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Ketone Reduction with Magnesium

Making catalysts from non-precious metals has been a challenge embraced by chemists over the past few decades. In particular, the ability of alkaline earth metal catalysts to be both highly stable and highly reactive has made them attractive for study. Magnesium complexes can catalyze hydroelementation across a carbon-oxygen double bond with a wide scope of substrates. However, it isn’t enough to just perform this reaction. For meaningful reactivity the catalyst would be enantioselective and prior to this report only one example of a stereoselective magnesium-based system existed in the literature. A persistent barrier to developing this reactivity has been the propensity of alkaline earth metal hydrides (a likely step in the catalytic cycle) to form contact-ion pairs with available anions. Based on this framework, researchers in Germany developed a catalyst motif involving borohydride-alkaline earth metal adduct formation to enhance stereoselectivity.

They drew on their precious work utilizing manganese complexes that demonstrated enantioselectivity, applying it to the magnesium system. They could straightforwardly create the magnesium alkyl complexes by mixing ligand and magnesium alkyl precursors. The complexes were tested for hydroelemetation using acetone as a model substrate. The rate and selectivity of the reaction was significantly impacted by the choice of reducing agent, with non-boron reductants producing low yields and low enantioselectivity. Altering the ligand backbone didn’t improve selectivity, but purifying an assembled precatalyst and slowly raising the temperature from -40 oC to RT increased enantiomeric excess to 96%. Even more exciting, using this general reaction format high enantioselectivity was seen for a wide range of aryl alkyl ketones screened with no adverse reactivity seen towards ester moieties (Figure 1). This system also catalyzes reactions with α-substituted, cyclic alkyl aryl, and dialkyl ketones, but with lowered enantioselectivity. However, the catalyst is tolerant of a range of functional groups and remains selective for carbonyls implying broad possible utility.

Figure 1. Substrates tested for hydroelementation using a magnesium catalyst with conversion and stereoselectivity numbers.

To more fully understand the system, the researchers undertook experiments to find possible intermediates in the catalytic cycle. The reactivity at room temperature required consistent low temperature work to prevent further progress of the intermediates along the catalytic cycle. In the absence of a boron-derived reducing agent, the catalyst forms a highly labile, and expected, alkoxide intermediate that is not observable under catalytic conditions. When the precatalyst is reacted with excess borane it forms a borohydride intermediate that was crystallographically observed.

Figure 2. Solid state molecular crystal structure of borohydride intermediate.

When this complex was reacted with a fluorinated ketone, variable temperature boron and fluorine NMR was used to identify transient species. Upon addition of the ketone immediate shifts in the boron signal to two species, one of which remains when the temperature increases to -60 oC. The unstable species can be attributed to a ketone borohydride complex, but the dynamic nature of the system makes crystallizing these intermediates very challenging. DFT calculations suggest a low-energy transition state that favors the (S)-product and accounts for the catalyst’s consistent stereoselectivity.

To find out more, please read:

Borohydride intermediates pave the way for magnesium-catalysed enantioselective ketone reduction

Vladislav Vasilenko, Clemens K. Blasius, Hubert Wadepohl and Lutz H. Gade

Chem. Commun., 2020, 56, 1203-1206

About the blogger:

Beth Mundy is a PhD candidate in chemistry in the Cossairt lab at the University of Washington in Seattle, Washington. Her research focuses on developing new and better ways to synthesize nanomaterials for energy applications. She is often spotted knitting in seminars or with her nose in a good book. You can find her on Twitter at @BethMundySci.

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Color switching with iridium complexes

Piezochromic luminescent materials (PCLMs) exhibit emissive behavior in response to mechanical stimuli and have captured extensive scientific attention, and not just because they’re awesome. Most display reversible bicolor switching, but a lack of universal design principles has hampered development of PCLMs with multiple colors. Utilizing the polymorphs of organic dyes, which generally have different luminescence profiles, is a promising approach towards creating multicolor switches. Iridium (III) complexes can be separately both phosphorescent and polymorphic, but no examples exhibiting both properties had been experimentally shown prior to this work.

Figure 1. Structure of the piezochromic iridium complex.

