New heteroleptic cyclometalated iridium(III) complexes containing 2-(2′,4′-difluorophenyl)-4-methylpyridine for organic light-emitting diode applications

Two phosphorescent iridium(III) complexes (dfpmpy)₂Ir(ppc) and (dfpmpy)₂Ir(prz) [dfpmpy=2-(2′,4′-difluorophenyl)-4-methylpyridine, ppc=pipecolinate, prz=2-pyrazine carboxylate] were synthesized from the reaction of the chloro-bridged dimeric complex [(dfpmpy)₂Ir(μ-Cl)]₂ and the ancillary ligand. Their structures and photoluminescence properties were investigated and device performances for application in organic light-emitting diodes (OLEDs) were studied. The complexes adopt a distorted, octahedral geometry around the iridium metal, exhibiting cis C-C and trans N-N arrangements. The photoluminescent (PL) properties reveal that (dfpmpy)₂Ir(ppc) emits in the blue-green region (λ_max=497 nm), whereas (dfpmpy)₂Ir(prz) shows red phosphorescence (λ_max=543 nm) in the film state (5% wt. doped in PMMA). The (dfpmpy)₂Ir(ppc)- and (dfpmpy)₂Ir(prz)-based OLEDs exhibited sky-blue and greenish-yellow electroluminescence with similar current-voltage characteristics, repectively. Maximum current efficiency of (dfpmpy)₂Ir(ppc) and (dfpmpy)₂Ir(prz) were 4.4 and 7.4 cd/A, respectively. Maximum luminance values were approximately 10,000 cd/m² for the both compounds.     

Synthetic studies on perophoramidine and the communesins: construction of the vicinal quaternary stereocenters

An efficient synthetic strategy for installation of the two vicinal quaternary carbon centers of the communesins is reported. Key steps include the O-allylation/Claisen rearrangement of spirolactone systems, which are formed by tandem intramolecular Heck cyclization/carbonylation. Substituent and solvent effects on the stereochemical outcome of the Claisen rearrangements have been examined. The stereochemical assignment of the allyl spirolactone previously reported as 17 has now been revised to 31, which has the communesin relative configuration at the quaternary carbons. Key C-allyl spirolactone 59 bearing functional handles required for the communesin core has been constructed with a 9.8:1 diastereomer ratio.     

Highly diastereoselective epimerization: stereodivergent synthesis of alpha-hydroxy-beta-amino isopentanoic acid

The high diastereoselectivity of the base-catalyzed epimerization of oxazolidin-2-ones 7 and 8 is shown to depend on the nature of the N-substituent (R group); when R = Bn, the 4,5-trans-product (4S,5R)-9 is formed, whereas when R = H the 4,5-cis-product (4S,5S)-10 is formed, both with >99:1 dr. The successful hydrolysis of the oxazolidin-2-one group in both cis- and trans-derivatives show this to be a stereodivergent route to enantiopure alpha-hydroxy-beta-amino isopentanoic acids (2R,3S)-1 and (2S,3S)-2.     

Strategy-proofness and public good provision using referenda based on unequal cost sharing

The paper studies strategy-proof cost sharing rules for public good provision based on referenda with different threshold quotas. By appropriately relaxing the assumptions of individual rationality and anonymity we provide a complete characterization of the family of quota rules with (possibly) unequal pricing. We prove that these quota rules are the only cost sharing rules satisfying four conditions: strategy-proofness, non-bossiness, weak continuity and weak anonymity. In addition, the specification of the degree to which individual rationality may be violated results in the selection of a specific “quota” for the referendum. While all these rules are “almost” always efficient when providing the public good and they are also almost everywhere coalitionally strategy-proof, only one family of rules from this class satisfies these two properties everywhere. The rules satisfying these two properties are Moulin’s Conservative Equal Costs Rule and unequal cost sharing variants of Moulin’s rule.     

