Ho-Jin Son

Generation of Blue Light-Emitting Zinc Complexes by Band-Gap Control of the Oxazolylphenolate Ligand System: Syntheses, Characterizations, and Organic Light Emitting Device Applications of 4-Coordinated Bis(2-oxazolylphenolate) Zinc(II) Complexes

Color-tunable Zn(II) complexes of the type Zn( N,O-OPh (OxZ)ArX) 2 ( 5), where the ligand consists of an oxazolylphenolate ion connected at the 4-position by a 2,4-substituted aryl functional group with X = NMe 2 a, OMe b, Ph c, Cl d, F 2 e, and CN f, were prepared. X-ray structural studies of 5a, 5b, and 5e showed that a zinc atom was positioned in a distorted tetrahedral coordination environment created by two oxazolylphenolate ligands with N,O-chelation. Hammet plots of absorption and emission maxima, respectively, in UV and photoluminescence (PL) spectra with respect to electron-donating and electron-withdrawing groups of the substituents indicate a direct correlation between the highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) band gaps and electronic alterations at the ligand sites. A similar correlation was also observed for the reduction and oxidation potentials in cyclic voltammograms (CVs). A gradual increase in the HOMO-LUMO band gap is seen from electron-donating to electron-withdrawing functional groups, NMe 2 < OMe < Ph < Cl < F 2 < CN. An emission peak with a maximum at 455 nm was achieved when the most electron-withdrawing group (cyano) was applied to the oxazolylphenolate ligand system. Density-functional theory (DFT) calculations on the HOMOs and LUMOs for this series lead to a conclusion similar to that arrived at from a blue-shift trend observed in UV data and trends in the CVs. The 4-coordinated zinc complex ( 5c) was shown to be a potential blue-emitting material, exhibiting a maximum efficiency of 1720 cd/m (2) at 17 V with 0.3 cd/A in a multilayered device structure of ITO/NPB/ 5c/BCP/Alq 3/LiF/Al. On the basis of the low HOMO level of this series, 5a was tested as a hole-transporting material; this resulted in the successful fabrication of a multilayered device of ITO/ 5a/DPVBI/Alq 3/LiF/Al with an efficiency of 7000 cd/m (2) at 13 V with 2.0 cd/A.

Emission Color Tuning and Deep Blue Dopant Materials Based on 1,6-Bis(N-phenyl-p-(R)-phenylamino)pyrene

Panchromatic 1,6-bis(N-phenyl-p-(R)-phenylamino)pyrenes, 2R, were obtained from Buchwald-Hartwig coupling reactions between N-phenyl-p-(R)-phenylamines and 1,6-dibromopyrene. The photophysical properties of 2R corresponded well to the electron-withdrawing and -donating nature of the diarylamine substituents, exhibiting a full color visible range between 454 and 620 nm. In particular, a deep blue 2CN showed a high radiative rate constant of 2.85 x 10(8) s(-1) with high emission quantum efficiency of 79%. Further applications of 2CN as a blue dopant were attempted using multilayer organic light-emitting devices. A maximum efficiency of 3.98 cd/A with CIE coordinates of x = 0.14, y = 0.10 were obtained.

Significance of hydrophilic characters of organic dyes in visible-light hydrogen generation based on TiO2.

A series of dyes were synthesized to examine the roles of the hydrophilic characteristics of R in sensitized hydrogen generation by dye-grafted Pt/TiO(2) under visible light irradiation. The hydrogen-generation efficiencies and optimum amounts of the dyes grafted to Pt/TiO(2) were affected substantially by the hydrophilic and steric effects of R; moderately hydrophilic DEO1 and DEO2 showed higher sensitization activity at a lower loading than hydrophobic D-H.

Stable Blue Phosphorescence Iridium(III) Cyclometalated Complexes Prompted by Intramolecular Hydrogen Bond in Ancillary Ligand

