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Noteworthy Chemisty

Tetrathienylethylenes emit deep blue light in the solid state. Tetraphenylethylene (TPE) has been extensively studied because it is the archetype of a luminogen that undergoes aggregation-induced emission (AIE). Tetraethienylethylene (TTE, 1) is an analogue of TPE that has five-membered thienyl rings in place of six-membered phenyl rings. TTE is expected to be another AIE generator with photophysical—especially luminescent—properties that result from the peculiar electronic properties of the thiophene ring.

C. Baldoli, E. Licandro, and colleagues at the University of Milan and at the CNR–Institute of Molecular Science and Technologies (Milan) developed a straightforward synthetic route to make a variety of nonsymmetrical TTE derivatives (29). The compounds contain functional groups that are incompatible with the conditions of the McMurry reaction that is usually used to make nonsymmetrical TTEs.

TTE is similar to TPE in that it has AIE behavior: It does not luminesce in solution, but it becomes emissive in the aggregate state. In contrast to TPE, however, solid TTE emits deep blue light at room temperature. This emission is 50 nm blue-shifted from its emission as a frozen glass at 77 K.

The emission properties of 29 are similar to those of 1. These findings demonstrate how a change in the molecular structure affects the emission behavior of an AIE generator. (Eur. J. Org. Chem. 2013, Early View; Ben Zhong Tang)

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Tetraethienylethylene and eight derivatives

What holds paper together? Paper, a centuries-old synthetic material, traditionally is used for information exchange and food storage. Recently, paper uses have been extended to flexible electronics, actuators, and sensors.

Despite paper’s widespread use and history, some of its mechanical properties, especially the bonding between pulp fibers, are not understood. Using atomic force microscopy (AFM), C. Teichert and colleagues at the University of Leoben and Graz University of Technology (both in Austria) explored the factors that determine the strength of paper on the nanometer scale.

Microscopically, paper consists of extensive networks of bonded cellulosic fibers, but the types of interactions that govern the mechanical strength of paper remain unclear. The authors separated dry cross-bonded fiber pairs (top fiber and lower fiber) and mounted them on a sample holder with nail polish. Load was then applied to the lower fiber by using a cantilever connected through a cantilever chip to the AFM. By analyzing plots of force versus distance throughout the bond failure process and in the broken-bond area, the authors identified the bridging effect of fibrils as a critical factor for the strength of paper.

Modifying pulp fibers by beating them in a mill significantly improves paper strength. Beating produces longer dangling fibrils that promote mechanical interlocking and bridging, and thus increases the number of fiber–fiber bonds. This understanding may lead to improvements in paper manufacturing processes. (Sci. Rep. 2013, 3, No. 2432; Xin Su)

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Stereoselectively convert tertiary alcohols to isonitriles. The bimolecular nucleophilic substitution (SN2) reaction is a well-known organic chemical transformation that predictably results in inversion of configuration at the targeted carbon atom. A drawback of the reaction is that nucleophilic attack on tertiary carbon atoms fails to give a reaction or produces a mixture of isomers.

S. V. Pronin, C. A. Reiher, and R. A. Shenvi* at the Scripps Research Institute (La Jolla, CA) report an SN2 reaction of tertiary alcohols that produces alkyl isonitriles and amines with high stereoselectivity. The reaction mimics the biosynthetic pathway to marine terpenoids.

The key components of the authors’ method are the use of the Lewis base Me3SiCN as the source of isonitrile, esters such as trifluoroacetates that provide Lewis-basic leaving groups, and a coordinating Lewis acid [Sc(CF3SO3)3] (see figure). Optical purities of up to 90% are obtained; and proximal unsaturation and Lewis-base substituents have little or no effect on stereoselectivity. With basic substituents, however, higher catalysts loadings are required.

Conversion of tertiary alcohols to isonitriles with inversion of configuration

The authors used their technique to prepare some marine terpenes in good yields. Cyclohexane substrates, however, gave low diastereomeric ratios. The isonitrile products can be hydrolyzed to produce amines.

The authors propose a mechanism that is based on the coordination of Sc(CF3SO3)3 with Me3SiCN before it activates the ester carbonyl group. The method is chemoselective for tertiary alcohols in presence of primary or secondary alcohols. The authors used it to transform terrestrial terpenoids into their marine counterparts. (Nature 2013, 501, 195–199; José C. Barros)

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Here’s how wind influences the mechanical properties of silk. I-M. Tso and collaborators at Tunghai University (Taichung), National Chung-Hsing University (Taichung), the Industrial Technology Research Institute (Hsinchu), and the National Synchrotron Radiation Research Center (Hsinchu, all in Taiwan) probed the relationship between various crystalline fractions in major ampullate (MA) silk from the spider Cyclosa mulmeinensis and measured the mechanical response of the silk samples to wind stimulation.

