SCIENCE & TECHNOLOGY CONCENTRATES
ELABORATING ON HETEROCYCLES
ADDING PEP TO PEPPSI The secret to the palladium-catalyzed cross-coupling reactions is often the judicious choice of catalyst ligand. Michael G. Organ’s group at York University, in Toronto, in collaboration with researchers at Dow Chemical and Eli Lilly & Co., have revealed a bit more of that secret by learning how to tweak the structure of a pyridine-N-heterocyclic-carbene-based ligand system called PEPPSI to carry out selective Negishi couplings of secondary alkyl groups with five-membered heterocyclic rings, a reaction not possible before (Angew. Chem. Int. Ed. 2015, DOI: 10.1002/ anie.201503941). Negishi coupling works for adding branched alkyl groups to sixmembered rings, but attempts to do the same with five-membered rings gave a mixture of linear and branched isomers. Organ’s group found that adjusting the structure of a commercial PEPPSI ligand they previously developed, by switching from isopentyl to isoheptyl substituents and adding chlorines, increases the bulk and alters the electronic properties to favor the desired branched isomers.—SR
Researchers have created an unexpectedly stable set of borylated dicarbonyl compounds that react to form borylated heterocyclic molecules that were previously difficult or impossible to make and that could be useful as building blocks in organic synthesis. The borylated dicarbonyls have multiple reactive groups in close O O proximity, leading chemists to think O O O they would break down too quickly OO O N2H4 to be useful as reaction intermediR B B N N ates. But Andrei K. Yudin of the N O CH3 N University of Toronto and coworkers H3C H O have found they are stable and can Borylated Borylated undergo double condensations that pyridazine dicarbonyl intermediate result in heterocyclic ring formation (Angew. Chem. Int. Ed. 2015, DOI: 10.1002/anie.201504271). The researchers make the dicarbonyl intermediates by combining photoredox catalysis and organocatalysis to couple brominated ketones with an N-methyliminodiacetic acid (MIDA)-boryl aldehyde. The stable dicarbonyls in turn react with nucleophiles to form MIDA-boryl pyrroles, furans, and pyridazines.—SB
mixture, water and the surfactant begin to separate from the organic solvent, forcing silver flakes to congregate into a conductive network at the ink’s surface. The surfactant also improves the silver’s affinity for the fluorinated polymer, allowing the ink to stay stretchy as it cures.—MD
CONDUCTIVE INK FOR ELASTIC ELECTRONICS
The ink used to print “Tokyo” remains conductive as it is stretched.
TUNABLE SLIM SEMICONDUCTORS
NAT. CO M M UN.
To create flexible electronics, researchers often have to make compromises. Boosting a device’s elasticity can mean incorporating materials that sap its conductivity. Nanostructured electronics can provide both flexibility and conductivity, but they tend to be difficult to fabricate and can be incompatible with certain soft substrates, such as textiles. A team of researchers led by Takao Someya of the University of Tokyo has now developed an ink that can be used to print highly conductive and elastic structures such as transistor arrays directly on a variety of materials (Nat. Commun. 2015, DOI: 10.1038/ncomms8461). Key to the ink’s versatility is a well-balanced chemical composition, which includes silver flakes, a fluorinated elastomer, a fluorinated surfactant, and an organic solvent. As the Tokyo team prints the
An international research team led by Chongwu Zhou of the University of Southern California has created a new family of layered, atomically thin black arsenic-phosphorous semiconductors (Adv. Mater. 2015, DOI: 10.1002/adma.201501758). The new materials possess tunable band gaps that span a previously unoccupied regime for atomically thin materials such as graphene, hexagonal boron nitride, and black phosphorus. An electronic material’s band gap represents the minimum energy input needed to allow charge carriers to conduct. The band gaps of the black arsenic-phosphorus materials correspond to the energy carried by long-wavelength infrared light and could be useful in applications including light radar (lidar), the team reports. The team created the layered semiconductors by heating arsenic and phosphorus compounds in the presence of a mineralizing agent. The researchers were CEN.ACS.ORG
41
JU LY 6, 2015
able to tune the band gap by controlling the ratio of the two elements in the final product. They then peeled off layers of black arsenic-phosphorus crystals using adhesive tape, getting sheets as thin as 1.3 nm.—MD
A QUANTUM DOT MINI SPECTROMETER Miniature spectrometers could make it easier to perform spectroscopy on the go. Jie Bao of Tsinghua University, in Beijing, and Moungi G. Bawendi of MIT have now developed a new way of creating one from an array of 195 filters made by printing different colloidal quantum dot inks on a microchip (Nature 2015, DOI: 10.1038/nature14576). The spectrometer, which covers the wavelength range of 390 to 690 nm, is the size of the charge-coupled device (CCD) sensor in a digital camera. Because each filter transmits light differently at different wavelengths, the intensity of light transmitted through the set can be combined to reconstruct the spectrum. Bao and Bawendi used their spectrometer to measure spectra generated by a white light source and a set of optical filters and to measure the emission spectra of fluorescent colloidal quantum dots. The researchers were able to resolve spectral peaks separated by 2 to 3 nm. They say the spectral range and resolution could be further improved by using additional quantum dot filters.—CHA
R
