Noteworthy Chemistry

January 16, 2012

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Make all-crystalline organic field-effect transistors from large mica single crystals. Single crystals are ideal charge-transport materials for fashioning organic field-effect transistors (OFETs) with no grain boundaries and few charge traps. Despite much OFET research, a method for producing all-crystalline devices remains elusive.

This goal has been attained by W. Hu and co-workers at the Chinese Academy of Sciences (Beijing). They used single-crystalline materials as components for all of the operating units in an OFET device.

The researchers developed a simple mechanical exfoliation process that generates ultrathin (<100 nm), large-area (A4-paper size) single crystals of mica. They used the crystals as insulators in the assembly of the first all-crystalline OFETs. The devices, which also contain copper phthalocyanine, have high mobility and low threshold and operating voltages. These properties are accessible because of the high quality of the single crystals and the perfect interfaces between the contacts. (Adv. Mater. 2011, 23, 5502–5507; Ben Zhong Tang)

Improve the properties of epoxy-based nanocomposites by adding graphene oxide. The atom-thin carbon material graphene has remarkable properties. Single-layer graphene is the thinnest known material and has the highest measured modulus and breaking strength of any substance. Therefore, graphene and its oxides are of interest as components in polymer matrices, such as epoxy, to form high-performance nanocomposites.

D. R. Bortz*, E. G. Heras, and I. Martin-Gullon at the University of Alicante (Spain) and Grupo Antolin IngenierÍa (Burgos, Spain) prepared and characterized epoxy–graphene oxide (GO) nanocomposites. To form the desired GO composition, they used established techniques to unravel and splay open helical-ribbon carbon nanofibers into graphene layers. This technique gives primarily monolayer GO sheets that are highly soluble in water and polar organic solvents.

The authors prepared the GO materials by suspending commercially available helical-ribbon carbon nanofibers in concd H2SO4. The suspension is oxidized with KMnO4 at high temperature. [Temperature is not specified in article.—Ed.] Subsequent treatment with 30% H2O2 and workup produces the desired GO. Transmission electron microscopy showed that the fibers are unraveled and GO sheets with lateral sheet dimensions of 0.5–10 μm are formed. Sheets that appear to have one layer measured ≈2 μm in the lateral dimension.

The authors formed the polymer matrix by adding a commercial epoxy to a GO–acetone suspension and then heating it to 60 °C for 12 h. Additional heating under vacuum completely removed the acetone. Uniform dispersion of GO particles is ensured by passing the mixture through a three-roll calendar mill. Mechanical test specimens are formed by combining the polymer blend with the curing agent 1,2-diaminocyclohexane (100:17 wt/wt) and heating the mixture at 60 °C for 15 h followed by 110 °C for 6 h.

Adding 0.1 wt% GO increases the tensile modulus of the epoxy by 12%. Larger amounts of GO do not further enhance this property. Flexural strengths can be increased, however, by adding more GO: 1 wt% GO increases flexural strength by 23% compared with the control epoxy with no GO.

Mode I fracture toughness measurements showed more significant improvements; critical stress intensity factor (K1c) values increased by as much as 63% when 0.5 wt% GO was added. Similarly, critical strain energy release rates improved to as much as 111% higher than control values. Unexpectedly, the mechanisms for toughening conventional matrix materials appear to be absent in the GO-toughened nanocomposites. The authors suggest that coarse, multiplane GO features on the composite fracture surface deflect propagating crack fronts that generate new fracture surfaces and increase the strain energy needed to continue the fracture process.

Results from uniaxial fatigue testing of the nanocomposites indicate a fatigue life 420% greater than the control at a stress level of 40 MPa. An even more dramatic fatigue life improvement—1580%—is seen at the 25-MPa stress level.

