Pushing Extended p-Quinodimethanes to The Limit: Stable Tetracyano- Oligo(N-annulated Perylene)quinodimethanes with Tunable Ground States
Largely extended p-quinodimethanes with tunable ground states! In a recent Article published in JACS (http://pubs.acs.org/doi/abs/10.1021/ja402467y), A/P Wu Jishan and his local/international collaborators reported the synthesis of a series of stable tetracyano- oligo(N-annulated perylene)quinodimethanes nPer-CN (n = 1-6), with the longest molecule having 12 para-linked benzenoid rings! These oligomers showed tunable ground states, varying from a closed-shell quinoidal structure for monomer, to a singlet biradical for dimer, trimer and tetramer, and to a triplet biradical for pentamer and hexamer. Large two-photon absorption cross-section values were observed in the near infrared range, which also exhibited a clear chain-length dependence.
Oxyanion Hole Stabilization by C-H▪▪▪O Interaction in Transition State - A Three-Point Interaction Model for Cinchona Alkaloid-Catalyzed Asymmetric Methanolysis of Meso-Cyclic Anhydrides
Prof. Richard Wong and Dr. Yang Hui have reported in the Journal of the American Chemical Society (DOI: 10.1021/ja4005893) an unconventional type of C-H oxyanion hole in organocatalysts. Oxyanion holes are commonly found in many enzyme structures. They are crucial for the stabilization of high-energy oxyanion intermediates or transition states through hydrogen bonding. Typical functionalities found in enzyme oxyanion holes or chemically designed oxyanion-hole mimics are N-H and O-H groups. Through DFT calculations, the authors show that asymmetric methanolysis of meso-cyclic anhydrides (AMMA) catalyzed by a class of cinchona alkaloid catalysts involves an oxyanion hole consisting of purely C-H functionality. This C-H oxyanion hole is found to play a pivotal role for stabilizing the developing oxyanion, via C-H∙∙∙O hydrogen bonds, in their newly proposed three-point interaction transition state model for AMMA reactions, and is the key reason for the catalyst to adopt the gauche-open conformation in the transition state. They believe that the C-H oxyanion hole will serve as a general protocol for future applications in organocatalysis, drug development, anion sensing, supramolecular chemistry, crystal engineering and medicine.
"A Hydrothermal Anvil made of Graphene Nanobubbles on Diamond" by Prof Loh KP's team has been scheduled for publication in Nature Communications 10.1038/ncomms2579.
What is the surface chemistry between graphene and diamond ? Candy Lim, NGS student, as well as Prof Loh KP, Department of Chemistry and Graphene Research centre, report a new way to generate static pressure by encapsulating single crystal diamond with graphene membrane, the latter is well known for its superior nanoindentation strength and in-plane rigidity. Heating the diamond-graphene interface to the reconstruction temperature of diamond (~1,275 K) produces a high density of nanobubbles on graphene which can trap water within. At high temperature, the chemical bonding between graphene and diamond is robust enough to allow the hybrid interface to act as a hydrothermal anvil cell due to the impermeability of graphene, which prevents the superheated water from escaping. Most surprisingly, superheated water trapped within the pressurized graphene nanobubbles was observed to etch the diamond surface to produce a high density of square-shaped voids. The molecular structure of superheated water trapped in the bubble was probed using vibrational spectroscopy and dynamic changes in the hydrogen bonding environment was observed.
The Role of van der Waals Forces in the Performance of Molecular Diodes
Nijhuis and Thompson et al. describe in a Letter to Nature Nanotechnology (DOI: 10.1038/NNANO.2012.238) that a change of just one CH2 unit in the alkyl chain of a molecular diode –a self-assembled monolayer of molecules with a thickness of exactly one molecule of the form S(CH2)nFc with Fc = ferrocene on a Ag bottom-electrode contacted with a liquid metal top-contact – is crucial in the device performance. Optimizing the van der Waals packing energies between the molecules, which are often considered to be weak and ignored, by only 0.5 kcal/mol by changing one CH2 unit, resulted in 10 times better performance of molecular diodes by lowering the leakage currents, 10% increase of yield in working devices, and increased the reproducibility 2-3 times. These results demonstrate that weak intermolecular interactions, besides strong interactions, such as, covalent bonds, hydrogen bonds, or p-p interactions, have to be an important part of the rational design of future organic devices.
Benzazepines are well-known structural design elements in medicinal chemistry. Angew. Chem. (ACIE; DOI: 10.1002/anie.201208076) by Prof. Wang Jian and Mr. Wang Lei, reports an efficient synthesis of benzazepine heterocycles.
The method utilizes simple and readily available isatins and alkynes, and employs direct Pd(II)-catalyzed oxidative cycloaddition. The heterocycles are well tolerated in the reaction, which allows access to a number of unique molecular structures. The significance of benzazepine scaffold as structural elements should render this method attractive for both synthetic and medicinal chemistry, paving the way for efficient synthesis of other complex biologically active heterocyclic systems.
Professor Loh Kian Ping won the American Chemical Society (ACS) Nano Lectureship award in 2013, being one of the three winners selected from Asia, Europe and USA. The award will be conferred in China Nano 2013. He has been conferred the Fellowship of the Royal Society of Chemistry (UK).