Teachable micromotors

Micromotors, for the uninitiated aka me, are a specific type of colloidal structure that harvests energy from their environment and turns it into motion. In order for them to be truly effective for possible applications though, they must be able to communicate and coordinate with one another. If you want to make microbots, they can’t just move about all willy-nilly with no regard for each other. Recently, scientists have created a mixed system where one structure sheds silver ions that cause other micromotors to accelerate, an approach that mimics natural systems like bee colonies.

Researchers in China developed a system where photochemically powered micromotors can spontaneously “teach” catalytic micromotors to oscillate without any external influence. The teachers are Janus-type microparticles composed of either polymethylmethacrylate (PMMA) or silicon dioxide particles half coated with silver. Under irradiation with KCl and H2O2, the silver can interconvert between Ag(0) and Ag(1), causing the oscillatory motion of the entire particle. In contrast, the two non-oscillatory micromotors, either polystyrene spheres half coated with platinum (PS-PT) or gold-rhodium microrods (Au-Rh), catalytically decompose H2O2 to move autonomously in standard Brownian motion. When the two types of micromotors are mixed under UV light, the motion of the non-oscillatory materials changes from random to clearly oscillating (Figure 1). The intensity of the motion change depends on the proximity of the learner to the teacher, with learners closer to the teacher displaying more intense oscillations.

Figure 1. Change in movement of non-oscillating micromotors when exposed to oscillating “teacher” structures.

In fact, the Au-Rh rods will demonstrate more intense oscillations than the PMMA-Ag particles. The researchers propose a mechanism where the PMMA-Ag particles release silver ions as they oscillate, which then deposit onto the Au-Rh rods. The silver increases the catalytic activity of the rods and then, given the operating conditions, undergoes the same redox process that causes oscillation in the PMMA-Ag system.

Figure 2. Proposed mechanism of silver release and adsorption onto Au-Rh rods.

This hypothesized mechanism is supported by the development of oscillatory behavior by the Au-Rh rods in under reaction conditions where the PMMA-Ag particles are replaced by silver ions in solution. The silver on the rod surface isn’t merely adsorbed – it forms into small silver nanoparticles which can be seen via electron microscopy, making a new trimetallic structure. These nanoparticles change the trajectories of the rods, causing them to move in circles. While this system isn’t perfect, the student structures have imperfect memories and cannot teach one another, it provides a strategy for working with groups of micromotors to move towards coordinated motion and further applications.

To find out more, please read:

Non-oscillatory micromotors ‘‘learn’’ to oscillate on-the-fly from oscillating Ag micromotors

Chao Zhou, Qizhang Wang, Xianglong Lv and Wei Wang

Chem. Commun., 2020, Advance Article

About the blogger:

Dr. Beth Mundy is a recent PhD in chemistry from the Cossairt lab at the University of Washington in Seattle, Washington. Her research focused 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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Potassium-Ion Batteries Welcome A New Electrode from Iron Compounds

A research team led by Jiang Zhou and Shuquan Liang of Central South University, China, recently identified iron oxyhydroxide (β-FeOOH) as a K-ion battery electrode material. It is reportedly the first iron-oxide-based compound to serve in K-ion batteries. The results have been published in Chemical Communications (DOI: 10.1039/d0cc01009j).

K-ion batteries are a new type of rechargeable battery emerging after Li-ion batteries, the charge-storage functionality of which is associated with the intercalation and de-intercalation of K+. Due to the relative abundance of K+ to Li+, K-ion batteries are poised as a promising alternative to Li-ion batteries.

The researchers made the electrode by a hydrothermal reaction. Specifically, they dispersed Super P® (SP), an electrically conductive additive, into FeCl3 aqueous solutions. The mixture was then heated at 150 °C for 10 h. The resultant powder (FeOOH-SP), comprised of uniformly mixed, crystalline β-FeOOH nanorods and Super P particles (Fig. 1), was directly used as an electrode material.

