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Noteworthy Chemistry

June 24, 2013

[We welcome Nancy McGuire to Noteworthy Chemistry. Dr. McGuire was formerly associate editor of the ACS publication Chemical Innovation.—Ed.]

Tensile-strained quantum dots luminesce below the bulk band gap. Many III–V semiconductors have band gap energies in the 0.6–1.0 eV range, slightly above the mid-IR region. Lowering the band gap energies by ≈0.5 eV would make these materials usable in mid-IR optoelectronic devices for detecting trace gases, in vivo diagnostics, environmental monitoring, and other applications. Growing self-assembled quantum dots (SAQDs) under 4% tensile strain could reduce the band gap energy by the desired amount, but difficulties in producing defect-free tensile dots hamper efforts to use this phenomenon.

P. J. Simmonds, M. L. Lee, and coauthors at Yale University (New Haven, CT), the University of California, Los Angeles, and the University of Arkansas (Fayetteville) propose a model for dislocation-free self-assembly of zinc blende–structured SAQDs under tensile strain that uses (110) and (111) crystal surfaces as substrates. They previously used this model to grow optically inactive tensile GaP nano-islands on GaAs(110). They now report the growth of optically active tensile GaAs SAQDs on In0.52Al0.48As/InP(110) with a band gap energy ≈0.2 eV less than the bulk value (see figure).

Lowering band gap by straining quantum dots

The authors used molecular beam epitaxy to grow the GaAs quantum dots, which have a 3.7% tensile lattice mismatch with the substrate. The authors believe that this is the first reported instance of photoluminescent emission from type I quantum dots in which the peak energy is lower than the bulk band gap. This energy decreases with increasing temperature and dot size.

The authors predict that dislocation-free, tensile self-assembly is possible on the (110) and (111) surfaces of all zinc blende and related materials, including III–V and group IV semiconductors. Because GaAs and germanium have almost the same lattice constant, the InAlAs buffers used here may also provide an important template for growing germanium SAQDs under ≈3.7% tensile strain. (ACS Nano 2013, 7, Article ASAP; Nancy McGuire)

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Turn cotton into an electrochemical supercapacitor. Cotton, first cultivated 7000 years ago, is one of the major sources of textile products. Because it is composed mainly of cellulose fibers, cotton is an excellent raw material for flexible, highly conductive synthetic fibers. L. Qu and co-workers at the Beijing Institute of Technology prepared a flexible, high-rate electrochemical supercapacitor from natural cotton.

The authors first formed natural cotton into 0.5 mm–thick cotton mats, which they annealed at 1000 ºC for 1 h under flowing argon. The carbonized mats (CCMs) are 0.4 mm thick and highly conductive, with a sheet resistance of <5 Ω/sq. The CCMs consist mainly of carbon (97 atom%) in the form of helical and twining tubular fibers; they have minimal oxygen content and maintain the flexibility of the original mats.

An all-cotton–derived electrochemical supercapacitor (allC-EC) is prepared by using two pieces of CCM as electrodes separated by a 0.4 mm–thick cotton mat. The assembly spontaneously absorbs 1 M Na2SO4 solution as the electrolyte. It is packed with plastic foils to make a prototype device. Cyclic voltammetry measurements showed that the allC-EC forms efficient electric double layers with high-rate capability.

The area-specific capacitance of the allC-EC is 0.7 mF/cm2 at a current density of 11.2 µA/cm2. Its mass-specific capacitance is 12–14 F/g, similar to capacitors made from carbon nanotubes and graphenes. The allC-EC features fast ion transport within the electrodes, with an estimated series resistance of 6 Ω. The allC-EC’s mechanical flexibility allows it to be arbitrarily deformed into a variety of shapes without affecting its performance as a supercapacitor. (Phys. Chem. Chem. Phys. 2013, 15, 8042–8045; Xin Su)

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A sila[7]helicene emits strong circularly polarized luminescence. Helicenes are chiral frameworks that consist of ortho-fused aromatic rings with screw-shaped structures. Helicenes’ optical properties, such as optical nonlinearity and circular dichroism, have been widely studied; their luminescence, however, has been investigated much less. Incorporating heteroatoms is expected to give helicenes new properties, but synthesizing heterohelicenes is challenging.

K. Nozaki and coauthors at the University of Tokyo, the Tokyo University of Agriculture and Technology, the Japan Science and Technology Agency (Saitama), Fukuoka University, the Tokyo University of Science (Chiba), the National Institute of Materials Science (Ibaraki), and Nara Institute of Science and Technology (all in Japan) developed a simple synthetic route to a silole-fused [7]helicene.

Last step of luminescent sila[7]helicene synthesis

The sila[7]helicene (1) is easily prepared by a Lewis acid–catalyzed double cyclization. Incorporating silole into the helicene framework produces strong circularly polarized luminescence. The sila[7]helicene has a high solid-state fluorescence quantum yield (17%) and a large dissymmetric factor (3.5 × 10–3). (Org. Lett. 2013, 15, 2104–2107; Ben Zhong Tang)

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What do you do when Suzuki and Negishi couplings don’t work? A. Witt, A. Minidis, and co-workers at AstraZeneca (Södertälje, Sweden) developed a synthetic route for scaling up the manufacture of a glycogen synthase kinase-3β inhibitor. In one reaction step, the standard palladium-catalyzed cross-coupling of metalated 1-(tetrahydro-2H-pyran-4-yl)-2-trifluoromethyl-1H-imidazole with 2,4-dichloro-5-fluoropyrimidine gave poor results under various combinations of solvent, ligand, and base.

