Yunkyoung Ha

Electrical properties of porphyrin-based switching devices

Abstract Molecular switching devices were reported utilizing Langmuir–Blodgett (LB) monolayer films containing the 5,10,15,20-Tetrakis-Octadecyloxymethylphenyl-Porphyrin-Zn(II) as a redox-active component. From current–voltage (I–V) characteristics, it was found that the devices exhibit outstanding switching diode and tunneling diode behavior at room temperature, which seems to be due to the asymmetric device structure. These electrical properties of the devices may be applicable to active components for the memory and/or logic circuits in the future.

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.

Theoretical study of Ir(III) complexes of fluorinated phenylbenzoquinoline as red phosphorescent material

In this study, Ir(III) complexes with fluorinated 4-benzoquinoline (pbq) ligands were designed and characterized theoretically. The Hartree–Fock (HF) method with the 3-21G(d) basis set and density functional theory (DFT) utilizing the B3LYP functional with the 6-31G(d) basis set were used for the geometry optimization and the energy level calculation of the ground state of these complexes, respectively. Excited triplet and singlet states are examined using the time-dependent density functional theory (TD-DFT). As a result, it was found that these complexes produced a pure red emission due to the elongated conjugation length. In particular, it is concluded that the Ir(III) complex with pbq-CF3 ligands exhibits the largest emission efficiency and emits light of the purest red wavelength.

Chlorine effect on- electroluminescence of Tb complexes

Abstract Terbium complexes are known to be the green-light-emitting materials. Electroluminescent (EL) characteristics of terbium complexes containing phenanthroline (Phen) and 3-chlorophenanthroline (Cl-Phen) as a ligand were evaluated, where the device structure of ITO/TPD/Tb complexes/Alq 3 or Bebq 2 /Li:Al was used. It was found that terbium complex containing Cl-Phen shows higher luminance efficiency compared to terbium complex containing Phen. Measurements of oxidation and reduction potential of these complexes have been carried out by using cyclic voltammetric method in order to explain the difference in luminance efficiency.

Luminescence Comparison of Iridium(III) Complexes Containing Symmetric vs. Asymmetric Quinolinate Ligands

We have focused our research on development of the iridium complexes, especially with respect to blue and red emission for OLED. Previously, we prepared bis-[2,3-bis(4-fluorophenyl)quinoxalinato]iridium(acetylacetonate) [Ir(2,3-dpqx-F2)2(acac)] where the main ligand, 2,3-dpqx-F2, itself is symmetric. The luminescence spectra of Ir(2,3-dpqx-F2)2(acac) revealed red emission with the peak at approximately 640 nm with the CIE coordinates of (0.684, 0.311). Recently, Johannes et al. reported the iridium complexes containing fluorophenylmethylquinoxaline derivatives (fpmqx) as asymmetric main ligands and investigated their luminescence properties. Herein, we synthesized symmetric Ir(2,3-dpqx-F2)2(AL) and asymmetric Ir(fpmqx)2(AL) and compared their emission patterns and characteristics where accetylacetonate (acac) and 2-(5-(trifluoromethyl)-2H-1,2,4-triazol-3-yl)pyridine (trzl-CH3) were introduced as ancillary ligands(AL). We investigated the emission properties of Ir(2,3-dpqx-F2)2(AL) and Ir(fpmqx)2(AL), and found that asymmetric Ir(fpmqx)2(AL) showed blue-shifted photoluminescence compared with symmetric Ir(2,3-dpqx-F2)2(AL). We also estimated the energy gap between the HOMO and LUMO of the complexes with electrochemical properties measured by cyclic voltammetry.

Synthesis and Luminescence Studies of Hydrocarbon-Branched Tris-Cyclometallated Iridium (III) Complexes

We synthesized the tris-cyclometallated iridium complexes containing the substituted styryl groups and their saturated analogs. As the styryl iridium complexes, Ir(F-ppy-4-CH=CHC6H4R)3 (where R = Me, NMe2, OMe) were prepared via direct functionalization of the methyl groups in the ppy (2-phenylpyridine) ligand at the iridium complexes. The corresponding saturated analogs, Ir(F-ppy-4-CH2CH2C6H4R)3 (where R = Me, NMe2), were synthesized during the two step reactions of the IrCl3·xH2O with F-ppy-4-CH=CHC6H4R via in situ hydrogenation. Their photophysical properties were investigated both in solution and in film. The longer π-conjugation in the cyclometallating ligands leads to the bathochromic shift in photoluminescence of their iridium complexes. Among R groups, the NMe2 end group had the strongest push-pull effect with the F group at the other end, and led to the effective control of the ILCT transition.

