Science Concentrates ▸ Single-atom heat engine created A German team has fulfilled a prediction made decades ago by physicist Richard Feynman: a heat engine composed of a single atom (Science 2016, DOI: 10.1126/ science.aad6320). Heat engines, which convert thermal energy to mechanical work, have been used in various forms for several hundred years. Over the past decade, scientists have designed ever-smaller heat engines, with the smallest being composed of a single molecule. Now, the German team, led by Kilian Singer of
Radio-frequency electrodes (silver) heat this trapped calcium ion (blue), which is the core of a single-atom heat engine. the University of Kassel and Johannes Rossnagel of the University of Mainz, has trapped a calcium ion (40Ca+) and alternately cooled and heated it with lasers and electric fields. The temperature differences produced by heating and cooling caused the atom to oscillate harmonically in an axial direction, “similar to the flywheel of a mechanical engine,” the authors say. They envision a wide variety of future applications, such as single-atom refrigerators and pumps.—ELIZABETH WILSON
2-D MATERIALS
▸ Sniffing single molecules with graphene Electronic sensors can now sniff out single gas molecules by using graphene, according to a research team from the Japan Advanced Institute of Science & Technology (Sci. Adv. 2016, DOI: 10.1126/ sciadv.1501518). Graphene has previously shown itself capable of single-molecule sensitivity, but this earlier feat required
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C&EN | CEN.ACS.ORG | APRIL 18, 2016
ORGANIC SYNTHESIS
Cyclopropanes built by trimerization Among synthetic methods available to chemists, cyclotrimerization reactions are an efficient approach to assembling complex cyclic molecules in a single step from three simple building blocks. One limitation of the process is that
O
O
O
CuI/2,2´-bipyridine, peroxide
3 F
–6H
F
F O
known examples only allow synthesis of aromatic or heterocyclic compounds, such as the [2+2+2] F cyclotrimerization of alkynes or acetophenones to make substituted benzenes or of aldehydes to make trioxanes. Srimanta Manna and Andrey P. Antonchick of the Max Planck Institute of Molecular Physiology have now expanded the cyclotrimerization strategy to make cyclopropanes (Angew. Chem. Int. Ed. 2016, DOI: 10.1002/anie.201600807). The team stitched together a variety of substituted acetophenones (one example shown) using a copper iodide/2,2´-bipyridine catalyst and a peroxide oxidant. The [1+1+1] cascade reaction proceeds through a previously unknown radical pathway in which a copper enolate intermediate functionalizes unactivated C–H methyl bonds of two acetophenone molecules to form a diketone. The diketone subsequently couples with a third acetophenone molecule leading to the cyclopropane ring. Overall, the new method is counter to the way chemists typically think about making cyclopropanes from olefins.—STEVE RITTER
Adsorbed CO2 gas molecules scatter electrons in a graphene film, bumping up its resistance. high magnetic fields. Now, researchers have observed single carbon dioxide molecules with a graphene-based sensor that operates at modest, readily supplied voltages. This work could lead to compact, highly sensitive devices for personal environmental monitoring, says team member Jian Sun. Sun and his colleagues developed their sensor using an unorthodox architecture. Usually, graphene in a sensor lies flat on a substrate, the team says. In this geometry, however, interactions between graphene and its support can mask interactions between graphene and gas molecules. So the researchers angled a twoatom-thick graphene ribbon between two
electrodes at different heights to lift the ribbon away from its silicon dioxide substrate. When a voltage is applied between the electrodes, each adsorbed CO2 molecule scatters electrons in the graphene, creating discrete but discernible changes in its resistance, the team reports.—MATT
DAVENPORT
ORGANIC SYNTHESIS
▸ Gold catalysis with less fuss In an advance that simplifies gold redox catalysis, a research team led by A. Stephen K. Hashmi of Heidelberg University, in
MIZUTA LAB/JAIST (GRAPHENE SCHEMATIC); COURTESY OF JOHANNES ROSSNAGEL (HEAT ENGINE)
PHYSICAL CHEMISTRY
+
Cl N Au
BF4–
NANOMATERIALS
P
Nanowires keep white LEDs flexible
