Soonnam Kwon

Carborane Photochemistry Triggered by Aryl Substitution: Carborane‐Based Dyads with Phenyl Carbazoles

A bright combination: a new type of donor-acceptor dyad, carbazolylaryl-substituted ortho-carboranes, which are conveniently prepared from the corresponding acetylenes and decaborane pathways, showed unique excited-state behavior associated with electron transfer unlike the meta- and para-counterparts.

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.

Asymmetric anthracene-based blue host materials: synthesis and electroluminescence properties of 9-(2-naphthyl)-10-arylanthracenes

A series of bulky aryl-substituted asymmetric anthracene blue host materials, 9-(2-naphthyl)-10-(3-(1-naphthyl)phenyl)anthracene, where phenyl was varied from H (5a), Me (5b), Ph (5e), and 1-Naph (5f) at the 6-position and Me (5c) at the 2-position, was synthesized by Suzuki coupling reaction between 10-(2-naphthyl)anthracene-9-boronic acid and 1-(3-iodophenyl)naphthalene derivatives. A less bulky aryl-substituted anthracene, 9-(2-naphthyl)-10-(2,5-diphenyl)phenyl)anthracene (5d), was also synthesized for comparison. All asymmetric anthracenes showed high glass transition temperatures in the range of 84–153 °C. The photophysical and electrochemical properties in solution showed that the substituent at the 10-positions of the anthracene unit did not influence the blue emission of 420 nm and HOMO–LUMO energy level (5.5–2.5 eV). However, a gradual decrease of bathochromic shift in solid state PL was observed from the increaseg of the substituent bulkiness, exhibiting 5e (18 nm) ≤ 5f (19 nm) < 5c (25 nm) ≤ 5b (26 nm) < 5a (30 nm) < 5d (41 nm), respectably. When 5a was used as a blue host material in the multilayered device structure of ITO/DNTPD/NPD/host:dopant (9%)/PyPySPyPy/LiF/Al, enhanced OLED device performance was observed, showing a luminous current efficiency of 9.9 cd A−1, power efficiency of 6.3 lm W−1 at 20 mA cm−2, deep blue color coordinates of (0.14, 0.18), and a 932 h device lifetime at L0 = 3000 cd m−2.

Effectiveness of nasal surgery alone on sleep quality, architecture, position, and sleep-disordered breathing in obstructive sleep apnea syndrome with nasal obstruction.

Background The goal of this study was to evaluate the effect of nasal surgery alone on sleep quality, architecture, position, and sleep-disordered breathing (SDB) (including obstructive apnea and snoring) in adult patients with obstructive sleep apnea syndrome (OSAS) and nasal obstruction. Methods A total of 22 consecutive male patients (mean age, 41.3 ± 10.9 years) with OSAS and nasal obstruction, who underwent nasal surgery alone, were enrolled in the study. We compared polysomnographic data related with sleep quality (sleep efficiency [SE] and arousal index [ArI]), sleep architecture (stages N [nonrapid eye movement], 1, 2, and 3, and R [rapid eye movement]), the distribution of sleep positions, and SDB (apnea–hypopnea index [AHI], apnea index [AI], minimum arterial oxygen saturation [SaO 2 ], and snoring) before and after nasal surgery. Results SE (from 86.6 ± 6.3% to 89.7 ± 7.1%; p = 0.039), stage R (from 15.3 ± 4.8% to 18.6 ± 5.4%; p = 0.016) and snoring (from 32.2 ± 16.4% to 25.8 ± 18.6%; p = 0.045) significantly changed after nasal surgery. However, there were no significant changes in ArI, other sleep stages, the proportion of sleep time spent in the supine position, AHI, AI, and minimum SaO 2 after nasal surgery. Conclusion Nasal surgery alone was partially effective in improving sleep quality, architecture, and snoring, but it had no effect on the change of the distribution of sleep positions and obstructive apnea in patients with OSAS and nasal obstruction.

Carborane Dyads for Photoinduced Electron Transfer: Photophysical Studies on Carbazole and Phenyl‐o‐carborane Molecular Assemblies

o-Carborane-based donor-acceptor dyads comprising an o-carboranyl phenyl unit combined with N-carbazole (1) or 4-phenyl-N-carbazole (2) were prepared, and their dyad characters were confirmed by steady-state photochemistry and photodynamic experiments as well as electrochemical studies. The absorption and electrochemical properties of the dyads were essentially the sum of those of the carbazole and o-carboranyl phenyl units; this indicates negligible interaction between the carbazole and o-carborane units in the ground state. However, the emission spectra of 1 and 2 indicated that carbazole fluorescence was effectively quenched and a new charge-transfer (CT) emission was observed in solvents, varying from hexane to acetonitrile, which exhibited large Stoke shifts. The CT emission properties of o-carborane-based dyads were further analyzed by using Lippert-Mataga plots to show that unit charge separation occurred to form a charge-separated species in the excited state, namely, 1⋅2. This excited-state species was confirmed by nanosecond transient absorption spectra and spectroelectrochemical measurements; the photoexcitation of carbazole generated the CT state in which a radical cation and anion were formed at the carbazole and o-carborane units, respectively, within a few nanoseconds. DFT calculations corroborated the presence of this CT species and showed localized populations of the highest singly occupied molecular orbital on 2 in the reduced anionic state. As a result, molecular assemblies formed by linking the carbazole group with the o-carborane cage through a phenylene or multi-phenylene spacer revealed that the photoinduced electron-transfer process occurred intramolecularly.

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.

The effect of energy level offset between Ir dopant and carbazole hosts on the emission efficiency

The interface energy offsets of various carbazole hosts and iridium(III) [bis(4,6-difluorophenyl) pyridinato-N,C2′]-tetrakis(1-pyrazolyl)borate (FIr6) and their influence on the efficiency were investigated. Vacuum level shifted by +0.12 eV for bis(4-(N-carbazolyl)phenyl)dimethylsilane (Me2SiCBP2) and FIr6 interface, which was negligible for other hosts and FIr6 interface. As a result, the hole injection from the host to the guest was almost barrier free to reach a high efficiency of 17.1% in the Me2SiCBP2 based device. It dropped to 14% when N, N′-dicarbazolyl-3,5-benzene (mCP) was used as the host, which is the reminiscent of the existence of a hole injection barrier (−0.16 eV).

Effects of intermolecular interaction on the energy distribution of valance electronic states of a carbazole-based material in amorphous thin films

Effects of intermolecular interactions on the occupied electronic structure of amorphous solid of a carbazole-based material were investigated under an assumption that the organic solid consists of randomly oriented assemblies of dimers. The electronic energy states were calculated on the ensemble of large number of random dimers, of which geometries are relaxed using semiempirical van der Waals density functional theory. Intermolecular interactions result in splitting of energy level, and further disorders occur by aggregation of randomly orientated molecules. As a result, frontier occupied energy states can be represented by a superposition of Gaussian distributions, including (i) a main distribution with full width at half maximum of 80-110 meV, depending on the methods of relaxation and (ii) shoulders separated from the center of the main distribution with a value as large as 150 meV. A possible origin for the appearance of these shoulders was ascribed to the presence of molecular assemblies consisting of more tightly bound dimers compared with the others.