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Head-to-tail Fashion (head-to-tail + fashion)
Selected AbstractsLuminescence Properties of Aminobenzanthrones and Their Application as Host Emitters in Organic Light-Emitting Devices,ADVANCED FUNCTIONAL MATERIALS, Issue 3 2007M.-X. Yu Abstract A series of aminobenzanthrone derivatives, possessing a keto and an amino group on the aromatic ring, are synthesized and their photoluminescence (PL) and electroluminescence (EL) properties are studied in detail. These compounds emit strongly in solution and in the solid state, with the emission maxima in the range of 528,668,nm resulting from charge-transfer transitions from the amino group to the keto moiety. The emission wavelength depends greatly on the polarity of the solvent. A red shift of nearly 100,nm is observed from n -hexane to dichloromethane for each of these compounds. The PL quantum yields of these molecules also depend tremendously on the solvent. The values are between 88 and 70,% in n- hexane and decrease as the polarity of the solvent increases. The single-crystal X-ray diffraction data reveal that the aminobenzanthrone planes of these molecules stack in the crystals in an antiparallel head-to-tail fashion. This strong dipole,dipole interaction accounts for the observed red-shifted emissions of the aminobenzanthrone molecules in powders and in films relative to those in nonpolar solvents. Electroluminescent devices using aminobenzanthrone derivatives as the host emitters or dopants emit orange to red light in the range 590,645,nm. High brightness, current efficiency, and power efficiency are observed for some of these devices. For example, the device using N -(4- t -butylphenyl)- N -biphenyl-3-benzanthronylamine as the emitter gives saturated red light with a current efficiency of 1.82,cd,A,1, brightness of 11,253,cd,m,2, and Commission Internationale de l'Éclairage (CIE) coordinates of (0.64,0.36); the device using N -(2-naphthyl)- N -phenyl-3-benzanthronylamine as the emitter gives orange,red light with a current efficiency of 3.52,cd,A,1, brightness of 25,000,cd,m,2, and CIE coordinates of (0.61,0.38). [source] Supramolecular aggregation in 4,4,-bipyridin-1,1,-ium dichloride, 4,4,-bipyridin-1,1,-ium dinitrate and 4,4,-bipyridin-1-ium bromideACTA CRYSTALLOGRAPHICA SECTION B, Issue 5 2003Peter Abeta Iyere 4,4,-Bipyridin-1,1,-ium dichloride [C10H10Cl2N2 (I)] and 4,4,-bipyridin-1,1,-ium dinitrate [C10H10N4O6 (II)] have been prepared and the crystal structures determined at 90.0,(2),K. Molecules of (I) are linked by two chlorine-bridged, three-centered N,H,Cl hydrogen bonds into chains along the b axis. The chains are coupled by weak C,H,Cl interactions into a molecular ladder along the c direction. In (II) each nitrate is coordinated to four bipyridinium ions through the interplay of the N,H,O and C,H,O contacts, resulting in a three-dimensional zigzag sheet on the ab plane. The sheets stack along the c axis. In 4,4,-bipyridin-1-ium bromide monohydrate [C10H9N2+·Br,·H2O (III)] the bipyridinium ions are linked by three-center N,H,N, hydrogen bonds in a head-to-tail fashion to form chains along the b axis. The chains are linked by C,H,Br and C,H,OH2 into a three-dimensional framework. [source] The neutral cluster amminehexa-,2 -chlorido-,4 -oxido-tris(1,4,6-triazabicyclo[3.3.0]oct-4-ene)tetracopper(II)ACTA CRYSTALLOGRAPHICA SECTION C, Issue 6 2009Gina M. Chiarella The title compound, [Cu4Cl6O(C5H9N3)3(NH3)], is a neutral conformationally chiral cluster which crystallizes under the conditions described in this paper as a racemic conglomerate. It contains four CuII atoms in a tetrahedral coordination with a central O atom lying on a crystallographic threefold axis. Six chloride anions bridge the four CuII atoms. Three CuII atoms are bound by an N atom of a monodentate 1,4,6-triazabicyclo[3.3.0]oct-4-ene (Htbo) ligand and the remaining CuII atom is bound by a terminal ammine ligand. The geometry at each copper center is trigonal