Rigidity of minimal submanifolds in hyperbolic space

We prove that if an n-dimensional complete minimal submanifold M in hyperbolic space has sufficiently small total scalar curvature then M has only one end. We also prove that for such M there exist no nontrivial L ² harmonic 1-forms on M.     

Relationship between defects and optical properties in Er-doped GaN

The density of the vacancy-type defect in Er doped GaN was measured by positron annihilation spectrometry (PAS) and the correlation between the intensity of the Er-related luminescence was studied. A luminescence peak at 558 nm originating from ⁴S_3/2 to ⁴I_15/2 transition of Er³⁺ was observed in Er-doped GaN. The intensity of the luminescence increased with increasing Er concentration and showed the maximum with the Er concentration of around 4.0 at%. The PAS measurements showed that the vacancy-type defect density increased with increasing Er concentration up to 4 at%, and around 4 at% of Er, the formation of defect complex such as V_Ga–V_N was suggested. The contribution of the defect to the radiative recombination of intra-4f transition of Er is discussed.     

Selective area growth of GaN nanorods on patterned W/SiO2/Si substrates by RF-MBE

We report on the selective area growth (SAG) of GaN nanorods on Si substrates masked with W or SiO₂ and also on bare Si substrates by RF plasma-assisted molecular beam epitaxy (RF-MBE). The growth of GaN (i.e. irradiation of Ga and RF plasma-activated N₂) on the W mask layer results in the appearance of a ring reflection high-energy electron diffraction (RHEED) pattern coming from α-W. In contrast, broken ring RHEED patterns from GaN nanorods are clearly observed on SiO₂ and Si surfaces. Ex-situ scanning Auger microscopy analysis confirms that no growth of GaN takes place on W. Utilizing this phenomenon, we have demonstrated the SAG of GaN nanorods on Si substrates partly masked with W. We will discuss this phenomenon in terms of the difference in the desorption energy of Ga on W, SiO₂, and Si.     

Local structural,magnetic, and optical properties of Zn1−xFexO thin films

High quality Zn_1−xFe_xO thin films were deposited on α-sapphire$\alpha \text{-sapphire}$ substrates by RF magnetron sputtering. X-ray absorption fine structure measurements showed that the chemical valence of Fe ions in the films was a mixture of 2+ and 3+ states, and Fe ions substituted mainly for the Zn sites in the films. DC-magnetization measurements revealed ferromagnetic properties from 5 to 300 K. The photoluminescence measurements at 15 K showed a sharp main transition peak at 3.35 eV along with a broad impurity peak at 2.45 eV. The structural and magnetization analyses of the Zn_1−xFe_xO films strongly suggested that the ferromagnetism was the intrinsic properties of the films.     

Synthesis of arginine-containing hydroxamate dipeptidomimetics

The syntheses of arginine-containing hydroxamates using various peptide coupling methods are described. Fmoc-Arg(NO₂)-Cl prepared at low temperature did not undergo intramolecular δ-lactam formation and effectively provided desired hydroxamates (8 and 10) in good yields. Fmoc and N-nitro protecting groups can be easily removed. Therefore, this report provides a facile synthesis of arginine-containing peptidomimetic compounds.     

Quencher-free molecular beacon systems with two pyrene units in the stem region

We appended pyrene units covalently onto adenosine and uridine nucleosides (forming A^P and U^P units, respectively) and then incorporated them into oligonucleotides such that they were positioned in complementary locations in opposite strands in the middle positions of hairpin stems. Systems 1 (A^PU^P) and 3 (A^PA^P) individually exhibit aromatic stacking between the opposing pyrene units in the stems of their hairpins and display in their spectra the photophysical properties of strongly red-shifted bands; in contrast, the U^PU^P system 2 exhibits quenching spectra. Systems 1 (A^PU^P) and 3 (A^PA^P) behave as effective molecular beacons (MBs) that change color from green to blue upon duplex formation, whereas 2 (U^PU^P) is an effective MB that changes the intensity of its fluorescence upon forming its perfectly matched duplex.