NUCLEAR CHEMISTRY
▸ Determining nuclear weapons yields from bomb debris Whether for treaty monitoring or historical interest, sometimes it is necessary to evaluate the yield of a nuclear weapons test long after the fact. A new method that involves measuring stable molybdenum isotopes could get around the time sensitivity of current approaches, which include monitoring seismic signals and measuring short-lived radionuclides (Proc. Natl. Acad. Sci. USA 2016, DOI: 10.1073/pnas.1602792113). Researchers from Los Alamos National Laboratory used mass spectrometry to measure the amounts of different molybdenum isotopes in debris samples from the 1945 Trinity nuclear weapons test, the world’s first. 95Mo and 97Mo are stable radionuclide daughters in the decay chains from plutonium fission, therefore the test debris has elevated 95Mo/96Mo and 97Mo/96Mo levels compared with natural abundances. The team used the molybdenum ratios and the amount of plutonium in the debris to calculate that the yield of the Trinity test was equivalent to 22,100 metric tons of TNT; the official Department of Energy estimate is 21,000 metric tons of TNT. Although the paper implies that the approach could be used to evaluate modern-era tests, the researchers declined to answer whether it could be used to evaluate underground tests, such as those done as recently as January by North Korea.—JYLLIAN KEMSLEY
CREDIT: NA N O L ET T . (SOLAR CELL); RAM SESHADRI (CRYSTAL STRUCTURE)
CHEMICAL BONDING
▸ Polyiodide chains resolve starch test mystery In the course of developing new semiconducting materials, a research team including Ram Seshadri and Fred Wudl of the University of California, Santa Barbara, has solved a historical puzzle regarding the structure of the starch-iodine complex (Angew. Chem. Int. Ed. 2016, DOI: 10.1002/ anie.201601585). Iodine dissolved in potassium iodide solution has long been used as a test for the presence
SOLAR POWER
Quantum dot solar cells hit record efficiency Researchers led by Edward H. (Ted) Sargent of the University of Toronto have built the first colloidal quantum dot solar cells certified to convert sunlight into electricity with greater than 10% power conversion efficiency (Nano Lett. 2016, DOI: 10.1021/acs.nanolett.6b01957). Although quantum dots have played a role in solar cells with efficiencies better than 11%, the nanoscopic spheres in those cases are used only as a light-absorbing material, says Oleksandr Voznyy, a researcher in Sargent’s group. The new cells use thin films of lead sulfide quantum
This SEM image shows how new record-setting quantum dot solar cells are built. dots deposited from solution as light-sensitive layers that also conduct electric charges between electrodes. Using these multitasking quantum dots unlocks simpler design approaches for this brand of solar cell, but the team will need to continue improving the efficiency to compete with or complement other promising materials, such as perovskites, Voznyy says. To beat the 10% benchmark, the researchers tweaked the surface chemistry of their quantum dots. The dots were initially capped with oleic acid to prevent clumping. But the organic compound traps charges, which undermines efficiency. By adding methylammonium iodide to the quantum dot solution before deposition, the team swapped out oleic acid for iodine, which inhibits charge trapping.—MATT DAVENPORT
of starch. The helical iodide chain that forms wraps around starch molecules (amylose) to create a complex that produces a deep purple color. Although scientists have speculated that polymeric iodide chains are involved, the exact structure has remained elusive. Chemists have reported other organic polyiodide complexes, but when characterized they have been found to have chain lengths shorter than
A crystal structure reveals that organiciodide complexes, such as this one of pyrroloperylene, contain helical polyiodide chains.
