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Noteworthy Chemistry

June 1, 2015


Switch mechanochromic emission between blue and near-IR. The change in the color of light emission from a solid-state fluorophore when mechanical stress (e.g., grinding, crushing, or milling) is applied is called mechanochromism. Mechanochromic materials should have a broad spectrum of applications, but few have been discovered. The ones that exist usually display rather small changes in emission wavelength in response to mechanical stimuli.

S. Kamino, A. Muranaka, S. Enomoto, and coauthors at Okayama University, RIKEN-CLST (Hyogo), RIKEN-CSRS (Saitama), Hiroshima University, the University of Tokyo, and Hitachi High-Tech Science Co. (Ibaraki, all in Japan) prepared a mechanochromic molecule based on the spiropyranoxanthene skeleton that can switch emission between blue and near-infrared (NIR) light, the largest shift observed for this class of materials.

The compound is an aminobenzopyranoxanthene derivative that the authors call ABPX010. They synthesized it as a mixture of cis- and trans-isomers by coupling 2-(4-diethylamino-2-hydroxybenzoyl)benzoic acid with resorcinol in methanesulfonic acid. The clathrate crystals of cis-ABPX010 (1 in the figure) and dichloromethane (CH2Cl2) exhibit blue and NIR fluorescence when excited at 365 nm. 

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NIR emission from cis-1 is not observed in CH2Cl2 solution. The authors attribute its solid-state emission to structural changes caused by photoexcitation that involve a transitional spiro-ring opening–closing process unique to the slip-stacked dimer packing mode.

The researchers also found that the dual emission of cis-ABPX010–CH2Cl2 is sensitive to mechanical grinding, which diminishes the NIR emission and enhances the blue emission. When the ground solid is subjected to solvent vapor fuming, the NIR emission is restored. The reversible switching can be repeated at least five times. This behavior is associated with the loss and recovery of CH2Cl2 from cis-ABPX010.

ABPX010’s large emission shifts may be useful in applications such as encryption and medical imaging and therapy. This study provides insight into the influence of crystal packing on photochemical reactions and luminescence processes. (J. Am. Chem. Soc. DOI: 10.1021/jacs.5b00877; Xin Su)

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Aggregates without “classical” luminophores emit efficiently. Polymers that do not contain aromatic rings normally do not emit visible light. Biogenic polymers in organized assemblies without traditional luminophores, however, autoluminesce in many colors with various degrees of efficiency, which suggests the existence of “heterodox” light-emitting structures.

R.-b. Wang, W.-z. Yuan, and X.-y. Zhu* at Shanghai Jiao Tong University found that pure aliphatic polymers without aromatic moieties emit bright visible light in the aggregated state. They propose a previously unknown emissive structure that is responsible for this unusual luminogen system.

The polymers are water-soluble, nonconjugated poly(amidoamine)s with linear and hyperbranched molecular structures, abbreviated as l- and hb-PAMAM, respectively. The polymers do not luminesce in aqueous media but become emissive when they aggregate in a “poor” solvent or in the solid state, an example of aggregation-induced emission (AIE).

The AIE effect of l-PAMAM is more pronounced than that of its hb-counterpart, possibly because of the more compact aggregated l-structure. The emission of the solid films of l- and hb-PAMAMs is red-shifted from the aggregated state with an increase in the excitation wavelength; this indicates the presence of several emitting species.

The authors believe that the periodic lone-pair electrons in the macromolecular chains form a variety of intra- and interchain clusters in the aggregates. The electronic conjugation is extended in the clusters with shared and overlapped electron clouds. In the aggregates, the radiationless relaxation pathways of the excitons are impeded by the rigidified chain conformation that result from the multiple intra- and intermolecular interactions, thus making the “cluster chromophores” highly luminescent.

The excitation wavelength–dependent emission of the aggregates suggests heterogeneity of the clusters, that is, the formation of clusters with different nanoparticle sizes and varying extent of electronic conjugation. (Chinese J. Polym. Sci. DOI: 10.1007/s10118-015-1635-x; Ben Zhong Tang)

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Make 2-substituted thiazolines without a metal catalyst. Thiazolines are important heterocyclic compounds that have activity as antibiotics, cancer drugs, anti-inflammatories, and antithrombotics. Thiazolines are prepared by the reaction between nitriles and cysteamines in the presence of electrophiles that activate the nitrile or copper catalyst.

C. S. J. Cazin and co-workers at the University of St. Andrews (UK) report a method for preparing thiazolines simply and inexpensively. While studying catalysis by copper N-heterocyclic carbenes, they found that subjecting neat benzonitrile (1 in the figure) and cysteamine hydrochloride (2) to only basic catalysts at 80 ºC gives 2-phenyl-4,5-dihydrothiazole (3). An initial screening of bases indicated that 20 mol% sodium hydroxide performed best.