Researchers in China and the UK synthesized a novel cationic Ir(III) complex with a ligand rationally designed for PCL behavior (Figure 1). The bulky ligand should disrupt close crystal packing and leave the material prone to collapse to an amorphous solid when under external pressure. The ligands also consist of heterocycles compounds, which are more likely to have polymorphs, possibly due to the range of non-covalent interactions between different atoms in the solid phase. These cyclic ligands are also quite rigid with limited intramolecular rotational motion, which in studies of aggregates showed that decreasing intramolecular vibrations and rotations can increase luminescence intensity. With all those design considerations put in place, the researchers generated three powder samples of the iridium complex with different emission profiles (blue, yellow, and green). Reproducibly switching the emission color is simple, done by either mechanical grinding or applying solvent.

Figure 2. Photographs and schematic of tricolor switching iridium complex.

The initial yellow complex showed low emission when solvated by acetonitrile, but altering the solvent profile to be 90% water induces aggregation and a significant increase in emission. Incorporating the complex into a rigid PMMA matrix similarly enhances its emission, which supports the hypothesis that the rigidity and corresponding diminishment of intramolecular rotations and vibrations serve to enhance the emission. All three solid polymorphs can be obtained directly via recrystallization from different solvents or by cycling through grinding and/or adding small amounts of solvent. Despite the clearly different optical properties, the samples all had identical 1H NMR spectra. The researchers turned to powder X-ray diffraction spectroscopy (PXRD) to gain additional information about the spatial arrangement of the atoms within the molecule and the overall crystallinity of the solids. They saw that the green and blue forms of the complex form distinct crystalline phases, whereas the yellow complex is completely amorphous. The blue crystals show the molecules in antiparallel coupling, while the molecules in the green crystal pack in a herringbone arrangement. However, both pack quite loosely and are thus easily collapsed to an amorphous form when under pressure. Molecular modeling used the obtained molecular confirmations to model the energy levels of the HOMO and LUMO. The models showed that the increased planarity of the phenyltriazole moieties and increase twist of the pyridine-based ligand cause the higher HOMO-LUMO gap, shifting the emission to higher energies.

Interestingly, the impact of solvent interactions with the molecules, particularly during crystallization, cannot be discounted. The researchers observed multiple hydrogen-bonding interactions with the iridium complex and either toluene or ethanol. Given the relative ease of switching, as a proof-of-concept the researchers patterned a tree and sky scene starting from the yellow form of the complex and adding ethanol or toluene with a capillary (Figure 3). The scene could be erased and repatterned with no obvious molecular performance degradation.

Figure 3. Photographs of the tree patterned from the three forms of the iridium complex.

To find out more, please read:

Reversible tricolour luminescence switching based on a piezochromic iridium(III) complex

Tianzhi Yang, Yue Wang, Xingman Liu, Guangfu Li, Weilong Che, Dongxia Zhu, Zhongmin Su and Martin R. Bryce

Chem. Commun., 2019, 55, 14582-14585.

About the blogger:

Beth Mundy is a PhD candidate in chemistry in the Cossairt lab at the University of Washington in Seattle, Washington. Her research focuses on developing new and better ways to synthesize nanomaterials for energy applications. She is often spotted knitting in seminars or with her nose in a good book. You can find her on Twitter at @BethMundySci.

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Tuning Zeolite Catalysis with Organic Molecules

Zeolites, a class of porous alumina-silicate materials, are industrially critical adsorbents and catalysts. Their highly robust nature and wide range of structural types (over 200!) make them suited to a range of applications. In particular, the general zeolite topology and pore size are selected to match and stabilize the intermediates of a chemical reaction. However, the tunability of zeolites is limited when compared to molecular catalysts, making them more like a solvent than, say, an enzyme. An active field of research is bridging the gap between the robust, scalable zeolites and highly controllable homogenous catalysts. Recent work identified organic residues maintained with the zeolite pores as key in the transformation of methanol to hydrocarbons. Previous fundamental studies demonstrated that a wide range of carbonyl and carbonyl derivative compounds promote the dehydration of methanol to dimethyl ether (DME).

Researchers at BP used methyl mono- and di-carboxylate esters to dehydrate methanol to DME at low temperatures. The mild reaction conditions allowed for high selectivity for DME while eliminating convoluting side reactions. They added either methyl formate or methyl n-hexanoate to a series of zeolite with pores ranging from narrow to wide. At a 5 mol% concentration relative to methanol they saw significant increases in DME production, particularly for the medium and wide pores. Systematic testing of carboxylate chain length found that increasing chain length increased turnovers occurred until methyl n-hexanoate, after which no further benefits were observed as the n-methyl hexanoate had already saturated the catalyst (Figure 1). All proved highly selective for converting methanol to DME with no observed hydrocarbon formation.