As an alternative, the authors used the Ziegler coupling reaction. They metalated the imidazole derivative with n-hexyllithium and then treated it with 2-chloro-5-fluoropyrimidine in THF. The intermediate lithio-dihydropyrimidine was quenched with HOAc–EtOAc, and water was added to form two layers.

The organic layer was solvent-switched to MeCN, catalytic Cu(OAc)2 and Et3N were added to the solution, and the dihydropyrimidine was aromatized by bubbling 5% oxygen in nitrogen through the solution. The desired coupling product was obtained in 61% yield after it was treated with ammonia to reduce the copper concentration from >2500 ppm to <50 ppm. (Org. Process Res. Dev. 2013, 17, 672–678; Will Watson)

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Do engineered nanoparticles poison algae? Engineered nanoparticles (ENPs)—those that are designed and produced intentionally rather than unintentionally or naturally—are used in numerous consumer products, for example, as antibacterial coatings. However, their fate and behavior in the environment, especially in aquatic ecosystems, is largely unknown.

Their small size gives ENPs unique physicochemical characteristics and versatile applications; but these characteristics can also affect living organisms by inhibiting photosynthesis, changing behavior and reproduction, and affecting the oxidation of lipids, proteins, and DNA. The hydrophilicity or hydrophobicity of ENPs affects their bioavailability and biological uptake and is a key factor in their ability to reach coastal waters.

A. Quigg and coauthors at Texas A&M University (Galveston and College Station), the University of California at Merced, and Nanjing University (China) reviewed the current information about the effects of ENPs on freshwater and marine algae. Surface algae account for about half of all photosynthetic activity, making them a driving force in sequestering CO2 from the atmosphere. About half of their photosynthetic production is released into the ocean as exopolymeric substances (EPS) to contribute to the dissolved organic carbon pool.

The researchers found that although the surface properties of ENPs control their aggregation behavior, and ionic strength controls their dissolution, their toxicity is determined by EPS produced by the algae. EPS production reduces the bioavailability and toxicity of ENPs and their ions, not by affecting aggregation or solubility, but by changing the stability of the ENPs. When algae are exposed to ENPs, EPS production increases, possibly as a protective mechanism.

Capping agents and surface coatings are applied to ENPs during manufacture to stabilize them in suspension and prevent aggregation or dissolution. Surface treatments also provide catalytic or pharmacological activity or specific binding sites. Stable ENP suspensions interact more efficiently with algae, but some surface coatings reduce or eliminate toxicity by preventing dissolution or direct contact between the organism and the interior of the ENP (see figure).

How coated and bare TiO2 nanoparticles affect a green alga

The figure contains transmission electron microscope images of the green alga Chlamydomonas reinhardtii in the presence of (a) 100 mg/L well-dispersed polyacrylate-coated TiO2 nanoparticles and (b) their aggregated bare counterparts. Arrows indicate the attachment of bare particles to the cell wall (CW). No coated particles were found on the cell wall or in the interior.

High molecular-weight natural organic matter (NOM) can promote ENP aggregation and sedimentation and reduce the bioavailability of the ENPs. Conversely, lower molecular-weight NOM can act as a surfactant and produce the opposite effect. (ACS Sustainable Chem. Eng. 2013, 1, Article ASAP; Nancy McGuire)

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A dithienophosphole fluorophore emits white light. White light–emitting organic fluorophores are highly desirable, but their emission must be achieved by color mixing and balancing different fluorophores via covalent or noncovalent interactions. H. V. Huynh, X. He, and T. Baumgartner* at the University of Calgary (AB) developed a halochromic fluorophore that emits white light at a specific pH.

The authors synthesized halochromic fluorophore 3 in one step by using a Stille coupling reaction between 2-piperidinothiophene 1 and 2,6-dibromo-dithienophosphole oxide 2, the donor and acceptor components respectively, of molecule 3. Compound 3 has a broad absorption spectrum (300–800 nm) that produces its deep red color. Its emission band spans the red visible spectrum (600–800 nm); it peaks at 657 nm.

Synthesis of dithienophosphole fluorophore

When CF3CO2H is added to an aqueous solution of 3, the molecule is diprotonated, and its two absorption peaks red-shift strongly. Acidification also blue-shifts the emission spectrum from red to green. When 3 is partially protonated, it exists in solution as an equilibrium mixture of the unprotonated, monoprotonated, and diprotonated species; and its emission profile spans the entire visible light range.

Consequently, in the presence of 500 equiv CF3CO2H, 3 emits white light when it is excited at 390 nm. Its Commission Internationale de l’Éclairage (CIE) coordinates are (0.34, 0.32). The coordinates of pure white light are defined as 0.33, 0.33). (Chem. Commun. 2013, 49, 4899–4901; Xin Su)

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