Metal-free reaction of nitriles with cysteamine

The researchers ran the reaction on several substrates and discovered that aromatic nitriles with electron-withdrawing or electron-donating substituents give good yields. Aliphatic and heterocyclic nitriles also can be used.

The reaction workup consists only of extraction with ethyl acetate–water to remove excess 2. This metal- and solvent-free reaction should be suitable for industrial-scale processes. (Green Chem. DOI: 10.1039/C5GC00286A; José C. Barros)

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How does fluoride in groundwater affect forage plants? Fluoride ion occurs naturally in groundwater and soil, but agricultural and industrial activities can elevate these levels considerably. Livestock that drink water and eat forage plants with high fluoride concentrations are at risk of developing fluorosis, a condition with symptoms ranging from a harmless discoloration of the teeth to brittle, malformed bones.

P. M. Kopittke and coauthors at the University of Queensland (St. Lucia) and Santos (Brisbane, both in Australia) conducted a laboratory and field study of soil fluoride levels and forage plant uptake in Australia’s Great Artesian Basin. They examined fluoride adsorption in soils, its release into the groundwater, plant root uptake and transport to leaf tissues, and accumulation of fluoride in the leaf tissues.

The seven soils in the study varied substantially in their capacity to adsorb fluoride. Enough fluoride reentered the groundwater to kill the plants growing in washed white quartz sand, the weakly-adsorbing control soil. Red ultisol (red clay soil) released an intermediate amount of fluorine; yellow ultisol and red vertisol (expansive clay) released the least. In red ultisol and sand, fluorine was present predominantly as free fluoride ion, but it occurred as Al–F complexes in the yellow ultisol.

Fluoride accumulation in plant leaf tissues was greatest for the sand and least for red vertisol. Leaf concentrations of fluoride did not depend on the total concentration of fluoride in the soil solution for Rhodes grass (Chloris gayana) or leucaena (Leucaena leucocephala), but there was a weak correlation for Lucerne (Medicago sativa).

The authors found no consistent relationship between fluoride speciation, soil solution concentration, and accumulation in leaf tissues across all of the examined soil types and plant species. Lucerne leaves, however, retained a significant amount of fluoride from overhead irrigation with fluoride-containing water. The other plant species retained more modest amounts. Lucerne tissues also showed the greatest loss of fluoride after being rinsed with deionized water.

Even in soils that contained the equivalent of irrigation with 0.26 mM fluoride solution at 5 ML/(ha·year) for 25 years, most plant tissues contained fluoride levels below the maximum tolerable level for mature beef cattle, sheep, chickens, and swine. For an 8-year-equivalent concentration, plant fluoride levels were still acceptable for young beef calves and heifers. The authors note that further studies are needed to evaluate other soils, plant species, and fluoride concentrations. (J. Agric. Food Chem. DOI: 10.1021/acs.jafc.5b01001; Nancy McGuire)

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Who receives compassionate use of experimental drugs? In an effort to make the decision process of who receives compassionate use of its investigational medicines more consistent and just, Johnson & Johnson (J&J, New Brunswick, NJ) formed a partnership with the Division of Medical Ethics at the New York University School of Medicine (New York City). Faculty member and bioethicist A. L. Caplan will convene a Compassionate-Use Advisory Committee (CompAC) consisting of 10 medical experts, bioethicists, and patient representatives. The independent panel will review patient requests and make recommendations to J&J.

In selected cases of serious diseases, the US Federal Drug Administration (FDA) allows the compassionate use of promising experimental drugs, devices, and biologics that haven’t yet been approved or shown to be effective and safe. FDA typically grants most compassionate use requests after weighing factors such as whether the potential benefits justify the potential risks. The drug company must also consider factors such as limited supply and possible interference with clinical trials before giving approval.

Companies have different processes for deciding which patients can receive experimental drugs under compassionate use. According to Caplan, the goal of CompAC is to ensure that the selection process is “guided by ethical principles” and is “thorough, transparent, and fair”. “We want to establish a model that will create a structured policy of allocation based on equality, need, and efficacy to ensure that the utility of our scarce resources are maximized,” he says.

CompAC hopes to consider requests in a manner similar to the way that UNOS (the United Network for Organ Sharing) determines the order of a transplant list. In an effort to “level the playing field”, only the pertinent medical facts of each case will be submitted to the committee.

CompAC will begin as a pilot program for one experimental drug that is in a late-stage clinical trial. If successful, it can be a model for when and to whom drugs will be provided for compassionate use. Says Caplan, “This is about getting every voice heard. How will we decide? I don’t know.” (Scientific American; Abigail Druck Shudofsky)

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Mechanochromic molecule with switchable blue and near-IR light emission