Figure 1. Transmission electron microscopy images of (a, b) FeOOH-SP and (c) FeOOH. (d) Elemental mappings of C, Fe, and O in FeOOH-SP.

The authors investigated the electrochemical properties of FeOOH-SP in K-ion batteries. At a current density of 100 mA/g, FeOOH-SP exhibited a stable specific capacity of ~200 mAh/g, approximately double and quadruple that of SP and FeOOH alone, respectively (Fig. 2a). The specific capacity of FeOOH-SP was maintained at ~100 mAh/g when the current density increased to 2000 mA/g (Fig. 2b), showing its fast-charging capability. Additionally, the authors observed that the crystalline β-FeOOH nanorods amorphized upon K+ intercalation after being discharged (Fig. 2c). Their crystallinity was only partially restored when being re-charged (Fig. 2d). The loss of crystallinity, however, did not undermine the charge-storage capacity of FeOOH-SP.

Figure 2. (a) Specific capacities of FeOOH-SP, SP, and FeOOH at different charge-discharge cycles. Current density: 100 mA/g. (b) Rate capability of FeOOH-SP. (c, d) Transmission electron microscopy images of (c) discharged and (d) charged FeOOH-SP. Red dashed boxes highlight crystalline regions. The electrolyte was a mixture of ethylene carbonate and diethyl carbonate containing 1 M potassium bis(fluorosulfonyl)imide.

Considering the low-cost of iron oxyhydroxide, FeOOH-SP could reduce the manufacturing cost of K-ion batteries and increase the affordability of electrochemical charge storage devices.

 

For expanded understanding, please read:

β-FeOOH: A New Anode for Potassium-Ion Batteries

Xiaodong Shi, Liping Qin, Guofu Xu, Shan Guo, Shuci Ma, Yunxiang Zhao, Jiang Zhou, and Shuquan Liang

Chem. Commun., 2020, 56, 3713-3716.

Tianyu Liu acknowledges Zacary Croft at Virginia Tech, U.S., for his careful proofreading of this post.

About the blogger:

Tianyu Liu obtained his Ph.D. (2017) in Chemistry from the University of California, Santa Cruz, in the United States. He is passionate about the communication of scientific endeavors to both the general public and other scientists with diverse research expertise to introduce cutting-edge research to broad audiences. He is a blog writer for Chem. Comm. and Chem. Sci. More information about him can be found at http://liutianyuresearch.weebly.com/.

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Aluminum-Based Liquid Coordination Complexes

Before I get into the research, I just have to say that I think ionic liquids (ILs) are just cool. An ionic compound that’s a liquid? Mind blown. Not only are they interesting, but those unique properties make them attractive for industrial use. One class of ionic liquids generating research focus is halometallate ionic liquids (HILs), generated by reacting a metal halide and an organic halide salt. In particular, chloro-alluminate ILs have been some of the most promising for applications in industrial acid catalysis and are composed of anionic aluminate species. However, recent work has found mixtures of aluminum chloride and N/O/S donors produce a liquid Lewis acidic compounds, referred to as liquid coordination complexes (LCCs). These LCCs are potential replacements for HILs, as they’re typically easier and cheaper to prepare. Further studies have found ionic species by 27Al NMR, drawing parallels between LCCs and HILs. By combining AlCl3 and polar organic solvents, researchers in the US screened for novel LCC or HIL reactivity in catalysis.

This straightforward approach allowed researchers to easily tune the ratio of AlCl3 and solvent to find mixtures with desired properties. They chose the nitrogen donor 1-methylimidazole, N-Mim, and an oxygen donor N-methyl-2-pyrrolidone (O-NMP) as their solvents for testing given their wide availability and relevance to organic chemical reactivity. The selected solvent and AlCl3 were mixed at room temperature and found in all cases to form heterogenous mixtures. When heated to 100 oC mixtures with molar fractions of AlCl3 between 0.33 and 0.6 formed viscous liquids, many of which became solids at room temperature. These compounds were then crystalized for x-ray crystallographic analysis, where their structures confirmed the association of the aluminum with the nitrogen (Figure 1) or oxygen.

