Michael Pittelkow

Organic Light‐Emitting Diodes from Symmetrical and Unsymmetrical π‐Extended Tetraoxa[8]circulenes

The understanding and development of molecular electronics depends on the discovery and exploitation of new pextended organic materials. One area of interest is the design of small molecules that are suitable as the fluorescent component in blue organic light-emitting diodes (OLEDs). OLEDs based on small organic molecules or conjugated polymers have been explored for applications in full-colour flat-panel displays, since the seminal work by the group at Kodak. Some classes of low-mass blue-emitting materials have been exploited, such as spirobifluorenes, oligofluorenes, siloles and distyrylarylenes, but the introduction of completely new structural motifs remain scarce. To this end, we report the acid-mediated condensation of a 2,3-diACHTUNGTRENNUNGalkyl-1,4-benzoquinone with 1,4-naphthoquinone to produce a series of highly soluble p-extended tetraoxa[8]circulenes (Figure 1) as well as the application of these materials in blue OLEDs. Tetraoxa[8]circulene (4 B) is a planar heteroaromatic compound formed in trace amounts by treating 1,4-benzoquinone with strong acids. The structure of the tetraoxa[8]circulene framework was deduced by means of mass spectrometry of the all-naphthalene tetraoxa[8]circulene (4 N) by Erdtman and Hçgberg, and was later confirmed by single-crystal X-ray crystallography. It became clear that when one side of the 1,4-benzoquinone was substituted, either by using 1,4-naphthoquinone or a 2,3-disubstituted1,4-benzoquinone, the tetraoxa[8]circulene was a major product upon acid treatment. More recently a series of liquid-crystalline tetraoxa[8]circulenes were synthesised by attachment of linear alkyl chains to the tetraoxa[8]circulene framework. That work also introduced a relatively mild method for the cyclisation reaction: treatment of the quinone with BF3·OEt2 in boiling CH2Cl2. The tetraoxa[8]circulene has been suggested as an intercalator for DNA. Rathore and co-workers used this synthetic protocol to prepare new stable radical cation salts based on tetraoxa[8]circulenes. From a synthetic and a materials point of view, the exploration of p-extended tetraoxa[8]circulenes is of interest. As the processing ability of large aromatic materials and polymers relies on solubility in organic solvents, the development of tetraoxa[8]circulenes based on 2,3-dialkyl-1,4-ben[a] Dr. C. B. Nielsen, T. Brock-Nannestad, Dr. T. K. Reenberg, Dr. P. Hammershøj, Dr. J. B. Christensen, Dr. M. Pittelkow Department of Chemistry, University of Copenhagen Universitetsparken 5, 2100 Copenhagen Ø (Denmark) E-mail : pittel@kiku.dk [b] Dr. J. W. Stouwdam Laboratory of Macromolecular and Organic Chemistry Eindhoven University of Technology, P. O. Box 513 5600 MB Eindhoven (The Netherlands) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201002261. Figure 1. Structure of symmetrical and unsymmetrical p-extended tetraACHTUNGTRENNUNGoxa[8]circulenes.

Biotin[6]uril Esters: Chloride-Selective Transmembrane Anion Carriers Employing C-H ... Anion Interactions

Biotin[6]uril hexaesters represent a new class of anionophores which operate solely through C-H···anion interactions. The use of soft H-bond donors favors the transport of less hydrophilic anions (e.g., Cl(-), NO3(-)) over hard, stongly hydrated anions (e.g., HCO3(-) and SO4(2-)). Especially relevant is the selectivity between chloride and bicarbonate, the major inorganic anions in biological systems.

Discovery of a cyclic 6 + 6 hexamer of D-biotin and formaldehyde

The discovery of receptors using templated synthesis enables the selection of strong receptors from complex mixtures. In this contribution we describe a study of the condensation of D-biotin and formaldehyde in acidic water. We have discovered that halide anions template the formation of a single isomer of a 6 + 6 macrocycle. The macrocycle (biotin[6]uril) is water-soluble, chiral and binds halide anions (iodide, bromide and chloride) with selectivity for iodide in water, and it can be isolated on a gram scale in a one-pot reaction in 63% yield.

Azatrioxa[8]circulenes: Planar Anti-Aromatic Cyclooctatetraenes

We describe herein the first synthesis of a new class of anti-aromatic planar cyclooctatetraenes: the azatrioxa[8]circulenes. This was achieved by treating a suitably functionalised 3,6-dihydroxycarbazole with 1,4-benzoquinones or a 1,4-naphthoquinone. We fully characterised the azatrioxa[8]circulenes by using optical, electrochemical and computational techniques as well as by single-crystal X-ray crystallography. The results of a computational study (NICS) suggest that the central planar cyclooctatetraene is anti-aromatic when the molecules are in neutral or oxidised states (2+), and that the corresponding dianions are aromatic. We discuss the aromatic/anti-aromatic nature of the planar cyclooctatetraenes and compare them with the isoelectronic tetraoxa[8]circulenes.

