Noteworthy Chemistry

November 14, 2011

Cyclo-p-phenylene emissions blue-shift as the ring size increases. The emission color of an inorganic semiconductor quantum dot shifts bathochromically with increasing particle size (see figure). Similarly, the luminescence color of a linear p-phenylene (LPP) oligomer red-shifts with increasing chain length. [n]Cyclo-p-phenylenes (CPPs) are cyclic forms of LPPs, or “nanohoops”. How does the emission color of CPPs change with ring size?

R. Jasti and co-workers at Boston University reported the synthesis of the first nanohoop, [12]CPP, in 2008 (Jasti, R., et al. J. Am. Chem. Soc. 2008, 130, 17646–17647). The group has now prepared [7]CPP, a much smaller nanohoop, via successive orthogonal Suzuki–Miyaura coupling reactions.

The authors compared [7]CPP with [12]CPP and the recently prepared [8]CPP and found that the CPP emission trend is the reverse of quantum dots and LPPs. CPPs [7], [8], and [12] emit at 592, 540, and 450 nm, respectively, showing that CPP emission blue-shifts with increasing ring size. (J. Am. Chem. Soc. 2011, 133, 15800–15802; Ben Zhong Tang)

Functional coatings give cellulose fibers a host of properties. A. Anthanassiou and colleagues at the Italian Institute of Technology (Lecce and Genoa) and the Italian National Research Council Institute of Nanoscience (Lecce) developed a simple, industrially scalable method for generating multifunctional waterproof cellulose fiber mats. They wetted cellulose fiber networks with biocompatible ethyl-2-cyanoacrylate (ECA) monomer solutions. ECA polymerization was initiated by atmospheric moisture and surface hydroxyl groups under ambient conditions.

The resulting recyclable cellulose sheets retained their fiber network morphology with poly(ethyl-2-cyanoacrylate) (PECA)–coated individual fibers. The PECA-functionalized cellulose sheets are water-resistant.

The degree of hydrophobicity can be modulated by adding plant wax or poly(tetrafluoroethylene) (PTFE) to the ECA solution to form a PECA cladding “decorated” with these hydrophobic particles. For example, including 20 wt% PTFE particles made the sheets superhydrophobic and self-cleaning.

The authors also showed that a plant wax–PECA coating on printing paper retains the dimensionality of the sheet and makes the paper water-repellent. They added other properties—luminescence, magnetic responsiveness, and bacterial resistance—to the cellulose sheets by incorporating quantum dots, magnetic nanoparticles, and silver nanoparticles, respectively, into the PECA fiber coating. (ACS Appl. Mater. Interfaces 2011, 3, 4024–4031; LaShanda Korley)

Should you use an acetyl or benzoyl protecting group in the synthesis of apricitabine? Apricitabine [2-(R)-hydroxymethyl-4-(R)-(cytosine-1’-yl)-1,3-oxathiolane] is an HIV reverse transcriptase polymerase inhibitor being developed by J. J. Deadman and coauthors at Advanced Medical Technologies (Scoresby, Australia), the University of Melbourne, and Avexa (Melbourne) to treat AIDS. The authors synthesized it from 2-(R)-benzoyloxymethyl-1,3-oxathiolane by oxidizing the sulfide group to a sulfoxide, coupling the sulfoxide with N-protected cytosine, and deprotecting the intermediate. The 2-(R) stereocenter comes from the starting material, but the 4-(R) center is generated during the nonstereospecific coupling step.

When N-acetylcytosine is used in the coupling step, the initial product must be deprotected and then converted to the p-toluenesulfonic acid (p-TsOH) salt, which crystallizes as a conglomerate. The N-benzoyl intermediate obtained from coupling with N-benzoylcytosine crystallizes as a conglomerate, which eliminates the need for making the p-TsOH salt. (Org. Process Res. Dev. 2011, 15, 763–773; Will Watson)

This ladder polymer takes up large amounts of hydrogen. Spiro(fluorene-9,9’-xanthene) (1) is easily prepared and has high thermal and oxidative stability. Because of its rigid, contorted (nonplanar) spiro structure, polymers based on 1 might be expected to have intrinsic microporosity and large specific surface areas.

B.-H. Han and coauthors at the National Center for Nanoscience and Technology (Beijing), Yangzhou University (China), and Central China Normal University (Wuhan) realized the potential of this structural scaffold for gas sorption. They report a series of four ladder-type copolymers synthesized from brominated monomers 2 and 3 with Suzuki or Sonogashira–Hagihara coupling techniques. Copolymer 4 showed the greatest hydrogen uptake ability in the test series.

The authors studied the gas sorption characteristics of the copolymers. The Brunauer–Emmett–Teller specific surface area for 4 is 965 m2/g, with a dominant pore size distribution of 0.6–0.8 nm. This result is consistent with reports that pore diameters in this range are optimal for hydrogen adsorption at low pressures.