This study addresses fundamental issues of interaction of polymeric matrices with atom-width fillers and begins to define the theoretical limits of nanocomposite properties. (Macromolecules 2012, 45, 238–245; W. Jerry Patterson)

Confine block copolymer nanostructures in nanobowl arrays. Q. Yan and colleagues at Tsinghua University (Beijing) studied the confinement of cylinder-forming polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA) copolymers in a silica nanobowl template. They prepared the template by infiltrating silica into the interstitial regions of a colloidal polystyrene array. PS-b-PMMA was then spin-coated onto the silica nanobowls and annealed under vacuum at 170 °C for 2 days. Scanning electron microscopy images show the confinement-induced morphologies after the PMMA phase is selectively etched via UV irradiation.

The morphologies of nanobowls with unmodified surfaces shift from a PMMA dotlike core with polystyrene sandwiched between a PMMA wetting layer to hexagonally packed PMMA dots with a polystyrene matrix adjacent to the PMMA surface as the confining diameter increases from 98 to 180 nm (images a through c in the figure). The authors discuss the relationship of packing frustration and degree of confinement to these morphologies.

The thickness of the PS-b-PMMA film influences the confined nanostructures. For example, in thicker regions near the center of 562-nm diam nanobowl substrates, PMMA cylinders align perpendicularly through the thickness, whereas concentric cylindrical domains are formed in the thinnest regions near the substrate edge. By modifying the surface with a PS-ran-PMMA brush, the authors eliminated the preferential segregation of PMMA to the silica wall to form PMMA dots within a polystyrene matrix.

When a hydroxyl-terminated polystyrene brush is used on the surface, the result is a hierarchically organized structure with three regions: a symmetric organization of PMMA dots within a polystyrene continuous domain in the inner layer; a middle layer consisting of a PMMA ring; and PMMA dots organized symmetrically within a polystyrene phase that wets the nanobowl as the outer region. (ACS Macro Letters 2012, 1, 62–66; LaShanda Korley)

Run a reductive alkylation with borane and chloroacetic acid. J. Chen and co-workers at Cephalon (Malvern, PA) developed a more efficient route to bendamustine, a blood cancer drug. A key step is attaching two chloroethyl groups to the amine nitrogen atom of an aniline derivative.

The authors discovered a reductive alkylation reaction in which the aniline is mixed with excess ClCH2CO2H in THF and treated with BH3–THF at room temperature. The mixture is heated to 60 °C to complete the reaction. Excess BH3 is quenched by adding MeOH. The resulting mixture is concentrated, diluted with water, and adjusted to pH 7–8 with K2CO3. The product precipitates and is filtered from the dissolved byproducts. (Org. Process Res. Dev. 2011, 15, 1063-1072; Will Watson)

Here are two stable unsubstituted thioheptacene isomers. Acenes with multiple fused aromatic rings are useful because of their substantial charge carrier mobility in organic thin-film transistors. The stability of these materials is limited, however; pentacene is the largest unsubstituted acene to be isolated and characterized. Longer analogues are prone to rapid oxidative degradation under ambient conditions. Greater stability can be conferred by appropriate substitution or introduction of sulfur-containing structures. Thioacenes with as many as seven fused systems have been reported.

P. K. De and D. C. Neckers* at Bowling Green State University (OH) synthesized two thioacene isomers (1 and 2) with seven fused rings, each containing a thiophene ring between acene fragments. Isomer 1 has seven linearly fused aromatic rings; in isomer 2, the fused rings are not linear. The remarkable feature is that both of these thioheptacenes are unsubstituted and oxidatively stable.

The authors prepared 1 by silylating commercially available o-xylylene dibromide (3) to form bis-silylated diyne 4. (DMPU is N,N’-dimethyl-N,N’-1,3-propyleneurea; DDQ is 2,3-dichloro-5,6-dicyano-1,4-benzoquinone.) The desired linearly fused heptacene scaffold is then formed via zirconium-mediated cyclization, followed by Takahishi coupling with periodothiophene, to give trimethylsilyl-substituted heptacyclic compound 5. (Cp is cyclopentadienyl.) The trimethylsilyl groups are removed with acid to form tetrahydroheptacene analogue 6. Aromatization gives target structure 1.

