In this episode of the Chemistry Literature Feature, we'll take a look at some new developments in molecular wires, track individual atoms through a catalyst that cleans your gasoline for you, meet some protein labels with interesting and useful functionalities, and more. But first, a quote from an education research seminar that recently happened in the department:
Overheard at Michigan:
"When we think about meaningful learning, we have to think about what we want our students to go out into the world and do. Do we want our students to be able to think about and tackle difficult problems? Or do we want them to play an awesome game of Trivial Pursuit?"Analytical - Visualizing the Stoichiometry of Industrial-Style Co-Mo-S Catalysts with Single-Atom Sensitivity
Hydrodesulfurization is a very important industrial process for purifying natural gas and petroleum products. Molybdenum disulfide is a well-known catalyst for this process, and the activity of MoS2 is enhanced by the addition of cobalt ions. The structural causes of that enhancement are not well-understood for this important class of catalysts. Using an extremely powerful microscopy technique called scanning tunneling microscopy (STM) in combination with electron energy loss spectroscopy (EELS), the authors of this Angewandte Chemie (GDCh) study ($) were able to determine the precise location of cobalt atoms within the MoS2 catalyst particles. Their conclusion that cobalt exists in a tetrahedrally-coordinated environment at the particle edges may help to design more effective catalysts in the future. However, the truly interesting part of the paper is the methodology, which allows the geography of small particles to be described on an atomic level.
Chemical Biology - De Novo-Designed Enzymes as Small-Molecule-Regulated Fluorescence Imaging Tags and Fluorescent Reporters
Living cells are really hard to understand. Proteins move around in immensely complicated ways, catalyzing a reaction here, moving a molecule across the membrane there, and so on and so forth. As a result, methods of understanding what's going on within a cell are always being developed. One such method is described by the authors of this study ($) in the Journal of the American Chemical Society (ACS). The authors use a de novo-designed protein (one which is not found in nature) to label a native protein they are interested in studying. The de novo protein tag has a number of advantages over other tags. Particularly interesting is the ability of the de novo tag to become fluorescent in the presence of a small molecule activator - all while preserving the reactivity of the original protein, which is very important. The fluorescence causes the protein-tag complex to light up, which means its location in the cells can be tracked, providing clues to its function.
Inorganic - Iridium Complexes of N-Heterocyclic Carbene Ligands: Investigation into the Energetic Requirements for Efficient Electrogenerated Chemiluminescence
There are a lot of ways to make a molecule light up. We've most often talked about fluorescence on this blog, but another phenomenon that causes chemicals to glow is electrochemiluminescence. During electrochemiluminescence, a luminescent molecule (luminophore) exists in an oxidized state. It then reacts with a reductant, which is either a reduced form of the luminophore or some sacrifical reductant, and the resulting release of energy causes the luminophore to glow. In this study ($), published in Organometallics (ACS), the authors study the electrochemiluminescent properties of a series of iridium N-heterocyclic carbene (NHC) complexes. Due to the easily-modified NHC ligand framework, five related iridium complexes were synthesized and their basic physical properties were measured alongside their luminescence activity. The authors are able to uncover some underlying factors controlling the brightness, color (wavelength), and mechanism of electrochemiluminescent materials. (Bonus points for a super dorky graphical abstract.)
Inorganic students might also be interested in the Materials paper, as they so often are.
Materials - Crystallization of Methyl Ammonium Lead Halide Perovskites: Implications for Photovoltaic Applications
An important aspect of solar cells is their crystallinity. Crystalline materials have few defects or particle boundaries for excited charges to bounce off of and get lost on their way to the electric circuit. For thin film devices that are compositionally complex, figuring out how to make uniform and comparatively crystalline layers of solar cell materials is a huge and important synthetic challenge. In a recent paper ($) published in the Journal of the American Chemical Society (ACS), the authors examine synthetic conditions that control the very beginnings of crystal growth, called nucleation, for the organic-inorganic perovskite solar cell materials methylammonium lead bromide and methylammonium lead iodide. They find via microscopy investigations that the addition of lead chloride makes for well-behaved nucleation, and then see the effects of the improved synthesis pay off in the increased performance of their solar cells.
Organic - Vanadium-Catalyzed Solvent-Free Synthesis of Quaternary α-Trifluoromethyl Nitriles by Electrophilic Trifluoromethylation The trifluoromethane functional group (-CF3) is an important structural component for a wide variety of interesting organic molecules, including prescription drugs and agricultural products. Methods for installing trifluoromethyl groups can include some harsh conditions and are constantly improving. A particularly rare feat for organic chemists thus far has been creating quaternary carbon centers (carbons with 4 carbon-carbon bonds) that contain -CF3 groups. A recent development ($) reported in Angewandte Chemie (GDCh) shows the activity of an oxovanadium catalyst towards installing -CF3 groups to form products with quaternary trifluoromethylated centers.
Physical - Electron transfer through rigid organic molecular wires enhanced by electronic and electron-vibration coupling
Certain biological systems are capable of moving electrons over surprisingly large distances, and there are research efforts in a variety of fields to figure out how that is and whether we can build systems to emulate it. The emerging field of molecular electronics is one such research area. The authors of this recent paper ($) published in Nature Chemistry (NPG) examines the charge transfer between a zinc porphyrin complex tethered to a C60 fullerene by means of both rigid and flexible molecular wires. Their data, taken from electrochemistry experiments as well as ultrafast electronic transient absorption spectroscopy, quantifies the rate of charge transfer from the zinc to the fullerene, and also the rate of the back electron transfer. The authors also identify a vibrational component to the charge transfer through rigid wires by which recombination (backwards electron transfer) of the charges is accelerated relative to flexible wires. While the recombination isn't the best, the fast initial charge transfer is an exciting prospect for future research.
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ACS - American Chemical Society
GDCh - Gesellschaft Deutscher Chemiker
NPG - Nature Publishing Group