The hydrogen uptake capacity of 4, based on gravimetric hydrogen adsorption isotherms, is 2.22 wt% at 1.0 bar and 77 K. This value is among the highest reported for porous organic polymers and is competitive with such high-efficiency porous materials as activated carbon (2.40 wt%) and metal–organic frameworks (2.59 wt%).

The authors suggest that the high hydrogen uptake of 4 is the result of its high specific surface area, the location of heteroatoms in the spiro scaffold, and a narrow pore size distribution. These features may be useful in future designs of high-efficiency porous materials for gas storage. (Macromolecules 2011, 44, 7987–7993; W. Jerry Patterson)

Biological materials inspire synthetic multiscale structures. Earth’s organisms adapt to their environment: They sense changes and respond to stimuli to survive. Materials that mimic biological adaptation are of increasing interest for many applications.

Our knowledge and understanding of hierarchical structures in organisms have been enriched significantly in recent years. The secrets of structure–property relationships in natural systems have been slowly unveiled with the assistance of improved characterization techniques at the micro- and nanoscales.

The “lotus effect” refers to the high degree of hydrophobicity exhibited by lotus leaves. The leaves have tiny papillae on their surfaces; each papilla is covered by a hydrophobic material that repels water droplets. Aqueous particles can roll freely with little association, collect debris, and clean the leaves. In contrast, papillae on rice leaves are assembled anisotropically, so that water can flow only in one direction. These natural assemblies and hydrophobic properties can be applied to materials design.

K. Liu and L. Jiang* at Beihang University and the Chinese Academy of Sciences (both in Beijing) summarize nature’s remarkable hydrophobic effects and the corresponding synthetic structures. They recommend that researchers continue to investigate unique biological properties and that they design multifunctional materials inspired by two or more biological materials. (ACS Nano 2011, 5, 6786−6790; Sally Peng Li)

Make white organic light-emitting diodes with long lifetimes by controlling charge recombination zones. A major concern in the manufacture of white organic light-emitting diodes (WOLEDs) is the products’ life spans. In spite of much research, it remains a difficult challenge. A team led by Y. Qiu at Tsinghua University (Beijing) developed an elegant strategy to make WOLEDs with unprecedentedly long lifetimes.

The key to the team’s strategy is controlling the recombination zones in WOLEDs. They deposited an extra blue-light–emitting layer (BLEL) on top of a mixed-host BLEL to prevent the holes from penetrating into the electron-transporting layer and to better confine carrier recombination. In this way, the researchers prepared a WOLED with a lifetime of >150,000 h at an initial brightness of 1000 cd/m2. They observed almost no color shift in the electroluminescent spectrum of the long-lived device after an accelerated aging test. (Adv. Funct. Mater. 2011, 21, 3540–3545; Ben Zhong Tang)

Core-extended terrylenes have optically active atropisomers. Atropisomeric chromophores, a class of chiral substances, can be defined as stereoisomers in which the element of chirality is not located on an atom, but on a molecular plane or axis. Most of these chiral molecules lack adequate conformational stability to be isolated as discrete isomers: For example, some perylenetetracarboxdiimides apparently form stereoisomers, but the racemization energy barrier is too low to form enantiomerically pure compounds.

C. Li, K. MÜllen, and coauthors at the Max Planck Institute for Polymer Research (Mainz, Germany), the Chinese Academy of Sciences (Qingdao), and the J. Gutenberg University of Mainz developed a procedure to extend the terrylenediimide aromatic core (1) and create unsubstituted tetranaphthoterrylenetetracarboxdiimides (2). (Terrylene is formally tribenzo[de,kl,rst]pentaphene; Tf is trifluoromethanesulfonyl; TMS is trimethylsilyl; dba is dibenzylideneacetone.)

Product 2 exists in three isomeric forms, two of which [(P)-2b and (M)-2b] are highly stable atropoenantiomers with enough conformational stability to allow complete enantiomeric separation. The authors separated structure 2a from the two atropoenantiomers by using standard silica chromatography. Atropisomers (P)-2b and (M)-2b were separated via chiral HPLC, in which they appeared as two distinct peaks on the chromatogram. Absolute configurations of these isomers were assigned on the basis of circular dichroism spectra and quantum chemical calculations.

The two atropisomers interconvert little even at temperatures as high as 80 °C. The unusual properties of these isomers originate from sterically induced bending of the four naphtho groups out of the molecular plane.

The authors suggest that (P)-2b and (M)-2b may be useful for chiral molecular switches. Another application may be single-molecule chiroptical spectroscopy because of the atropisomers’ strong absorption and fluorescence of visible light and their conformational stability. (Org. Lett. 2011, 13, 5528–5531; W. Jerry Patterson)

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