triazine-based compounds. Result and DiscussionStarting material 1 was synthesized by a slight modification of a previous synthetic methodology reported by Kostas et al., as shown in Scheme 1. (See experimental section)7 By cyclotrimerization of 4-bromo-3-methylbenzonitrile with trifluoromethanesulfonic acid in dry chloroform, compound 1 was obtained in a high yield. TTPT was then synthesized using a Suzuki coupling by the reaction of 2-bromothiophene with 2,4,6-tris(3-methyl-4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)-1,3,5-triazine and was obtained in good yields (~85%). All compounds were fully characterized by varied spectroscopic metho...