Electrophosphorescent Light Emitting Devices Using New Iridium(III) Complexes
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Electrophosphorescent Light Emitting Devices Using New Iridium(III) Complexes Rupasree Ragini Das, Chang-Lyoul Lee, Jang-Joo Kim* Kwangju Institute of Science and Technology, Department of Materials Science and Engineering, Kwangju, Korea ABSTRACT We have synthesized a new tris-orthometalated Ir(mpp)3 and several bis-orthometalated Ir(III) complexes of 3-methyl-2-phenylpyridine(Hmpp). The influence of the methyl substituent in Hmpp and different ligand environments on the optical, electrochemical and electroluminescence properties of the orthometalated complexes are examined by varying the donor acceptor properties. This includes several uninegative bidentate ligands consisting of different organic functional groups. Absorption, excitation and emission spectra in solution and solid state of these complexes agree with a strong electronic excitation to 1MLCT in Ir(mpp)3 and preferably to 3MLCT in other complexes. These complexes demonstrate strong phosphorescence at room temperature with a lifetime of 1-2 µs. Multilayer organic light emitting devices using these complexes as the triplet emitter are studied using PVK (poly(9-vinyl carbazole)), CBP (4,4’-N,N’-dicarbazole-biphenyl) and TAZ (3-phenyl-4-(1’-napthyl)-5-phenyl1,2,4-triazole) as the host materials. Ir(mpp)3 exhibits quite different behavior in different host materials. INTRODUCTION Ir(III) complexes with the orthometalating ligands constitute one of the important paradigms of the device configuration for their potential of harnessing 100% internal quantum yield. The investigation of the devices using these complexes not only monitors the inorganic excited processes[1] but also necessitates the chemical modulation of the structure of the complexes by extensive, coherent synthetic approaches[2,3] in order to tune the color[3,4,5], mode of energy transfer [5,6], inhibition of aggregation and luminescence quenching [2, 3], and above all device performance. Role of substituent is largely manifested in the electronic effects on the σdonor and π-acceptor properties of these heterocyclic ligands as regards to the ground and excited state properties of the complexes. Structural modulation by chemical rough tuning brings about major changes in the luminescence character of a complex[7], whereas fine-tuning by the introduction of substituents in the ligand is capable of causing small alterations in absorption and emission spectra without grossly changing the basic nature of the transitions responsible for these spectra. But it can reduce the deactivation of the emitting state and increase the quantum yield[8,9]. Steric interactions originating from a substituent are also anticipated to affect the energies of the excited states involving π-symmetry ligand orbitals owing to a change in the delocalization between the rings. In order to evaluate and emphasize the effect of the nature and position of the substituent upon both the ground- and excited-state properties, the present report aims at designing a new tris-orthometalated Ir(III) complex as a triplet emitter in phosph
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