Noteworthy Chemistry

June 11, 2012

Pyrene–fluorene hybrids may form OLED materials. Organic compounds with extended π-conjugation have unique properties that make them potential materials for use in electronic devices such as organic light-emitting diodes (OLEDs) that are used in television and mobile phone screens. Pyrene and fluorene compounds have promising optical and electrochemical properties and have sparked increasing interest in using them as active ingredients in OLEDs.

K. R. J. Thomas and coauthors at the Indian Institute of Technology Roorkee (India), and National Tsing Hua University (Hsinchu, Taiwan) combined the two structures to form a new class of materials. They synthesized a series of pyrene–fluorene hybrids and investigated their optical properties. They found that using an acetylene group as the linker between the pyrene and fluorene segments improved the hybrids’ properties by extending π-conjugation.

The number of fluorene groups on the pyrene core influences the absorption properties of the compounds: The monosubstituted derivatives absorb at the shortest wavelengths, and the tetrasubstituted at the longest. When they are exposed to visible light, the monosubstituted analogues emit bright blue light, and the tetrasubstituted molecules emit yellow light.

The series of pyrene–fluorene hybrids are efficient dopants in multilayered OLEDs. (J. Org. Chem., 2012, 77, 3921–3932, Chaya Pooput)

Use valence-bond calculations to explain isocyanide structures. W. Lieke first described isocyanides in the mid-19th century, but only with the advent of multicomponent reactions (e.g., the Ugi reaction) did they become useful. Several studies have compared two proposed structures for isocyanides: a carbene and a zwitterion.

B. Braïda, P. Fleurat-Lessard, and coauthors at École Normale SupÉrieure de Lyon, École Nationale SupÉrieure de Techniques AvancÉes (Paris), Pierre and Marie Curie University (Paris), and UniversitÉ de Paris-Sud (Orsay, France) report a valence-bond technique to describe isocyanides. They used the breathing orbital valence bond (BOVB) method with XMVB ab initio software to model the π-system of four possible mesomeric isocyanide forms (14).

The authors showed that structure 1 (the carbene form) is the dominant component (50–55%) and that the structure of the R group (e.g., alkyl, vinyl, and aryl) does not affect this result. Zwitterionic form 2 is second (24–29%), and structures 3 and 4 contribute ≈10% each. There is no significant change in the isocyanide structure in the gas phase or in solvents such as water or CH2Cl2.

The authors also calculated resonance and stabilization energies that indicated that structure 2 stabilizes carbene form 1 by nitrogen lone pair delocalization. Linear geometry is necessary for the dominant carbene form to mix efficiently with the triply bonded zwitterionic form and to maximize the resonance energy arising from this mixing. (New J. Chem. 2012, 36, 1137–1140; JosÉ C. Barros)

A cholesteric liquid crystal–based probe senses moisture. Battery-free humidity sensors with visible indicators would be useful tools for the pharmaceutical and food-packing industries.

Because of their unique structures, cholesteric liquid crystals (CLCs) can reflect circularly polarized light at a specific wavelength. A. P. H. J. Schenning and coauthors at Eindhoven University of Technology (The Netherlands) and Queen Mary University of London used this CLC property to make printable moisture sensors that change color in response to changes in environmental humidity.

The authors first selected building blocks for the CLC polymer film. Molecules 1 and 2 have reactive cross-linkable ends and chiral centers, whereas the carboxylic acid groups in 3 and 4 provide hydrogen-bonding sites. Compound 5 was added to lower the crystalline–nematic phase transition temperature.

The researchers bar-sorted a solution of the monomers onto a polyimide-coated glass slide, and the monomers were photopolymerized. Treating the polymer with KOH solution converted it into a hygroscopic potassium salt form. The dried hygroscopic film showed a green-selective reflection band (SRB). When it absorbed water, its color gradually changed to red as a result of a change in its helical aggregation. The SRB shift can be correlated to relative humidity (RH). UV–vis transmission spectroscopy showed a wide detection range of 3−83% RH. The authors studied SRB film changes at different temperatures and observed distinct responses.