Diazadioxa(8)circulenes: Planar Antiaromatic Cyclooctatetraenes

In this paper we describe a new class of antiaromatic planar cyclooctatetraenes: the diazadioxa[8]circulenes. The synthesis was achieved by means of a new acid-mediated oxidative dimerization of 3,6-dihydroxycarbazoles to yield the diazadioxa[8]circulenes in high yields. The synthetic protocol appears to be general, and is a one-pot transformation in which two C-C bonds and two C-O bonds are formed with the loss of two molecules of water. We also present a detailed characterization of the optical and electrochemical properties of this new class of stable planar cyclooctatetraenes. The properties of the diazadioxa[8]circulenes are compared with the properties of isoelectronic tetraoxa[8]circulenes and azatrioxa[8]circulenes. We discuss the antiaromatic nature of the planar central cyclooctatetraene moiety. The antiaromatic nature of the planar cyclooctatetraenes was studied by using computational methods (NICS calculations), and these calculations reveal that the central eight-membered ring has antiaromatic character.

New organocatalyst scaffolds with high activity in promoting hydrazone and oxime formation at neutral pH.

The discovery of two new classes of catalysts for hydrazone and oxime formation in water at neutral pH, namely 2-aminophenols and 2-(aminomethyl)benzimidazoles, is reported. Kinetics studies in aqueous solutions at pH 7.4 revealed rate enhancements up to 7-fold greater than with classic aniline catalysis. 2-(Aminomethyl)benzimidazoles were found to be effective catalysts with otherwise challenging aryl ketone substrates.

Nucleus-independent chemical shift criterion for aromaticity in π-extended tetraoxa[8]circulenes

AbstractRecently synthesized π-extended symmetrical tetraoxa[8]circulenes that exhibit electroluminescent properties were calculated at the density functional theory (DFT) level using the quantum theory of atoms in molecules (QTAIM) approach to electron density distribution analysis. Nucleus-independent chemical shift (NICS) indices were used to characterize the aromaticity of the studied molecules. The tetraoxa[8]circulene molecules were found to consist of two antiaromatic perimeters (according to the Hückel “4n” antiaromaticity rule) that include 8 and 24 π-electrons. Conversely, NICS calculations demonstrated the existence of a common π-extended system (distributed like a flat ribbon) in the studied tetraoxa[8]circulene molecules. Thus, these symmetrical tetraoxa[8]circulene molecules provide examples of diatropic systems characterized by the presence of induced diatropic ring currents. FigureSpecial aromaticity of the tetraoxa[8]circulenes

Chemical Signals Turn On Guest Binding through Structural Reconfiguration of Triangular Helicates

Be my guest: The function of a system based on self-assembled Zn(II) complexes can be controlled by external chemical stimuli. The complexes are based on a C3 -symmetric ligand that forms a unique triangular triple helicate structure 1. Upon subcomponent substitution, 1 is able to transform into a triangular double helicate 2 which, unlike 1, can encapsulate guests.

From static to dynamic: escaping kinetic traps in hydrazone-based dynamic combinatorial libraries

Thermodynamic control over kinetically-trapped mixtures of hydrazone-based macrocycles is achieved by addition of an aromatic monohydrazide to generate dynamic combinatorial libraries (DCLs) of linear and macrocyclic oligomers.

Synthetic Receptors for the High-Affinity Recognition of O-GlcNAc Derivatives

Abstract The combination of a pyrenyl tetraamine with an isophthaloyl spacer has led to two new water‐soluble carbohydrate receptors (“synthetic lectins”). Both systems show outstanding affinities for derivatives of N‐acetylglucosamine (GlcNAc) in aqueous solution. One receptor binds the methyl glycoside GlcNAc‐β‐OMe with K a≈20 000 m −1, whereas the other one binds an O‐GlcNAcylated peptide with K a≈70 000 m −1. These values substantially exceed those usually measured for GlcNAc‐binding lectins. Slow exchange on the NMR timescale enabled structural determinations for several complexes. As expected, the carbohydrate units are sandwiched between the pyrenes, with the alkoxy and NHAc groups emerging at the sides. The high affinity of the GlcNAcyl–peptide complex can be explained by extra‐cavity interactions, raising the possibility of a family of complementary receptors for O‐GlcNAc in different contexts.