Noteworthy Chemistry

December 2, 2013

Why do different crystals of a fluorogen emit light differently? Fluorogen molecules can pack into crystal forms that emit differently colored light with different emission efficiency. W. Liu and co-workers at Lanzhou University (China) report an example of this crystallochromism effect. Fluorogen 1 adopts three crystal structures: Crystal forms I and II emit blue and green light with fluorescence quantum yields (FF) of 0.18 and 0.15, respectively, whereas crystal form III is practically nonemissive (FF < 0.01).

Fluorogen whose crystals emit light differently

Based on their analysis of the crystal structures of 1, the researchers believe that the blue and green emission of crystals I and II are associated with the bianthracene and anthracenylphenyldiphenylacrylonitrile units, respectively. The layer thickness of crystal III is the largest of the three crystal forms. This loose packing allows the molecules of 1 to undergo dynamic intramolecular rotation, which quenches the emission of crystal III. Cooling crystal III to 77 K “freezes” the rotation and turns the emission on. (Adv. Mater. 2013, 25, 6112–6116; Ben Zhong Tang)

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Assemble iridescent chiral nematic nanocrystal–polymer composites in organic solvents. M .J. MacLachlan and colleagues at the University of British Columbia and FPInnovations (both in Vancouver) describe a simple way to develop hydrophilic cellulose nanocrystal (CNC)–polymer nanocomposites that have unique, tailored photonic behavior. The key to their method is neutralizing acidic CNCs (CNC-Hs) to form CNC-Xs, in which X+ is an alkali metal cation (Li+, Na+, or K+) or a quaternary ammonium ion (NH4+, NMe4+, or NBu4+). The CNC-Xs are readily dispersed in solvents such as DMF.

The authors prepared iridescent cast films by slowly evaporating the solvent. The chiral nematic CNC phases of the films have layered, helical structures normal to the film’s surface and throughout its thickness. The films’ photonics can be tuned as a function of cation size and hydrophobicity. For example, increasing the alkali metal cation size results in blue-shifting the UV spectrum.

With CNC-Na, the authors obtained homogenous, optically clear polymer composites in polystyrene or poly(methyl methacrylate) matrices when the films were prepared under dry conditions. The chiral, nematic CNC polymer composites have enhanced thermal properties and strong CNC–matrix interactions without changes in CNC crystallinity.

For a given polymer concentration, the authors observed that the type of polymer matrix influences the reflected color. This optical tuning is also influenced by the polymer content and the addition of salts such as LiCl. Above a certain matrix loading, the chiral nematic morphology is disturbed, and no color is reflected. (ACS Macro Lett. 2013, 2, 1016–1020; LaShanda Korley)

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Produce drugs continuously from synthesis to formulation. Continuous manufacturing is a rapidly emerging technique in the pharmaceutical industry. This method allows higher production rates, smaller inventories, and on-line monitoring. B. L. Trout and co-workers at MIT (Cambridge, MA) report a process in which synthesis, purification, and formulation are integrated into a continuous-flow system.

The authors chose aliskiren hemifumarate (1), a hypertension drug, to test their method. The drug is prepared in three steps and formulated as tablets that contain 112 mg of the active pharmaceutical ingredient (API). The chemical synthesis steps (lactone cleavage, tert-butoxycarbonyl protecting group removal, and hemifumarate formation), crystallization, liquid–liquid separation, and membrane filtration were performed under flow conditions. In the formulation step, excipients SiO2 and poly(ethylene glycol) were mixed with the API, melted, and extruded to form tablets with the size and dosage comparable with the commercial product. 

Aliskiren hemifumarate structure

During process design, the authors adjusted many steps to allow the entire process to run continuously. The manufacturing unit is compact (2.4 m x 7.3 m), operates 8 h/day for 10 days, and produces 4.5 g/h of the API, corresponding to 2.7 million tablets per year. This research will assist in the transition of new continuous processes from the laboratory to the industrial scale. (Angew. Chem., Int. Ed. 2013, 20, 12359–12363; José C. Barros)

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Reduce foaming problems during a bromination reaction. H. N. Pati and co-workers at Advinus Therapeutics (Karnataka, India) improved the way to scale up the synthesis of 2-bromo-4-nitro-1H-imidazole, a key building block for nitroimidazole drugs. In the first step, the dibromination of 4-nitroimidazole by adding bromine to an aqueous solution of the imidazole and NaHCO3 at 25 ºC was accompanied by excessive foaming that emitted bromine fumes.

Alternative bases such as NaOH resulted in low yields, so the authors investigated the effect of temperature and addition time on foaming, with NaHCO3 as the acid scavenger. Running the reaction at 5 ºC  and adding bromine over 4–5 h, followed by stirring at 5 ºC for 1 h and heating to 65 ºC for 6 h, gave 2,5-dibromo-4-nitro-1H-imidazole in 75% yield and no foaming problems. (Org. Process Res. Dev. 2013, 17, 1149–1165; Will Watson)

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Why are sulfone-based electrolytes stable at high voltages? High-voltage cycling of batteries or capacitors can oxidatively decompose the electrolyte and degrade the device’s performance. In lithium-ion batteries, traditional organic solvent–based electrolytes have oxidation potentials (vs Li/Li+) of ≈4.3–4.5 V. Sulfone-based electrolytes are stable at 5 V or higher, which makes them an attractive alternative.

In contrast to the experimentally observed stability of sulfone electrolytes, the ab initio–calculated oxidation potentials (Eox) for isolated sulfones are lower than those for carbonates. W. Li and coauthors at South China Normal University (Guangzhou) and the University of Utah (Salt Lake City) investigated this apparent contradiction by using density functional theory (DFT) calculations to compare several sulfones and carbonates.

Single-molecule DFT calculations that use an implicit solvent shell produce oxidation stabilities for the investigated sulfones that are lower than for the investigated carbonates. This is inconsistent with experimental observations, which show higher anodic stability for sulfone-based electrolytes. The inconsistency is resolved by including anions or additional neighboring solvent molecules in the calculation. The molecules in the first coordination shell strongly influence the potential and stability of the electrolyte molecules.

Anions and additional solvent molecules generally lower the oxidation stability of an electrolyte by participating in oxidation and decomposition reactions. For some of the sulfones, the calculated oxidation potentials are dramatically lowered in the presence of anions (PF6, ClO4, or BF4) or neighboring solvent molecules, which is similar to the carbonates’ behavior. Other sulfones, however, are surprisingly stable in the presence of anions and neighboring solvent molecules, resulting in overall higher calculated oxidation potentials than those of carbonates.

Influence of anions on carbonate and sulfone electrolyte stability

When the authors compared Eox for a series of solvent–anion clusters, they found that LiPF6 is a more suitable lithium salt than LiBF4 or LiClO4 for a high-voltage electrolyte. LiClO4 actually reduces the oxidation stability of the electrolyte and thus was the least suitable.

The authors also were able to design new oxidation-stable sulfones by modifying their functional groups. This is an important development in designing high energy-density lithium-ion batteries. (J. Phys. Chem. Lett. 2013, 4, 3992–3999; Nancy McGuire)

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