Jonathan G. Weis

Rapid prototyping of carbon-based chemiresistive gas sensors on paper

Significance This paper describes a rapid, solvent-free, two-step procedure for the fabrication of selective gas and vapor sensors from carbon nanotubes and graphite on the surface of paper that overcomes challenges associated with solvent-assisted chemical functionalization and integration of these materials into devices. The first step generates solid composites from carbon nanotubes (or graphite) and small molecules (chosen to interact with specific types of gases and vapors) by mechanical mixing and subsequent compression into a form similar to a conventional pencil “lead”. The second step uses mechanical abrasion (“drawing”) of these solid composites on the surface of paper to generate functional devices. The use of diverse composites yields sensing arrays capable of detecting and differentiating gases and vapors and part-per-million concentrations. Chemically functionalized carbon nanotubes (CNTs) are promising materials for sensing of gases and volatile organic compounds. However, the poor solubility of carbon nanotubes hinders their chemical functionalization and the subsequent integration of these materials into devices. This manuscript describes a solvent-free procedure for rapid prototyping of selective chemiresistors from CNTs and graphite on the surface of paper. This procedure enables fabrication of functional gas sensors from commercially available starting materials in less than 15 min. The first step of this procedure involves the generation of solid composites of CNTs or graphite with small molecule selectors—designed to interact with specific classes of gaseous analytes—by solvent-free mechanical mixing in a ball mill and subsequent compression. The second step involves deposition of chemiresistive sensors by mechanical abrasion of these solid composites onto the surface of paper. Parallel fabrication of multiple chemiresistors from diverse composites rapidly generates cross-reactive arrays capable of sensing and differentiating gases and volatile organic compounds at part-per-million and part-per-thousand concentrations.

Stereospecific Ring-Opening Metathesis Polymerization of Norbornadienes Employing Tungsten Oxo Alkylidene Initiators

We report here the polymerization of several 7-isopropylidene-2,3-disubstituted norbornadienes, 7-oxa-2,3-dicarboalkoxynorbornadienes, and 11-oxa-benzonorbornadienes with a single tungsten oxo alkylidene catalyst, W(O)(CH-t-Bu)(OHMT)(Me2Pyr) (OHMT = 2,6-dimesitylphenoxide; Me2Pyr = 2,5-dimethylpyrrolide) to give cis, stereoregular polymers. The tacticities of the menthyl ester derivatives of two polymers were determined for two types. For poly(7-isopropylidene-2,3-dicarbomenthoxynorbornadiene) the structure was shown to be cis,isotactic, while for poly(7-oxa-2,3-dicarbomenthoxynorbornadiene) the structure was shown to be cis,syndiotactic. A bis-trifluoromethyl-7-isopropylidene norbornadiene was not polymerized stereoregularly with W(O)(CHCMe2Ph)(Me2Pyr)(OHMT) alone, but a cis, stereoregular polymer was formed in the presence of 1 equiv of B(C6F5)3.

Transition Metal-Oxide Free Perovskite Solar Cells Enabled by a New Organic Charge Transport Layer

Various electron and hole transport layers have been used to develop high-efficiency perovskite solar cells. To achieve low-temperature solution processing of perovskite solar cells, organic n-type materials are employed to replace the metal oxide electron transport layer (ETL). Although PCBM (phenyl-C61-butyric acid methyl ester) has been widely used for this application, its morphological instability in films (i.e., aggregation) is detrimental. Herein, we demonstrate the synthesis of a new fullerene derivative (isobenzofulvene-C60-epoxide, IBF-Ep) that serves as an electron transporting material for methylammonium mixed lead halide-based perovskite (CH3NH3PbI(3-x)Cl(x)) solar cells, both in the normal and inverted device configurations. We demonstrate that IBF-Ep has superior morphological stability compared to the conventional acceptor, PCBM. IBF-Ep provides higher photovoltaic device performance as compared to PCBM (6.9% vs 2.5% in the normal and 9.0% vs 5.3% in the inverted device configuration). Moreover, IBF-Ep devices show superior tolerance to high humidity (90%) in air. By reaching power conversion efficiencies up to 9.0% for the inverted devices with IBF-Ep as the ETL, we demonstrate the potential of this new material as an alternative to metal oxides for perovskite solar cells processed in air.

Quinoxalines Directly from Electron-Deficient Alkynes

Significance: The authors report the metal-free oxidative [4+2] annulation of electron-deficient alkynes and ortho-phenylenediamines to synthesize quinoxalines, which are most commonly prepared by condensing diamines with α-diketones. The reactions with electron-rich, -neutral, and -poor substrates all proceed with high yield. The synthesis of anhydride 4 demonstrates the utility of this method, as such electron-deficient quinoxalines are challenging to prepare by traditional methods. Comment: Initial results with tert-butyl hypoiodite predominately yielded the dearomatized cis,cismucononitrile. Phenyliodine diacetate (PIDA) proved to be crucial, despite attempts with several other hypervalent iodine reagents. The reaction is also highly solvent-dependent, proceeding more effectively with increasing solvent polarity. NH2 NH2 R3 R2 N N

Superacid Anions from Tetracyanothiophene

Significance: The authors report an improved synthesis of tetracyanocyclopentadienyl anions, a class of superacid anions with functional handles. By including the leaving group on the nucleophile, product 3 is accessed more directly from thiophene 1 and sulfone 2. The researchers demonstrate the reaction’s utility with a variety of sulfones and successful exchange of cations. Comment: Optimization of the reaction conditions revealed a strong dependence on base, with NaH outperforming alkoxide, silazide, and non-ionic nitrogen bases. Using phenylsulfones as the leaving groups proved more facile than using halides. Ketones could not be directly synthesized with this method but can be obtained from the Weinreb amide derivative 3c. S

Precisely Defined Electron-Rich Oligopyrroles

Significance: The authors report the synthesis of 3,3′-linked oligopyrroles through the domino ringenlargement of cyclopropanes. In one step, furan is converted into 2, which is subsequently transformed into diketone 3 via Weinreb-ketone synthesis. Refluxing with catalytic acid in benzene with an aniline, yielded the desired bispyrrole products 4. Unsurprisingly, the electron-rich anilines provided significantly higher yields than the electronpoor anilines. Comment: Extended oligoacetalic diketone 5 was synthesized and subsequently converted into quarter-pyrrole 6 in 25% yield. This is the first electron-rich, precisely defined oligopyrrole reported and it is noteworthy due the inherent instability of electron-rich oligopyrrolic systems. O COOEt R1