Synthesis of new boron complexes containing aromatic moieties and their application to organic electroluminescent devices

Abstract The boron complexes coordinated with the anion of 2,3-dihydroxynaphthalene (DHN) or 2,3-dihydroxyquinoxaline (DHQx) were synthesized and applied to the organic electroluminescent devices as a blue light emitting layer. Emission peaks were observed at 480 and 430 nm in photoluminescence spectra of (DHQx) 3 B and Ph 2 B(DHN), respectively. The electroluminescent devices containing the emitting layer of Ph 2 B(DHN) showed greenish blue luminescence with the maximum emission peak at 498 nm. The ionization potential, electron affinity and electrochemical gap of (DHQx) 3 B and of Ph 2 B(DHN) were investigated with cyclic voltammetry and found to be consistent with their UV–vis absorption spectral band gap.

Deep Blue Phosphorescence of the Iridium(III) Complexes Containing N-Heterocyclic Carbene Ligands

Previously, the iridium complexes containing N-Heterocyclic carbene (NHC) ligands have caught attention because of their phosphorescence for organic light-emitting diodes (OLEDs). It was reported that the use of strong-field ligands such as carbenes should result in an increase of the blue phosphorescent efficiency of their metal complexes. In this study, the new NHC ligands, 1-(4-fluorophenyl)-3-methyl benzimidazolate (fpmb) and 1-naphthyl-3-methyl benzimidazolate (nmb), were introduced to the iridium complexes to develop phosphorescent dyes for OLED. We investigated photoabsorption and photoluminescence (PL) properties of the iridium complexes and studied their bandgaps with cyclic voltammetry (CV). Deep blue and green phosphorescence was observed with these complexes, and the bandgaps between their highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals (LUMOs) were correlated with CV data. We also studied the electroluminescence (EL) properties of the new iridium complexes as possible phosphors for the application to OLED.

Synthesis and the Luminescent Study of the Iridium Complexes Containing 2,3-Diphenylquinoline Derivatives and the New Ancillary Ligand for OLED

We previously reported that Ir(6-F-2,3-dpqx-(OMe)2)2(acac) (6-F-2,3-dpqx-(OMe)2 = 6-fluoro-2,3-bis(4-methoxylphenyl)quinoxaline) exhibited photoluminescence(PL) and electroluminescence (EL) emission at 645 and 667 nm, respectively. To modulate the emission maxima toward the saturated red chromacity, a new main ligand and its iridium complexes were prepared, and their photophysical properties were investigated. As a main ligand, 6-OMe-2,3-dpqx-(OMe)2 (6-OMe-2,3-dpqx-(OMe)2 = 6-methoxy-2,3-bis(4-methoxylphenyl)quinoxaline), were designed and synthesized. Acetylacetonate (acac) and pyrazolonates (przls) were also chelated to the iridium center as an ancillary ligand. The resulting complexes prepared herein were Ir(6-OMe-2,3-dpqx-(OMe)2)2(acac), Ir(6-OMe-2,3-dpqx-(OMe)2)2(przl1) and Ir(6-OMe-2,3-dpqx-(OMe)2)2(przl2). The photoluminescence (PL) of these complexes were observed around 660 nm, and their electroluminescence maxima were observed around 650 nm. The PL investigation of the complexes in PMMA (PMMA = poly(methylmetacrylate)) for the polymer solution process revealed that the emission peaks became broader with the range of 600∼850 nm than those of the complexes only.

The New Iridium(III) Pyridyltetrazolate Complexes for Blue Phosphorescence

A series of new blue-phosphorescent iridium complexes containing phenypyridine (ppy) derivatives and pyridyltetrazole were synthesized and their photophysical and electroluminescent properties were investigated. In the complex, the phenyl moiety which is mostly the highest occupied molecular orbital (HOMO) site was modified with the electron-withdrawing groups F and CF3. By doing so, the emission maxima were expected to shift hypsochromically due to the HOMO level decrease. Furthermore, addition of the modest electron donating group, CH3, to the pyridyl moiety in the ligand could slight raise the lowest unoccupied molecular orbital (LUMO) level of the complex. Therefore, the energy gap increase of the main ligands, phenylpyridine derivatives, might lead to blue emission of their iridium complexes. Previously reported pyridyltetrazole, an ancillary ligand, were also introduced to the iridium complexes for efficient blue phosphorescence and charge balance. The complexes prepared herein exhibited the blue emission at 467 and 486 nm with a shoulder peak at the longer wavelengths, respectively. We also investigated the luminescence properties of the complexes in a polymer film of PMMA (poly(methylmetacrylate)) for their application to the solution process. The photoemission of the complex in PMMA showed similar pattern with that of the complex itself.