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Noteworthy Chemistry

September 16, 2013


This acid–base indicator works in nonpolar organic solvents. Acid–base indicators consist of a broad range of halochromic compounds, most of which indicate pH only in aqueous solutions. pH indicators for visualizing acid–base equilibria in nonpolar organic solvents are scarce.

To tackle this problem, K. Ariga, J. P. Hill, and colleagues at the National Institute for Materials Science (Ibaraki), Tokyo University of Science, and JST-CREST (Tsukuba, all in Japan) developed a calix[4]pyrrole-based indicator that can be used to qualitatively measure the acidity or basicity of materials dissolved in nonpolar media.

The authors chose oxoporphyrinogen (OxP) 1 as the indicator because its pyrrole protons have highly sensitive colorimetric responses to hydrogen bonding. It is highly soluble in nonpolar solvents because of its peripheral tert-butyl groups.

Acid–base indicating oxoporphyrinogen

In the presence of 0.1 equiv 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), the color of the OxP in CH2Cl2 solution is blue. As CF3CO2H is added incrementally, the solution color changes to purple (the same as the neutral OxP), red, and eventually green.

The authors found that when CF3CO2H is added after DBU is neutralized, its protonated form transfers to the OxP to form the dication OxPH22+. Adding more CF3CO2H converts OxPH22+ to the tetracation OxPH44+. They showed that OxPH22+ and OxPH44+ are responsible for the red and green colors, respectively. The OxP also interacts with anions to form mono- and dianionic complexes.

The authors created an OxP-doped polymer matrix for qualitatively indicating acidity. When the matrix is exposed to varying amounts of organic acid vapors, the compounds’ acidities correspond to characteristic color combinations that can be differentiated by the naked eye. (Chem. Commun. 2013, 49, 6870–6872; Xin Su)

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Explore liquid-crystal confinement in electrospun nanofibers. E. Enz (Martin Luther-University, Halle-Wittenberg, Germany), V. La Ferrara (ENEA Portici Research Center (Italy), and G. Scalia* (Seoul National University) investigated the assembly and optical behavior of cholesteric liquid crystals (LCs) confined in electrospun nanofibers.

The authors prepared continuous, coaxial electrospun nanofibers with a chiral LC core and a poly(vinylpyrrolidone) (PVP) shell. The LC aligns along the long axis of the nanofiber; the helical structure of the LC is perpendicular to the fiber surface.

Using focused ion beam (FIB) etching, the authors determined that the nanofiber surface is noncylindrical as the result of wetting caused by the spinning conditions. This fiber structure greatly influences the LC confinement response. Because of the asymmetry in the fiber diameter, the optical behavior (i.e., the reflection wavelength) is dictated by the confining dimension below ≈500 nm and by defects in fiber thickness and length, which compress or expand the LC helix.

The authors note that this study highlights the structural complexity of the confinement-induced organization of LCs and reveals how FIB etching and current response, coupled with polarizing optical microscopy, can be used in other applications of fiber technology. (ACS Nano 2013, 7, 6627–6635; LaShanda Korley)

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Dithienostannole structures determine how they photoluminesce. Electronic communication, such as in-phase interactions between metal σ* orbitals and bithiophene π* orbitals in heteroatom-bridged 2,2’-bithiophene derivatives, lowers the structures’ LUMO energy levels and reduces their HOMO–LUMO energy gaps. Silicon- and germanium-bridged 2,2’-bithiophenes are used as building blocks for functional materials, but much less is known about tin-bridged bithiophenes (dithienostannoles or DTSs).

J. Ohshita and coauthors at Hiroshima University (Higashi-Hiroshima), Sumitomo Chemical (Tsukuba), and Hokkaido University (Sapporo, all in Japan) synthesized DTS derivatives 1 and 2 and found that their luminescence behaviors are dramatically altered by their molecular structures.

Dithienostannoles with different luminescence properties

In THF solution, dynamic intramolecular motions of the trimethylsilyl and phenyl rotors in 1 provide a path for nonradiative relaxation, which renders 1 nonemissive in the solution state. Its photoluminescence quantum yield (ΦPL) is as low as 0.009. Intramolecular motion becomes difficult when the molecules are packed; thus 1 becomes emissive in the crystalline state (ΦPL = 0.556).

The two benzoannulated units in 2 hinder the motion of the phenyl rotors on the stannole ring, making 2 more emissive (ΦPL = 0.296) than 1. The intermolecular π–π contacts between the large benzoannulated dithienostannole plates in the crystals of 2 compete with the conformational rigidification effect of crystallization. As a result, little change in emission efficiency is observed between its solution and its crystals. (Organometallics 2013, 32, 4136–4141; Ben Zhong Tang)

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Costing and process studies optimize a biocatalytic reduction. 2-Octanone can be reduced to (R)-2-octanol with the enzyme Lactobacillus brevis alcohol dehydrogenase in a cascade of two enzyme-membrane reactors. NADPH is the cofactor, and glucose dehydrogenase from Bacillus spp. is used to regenerate NADPH.