Figure 1. Production of DME on a medium-pore zeolite with methyl carboxylate esters of varying chain lengths.

The experimental results were coupled with theoretical work modeling the energetics of the adsorption of the ester onto the zeolite. The calculations showed an increase in adsorption energy with increased chain length, attributed to van der Waals interactions.

Figure 2. Transition state predicted by molecular modeling with methanol attacking the organic promoter adsorbed on the zeolite catalyst.

They also gave even higher energies to molecules with two carboxylate esters, like dimethyl adipate. In fact, the strongly binding molecules produced increased catalysis at loadings as low as 0.001% with respect to methanol. The promoters can be easily switched by changing the input, demonstrating the reversibility of binding at the active site. Additional molecular modeling was used to study possible transition states to develop a catalytic cycle. A proposed transition state involves a direct reaction between the methanol and the organic promotor, however specific evidence has yet to be seen. Additional work examining the role of the water present as a co-adsorbate and its impacts on transition states has yet to be done. Overall, the use of various organic molecules as promotors for the dehydration of methanol to DME on various zeolite catalysts was explored. This represents exciting fundamental study of industrially-relevant chemistry with significant room for future work.

To find out more, please read:

Getting zeolite catalysts to play your tune: methyl carboxylate esters as switchable promoters for methanol dehydration to DME

Benjamin J. Dennis-Smither, Zhiqiang Yang, Corneliu Buda, Xuebin Liu, Neil Sainty, Xingzhi Tan and Glenn J. Sunley

Chem. Commun., 2019, 55, 13804-13807.

About the blogger:

Beth Mundy is a PhD candidate in chemistry in the Cossairt lab at the University of Washington in Seattle, Washington. Her research focuses on developing new and better ways to synthesize nanomaterials for energy applications. She is often spotted knitting in seminars or with her nose in a good book. You can find her on Twitter at @BethMundySci.

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Designing Syntheses with Machine Learning

I don’t know if you’ve looked at the structure of pharmaceuticals recently, but most novel drugs are rather complicated. Identifying promising new targets is just the start for synthetic chemists; they then need to figure out how to use a series of reactions to take simple (and commercially available) molecules and transform them into a new drug. They also must predict all possible side reactions and products given a set of reaction conditions, particularly when a range of functional groups are involved. Historic approaches involved manual curation of reaction rules, limited by personal experience and the state of the accessed chemical literature. Newer approaches seek to create templates directly from data but are defined by available data sets and cannot reliably extrapolate. The emergence of machine learning offers the opportunity to move beyond traditional templating and atom mapping of reactants to products. It also offers to take full advantage of novel technologies and address problems with dataset bias and ineffective modeling systems.

In a collaboration between academics in the UK and industrial scientists in the US, researchers used Molecular Transformer, an attention-based machine translation model, to perform both reaction prediction and retrosynthesis analysis after training on a publicly available dataset. Instead of atom mapping, which moves atoms from the reactants to the products, Molecular Transformer (MT) relies on SMILES text strings, which represent structures in a line format. A unique aspect of this work is the validation and training performed using proprietary data of drug targets from Pfizer. They used three datasets: the first a literature standard from the US Patent and Trade Office (USPTO), the second from internal medicinal chemistry projects in Pfizer, and the final a diverse range of 50,000 reactions from US patents (USPTO-R). Building on previous research from the authors, they trained the MT on both the Pfizer data and the initial USPTO data sets. They found that the Pfizer data provided the most accurate product predictions and that the MT could also return a confidence rating to determine the probability the prediction is correct.

Figure 1. Sample syntheses predicted by Molecular Transformer for various bioactive molecules of interest.

While synthesis predictions can easily be checked, it’s harder to confirm accuracy with retrosynthesis since there is not a single correct answer. The researchers used the broad USPTO-R to train MT, which consistently outperformed both a benchmark template-based program and another literature machine learning method also trained on USPTO-R. When tested on the Pfizer dataset, the MT performed best with 31.5% accuracy despite the datasets coming from different regions of chemical space (which increased to 91% when MT was trained on Pfizer data). Figure 1 shows several predicted routes for the synthesis of bioactive molecules as predicted by MT, which generally agree with established syntheses. These data suggest the highly generalizable nature of MT as a tool for developing novel pharmaceutically interesting molecules.