10 iodide units and contain molecular iodine (I2) within the chain. The perylene-iodine complex, for example, made up of stacks of the planar polycyclic aromatic hydrocarbon perylene interspersed with iodide strands, was one of the first known organic conducting materials. But like the starch-iodine complex, its structure has never been fully resolved either. Seshadri, Wudl, and coworkers prepared a similar conductive material made from an iodine solution with pyrroloperylene and were able to get a crystal structure. The team concludes that stacks of pyrroloperylene molecules are interspersed with helical polyiodide strands of infinite length with no neutral I2 units. The researchers have shown that the starch-iodine complex also contains these extended polyiodide chains.—STEVE RITTER JULY 11, 2016 | CEN.ACS.ORG | C&EN
9
Science Concentrates POLYMERS MOLECULAR ELECTRONICS
One-pot method recycles unwanted polycarbonates
▸ N-heterocyclic carbenes aminate silicon surfaces
In the classification scheme for recycling polymers, polycarbonates typically fall into the forsaken “other” category. That means millions of tons of polycarbonates produced each year for compact disks, headlight lenses, and other applications can’t be easily recycled by conventional methods. Gavin O. Jones, Jeannette M. Garcia, and coworkers at IBM O Research-Almaden H have come up with O O O OH a process that could n help avoid relegatPolycarbonate ing polycarbonates O O to landfills. Their S method allows the easy conversion K2CO3 F F of polycarbonates into poly(aryl ether O O sulfone)s in the presS ence of a carbonate H salt and a bis(aryl O O OH fluoride) (Proc. n Natl. Acad. Sci. USA Polysulfone 2016, DOI: 10.1073/ pnas.1600924113). In this reaction, a carbonate salt degrades The carbonate salt polycarbonate, with the reactive intermediates initiates depolymercondensed by a sulfone-containing aryl fluoride to ization of the polyform value-added poly(aryl ether sulfone). carbonate, and the sulfone-containing aryl fluoride condenses the resulting reactive phenoxide intermediates into poly(aryl ether sulfone)s, with the loss of carbon dioxide as a by-product. The researchers performed computational studies to determine that the carbonate salt’s role is twofold: It decomposes the polycarbonate by nucleophilic attack and promotes the reaction of subsequently formed phenolate dimers with the aryl fluoride.—CELIA ARNAUD
A synthetic approach based on N-heterocyclic carbenes provides a way to covalently link amines and aminals in close proximity to silicon surfaces. The technique could be useful for modulating the electronic properties of silicon chips used in the semiconductor industry. Jeremiah A. Johnson of Massachusetts Institute of Technology and coworkers developed the strategy in which they functionalize silicon by inserting the persistent carbene into Si–H surface bonds (J. Am. Chem. Soc. 2016, DOI: 10.1021/ jacs.6b04962). Silicon chips could already be aminated in other ways. But the carbene-insertion technique is the first to attach amine groups only one carbon away from the surface, rather than through a typically long spacer group. A one-carbon separation positions nitrogen close enough to the silicon surface to modify how readily electrons can escape, potentially easing customization for specific microelectronics applications. In addition, the well-controlled reactivity of carbenes reduces problematic side-reactions, and the technique derivatizes surfaces with better site-selectivity than some existing amine-insertion techniques. Johnson says his group plans to investigate the performance of these novel carbene-derived monolayers on silicon in solar cells and to explore substrates beyond silicon.—STU
BORMAN
REACTION MECHANISMS
▸ Arylation reaction fails ascorbic acid test Ascorbic acid is familiar to many organic chemists as a useful reducing agent to help promote reactions. For example, researchers have thought that ascorbic acid is needed in conjunction with tert-butyl nitrite to carry out C–H activation and radical arylation of heterocyclic N-oxides to make biaryl compounds. Richard A. J. Horan of GlaxoSmithKline and coworkers published a paper in Organic Process Research & Development last year expanding on the selective metal-free coupling reaction. But it wasn’t until the researchers later ran control experiments omitting ascorbic acid as part
10
C&EN | CEN.ACS.ORG | JULY 11, 2016
of their ongoing investigations that they discovered they could get the same results without it. “This was a complete surprise, as ascorbic acid is a well-precedented additive,” Horan says. “As a result, we thought it appropriate to retract our paper at the galley proof stage and investigate the mech-
Contrary to previous assumptions, ascorbic acid isn’t needed to promote this cross-coupling reaction. NH2 O O +
OH O
OH OH HO OH L-Ascorbic acid ONOC(CH3)3
O
O O
+N
O–
nism in greater depth” (Org. Process Res. Dev. 2015, DOI: 10.1021/acs.oprd.5b00231). The team has now published a new version of the paper, with a warning for fellow chemists to not take the role of ascorbic acid for granted (Org. Process Res. Dev. 2016, DOI: 10.1021/acs.oprd.6b00117). “This has highlighted the importance of mechanistic understanding in organic chemistry, and we hope that our work will encourage further research in this area,” Horan says. “In my mind, this case shows how science should work,” adds Kai Rossen, a research director at Sanofi and editor-in-chief of OPR&D. “The next +N question will be how the – O common assumptions about this chemistry will be corrected.”—STEVE RITTER
