Theis Brock-Nannestad

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.

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.

Single-Ion Anisotropy and Exchange Interactions in the Cyano-Bridged Trimers MnIII2MIII(CN)6 (MIII = Co, Cr, Fe) Species Incorporating [Mn(5-Brsalen)]+ Units: An Inelastic Neutron Scattering and Magnetic Susceptibility Study

The electronic structures of the compounds K[(5-Brsalen)(2)(H(2)O)(2)-Mn(2)M(III)(CN)(6)].2H(2)O (M(III) = Co(III), Cr(III), Fe(III)) have been determined by inelastic neutron scattering (INS) and magnetic susceptibility studies, revealing the manganese(III) single-ion anisotropy and exchange interactions that define the low-lying states of the Mn-M(III)-Mn trimeric units. Despite the presence of an antiferromagnetic intertrimer interaction, the experimental evidence supports the classification of both the Cr(III) and Fe(III) compounds as single-molecule magnets. The value of 17(2) cm(-1) established from AC susceptibility measurements for a spin-reversal barrier of K[(5-Brsalen)(2)(H(2)O)(2)-Mn(2)Cr(CN)(6)].2H(2)O may be readily rationalized in terms of the energy level diagram determined directly by INS. AC susceptibility measurements on samples of K[(5-Brsalen)(2)(H(2)O)(2)-Mn(2)Fe(CN)(6)].2H(2)O are contrary to those previously reported, exhibiting but the onset of peaks below temperatures of 1.8 K at oscillating frequencies in the range of 100-800 Hz. INS measurements reveal an anisotropic ferromagnetic manganese(III)-iron(III) exchange interaction, in accordance with theoretical expectations based on the unquenched orbital angular momentum of the [Fe(CN)(6)](3-) anion, giving rise to an M(s) approximately +/-9/2 ground state, isolated by approximately 11.5 cm(-1) from the higher-lying levels. The reported INS and magnetic data should now serve as a benchmark against which theoretical models that aim to inter-relate the electronic and molecular structure of molecular magnets should be tested.

Combining metabolic engineering and biocompatible chemistry for high-yield production of homo-diacetyl and homo-(S,S)-2,3-butanediol

Biocompatible chemistry is gaining increasing attention because of its potential within biotechnology for expanding the repertoire of biological transformations carried out by enzymes. Here we demonstrate how biocompatible chemistry can be used for synthesizing valuable compounds as well as for linking metabolic pathways to achieve redox balance and rescued growth. By comprehensive rerouting of metabolism, activation of respiration, and finally metal ion catalysis, we successfully managed to convert the homolactic bacterium Lactococcus lactis into a homo-diacetyl producer with high titer (95mM or 8.2g/L) and high yield (87% of the theoretical maximum). Subsequently, the pathway was extended to (S,S)-2,3-butanediol (S-BDO) through efficiently linking two metabolic pathways via chemical catalysis. This resulted in efficient homo-S-BDO production with a titer of 74mM (6.7g/L) S-BDO and a yield of 82%. The diacetyl and S-BDO production rates and yields obtained are the highest ever reported, demonstrating the promising combination of metabolic engineering and biocompatible chemistry as well as the great potential of L. lactis as a new production platform.

Mixed valence radical cations and intermolecular complexes derived from indenofluorene-extended tetrathiafulvalenes

Engineering of mixed-valence (MV) radical cations and intermolecular complexes based on π-extended tetrathiafulvalenes (TTFs) is central for the development of organic conductors. On another front, redox-controlled dimerization of radical cations has recently been recognized as an important tool in supramolecular chemistry. Here we show that π-extended TTFs based on the indenofluorene core, prepared by Horner–Wadsworth–Emmons reactions, undergo reversible and stepwise one-electron oxidations and that the detectable, intermediate radical cation forms remarkably strong intermolecular MV ([neutral·cation]) and π-dimer ([cation·cation]) complexes with near-infrared radical cation absorptions. The radical cation itself seems to be a so-called Class III MV species in the Robin–Day classification. The formation of MV dimers was corroborated by ESR spectroelectrochemical studies, revealing two slightly different ESR signals upon oxidation, one assigned to the MV dimer and the other to the cation monomer. Crystals of the radical cation with different anions (PF6−, BF4−, and TaF6−) were grown by electrocrystallization. Conductance studies revealed that the salts behave as semiconductors with the hexafluorotantalate salt exhibiting the highest conductance. Using a custom-built ESR spectrometer with sub-femtomole sensitivity, the magnetic properties of one crystal were investigated. While the spin-to-spin interaction between radical cations was negligible, a high cooperativity coupling to the microwave field was observed – as a result of an exceptionally narrow spin line width and high spin density. This could have great potential for applications in quantum computation where crystalline spin ensembles are exploited for their long coherence times.

