Noteworthy Chemistry

May 28, 2012

Tetraphenylethylene makes solid-state luminophores emissive. Conventional luminophores often emit in solution but become weakly luminescent or even nonemissive in the aggregated or solid state. Intermolecular interactions promote the formation of detrimental species such as excimers and exciplexes. This phenomenon is the notorious aggregation-caused quenching (ACQ) of light emission.

Oligofluorenes are one type of ACQ luminophore whose solutions are much more emissive than their solids. A team led by A. D. Q. Li at Washington State University (Pullman), D. Ma at the Changchun Institute of Applied Chemistry (Jilin, China), and M.-Q. Zhu at the Huazhong University of Science and Technology (Wuhan, China) reversed this effect by making oligofluorenes nonemissive in solution but highly luminescent in the solid state.

The researchers used tetraphenylethylene, an archetypal luminogen with the property of aggregation-induced emission (AIE), as their “magic stick”. They attached tetraphenylethylene units to oligofluorenes as end-capping groups to generate a series of adducts with one to five repeat units (1). The adducts are weakly fluorescent in the solution state (ΦF as low as 0.3%), but their nanoparticles or thin films emit intensely (ΦF up to 68%) with AIE effects up to >125-fold.

The luminogens are excellent solid-state chemosensors for detecting volatile organic compounds with a clear fluorescence on–off response. The adducts are much more thermally stable than their oligofluorene analogues, with decomposition temperatures as high as 435 °C. (J. Mater. Chem. 2012, 22, 7515–7528; Ben Zhong Tang)

[Also see Noteworthy Chemistry for April 30, 2012.—Ed.]

Triazole–pyridine compounds may be the next fountain of youth. The aging process is closely associated with increasing levels of reactive oxygen species (ROS) and free radicals in the body. ROS formation and oxidative damage correlate inversely with longevity across a range of species. Antioxidants may play an important role in slowing aging by interfering with the generation of radicals or by scavenging them. Antioxidants prolong the lifespan of the nematode Caenorhabditis elegans, and these worms are popular models for aging and longevity studies because of their short lifespan and experimental flexibility.

R. A. Mekheimer*, A. A. R. Sayed, and E. A. Ahmed at King Abdulaziz University (Jeddah, Saudi Arabia), El-Minia University (Egypt), and AlFashir University (Sudan) designed a new class of antioxidants. They synthesized a series of 1,2,4-triazolo[1,5,a]pyridine derivatives and studied their biological effects on C. elegans.

Among the analogues they synthesized, compound 1 significantly extends, in a dose-dependent manner, the nematode’s lifespan. While trying to understand the effects of 1, the authors found that it greatly increases the worm’s tolerance to high oxidative stress and heat, its two principal aging factors. They also found that 1 reduces the production of molecules associated with ROS and free-radical formation and also may scavenge the molecules .

Compound 1 is a promising antioxidant drug. Additional studies are needed to determine its exact mechanism. (J. Med. Chem. 2012, 55, 4169–4177, Chaya Pooput)

Trigger drug delivery by irradiating spiropyran nanoparticles. D. S. Kohane and collaborators at MIT (Cambridge, MA) and Harvard Medical School (Boston) prepared nanoparticles that consist of low-toxicity spiropyran derivatives with alkyl chains of different lengths (C7,C9, C18) and poly(ethylene glycol) (PEG)-functionalized lipids that can be triggered by UV irradiation for on-demand therapeutic delivery. The spiropyran structure undergoes a UV-initiated ring opening to the less stable merocyanine zwitterion with an accompanying volume change. Merocyanine reverts to the hydrophobic spiropyran form spontaneously in the dark; this process can be accelerated under visible light.

Spiropyran nanoparticles prepared via nanoprecipitation in the absence of PEG-based lipids form aggregated structures, produce a bimodal size distribution upon UV exposure, and exhibit low drug loadings and efficiencies. The authors prepared hybrid nanoparticles consisting of derivatized spiropyrans and a PEG ligand via ultrasonication to yield monodisperse, photoresponsive drug carriers with neutral shell surfaces. These drug carriers are stable in phosphate-buffered saline solution for >1 month and can be used to encapsulate drugs and dyes. Surface functionalization with cell-targeting peptides enhances their therapeutic efficacy.

The authors present evidence of drug penetration assisted by the UV-induced size changes of the nanoparticles. The spiropyran nanoparticle hybrids are fluorescent in living cells after irradiation, which suggests a potential tracking mechanism. The key to the usefulness of this research is the ability to control the location and length of therapeutic delivery. (J. Am. Chem. Soc. 2012, 134, Article ASAP DOI: 10.1021/ja211888a; LaShanda Korley)

Here’s a practical dynamic kinetic resolution of ibuprofen. Ibuprofen is a nonsteroidal anti-inflammatory whose (S)-enantiomer is 100 times more active than the (R)-enantiomer and has fewer side effects. D. Chavez-Flores* and J. M. Salvador at the University of Texas at El Paso describe an unconventional dynamic kinetic resolution (DKR) of racemic ibuprofen (1) to produce the desired enantiomer.