Arylgold(III) catalyst intermediate Germany, has discovered a means for carrying out visible-light-mediated reactions without the need for an auxiliary oxidant or a ruthenium or iridium photosensitizer. The additional reagents are typically needed to promote oxidation of gold(I) to gold(III) as part of the catalytic cycle. The Hashmi group determined that a phosphine gold(I) chloride catalyst can function as its own photosensitizer when irradiated with visible light. When paired with an aryldiazonium salt, the gold(I) species is oxidized to a gold(III) species that can complete the difunctionalization of alkynes to make α-aryl ketones (Angew. Chem. Int. Ed. 2016, DOI: 10.1002/anie.201511487). What’s more, by using a P,N-bidentate ligand, the researchers were able to isolate and obtain the X-ray crystal structure of the gold(III) species (shown above). Chemists have debated whether gold(III) intermediates are involved given gold’s high oxidation potential. The Heidelberg group’s effort provides the first direct evidence to satisfy that curiosity, and further studies show that the group’s approach is a general method for generating arylgold(III) complexes (Chem. Commun. 2016, DOI: 10.1039/c6cc02199a).—
Flexible light-emitting diodes allow designers to create wearable displays, flexible screens, and bendable biomedical devices. Today’s best technology for flexible light sources is organic LEDs. But OLEDs have relatively short lifetimes,
and bright ones aren’t very energy Bending this white-light efficient. Now, researchers have nanowire LED does not impede its shown the potential to overcome performance. those limitations by building flexible white LEDs out of a more robust, efficient inorganic semiconductor—gallium nitride (ACS Photonics 2016, DOI: 10.1021/acsphotonics.5b00696). Joël Eymery of France’s Alternative Energies & Atomic Energy Commission; Maria Tchernycheva of the University of Paris, Saclay; and colleagues grew GaN nanowires on a sapphire substrate and embedded them in polydimethylsiloxane laced with a commercially available phosphor, yttrium aluminum garnet doped with cerium. The team peeled the material from the substrate and sandwiched it between a silver nanowire mesh and a thin metal foil, which serve as electrodes. The device’s conversion efficiency—the ratio of electrons in to photons out—reached 9.3%. That’s low, but flexible devices’ efficiencies don’t need to be as high as those of general lighting applications, Eymery says. The devices could be bent to a radius of 5 mm without any reduction in performance.—NEIL SAVAGE, special to C&EN
STEVE RITTER
NATURAL PRODUCTS
▸ Fungi make isoquinolines
ACS PHOTONICS (FLEXIBLE DEVICE)
of Wisconsin, Madison, has discovered that the microorganism unexpectedly By peering at a mysterious biosynthetic gene cluster in the common compost heap makes a variety of isoquinolines. Scientists long thought that these natural prodfungus Aspergillus fumigatus, a team of reucts—based on a heterocyclic scaffold searchers led by Cornell O composed of a benzene ring fused University’s Frank to a pyridine ring—were produced C. Schroeder and OH primarily by plants. Isoquinolines Nancy P. Keller NH2 are incredibly useful scaffolds: of the UniHO Members of the family are versity (S)-Tyrosine used as anesthetics, Fungal pathway
Plant pathway
HO H OH N
HO OH
H
NH2 O
Fumisoquin A
OH
N
CH3O OH
(S)-Scoulerine
OCH3
Plant and fungi both build isoquinolines from the amino acid tyrosine through similar but evolutionarily independent mechanisms.
vasodilators, antifungal agents, and disinfectants. Researchers may want to mine other fungi—many of which also possess similar biosynthetic gene clusters as the one in A. fumigatus—for new, potentially useful isoquinolines or harness fungi to produce existing ones. The team found that the isoquinolines—called fumisoquin A, B, and C—made by A. fumigatus were synthesized from the amino acid tyrosine, through a sequence of phenol hydroxylation, N-methylation, and oxidative cyclization steps reminiscent of plant isoquinoline biosynthesis (Nat. Chem. Biol. 2016, DOI: 10.1038/nchembio.2061). However, the plant and fungal biosynthetic genes show no homology, suggesting that the ability to make these compounds evolved independently, yet settled on the same synthetic strategy—an example of convergent evolution.—SARAH EVERTS APRIL 18, 2016 | CEN.ACS.ORG | C&EN
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