To find out more, please read:

Molecular Transformer unifies reaction prediction and retrosynthesis across pharma chemical space

Alpha A. Lee, Qingyi Yang, Vishnu Sresht, Peter Bolgar, Xinjun Hou, Jacquelyn L. Klug-McLeod and Christopher R. Butler

Chem. Commun., 2019, 55, 12152-12155.

About the blogger:

Beth Mundy is a PhD candidate in chemistry in the Cossairt lab at the University of Washington in Seattle, Washington. Her research focuses on developing new and better ways to synthesize nanomaterials for energy applications. She is often spotted knitting in seminars or with her nose in a good book. You can find her on Twitter at @BethMundySci.

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MOF-Derived Solid-State Lithium-Oxygen Batteries

Just in case you weren’t aware, it turns out that lithium-based batteries are kind of a big deal. While the Nobel-winning batteries have already revolutionized consumer electronics, further development requires batteries with even higher energy densities. Enter: lithium-oxygen batteries (LOBs) with theoretical energy densities of 3500 W h/kg. LOBs come in non-aqueous, aqueous, hybrid, and solid-state varieties based on their electrolytes. Given the previous safety issues for lithium-based batteries with liquid electrolytes (remember the exploding phones?), solid-state electrolytes have attracted substantial research attention. Specifically, Li1+xAlxGe2x(PO4)3, or LAGP, shows promise given its high Li+ transport number and electrochemical stability over a wide window. These solid-state electrolytes need to be combined with new catalytically active high surface area cathode materials that will not react with the lithium and degrade, a persistent issue with MOFs.

Figure 1. Schematic of an assembled all solid-state lithium-oxygen battery.

Researchers in China and Japan have combined LAGP electrolyte with NiCo2O4 (NCO) nanoflakes as the catalytically active cathode material. They then assembled full solid-state batteries, the structure of which is shown in Figure 1, for electrochemical and stability testing. The LAGP was prepared using previously established methods and found to exhibit the expected high stability and lithium mobility. To prepare the nanoflakes, the researchers annealed cobalt-based MOFs on a sacrificial carbon substrate then dipped them in a Ni(NO3)2 solution for nickel doping and annealed once more. This leaves the final nanostructured metal oxide, with the elemental composition confirmed by TEM elemental mapping. As a conveniently freestanding electrode material, the nanoflakes were then loaded in as the cathode.

Once assembled, the researchers tested the full all solid-state LOBs for stability and performance. They demonstrated high discharge capacity and electron transfer efficiency with charge and discharge potentials well within the electrochemical window of the LAGP electrolyte. These are attributable to the high lithium ion mobility and the porous bimetallic nature of the cathode. To confirm that the incorporation of nickel impacted the overall device performance, the pure cobalt nanoflakes were used as the cathode.

Figure 2. Cycling performance of cobalt (left) and cobalt-nickel cathodes (right) at a current density of 100 mA/g.

As seen in Figure 2, the cobalt-only batteries exhibit significant capacity loss in only 35 cycles whereas the NCO cathodes showed no degradation after 90 cycles. While cycling the NCO electrodes, the reversible formation of Li2O2, a common discharge product, occurred in the open pores of the cathode. These pores allow the 500 nm Li2O2 particles to form and dissolve without disrupting the structure of the cathode and give a more stable battery. This research brings completely solid-state lithium-oxygen batteries one step closer to reality.

To find out more, please read:

All solid-state lithium–oxygen batteries with MOF-derived nickel cobaltate nanoflake arrays as high-performance oxygen cathodes

Hao Gong, Hairong Xue, Xueyi Lu, Bin Gao, Tao Wang, Jianping He and Renzhi Ma

Chem. Commun., 2019, 55, 10689-10692.

About the blogger:

Beth Mundy is a PhD candidate in chemistry in the Cossairt lab at the University of Washington in Seattle, Washington. Her research focuses on developing new and better ways to synthesize nanomaterials for energy applications. She is often spotted knitting in seminars or with her nose in a good book. You can find her on Twitter at @BethMundySci.