J. M. Woodley and coauthors at DECHEMA Research Institute (Frankfurt am Main, Germany), the Technical University of Denmark (Lyngby), and Mannheim University of Applied Sciences (Germany) carried out a costing study and a preliminary environmental assessment on the base-case reaction. The studies showed that the overall cost was €149/kg. The main cost contributors were the NADPH cofactor (36.7%), the N-(2-acetamido)iminodiacetic acid buffer (26.3%), and an ionic-liquid cosolvent (20.4%). The environmental performance consisted of a process mass intensity (PMI) of 133 and an E-factor (kgwaste/kgproduct [including H2O waste]) of 132.

The authors developed a process model based on initial reaction rates and used it to assess the effect of changing parameters such as cofactor loading. The optimized process, which includes recycling of 90% of the aqueous stream, reduced the cofactor level, adjusted the pH level to improve the stability of the cofactor. The resulting cost reduction was 65%, the PMI was 18, and the E-factor was 17. (Org. Process Res. Dev. 2013, 17, 1027–1035; Will Watson)

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This magnetic stir bar is designed for microwave reactors. Running chemical reactions in microwave reactors usually produces higher yields in shorter times than conventional heating. The reactors, however, require special glassware such glass or quartz tubes. Because of this geometry, traditional magnetic stirring bars do not perform well; and some parts of the reactor may not be properly stirred, generating hot spots or lowering reaction yields.

D. Obermayer, M. Damm, and C. O. Kappe* at the University of Graz (Austria) developed a new magnetic stirrer for use in microwave vessels that performs much better than traditional stirrers.

The authors first replaced the AlNiCo and ferrite alloys used in common stirring bars with the rare-earth alloy Sm2Co17. They used this material, covered with poly(tetrafluoroethylene) (PTFE) to make a cylindrical stirrer. Sm2Co17 is attractive because it has a maximum service temperature in the range 330–500 ºC and an energy product (the stored energy in a magnet) several times greater than that of standard alloys.

The researchers then added a PTFE blade extension vertical to the magnetic stirrer to increase the cross-sectional area of the stirrer and therefore the centrifugal forces inside the vessel. They compared the blade-extended stirrer to the nonextended (cylindrical) stirrer in three reactions:

  1. An SNAr reaction under viscous conditions that uses solid K2CO3 as the base and N,N-dimethylacetamide as the solvent;
  2. a biphasic reaction in which cyclohexene is oxidized by aq H2O2 to produce adipic acid; and
  3. a ring-opening polymerization of ε-caprolactone.

In all cases, the extended stir bar worked better than the cylindrical one. It gave a higher yield (reaction 1), a smaller temperature increase (i.e., a safer reaction) (reaction 2), and a longer stirring time before the stirrer was “frozen” by the polymer product (reaction 3). (Org. Biomol. Chem. 2013, 11, 4949–4956; José C. Barros)

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Coated graphene quantum dots shine on and on in vivo. Medical imaging agents that are based on photoluminescent inorganic semiconductor quantum dots (QDs) give higher quantum yields and less photobleaching than organic dyes. The QDs. however, tend to ionize in vivo, causing toxic side effects.

Photoluminescent graphene quantum dots (GQDs) are less toxic, but the uncoated GQDs rapidly lose their photoluminescence. Coating the GQDs preserves their photoluminescence, improves their stability in water, and further reduces their toxicity.

Y.-k. Lee and co-workers at the Korea National University of Transportation (Chungju) investigated GQDs coated with polydopamine (pDA), a derivative of the adhesive compound 3,4-dihydroxyphenylalanine (DOPA) used by mussels. A previous study (Hong, S., et al. Nanomedicine 2011, 6, 793–801) showed that pDA coatings greatly reduce inflammatory and immunological responses to QDs in the bloodstream.

The authors observed pDA-coated and uncoated GQDs in pH buffer solutions (pH 5, 7, 9, and 11) and in a 2% NaCl solution over a period of 14 days. The uncoated GQDs lost 45% of their photoluminescence in the pH 5 and 7 solutions over this period, possibly because protons interacted with the negatively charged GQDs. All of the coated GQDs retained their photoluminescence over the test period. Increasing particle size promoted aggregation but did not cause the GQDs to precipitate.

The researchers tested cytotoxicity toward KB cancer cells in vitro. The cells took up both coated and uncoated QDs, and a green photoluminescence was observed from the cytoplasm and the cell membrane. The uncoated QDs were only slightly toxic; the authors attribute this to large amounts of adsorbed oxygen, which acts as a coating. pDA-coated GQDs had no detectable toxicity.

The authors injected coated and uncoated GQDs into nude mice to observe the biodistribution of the particles and the duration of the photoluminescence. The figure shows optical images of nude mice and their isolated organs: (a) saline-treated control mice; (b) uncoated GQD–treated mice; (c–e) mice treated with GQDs coated with dopamine polymerized for 3, 6, and 12 h, respectively; and (f) photoluminescence intensities of organ tissues after treatment with coated and uncoated GQDs.

In vivo biodistribution and bioimaging of uncoated and coated GQDs.

Both types of GQDs were distributed to all of the organs examined. Most of the uncoated GQDs in the size range 3–6 nm were expelled through the kidneys within 4 h of injection. Coated GQDs remained in the body longer, and showed stronger photoluminescence in all organs except the lungs. Increasing the particle size increases the photoluminescence in the liver, possibly because larger particles are not excreted as rapidly. (ACS Appl. Mater. Interfaces 2013, 5, 8246–8253; Nancy McGuire)

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