The authors observed that the ibuprofen methyl ester easily racemizes in refluxing base in aq DMSO, that a 20% aq solution of DMSO enhances the selectivity of Candida rugosa lipase in the resolution process, and that the H2O–DMSO system increases the solubility of ibuprofen esters in alkaline media so that the reaction can proceed in one phase.

Based on these observations, they converted racemic ibuprofen to its methyl ester (2) under Fischer esterification conditions and then subjected 2 to DKR with C. rugosa lipase in aq DMSO with a pH 10 buffer. They obtained (S)-ibuprofen (3) in 94% yield and 94% ee. Purification by means of careful crystallization of the racemate followed by recovery of the (S)-enantiomer in the mother liquor improved the selectivity to 99.7% ee. Kinetic experiments showed that the base and DMSO act together to modify lipase activity so that only the (R)-ester is racemized. (Tetrahedron Asymm. 2012, 23, 237–239; JosÉ C. Barros)

Make your own “glassware” by using 3-D printing. Most reactions in chemistry labs are carried out in various types of glass vessels. Because of the intrinsic limitations of glassware manufacture, it is not easy to customize glassware on demand.

Despite its popularity in engineering design and production, 3-D printing technology has not been used in chemistry applications. But now L. Cronin and coauthors at the University of Glasgow (UK) and Uformia AS (Furuflaten, Norway) have made 3-D–printed chemical reaction vessels.

The authors used a low-cost (US$2,000) Fab@Home robocasting platform obtained from the NextFab Store (Albuquerque, NM). They chose the robust, easily curable acetoxysilicone polymer as the building material. The major part of a reaction vessel can be printed as designed. The nonprintable components can be inserted during scheduled pauses.

Using this technique, the authors demonstrated organic and inorganic syntheses with their home-printed reaction ware. In addition, some of the reactions can be monitored in situ.

When they studied the reaction between 5-(2-bromoethyl)phenanthridinium bromide (1) and p-methoxyaniline (2), the authors found that they could selectively synthesize products 3 and 4 by adjusting the volume of the reaction chamber. With a 9.5-mL chamber, the selectivity to product 3 was 100%. A 2.0-mL chamber gave a 4:1 4/3 yield ratio.

In a phosphomolybdic acid reduction, they monitored the reaction progress by using cyclic voltammetry with an electrochemical cell that consisted of electrodes and an indium tin oxide glass slide. The apparatus also allowed real-time UV–vis spectroscopy.

In the catalytic hydrogenation of styrene, the Pd/C catalyst was incorporated into the reaction ware by a preformed Pd/C polymer mixture. The catalyst efficiently promoted the hydrogenation, and the Pd/C material did not leach into the reaction medium. (Nat. Chem. 2012, 4, 349–354; Xin Su)

These liquid hypergolic fuels may replace hydrazine. Hypergolic ionic liquids (ILs)—fuels that undergo spontaneous ignition with an oxidizer—are promising because they offer simplified systems, less in-flight thermal management, increased propellant energy density, and reduced handling precautions compared with traditional in-space propellants such as hydrazine and N2O4. Using ILs as hypergolic propellants is challenging, however, because not all hypergolic ILs meet desirable criteria such as high thermal and hydrolytic stability, wide liquid temperature ranges, low viscosity, and short ignition delay times. In addition, the experimentation needed to design ILs for hypergolic fuels with the desired properties can be an expensive, tedious, time-consuming process.

H. Gao at China Agricultural University (Beijing) and J. M. Shreeve* at the University of Idaho (Moscow) took a new tack. They dissolved solid NH3·BH3, N2H4·BH3, and N2H4·2BH3—established hydrogen-storage materials—in ILs that were themselves hypergolic. The saturated borane solutions exhibited shorter ignition delay times in response to white fuming HNO3 as the oxidizer than the solute or solvent alone, making them superior to known hypergolic ILs.

This route provides a convenient, controllable method for introducing hypergolic solids as fuels into oxidizer systems. The fuel and IL components are easily and inexpensively synthesized and present a likely route to environmentally friendly systems. The borane–IL solutions appear to be the brightest hope so far for replacing hydrazine and its derivatives as fuels in hypergolic IL propellant systems and provide a giant step toward practical applications. (J. Mater. Chem. 2012, 22, Advance Article DOI: 10.1039/C2JM31627G; Gary A. Baker)

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