Gui Youn Park

Iridium Complexes Containing Three Different Ligands as White OLED Dopants

Previously, we studied the luminescence characteristics of Ir(ppy)2(F2-ppy) and Ir(ppy)2(piq-F), which are heteroleptic iridium complexes involving two kinds of ligands, where F2-ppy, ppy and piq-F represent 2-(2,4-difluoro-phenyl)-pyridine, 2-phenylpyridine and 1-(4′-fluoroyphenyl)isoquinoline, respectively. Photoluminescence (PL) spectrum of Ir(ppy)2(F2-ppy) showed an emission peak at 495 nm in bluish green area. Ir(ppy)2(piq-F) showed two peaks at 513 nm and 600 nm in the PL spectrum. In order to make a white phosphorescent emitter for OLED, we herein designed a heteroleptic iridium complex containing three different ligands. We thought that reaction of F2-ppy, ppy and piq-F ligands all together with Ir(acac)3 might lead to Ir(F2-ppy)(ppy)(piq-F) among the mixture, displaying red and bluish green emission simultaneously. Ir(F2-ppy)(ppy)(piq-F) and other double-heteroleptic complex mixture are prepared from various concentration combination of ligands. The heteroleptic iridium complex mixture displays a variety of emission color, depending upon the combination ratio of ligands in the iridium complex synthesis. UV-vis absorption and photoluminescence (PL) spectra of the mixtures are analyzed and compared with the double-heteroleptic complexes. The white-yellow emission is observed when the mole ratio of ligands is 5:1:3 for F2-ppy, ppy and piq-F, respectively.

Heteroleptic Iridium(III) Complexes with Phenylpridine and Diphenylquinoline Derivative Ligands

The synthesis and photophysical study of efficient phosphorescent iridium(III) complexes having two different (C ˆ N) ligands are reported. In order to improve the luminescence efficiency by avoiding triplet-triplet (T-T) annihilation, the iridium complexes, Ir(ppy)2(dpq) and Ir(ppy)2(dpq-3-F), are designed and prepared where ppy, dpq and dpq-3-F represent 2-phenylpyridine, 2,4-diphenylquinoline and 2-(3-fluorophenyl)-4-phenylquinoline, respectively. Since Ir(ppy)3, Ir(dpq)3 and Ir(dpq-3-F)3 can be placed in the metal-to-ligand charge transfer (MLCT) excited state, they absorb light effectively. Thus, ppy ligands and a dpq derivative can act as a source of energy supply. When Ir(ppy)2(dpq-3-F) is placed in the lowest excited state, the excitation energy is not quenched nor deactivated but quickly transferred intramolecularly from two ppy ligands to one luminescent dpq-3-F ligand. Such transfer can occur because the triplet energy level of ppy is higher than that of dpq-3-F and light is emitted from dpq-3-F ligand in the end. Thus, Ir(ppy)2(dpq-3-F) shows strong photoluminescence from dpq-3-F ligand. To analyze luminescent mechanism, we calculated these complexes theoretically by using computational method.

Synthesis and Photophysical Studies of Heteroleptic Tris-Cyclometalated Ir(III) Complex for Red OLED

The synthesis and photophysical study of efficient phosphorescent iridium(III) complex having two different (C∧N) ligands are reported. In order to improve the luminescence efficiency by avoiding triplet–triplet (T–T) annihilation and study luminescent mechanism of the heteroleptic iridium complex, Ir(piq-3F)2 (dpq-4F) is designed and prepared where piq-3F and dpq-4F represent 1-(4′-fluorophenyl)isoquinoline and 2-(3′-fluorophenyl)-4-phenylquinoline, respectively. We thought that both piq-3F and dpq-4F ligands can act as a source of energy supply because of the relative HOMO energy level and the luminescence lifetime of homoleptic Ir complexes Ir(piq-3F)3 Ir(dpq-4F)3. However the UV-vis absorption spectra and the PL spectra of Ir(piq-3F)2 (dpq-4F) is more similar to that of Ir(piq-3F)3. In spite of lower triplet energy level of dpq-4F, the excitation energy is not intramolecular transferred from two piq-3F ligands to one dpq-4F ligand. The luminescence occurs mainly at piq-3F ligand. It is due to the strong MLCT characteristic of piq-3F ligand. To analyze luminescent mechanism, we calculated these complexes theoretically by using computational method.

Iridium(III) Complexes with 6-Pentafluorophenyl-2,4-Diphenylquinolines for Red OLEDs

Novel red electrophosphorescent devices were fabricated by doping perfluorophenyl substituted iridium(III) complex, bis[1-(6-Pentafluorophenyl-2,4-diphenyl)quinolinato-N,C2′] iridium(III) (acetylacetonate) [Ir(PF-dpq)2(acac)] for the application in organic light-emitting diodes (OLEDs). The maximum electroluminescent (EL) wavelengths of Ir(dpq)2(acac) and Ir(PF-dpq)2(acac) have shown at 614 nm and 620 nm, respectively. The device using Ir(PF-dpq)2(acac) showed red emission with 1931 CIE chromaticity coordinates (x = 0.640, y = 0.342) at 12 V. The perfluorophenyl substituent on the quinoline ring as electron withdrawing group decreased the lowest unoccupied molecular orbital (LOMO). As a result, the energy gap is reduced, leading to red-shift the emission wavelength. The ab initio calculation using the time-dependent density function theory (DFT) showed in agreement with the experimental results. However, the iridium complex of PF-dpq underwent a weak MLCT transition because of the weak coupling between the 5d-orbital of the iridium atom and HOMO of the substituted ligand. Thus, the luminous efficiency of the device using Ir(dpq)2(acac) and Ir(PF-dpq)2(acac) are 4.36 cd/A and 3.13 cd/A, respectively, at the current density of 3.38 mA/cm2 and 1.11 mA/cm2.

Ab Initio Study of Substituted Pyrenes for Blue Organic Light-Emitting Diodes

Luminescence efficiency of pyrene molecule is very low because of the aggregation effect of planar pyrene molecules. However, 1,3,6,8-tetra-substituted pyrenes with large electron donating group were reported to give a bright blue fluorescence [1]. In this paper, 1,6-bi-substituted and 1,4,6,9-tetra-substituted pyrenes as well as 1,3,6,8-tetra-substituted pyrenes were studied to find out the possibilities as the blue fluorescent materials of organic light-emitting diodes (OLEDs) [2-4]. Geometrical and electrical calculations were performed by ab initio methods. HF/3-21G(d) basis set was used for the geometry optimization of the ground electronic states of those compounds. The geometry of the low-lying excited electronic state was optimized using configuration interaction with single excitation (CIS) method. The vertical and adiabatic transition energies were calculated by time-dependent density functional theory (TD-DFT) using the B1LYP functional with 6-31G(d) basis set. From calculational results, it was explained that the change in fluorescence wavelength was affected by the position and the number of substituents, through analyzing the change of energy levels of the highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals (LUMOs) of pyrene. Some of substituted pyrenes showed possibilities as stronger fluorescent materials. New efficient emitting materials for OLEDs were proposed from the calculation results obtained by tuning the position, the number of substitution and the species of substituting moiety.