Noteworthy Chemistry

June 16, 2014

What gives blue sapphires their color? The mineral corundum consists mainly of α-Al2O3. In its pure form, it is a transparent ceramic material; but its crystals become blue sapphires in the presence of iron and titanium impurities. The effect of iron and titanium has long been known, but the optical origin of the color remains unexplained. A. Walsh and colleagues at the University of Bath (UK) and University College London used atomistic modeling to clarify the optical process in blue sapphires.

According to previous work, there are two possible optical mechanisms:

  • intervalence charge transfer (IVCT) between Ti(IV) and Fe(II) [derived from Ti(III) and Fe(III)] and
  • intra-atomic d–d orbital transitions in the metal impurities.

The authors used a polarizable interatomic potential to model the charge transfer process and the angular overlap model to probe crystal field transitions.

The authors calculated that for the reaction Fe(III) + Ti(III) → Fe(II) + Ti(IV) to proceed, the optical absorption energy of the face-sharing nearest-neighbor pairs of iron and titanium should be 1.85 eV (equivalent to a wavelength of 670 nm). This value is consistent with results measured with absorption spectroscopy. The IVCT pathway is also supported by the difference in binding energies between the Ti(IV)–Fe(II) and Ti(III)–Fe(III) ion pairs.

In contrast, the authors showed that the d–d transitions in the metal impurities are outside the appropriate spectrum range or too weak to cause the intense coloration in blue sapphires. This finding excludes the possibility of intra-atomic d–d transitions. (Chem. Commun. 2013, 49, 5259–5261Xin Su)

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Colorful colloid crystals combat counterfeiters. Anticounterfeiting efforts often rely on materials whose colors arise from physical rather than chemical properties. Iridescence, dichroism, diffraction, and other color properties are difficult to imitate convincingly. Colloidal crystal films, thin films formed from monodisperse colloidal particles self-assembled into periodic lattices, would seem to be ideal candidates for this type of application. Difficulties in production and the fragility of the resulting films, however, present obstacles to practical applications. Freestanding films and micropatterns often require complicated preparation processes. Strong light scattering makes the films milky or opaque, limiting the ability to mix colors using multiple layers.

S.-M. Yang, S.-H. Kim, and co-workers at the Korea Advanced Institute of Science and Technology (Daejeon) developed a practical method for creating highly transparent colloidal photonic crystal films in a variety of colors. They dispersed colloidal silica particles with the desired size in a photocurable ethoxylated trimethylolpropane triacrylate resin. To reduce light scattering and increase the transparency of the resulting film, the authors chose a resin with a similar refractive index to the particles.

The suspension was drawn between two glass plates by capillary forces and then photopolymerized under UV light. The resulting crystalline films can be micropatterned by using conventional photolithography.

Because the films are transparent, multicolored patterns can be made by superimposing two or more colored layers. Combining red, green, and blue structural colors produces the secondary colors cyan, magenta, yellow, and white. The colors produced lie within a narrow spectral range to provide a high degree of specificity, and they vary with the viewing angle and the size of the colloidal particles. The films are highly transparent under ambient light, but when the incident angle of the light is the same as the angle of observation, the films appear to be brightly reflective.

In the figure, the optical images (b) of the colloidal crystal pattern composed of letter Ks show a reflection color that strongly depends on the angle of incidence of the light, which is given in each image. Optical image (c) is a freestanding film that contains a pattern of Ks. Image (d) is a patterned photonic crystal film on a Korean bank note. The pattern exhibits a bright green color under normal reflection (main image), whereas it displays a blue color for high incident and reflection angles (top right). The film is highly transparent and difficult to distinguish when the viewing angle is different from the angle of incidence of the light (bottom right).

The authors note that their colored films can also be used in decorative coatings, paints, or cosmetics. (Chem. Mater. 2013, 25, 2684–2690Nancy McGuire)

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Metallacycles show rare Möbius aromaticity and aggregation-enhanced emission. The desire to set new scientific records drives researchers to design and synthesize molecules with ever-more exotic structures and abnormal properties. Antiaromatic compounds such as pentalyne (1) are difficult to prepare and isolate because of their high energy and reactivity. J. Zhu, H. Xia, and colleagues at Xiamen University (China) and the University of Georgia (Athens) took an elegant route to synthesizing this exotic cyclic structure. They stabilized the elusive pentalyne by incorporating a transition-metal atom into the ring system.

The researchers prepared an osmapentalyne complex (2) in high yield via a one-step coupling of an alkyl propiolate with an osmium alkenyl complex at room temperature. The complex features the smallest carbyne angle observed to date and full planarity throughout the pentalyne unit.

Incorporating the metal atom into the ring system relieves considerable strain and results in rarely achieved Craig-type Möbius aromatic stabilization. Whereas molecular luminescence is often weakened by aggregation, 2 exhibits extraordinary aggregation-enhanced near-IR photoluminescence with a large Stokes shift and a long emission lifetime. (Nat. Chem. 2013, 5, 698–703; Ben Zhong Tang)

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The structure of the 2-norbornyl cation is finally resolved. For more than 50 years, there has been a vigorous debate over the structure of the 2-norbornyl cation. S. Winstein and P. D. Bartlett, among others, argued for the nonclassical structure (1), whereas H. C. Brown proposed rapidly equilibrating enantiomers (2). I. Krossing and colleagues at the University of Freiburg (Germany), the University of Erlangen-Nuremberg (Erlangen, Germany), and the University of Georgia (Athens) have finally pinned down the cation’s structure by X-ray diffraction (XRD) analysis.

Proposed structures for the 2-norbornyl cation and the complex used to study its crystal structure

A key step was the use of “soft” bromoaluminate anions (Al2Br7) to stabilize the carbocation. The authors treated 2-exo-norbornyl bromide (3) with Al2Br6 in the presence of CH2Br2 to obtain solvated [C7H11]+[Al2Br7]·CH2Br2 crystals (4). They observed phases in XRD structures at >86 K that contained ordered anions but disordered cations. They attribute this result to the near-spherical shape of the cations and 6,1,2-hydride shifts within them. When they cooled the material to ≈50 K with careful annealing, they obtained crystals suitable for analysis.

The experimental crystal structure agreed with computed structures of the cation and confirmed the nonclassical geometry. The authors believe that their new tool, the bromoaluminate anion, should be used to stabilize other carbocations so that their crystals can be studied. (Science 2013, 341, 62–64; José C. Barros)

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