Heterobimetallic Nitride Complexes from Terminal Chromium(V) Nitride Complexes: Hyperfine Coupling Increases with Distance†

Terminal nitride complexes of rhenium, osmium and molybdenum can form complexes with either alkylating agents, Lewis acidic metal halides, or low-valent, coordinatively unsaturated metal complexes. The few reactions of this type with a first-row transition-metal complex are limited to vanadium. Recently, the nitride chemistry of the chromium(V) cation has been significantly expanded by introduction of a preparative route which is based on nitrogen transfer from [Mn(N)(salen)] (salen=N,N’-bis(salicylidene)ethylenediamine) to the chromium(V) cation. With a range of chromium nitride complexes at hand we have investigated their reactivity and found that nucleophilicity is a general property which can be observed during formation of imide complexes with, for example, the trityl cation, tris(pentafluorophenyl)boron, and methyl triflate. In addition we report that terminal chromium(V) nitride complexes coordinate through the nitride ligand to low-valent complexes of the platinum metals. These compounds are possible precursors to bimetallic nitride phases which are gaining in importance as heterogeneous catalysts in, for example, the Haber–Bosch process. Solutions of terminal chromium nitride complexes in noncoordinating solvents treated with electrophiles such as B(C6F5)3 or C(C6H5)3 + quickly yield intensely colored orangered or green solutions. The reactions proceed cleanly as shown by EPR spectra which display a signal from a single S= =2 spin species. Similar reactivity was observed in reactions with either [Rh(cod)Cl]2 or cis-[PtCl2(dmso)2] (cod= 1,5cyclooctadiene, dmso= dimethyl sulfoxide). Structures of some of these systems, characterized by single-crystal X-ray diffraction, are shown in Scheme 1. Experimental and crystallographic details such as ORTEP drawings andmetric parameters of complexes 1–5 (Scheme 1) are available in the Supporting Information (Tables S1 and S1a). Inspection of the structures reveals a number of general aspects: there is a strong propensity for the chromium center to increase its coordination number from five to six upon coordination of the nitride ligand. This propensity is expected and a consequence of the trans influence of either an imide or a bridging nitride ligand which is significantly lower than that of a terminal nitride ligand. Accompanying this, the displacement of Cr out of the plane spanned by the equatorial ligators is diminished from about 0.5 to about 0.2 . The Cr N bond length is elongated from 1.55 in the terminal nitride complexes to approximately 1.60–1.62 in the functionalized systems. Comparison of structure 1 with that of [Cr(N)(salen)] reveals that the metal–salen ligand bonds are significantly shorter when the nitride ligand is functionalized, as expected when two ligands compete for electron donation. However, for the systems derived from [Cr(N)(dbm)2] the situation is less clear (dbm= dibenzoylmethanolate). In complex 2 all the Cr–dbm bonds are longer than in the parent terminal nitride complex, while they are shorter or similar within the limits of uncertainty in complex 5. The B N and C N bonds in 1, 4, and 5 are unexceptional but the N Rh and N Pt bond lengths in 2 and 3 are at about 1.970 and 1.906 , respectively, and very short; the first value belongs to the top 5% of the shortest Rh N bonds and the second belongs to the top 1% of the shortest Pt N bonds. Table 1 compares the Pt N bond of 3 with Pt N bonds of other cis[PtCl2(dmso)L] structures. Scheme 1. Schematic representation of the chromium(V) imide and chromium(V) bridging-nitride complexes.

Frozen-solution magnetisation dynamics of hexanuclear oxime-based MnIII Single-Molecule Magnets

Frozen solution SQUID measurements of the hexanuclear Single-Molecule Magnets [Mn6O2(Et-sao)6(EtOH)6(Me2benz)2] (1) and [Mn6O2(Et-sao)6(EtOH)4(H2O)2(benz)2] (2) allow the molecular and solid state contributions to the magnetic properties to be quantified.

A Triptycene‐Based Approach to Solubilising Carbon Nanotubes and C60

We describe herein the synthesis of a triptycene-based surfactant designed with the ability to solubilise single-walled carbon nanotubes (SWNTs) and C(60) in water through non-covalent interactions. Furthermore, an amphiphilic naphthalene-based surfactant with the same ability to solubilise SWNTs and C(60) has also been prepared. The compounds synthesised were designed with either two ionic or non-ionic tails to ensure a large number of supramolecular interactions with the solvent, thereby promoting strong solubilisation. The surfactants produced stable suspensions in which the SWNTs are dispersed and the surfactant/SWNT complexes formed are stable for more than one year. UV/Vis/NIR absorption spectroscopy, TEM and AFM were employed to probe the solubilisation properties of the dispersion of surfactants and SWNTs in water.

Stabilizing Coordinated Radicals via Metal–Ligand Covalency: A Structural, Spectroscopic, and Theoretical Investigation of Group 9 Tris(dithiolene) Complexes

Proper assignment of redox loci in coordination complexes with redox-active ligands to either the metal or the ligand is essential for rationalization of their chemical reactivity. However, the high covalency endemic to complexes of late, third-row transition metals complicates such assignments. Herein, we systematically explore the redox behavior of a series of group 9 tris(dithiolene) complexes, [M(mnt)3]3– (M = Ir, Rh, Co; mnt = maleonitriledithiolate). The Ir species described comprise the first examples of homoleptic Ir dithiolene complexes. The enhanced metal–ligand covalency of the Ir–S interaction leads to remarkable reactivity of [Ir(mnt)3]3– and stabilization of mononuclear [Ir(mnt)3]2– complex ions as well as dimerized versions featuring weak, covalent, intermolecular S–S bonds. The dianionic Rh and Co analogues are, in contrast, highly unstable, resulting in the rapid formation of [Rh2(mnt)5]4– and [Co(mnt)2]22–, respectively. The synthesized complexes were studied by single-crystal X-ray diffraction, X-ray absorption spectroscopy, optical spectroscopy, magnetometry, density functional theory, and spectroscopy-oriented configuration interaction calculations. Spectroscopic and theoretical analyses suggest that the stability of [Ir(mnt)3]2– may be attributed to dilution of ligand radical character by a high degree of Ir 5d character in the singly occupied molecular orbital.