Improvement of the stability of blue phosphorescent dopant material is one of the key factors for real application of organic light-emitting diodes (OLEDs). In this study, we found that the intramolecular hydrogen bonding in an ancillary ligand from a heteroleptic Ir(III) complex can play an important role in the stability of blue phosphorescence. To rationalize the role of intramolecular hydrogen bonding, a series of Ir(III) complexes is designed and prepared: Ir(dfppy)2(pic-OH) (1a), Ir(dfppy)2(pic-OMe) (1b), Ir(ppy)2(pic-OH) (2a), and Ir(ppy)2(pic-OMe) (2b). The emission lifetime of Ir(dfppy)2(pic-OH) (1a) (τem = 3.19 μs) in dichloromethane solution was found to be significantly longer than that of Ir(dfppy)2(pic-OMe) (1b) (τem = 0.94 μs), because of a substantial difference in the nonradiative decay rate (knr = 0.28 × 10(5) s(-1) for (1a) vs 2.99 × 10(5) s(-1) for (1b)). These results were attributed to the intramolecular OH···O═C hydrogen bond of the 3-hydroxy-picolinato ligand. Finally, device lifetime was significantly improved when 1a was used as the dopant compared to FIrpic, a well-known blue dopant. Device III (1a as dopant) achieved an operational lifetime of 34.3 h for an initial luminance of 400 nits compared to that of device IV (FIrpic as dopant), a value of 20.1 h, indicating that the intramolecular hydrogen bond in ancillary ligand is playing an important role in device stability.

Electronic Alteration on Oligothiophenes by o-Carborane: Electron Acceptor Character of o-Carborane in Oligothiophene Frameworks with Dicyano-Vinyl End-On Group.

We studied electronic change in oligothiophenes by employing o-carborane into a molecular array in which one or both end(s) were substituted by electron-withdrawing dicyano-vinyl group(s). Depending on mono- or bis-substitution at the o-carborane, a series of linear A1-D-A2 (1a-1c) or V-shaped A1-D-A2-D-A1 (2a-2c) oligothiophene chain structures of variable length were prepared; A1, D, and A2, represent dicyano-vinyl, oligothiophenyl, and o-carboranyl groups, respectively. Among this series, 2a shows strong electron-acceptor capability of o-carborane comparable to that of the dicyano-vinyl substituent, which can be elaborated by a conformational effect driven by cage σ*-π* interaction. As a result, electronic communications between o-carborane and dicyano-vinyl groups are successfully achieved in 2a.

Turning on fluorescent emission from C-alkylation on quinoxaline derivatives.

Reduction on imine moiety (C=N) of quinoxalines by alkyl-/aryllithiums led to a geometrical change on the quinoxaline ring, thereby perturbing the electronic structure to turn on fluorescence emission. Such a structural change resulted in interrupted cyclic-ring systems with electron-donating amine (sp(3)-type) and electron-accepting imine (sp(2)-type) units bridged by a phenylene unit. Through either alkylation or arylation, a highly polarized electron donor-electron acceptor bipolar system was established in a single molecule with dramatically enhanced PL efficiency (up to 60%).

Fluorescence Control on Panchromatic Spectra via C-Alkylation on Arylated Quinoxalines

A coherent green fluorescence was obtained by butylation at the 2-position of panchromatic 2,3-diaryl-5,8-diarylquinoxalines (2) to give corresponding 2-butyl-2,3-diaryl-5,8-diaryl-1H-quinoxalines (3). Full color quinoxaline derivatives (2) were prepared from electronic modification at either the 2,3- or 5,8-positions at the peripheral ArX group or X group (X = -H, -OMe, -NPh(2), -NMe(2), -NMePh) of the quinoxalines. 2-Butylation converted one imine unit of the pyrazine ring to an amine group, which effectively altered the electron donor and acceptor functions to produce a coherent green fluorescence.

Intermolecular peripheral 2,5-bipyridyl interactions by cyclization of 1,1′-silanylene unit of 2,3,4,5-aryl substituted siloles: enhanced thermal stability, high charge carrier mobility, and their application to electron transporting layers for OLEDs

Two 2,5-bipyridyl substituted 1,1′-silanylene unit cyclized siloles with 1,1-silacyclopentyl- or 1,1-silacyclohexyl groups (3-Cy5 and 3-Cy6) were prepared from the intramolecular reductive cyclization of silacycloalkyl bis(phenylethynyl)silane with lithium naphthalenide and a a subsequent Pd-catalyzed cross-coupling reaction. Non-cyclized bipyridyl substituted dimethylsilole, 2,5-bis(2′,2′′-dipyridin-6-yl)-1,1-dimethyl-3,4-diphenylsilacyclopentadiene (PyPySPyPy), was also synthesized for comparison. All three siloles were characterized by X-ray structural studies. The results showed that the major contribution of three dimensional ordering found in crystal packing originated from distinctive intermolecular C–H⋯π interactions within the distance range 3.30–3.65 A. Even higher ordering was apparent in the crystal packing due to the additional plane-to-plane packing interactions between the peripheral bipyridyl units in cyclized siloles, 3-Cy5 and 3-Cy6. In accordance with such an increase in the peripheral intermolecular interactions, 3-Cy5 and 3-Cy6 exhibited higher Tg values (95 and 86 °C, respectively) than the acyclic analogue, PyPySPyPy (77 °C). In particular, 3-Cy5 showed a higher electron mobility of 6.9 × 10−4 cm2V−1 s−1 at E = 0.581 MV cm−1 in the solid films. As a result, enhanced OLED performance was observed when 3-Cy5 was used as an electron transporting layer in the multilayered device structure of ITO/PEDOT·PSS/NPB/Alq3/3-Cy5/LiF/Al, showing a maximum luminance of 17430 cd m−2 at 11.5 V and a current efficiency of 4.52 cd A−1 at 69.4 mA cm−2 with a turn-on voltage of 2.6 V.