bipyramidal, produced by the bound N atom of Htbo or ammonia, the O atom in the axial position, and three chloride ions in the equatorial plane. The chloride anions form an octahedron about the oxygen center. The copper,ammonia bond lies along the crystallographic threefold axis, along which the molecules are packed in a polar head-to-tail fashion. [source] Features of homotetrameric molecular association in protein crystalsACTA CRYSTALLOGRAPHICA SECTION D, Issue 1 2009Uma V. Katre The crystal structures of proteins showing homotetrameric association, a common feature observed in many lectins, have been analyzed in order to understand the characteristics of tetrameric association in terms of the arrangement of subunits and their biological significance. The analysis could group the tetramer units into the following four categories. (i) Tetrahedral molecules, in which the four monomers form a nearly perfect tetrahedral arrangement. The angle between the axes of any two monomers is ,109°. (ii) Molecules that form a sandwiched dimer of dimers in which the two dimers are arranged perpendicular to each other, one upon the other. (iii) Planar molecules, in which the four monomers lie in one plane and the corresponding sides of adjacent monomers face in opposite directions. This can be considered as a flattened tetrahedral shape. (iv) Planar closed molecules, in which all four monomers lie in one plane arranged in a head-to-tail fashion in a square. The first group and its variant, the third group, are the most commonly found arrangements in crystal structures. Each arrangement has its own importance for biological function. Some tetrameric assemblies that deviate from the majority described above also have relevance to their biological function. [source] First look at RNA in l -configurationACTA CRYSTALLOGRAPHICA SECTION D, Issue 1 2004RNA in l -configuration Nucleic acid molecules in the mirror image or l -configuration are unknown in nature and are extraordinarily resistant to biological degradation. The identification of functional l -oligonucleotides called Spiegelmers offers a novel approach for drug discovery based on RNA. The sequence r(CUGGGCGG)·r(CCGCCUGG) was chosen as a model system for structural analysis of helices in the l -configuration as the structure of the d -form of this sequence has previously been determined in structural studies of 5S RNA domains, in particular domain E of the Thermus flavus 5S rRNA [Perbandt et al. (2001), Acta Cryst. D57, 219,224]. Unexpectedly, the results of crystallization trials showed little similarity between the d - and the l -forms of the duplex in either the crystallization hits or the diffraction performance. The crystal structure of this l -RNA duplex has been determined at 1.9,Å resolution with Rwork and Rfree of 23.8 and 28.6%, respectively. The crystals belong to space group R32, with unit-cell parameters a = 45.7, c = 264.6,Å. Although there are two molecules in the asymmetric unit rather than one, the structure of the l -form arranges helical pairs in a head-to-tail fashion to form pseudo-continuous infinite helices in the crystal as in the d -form. On the other hand, the wobble-like G·C+ base pair seen in the D-RNA analogue does not appear in the l -RNA duplex, which forms a regular double-helical structure with typical Watson,Crick base pairing. [source] Practical asymmetric synthetic route to 4,4,4-trifluoro-3-hydroxybutyrate: Head-to-tail and head-to-head crystallizations through double and single hydrogen bonds of hetero- and homochiral 4,4,4-trifluoro-3-hydroxybutyrophenonesCHIRALITY, Issue 9 2002Akihiro Ishii Abstract A practical asymmetric synthetic route to 4,4,4-trifluoro-3-hydroxybutyrophenone and the butyric acid phenyl ester is described using heterochiral crystallization through double hydrogen bonding assembly in head-to-tail fashion and sequential Baeyer-Villiger oxidation reaction by trifluoroperacetic acid. Chirality 14:709,712, 2002. © 2002 Wiley-Liss, Inc. [source] | ||||||||||