Nonlinearly fused isomer 2 is made by using a photocyclization strategy that begins with the conversion of commercially available 2-bromomethylnaphthalene (7) to the corresponding phosphonium ylide 8. Treating 8 with 2,5-thiophenedicarboxaldehyde gives conjugated thiophene derivative 9, which underwent iodine-mediated photocyclization to create nonlinearly fused isomer 2. The mechanism of this step includes a trans-to-cis conversion that forms a tetrahydrothioheptacene via photoinduced electrocyclization; oxidative aromatization then produces target structure 2.

Solution spectra of 1 and 2 show λmax values of ≈400–450 nm. The authors note that these absorption spectra correspond to that of anthra[2,3-b]thiophene, implying that the thioheptacenes and anthrathiophene have similar conjugation characteristics. In this sense, 1 and 2 can be viewed as two anthra[2,3-b]thiophene units. Because the structures of 1 and 2 are analogous to that of a heptacene, however, they may have enhanced 2-D interactions in the solid state with the potential for improved charge carrier mobility in thin-film transistors.

An additional useful feature is the partial solubility of 1 and 2 in most common solvents, which allows convenient solution processing for transistor devices. The authors are evaluating the potentially increased charge carrier mobility in test organic thin-film resistor devices. (Org. Lett. 2012, 14, 78–81; W. Jerry Patterson)

Paint Congo red–barium sulfate on an umbrella to detect acid rain. Acid rain occurs when large amounts of gases such as SO2 and NO2 are released into the atmosphere. When the gases are taken up by rainwater, they form acids that can cause breathing and lung problems; damage forests; pollute soil; and corrode buildings, statues, and sculptures. Acid rainfall is difficult to detect as it is falling; H.-W. Gao* and X.-H. Xu at Tongji University (Shanghai), however, have developed a simple acid rain–indicating material.

Their detector is an organic–inorganic hybrid made from Congo red (CR, an acid–base indicator that is blue when protonated and red when deprotonated) and BaSO4. CR@BaSO4 is a dark red semifluid that is insoluble in water. CR is confined between adjacent BaSO4 layers, which prevents it from leaching.

The authors painted the material on parts of an umbrella that was exposed to simulated acid rain over a range of pH values. The indicator detected acid rain with pH <5. The umbrella can be subjected to at least three acid rain cycles without losing activity. The blue color develops after 5 min, and the original red color reappears 1 h after the rain stops. (Chem. Commun. 2011, 47, 12810–12812; JosÉ C. Barros)

Polyiodides persist in the gas phase. The diverse variety of polyiodide structures fascinates chemists. With the notable exception of the triiodide ion (I3), however, compelling evidence for their existence in phases other than the solid state has been difficult to discover.

P. J. Dyson at EPFL Lausanne (Switzerland), S. A. Katsyuba at the Russian Academy of Sciences (Kazan), J. S. McIndoe at the University of Victoria (BC), and coauthors examined iodine-rich MeCN solutions of 1-(1-propyl)-3-methylimidazolium iodide by using electrospray-ionization mass spectrometry (EI-MS). They detected a host of higher polyiodides, including the monoanionic series In (n = 3, 5, 7, 9, 11, 13, and 15) and the dianionic series I2n2– (n = 2, 4, and 6). The figure shows a portion of the mass spectrum of the monoanionic series.

The isotope patterns of all of the ions are characteristically monoisotopic, and the authors obtained additional evidence for their identity from MS-MS studies. Each In ion fragments by the sequential loss of neutral I2. The dianions split into two monoanions, which also break down by losing I2.

Structures calculated for each of the mass spectrometrically characterized monoanions indicate that branched species are favored and that the longest bond in each structure always ends with a terminal I2 unit. This observation is consistent with the MS-MS results, in which the most favored fragment is always the result of I2 loss (rather than the loss of I4 or I6, which would suggest internal bond cleavage). (Inorg. Chem. 2011, 50, 9728–9733; Gary A. Baker)

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