Efficient light harvesting and energy transfer in a red phosphorescent iridium dendrimer.

A series of red phosphorescent iridium dendrimers of the type [Ir(btp)2(pic-PCn)] (Ir-Gn; n = 0, 1, 2, and 3) with two 2-(benzo[b]thiophen-2-yl)pyridines (btp) and 3-hydroxypicolinate (pic) as the cyclometalating and ancillary ligands were prepared in good yields. Dendritic generation was grown at the 3 position of the pic ligand with 4-(9H-carbazolyl)phenyl dendrons connected to 3,5-bis(methyleneoxy)benzyloxy branches (PCn; n = 0, 2, 4, and 8). The harvesting photons on the PCn dendrons followed by efficient energy transfer to the iridium center resulted in high red emissions at ∼600 nm by metal-to-ligand charge transfer. The intensity of the phosphorescence gradually increased with increasing dendrimer generation. Steady-state and time-resolved spectroscopy were used to investigate the energy-transfer mechanism. On the basis of the fluorescence quenching rate constants of the PCn dendrons, the energy-transfer efficiencies for Ir-G1, Ir-G2, and Ir-G3 were 99, 98, and 96%, respectively. The energy-transfer efficiency for higher-generation dendrimers decreased slightly because of the longer distance between the PC dendrons and the core iridium(III) complex, indicating that energy transfer in Ir-Gn is a Förster-type energy transfer. Finally, the light-harvesting efficiencies for Ir-G1, Ir-G2, and Ir-G3 were determined to be 162, 223, and 334%, respectively.

Sterically Less‐Hindered Half‐Titanocene(IV) Phenoxides: Ancillary‐Ligand Effect on Mono‐, Bis‐, and Tris(2‐Alkyl‐/arylphenoxy) Titanium(IV) Chloride Complexes

A series of mono-, bis-, and tris(phenoxy)-titanium(IV) chlorides of the type [Cp*Ti(2-R-PhO)(n)Cl(3-n)] (n=1-3; Cp*=pentamethylcyclopentadienyl) was prepared, in which R=Me, iPr, tBu, and Ph. The formation of each mono-, bis-, and tris(2-alkyl-/arylphenoxy) series was authenticated by structural studies on representative examples of the phenyl series including [Cp*Ti(2-Ph-PhO)Cl(2)] (1 PhCl2), [Cp*Ti(2-Ph-PhO)(2)Cl] (2 PhCl), and [Cp*Ti(2-Ph-PhO)(3)] (3 Ph). The metal-coordination geometry of each compound is best described as pseudotetrahedral with the Cp* ring and the 2-Ph-PhO and chloride ligands occupying three leg positions in a piano-stool geometry. The mean Ti-O distances, observed with an increasing number of 2-Ph-PhO groups, are 1.784(3), 1.802(4), and 1.799(3) A for 1 PhCl2, 2 PhCl, and 3 Ph, respectively. All four alkyl/aryl series with Me, iPr, tBu, and Ph substituents were tested for ethylene homopolymerization after activation with Ph(3)C(+)[B(C(6)F(5))(4)](-) and modified methyaluminoxane (7% aluminum in isopar E; mMAO-7) at 140 degrees C. The phenyl series showed much higher catalytic activity, which ranged from 43.2 and 65.4 kg (mmol of Ti x h)(-1), than the Me, iPr, and tBu series (19.2 and 36.6 kg (mmol of Ti x h)(-1)). Among the phenyl series, the bis(phenoxide) complex of 2 PhCl showed the highest activity of 65.4 kg (mmol of Ti x h)(-1). Therefore, the catalyst precursors of the phenyl series were examined by treating them with a variety of alkylating reagents, such as trimethylaluminum (TMA), triisobutylaluminum (TIBA), and methylaluminoxane (MAO). In all cases, 2 PhCl produced the most catalytically active alkylated species, [Cp*Ti(2-Ph--PhO)MeCl]. This enhancement was further supported by DFT calculations based on the simplified model with TMA.