The CLC mixture was printed on glass and foil substrates with an inkjet printer and then polymerized. The printed films exhibited even faster responses to humidity changes. (J. Am. Chem. Soc. 2012, 134, 7608–7611; Xin Su)

An acyclic container enhances drug solubility. An estimated 40–70% of new drug candidates are so poorly soluble that they cannot be formulated. Among several strategies being tried to improve solubility are reducing crystal size and using molecular containers such as cyclodextrins. Molecular containers, however, usually complex only small molecules. L. Isaacs and co-workers at University of Maryland (College Park) describe acyclic curcubit[n]uril molecular containers that can encapsulate larger molecules.

The authors synthesized curcubit[n]urils 1 and 2 on a gram scale in moderate yields from inexpensive starting materials. They are soluble in water and sodium phosphate buffer and are nontoxic. X-ray crystallography showed that they are acyclic and adopt a C-shaped form with a hydrophobic cavity. The containers were tested for encapsulating several poorly soluble drugs. NMR signals of the drugs in the liquid phase indicated the concentration of the drugs within the containers.

The authors tested 10 drugs in various classes. The solubilities of the drugs increased by 23- to 2750-fold. Paclitaxel showed the greatest solubility increase, and it had higher activity against cancer cells when encapsulated. These molecular containers should be superior to cyclodextrins for delivering poorly soluble drugs. (Nature Chem. 2012, 4, 503-510, JosÉ C. Barros)

Luminogen aggregates emit red light in aqueous media. In biological applications, red-light emission is desirable because it penetrates tissues deeply, in contrast to biosubstrate autofluorescence. In aqueous media or physiological buffers, red-light emission is often quenched because solvation by polar water molecules leads to nonradiative deactivation of the polarized excited states.

A team led by T. Ishi-i at Kurume National College of Technology (Japan) developed a series of efficient red emitters in aqueous media by suppressing solvation-induced quenching by the self-assembling aggregation of donor–acceptor molecules.

An example of the red-light emitters developed by the researchers is compound 1. In THF solution, 1 emits strong red light with a fluorescence quantum yield (FF) of 71%. When water is added, the emission weakens. At a water fraction (fw) of 0.6 (v/v), the FF decreases to 8%. Emission quenching results from the solvation of the polarized excited states of 1.

At ³0.7 fw, however, emission intensity recovers and intensifies until FF is as high as 80% at 0.9 fw. In aqueous media with high fws, the luminogen molecules form hydrophobic aggregates that suppress the solvation-caused nonradiative deactivation and lead to emission recovery. (Chem. Asian J. 2012, 7, Early View DOI: 10.1002/asia.201200136; Ben Zhong Tang)

Carbon nanotube-based devices know when fruits are ripe. As the simplest plant hormone, ethylene is important for plant physiology, especially fruit ripening. Despite the need for simple, inexpensive ethylene detection, current techniques still rely on complicated, expensive instrumentation. B. Esser, J. M. Schnorr, and T. M. Swager* at MIT (Cambridge, MA) invented a chemoresistive sensor based on carbon nanotubes that might provide an alternative.

Cu(I) is the key cofactor in the receptor ETR1 that triggers the ripening process when it binds to ethylene. The authors prepared Cu(I) complex 1, which binds to ethylene to give ethylene adduct 2, the most stable adduct to date (Dias, H. V. R., et al. Organometallics 2002, 21, 1466–1473). They next mixed 1 with single-wall nanotubes (SWNTs). Complex 1 can interact with SWNTs through its trifluoromethyl groups and change the SWNTs’ electrical resistance. When 2 forms, it weakens the interaction between the complex and the SWNTs and increases their resistance.

The authors built the device by drop-casting the suspended mixture onto a glass slide equipped with electrodes. The device was connected to a potentiostat, and measurements were carried out in a closed chamber with adjustable gas flow.

The device could detect sub-ppm ethylene concentrations and had a wide linear response range (0.5–50 ppm). The authors measured ethylene emissions from some common fruits using the device. They also characterized the climacteric properties of the fruits by using time-dependent measurements.

The authors screened various SWNTs and found that SWNTs with small diameters performed best. The sensitivity of the device can be enhanced by adding polystyrene particles. (Angew. Chem., Int. Ed. 2012, 51, Early View DOI: 10.1002/anie.201201042; Xin Su)

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