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Mechanical Stress Turns These Dendrimers Blue

We all know what happens when materials take too much mechanical stress – they eventually break.

What if you could easily tell when something like a support was close to its maximum stress, before it undergoes a catastrophic event, just by looking at it? One option is to incorporate a mechanochromic polymer, a polymer that changes color when under sufficient mechanical stress, to provide a visual indicator that a material has reached a specific stress threshold. The polymers don’t need to be entirely composed of mechanochromically active moieties to exhibit useful properties; many studies have focused on a single active mechanophore at the center of a large polymer chain. In fact, the mechanical force is greatest at the center of a chain and is directly proportional to the length of the chains. This holds for polymers in solution but hasn’t been extensively studied in the types of bulk systems useful for applications.

Recently, researchers in Japan set out to characterize the effects of chain length and branching on mechanochromic dendrimers, polymers with monodisperse and regularly branched globular structures. Showing that dendrimers exhibit mechanochromism is already a novel result, but their well-defined nature allowed the researchers to draw correlations between structure and bulk responsiveness. They employed diarylbibenzylfuranone (DABBF) as the mechanochromic moiety since it generates arylbenzofuranone (ABF) radicals, which are blue, air-stable, and electron paramagnetic resonance spectroscopy (EPR) active, when exposed to mechanical force (Figure 1).

Figure 1. Structure of the DABBF moiety and the active ABF radicals generated by its dissociation.

These characteristics allow for straightforward qualitative and quantitative analysis. The team coupled the DABBF moiety with two series of dendrimers, with increasing generations having larger and more highly branched monomer units, to create a range of molecular weights and degrees of branching for study. The dendrimers showed a color change from white to blue (Figure 2) when ground in a ball mill, which was used to ensure the reproducibility of the force applied to all samples.

Figure 2. Photographs of the first (top) and second (bottom) mechanochromic dendrimers before and after grinding, showing the color change associated with the generation of ABF radicals.

EPR measurements confirmed the presence of the ABF radicals in the samples after milling, demonstrating that the color change is due to the cleavage of the DABBF. The integrated EPR spectra were used to quantitatively determine the percentage of DABBF moieties that dissociated. The responsiveness of the dendrimers increased exponentially with increasing generation and branching. However, the primary factor governing ABF generation was found to be molecular weight. Two dendrimers with different levels of chain entanglement, but similar molecular weights, exhibited comparable cleavage ratios.  The question then became does molecular weight increase the transfer efficiency of force to the DABBF or does the increased steric bulk make it harder for the ABF radicals to recombine? To probe the kinetics of this process, the researchers varied the grinding time and saw that within 5 minutes all the highly branched samples reached their maximum dissociation level. Additionally, monitoring the ABF recombination showed that even after 6 hours approximately 95% of the radicals remained dissociated in all 3rd and 4th generation dendrimers. These data suggest that the enhancement in responsiveness can be attributed to better force transmission to the DABBF.

This work shows mechanoresponsiveness in a range of dendrimers with varying degrees of branching and rigidity. Not only did they demonstrate novel activity, but the researchers also probed the mechanism of the enhanced activity with increasing molecular weight. This initial study opens avenues to explore polymer rigidity, surface functionality, and other dendrimer features to design new, functional materials.

To find out more, please read:

Mechanochromic dendrimers: the relationship between primary structure and mechanochromic properties in the bulk

Takuma Watabe, Kuniaki Ishizuki, Daisuke Aoki, and Hideyuki Otsuka

Chem. Commun., 2019, 55, 6831-6834.

About the blogger:

Beth Mundy is a PhD candidate in chemistry in the Cossairt lab at the University of Washington in Seattle, Washington. Her research focuses on developing new and better ways to synthesize nanomaterials for energy applications. She is often spotted knitting in seminars or with her nose in a good book. You can find her on Twitter at @BethMundySci.

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Guiding Light with Molecular Crystals

We’re all used to communications and computing happening at high, and seemingly ever-increasing speeds. Continuing on this trajectory requires the development of materials capable of acting as micro/nanoscale waveguides that don’t experience interference effects from strong external electromagnetic fields. Molecular crystals represent an exciting but relatively under-explored materials class due to their inherently limited emission and absorption properties. However, an international group of researchers recently combined two different crystalline materials with complementary optical properties in a filled-hollow crystal architecture, involving no binding materials or polymer matrices.