There was no significant variation in amino-acid sequences in the evaluated silk samples. Exposure to wind treatment resulted in lower β-sheet density and smaller crystallite dimensions. But greater alignment of β-sheet domains along the fiber axis enhanced the silk’s ultimate mechanical properties.

Unlike the supercontracted state of the native spider silk, the wind-treated silk’s strength and extensibility increased simultaneously. The authors believe that these enhancements result from changes in crystalline β-sheet density, secondary structure, and chain alignment of crystalline and amorphous regions. (Biomacromolecules 2013, 14, 3484–3490; LaShanda Korley)

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Silicon nanowires form flexible, transparent, free-standing sheets. Current flexible, transparent electronic-device components are largely the domain of organic light-emitting diodes, but these materials are expensive and have limited life spans. Silicon, the mainstay of the conventional semiconductor industry, is brittle and opaque at the macro scale; but it becomes highly flexible at the nanoscale. Nanostructured silicon may be used in applications such as flexible, touch-sensitive display screens and wearable electronic devices.

C. Wang and co-workers at Sun Yat-sen (Zhongshan) University (Guangzhou, China) developed a simple method for making free-standing sheets of interlocking 10-nm–diam silicon nanowires. They heated metastable SiO powder in a vertical high-frequency induction furnace at 1600 ºC under a stream of high-purity argon for 1 h [(a) in the figure]. The carrier gas transports the resulting silicon and SiO2 vapors to the low-temperature zone of the graphite furnace tube. The denser SiO2 forms a powder on the upper inner wall of the tube. The silicon vapor is carried to the upper opening of the tube, where it forms nanowires [(b) and (c) in the figure].

Schematic of apparatus and formation of silicon nanowires and Li+ electrodes

Because the authors found no evidence of metal catalysts or impurities on the tips of the nanowire nuclei, they believe that the growth of the nanowires is assisted by a small amount of SiO2 that forms a matrix at the opening of the tube.

Each wire consists of a crystalline core surrounded by an amorphous sheath. The carrier gas orients the nanowires in the direction of the gas flow, and the wires spontaneously interlock with each other as they grow. This process forms a self-supporting cylindrical network structure ≈2 cm high and ≈2 cm diam. The sheets are flexible, transparent, and highly porous; and they could serve as hosts to other functional materials.

Coating the nanowires with graphene [(d) in the figure] produces flexible, transparent lithium-ion battery electrodes with excellent lithium-storage performance, high storage capacity, and good cycling stability. The graphene sheaths insulate the nanowires from direct contact with the electrolyte. Graphene gives a slightly black color to the sheets, but it does not block light transmittance significantly. After the residual SiO2 is removed with HF, there is ample space between the silicon nanowires and their graphene sheaths to allow the nanowires to expand during lithiation. (Nano Lett. 2013, 13, 4708–4714; Nancy McGuire)

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Make aromatic nitriles from methylarenes. Aromatic nitriles are versatile organic compounds that are easily converted to a variety of synthetic building blocks. The conventional synthesis of aromatic nitriles usually requires toxic cyanide salts as the sources of the nitrile groups. Y. Zhang, J. Wang, and coauthors at Peking University (Beijing) and the Chinese Academy of Sciences (Shanghai) developed a method for preparing aromatic nitriles directly from the ammoxidation of methylarenes with t-BuONO.

The authors first explored the conversion of p-methylanisole to p-methoxybenzonitrile (see figure). They used t-BuONO as the oxidant and nitrogen source and obtained yields as high as 89% with 5 mol% Pd(OAc)2 and 30 mol% N-hydroxyphthalimide (NHPI) as the catalysts. The reaction was run with 3 equiv t-BuONO in MeCN at 80 ºC for 8 h. When they replaced Pd(OAc)2 with other Lewis acids, they produced only the corresponding aldehyde.

Nitrile synthesis without cyanide salts

This method is compatible with a wide range of substituted methylarenes and several polycyclic and heterocyclic methylarenes, with moderate-to-high yields (41–99%). The authors propose a mechanism in which t-BuONO decomposes to t-BuOH and a nitroxyl radical, which generates phthalimide N-oxyl (PINO) from NHPI. PINO reacts with the methylarene to form a benzylic radical that couples with another nitroxyl. The coupling product isomerizes to an aldoxime that, catalyzed by Pd(OAc)2, loses water to form the nitrile. (Angew. Chem., Int. Ed. 2013, 52, 10573–10576; Xin Su)

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