Figure 1. Spectra and structure of DCA (left) and PDI (right).

The group used 9,10-dicyanoanthracine (DCA) as the hollow outer crystal, with a perylene diimide derivative (PDI) as the interior compound (Figure 1). When combined, these two compounds exhibit fluorescence that covers the visible and near-IR portions of the electromagnetic spectrum. The researchers grew hollow crystals of DCA with diameters ranging from 50-400 μm in diameter with pores of 10-200 μm and filled them with 1-50 μm PDI crystal fibrils manually by hand(!) (Figure 2) (I honestly can’t imagine how many crystals ended up broken during that experimental learning curve!). The assembled structure for study had a single hollow DCA crystal filled with 18 individual PDI fibrils to create the waveguide.

Figure 2. Schematic of hollow crystal architecture (top) with demonstration of construction (bottom).

When the researchers excited the full structure with a 365 nm continuous wavelength LED, both crystal components emitted light that was guided down to the opposite end. The specific makeup of the spectrum depends on the point of illumination; only the excited compounds emit. This supports the active waveguiding capabilities of the materials. The emissive properties can also be controlled by changing the excitation wavelengths to exclude the absorbance of one of the molecular crystals. PDI can be selectively excited using light above 550 nm and both PDI and DCA act simply as passive waveguides for light in the infrared region of the spectrum, of particular importance for wireless communication. This study represents an exciting next step for organic molecular materials as optical waveguides with a new architecture for devices.

To find out more please read:

A filled organic crystal as a hybrid large-bandwidth optical waveguide

Luca Catalano, Patrick Commins, Stefan Schramm, Durga Prasad Karothu, Rachid Rezgui, Kawther Hadef and Panče Naumov

Chem. Commun, 2019, 55, 4921-4924.

About the blogger:

Beth Mundy is a PhD candidate in chemistry in the Cossairt lab at the University of Washington in Seattle, Washington. Her research focuses on developing new and better ways to synthesize nanomaterials for energy applications. She is often spotted knitting in seminars or with her nose in a good book. You can find her on Twitter at @BethMundySci.

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Targeting the Powerhouse of the Cell to Fight Cancer

Everyone knows that cancer as a disease is awful, but the side effects of currently utilized chemotherapies have their own horrors. Research into natural products as therapies have found some promising compounds, but they face barriers to practical use in patients. One particular molecule, artesunate (ART), recently showed high potential for anticancer activity when in the presence of iron. Unfortunately, ART has major problems that limit its current applicability, including low solubility in water and high instability in biologically relevant conditions.

One approach to get around these issues is to encapsulate the drug (pun intended) in a nanoparticle-based carrier. A carrier with a hydrophobic interior and hydrophilic exterior can bring higher concentrations of drugs with low solubility into a cell and protect them from deleterious conditions in the body. An additional benefit is the relative ease of incorporating targeting ligands into the particles during synthesis. This allows the drugs to only interact with specific cells or, in this specific case, the mitochondria within cells.

Figure 1. Schematic of the nanoparticle synthesis process complete with targeting ligand molecules. The anticancer agent is activated in the presence of iron.

Researchers in China have prepared approximately 200 nm nanoparticle carriers for ART (Figure 1) using triphenyl phosphonium (TPP) as a mitochondrial targeting ligand. These nanoparticles remained stable in biologically relevant conditions for a week, sufficient for in-vitro studies. The studies showed significant decreases in cancer cell growth when the nanoparticles were used compared to the ART alone. The nanoparticles with TPP on the surface showed the highest efficacy, particularly when coupled with iron treatment to activate the ART.

Figure 2. Images of cells exposed to nanoparticles with (bottom) and without (top) a targeting ligand filled with different fluorescent dyes. The increased brightness corresponds to higher uptake of the nanoparticles by the cells.

To further investigate the cell uptake pathway of the nanoparticles, the researchers added fluorescent dye molecules to the inside of the particles. Once the cells took up and ruptured the nanoparticles, the dyes were released and became visible to the researchers (Figure 2). The fluorescence was twice as great in cells exposed to the nanoparticles treated with the TPP targeting ligand, showing its value for cell uptake. The researchers also used fluorescent dyes that react with reactive oxygen species (ROSs), as their generation is how ART kills cancer cells. The in-vitro studies showed an over three-fold increase in fluorescence from reactions with ROSs which, combined with data showing higher rates of cell death, supports the increased activity of ART when combined with this nanoparticle architecture.

To find out more please read:

A mitochondria targeting artesunate prodrug-loaded nanoparticle exerting anticancer activity via iron-mediated generation of the reactive oxygen species

Zhigang Chen, Xiaoxu Kang, Yixin Wu, Haihua Xiao, Xuzi Cai, Shihou Seng, Xuefeng Wang and Shiguo Chen

Chem. Commun., 2019, 55, 4781 – 4784.

About the blogger:

 

Beth Mundy is a PhD candidate in chemistry in the Cossairt lab at the University of Washington in Seattle, Washington. Her research focuses on developing new and better ways to synthesize nanomaterials for energy applications. She is often spotted knitting in seminars or with her nose in a good book. You can find her on Twitter at @BethMundySci.

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Using Carbon to Make a Better Solar Cell

Maybe I’m stating the obvious, but solar cells are incredibly complex devices with more components than just the light absorber.

While the focus on the active layer by chemists looking to develop new materials is understandable, in order to truly create next-generation solar cells the other components of the architecture must be improved.  Creating the crack-resistant or resilient layers necessary for functional flexible solar cells is a major challenge currently being addressed. These new materials and approaches also need to work within the general framework of fabrication techniques used for the other layers – ideally at low temperature and solution processible.

An often-neglected piece of the puzzle is the electrode. Electrodes are traditionally composed of a thin metal layer, which is often vapor deposited at high temperatures and low pressures. This class of electrodes is expensive, susceptible to degradation, and can damage the critical hole transport or active layers. One emerging alternative is carbon-based electrodes, applied as pastes. These low-cost, highly stable, and hydrophobic materials are attractive given their compatibility with emerging photovoltaic technologies, particularly perovskites. Their broad application has been limited by the necessity of toxic solvents to create the pastes, but researchers in China have developed a low-temperature, highly conductive carbon paste that can be screen printed onto perovskite solar cells without using toxic solvents.

Fabrication schematic and cross sectional SEM for a perovskite solar cell with a carbon electrode

Figure 1. (a) Fabrication schematic for perovskite solar cells with carbon electrodes and hole transport layers. (b) Cross sectional SEM image of a device.

Not only are the solvents more environmentally friendly compared to those previously used, they also increase the mechanical strength of the final film and, under fabrication conditions, do not damage the perovskite active layer or organic hole transport layer. While the hole transport layer isn’t strictly necessary to create a working device, it has been shown to increase the champion efficiency from 11.7% to 14.55%. This is likely due to poor contact between the perovskite and carbon electrode, which the thin hole transport layer (PEDOT:PSS) helps remedy.

Carbon-based electrode undergoing a bending test and sheet resistivity data

Figure 2. (a) A sample undergoing a bending test. (b) The electrode sheet resistance before and after 100 bends.

The most exciting aspect of these electrodes is their resilience when subjected to a bending test. After 100 bends, the researchers saw no visible film damage or increase in the sheet resistance when compared to the initial sample. Actual flexible solar cells fabricated and studied did show a decrease in performance after 1,000 bends, but this was attributed to known robustness issues in the base ITO layer. This work with carbon-based electrode materials could lead to simpler manufacturing for fabricating perovskite solar cells at a commercial level.

 

To find out more please read:

A low-temperature carbon electrode with good perovskite compatibility and high flexibility in carbon based perovskite solar cells

Shiyu Wang, Pei Jiang, Wenjian Shen, Anyi Mei, Sixing Xiong, Xueshi Jiang, Yaoguang Rong, Yiwen Tang, Yue Hu & Hongwei Han

Chem. Commun., 2019, 55, 2765-2768

This article is also part of the Chemical CommunicationsPerovskites‘ themed collection.

About the blogger:

 

Beth Mundy is a PhD candidate in chemistry in the Cossairt lab at the University of Washington in Seattle, Washington. Her research focuses on developing new and better ways to synthesize nanomaterials for energy applications. She is often spotted knitting in seminars or with her nose in a good book. You can find her on Twitter at @BethMundySci.

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