Figure 1. 50% probability ellipsoid plot of AlCl3(N-Mim), showing coordination between the aluminum and nitrogen

In the N-Mim system, 27Al NMR showed the formation of a single aluminum-containing species at AlCl3 molar fractions of 0.5 and below, with exchange occurring when higher concentrations of N-Mim are present. The O-NMP system proved more challenging to characterize crystallographically, potentially due to the formation of larger oligomeric complexes in the solid phase and increased disorder in the O-NMP ligand. However, 27Al NMR proved insightful and showed the presence of multiple aluminum-containing species, including several different stoichiometries of aluminum-solvent adducts.

Figure 2. Aluminum NMR spectra of aluminum/O-NMP complexes showing speciation over a range of different stoichiometries.

When the two systems were side-by-side compared for Lewis acidity and catalytic activity for the alkylization of benzene, the clear winner was the O-NMP system. The O-NMP-AlCl3 complex with an aluminum molar fraction of 0.6 was both the most Lewis acidic, determined by an acetonitrile IR probe, and the most catalytically active. It gave full conversion with a selectivity of almost 80%, while the N-Mim complex with the same mole fractions produced only a 32% conversion with no significant increase in selectivity. Complexes with less aluminum showed no signs of catalytic activity and were less Lewis acidic. The high activity of the AlCl3/O-NMP system can be explained by its possession of both a highly Lewis acidic cation [AlCl2(O-NMP)2]+ and a highly Lewis acidic anion [Al2Cl7], whose presence was identified in the NMR experiments. This work demonstrated a straightforward method to synthesize LCC-based catalysts with high activity, while providing some general guidance on the suitability of O-donor ligands for further study.

To find out more, please read:

Are ionic liquids and liquid coordination complexes really different? – Synthesis, characterization, and catalytic activity of AlCl3/base catalysts

Rajkumar Kore, Steven P. Kelley, Anand D. Sawant, Manish Kumar Mishra and Robin D. Rogers

Chem. Commun., 2020, Advance Article

About the blogger:

Dr. Beth Mundy is a recent PhD in chemistry from the Cossairt lab at the University of Washington in Seattle, Washington. Her research focused 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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Finding early warnings in chemical systems

Our world runs on systems of ever-increasing complexity, be they natural or human created. Their behavior can generally be modeled and operates within a normal range… until it hits a tipping point at a border between two behavioral regimes. Once that threshold is reached the system acts in dramatically different, and often destructive, ways. However, there are often warning signs right around the tipping point that can serve to alert careful observers that the danger zone is near. By finding ways to identify and monitor this behavior either contingency plans can be put in place or the problem corrected before it becomes catastrophic. While much of this has been studied for large systems like the stock market or ecosystems, researchers recently applied this to chemical systems.

The utility of identifying early warning signals isn’t limited to preventing massive changes; it can also help bound regions of different functionality in chemical systems. To explore this, researchers used a trypsin oscillator system they previously developed that has two modes of flow (Figure 1), either sustained oscillations in trypsin concentration or dampened oscillations that eventually lead to a steady concentration of trypsin. By changing flow rate, temperature, or reagent concentration the researchers could tune the behavior of the system to test either active or passive monitoring schemes to find early warning signs. Since they knew what conditions produced each mode, they could operate the system right at the boundary and watch its behavior.

Figure 1. Schematic of trypsin system with examples of oscillation patterns.

In the “active” experiments the researchers intervened in the system’s normal functioning and watched how its response, the “critical slow down phenomena,” changed as the conditions were brought closer to the boundary. In “passive” experiments, the researchers simply brought the system close to the boundary and monitored the shape of the oscillation waves focusing on their full-width half maxima (FWHM). In the series of active sensing experiments, the researchers waited to see how long it took the system to re-establish sustained oscillations after a change in temperature. They expected to see this recovery time increase as the system was brought closer to the edge, aka the “critical slow down phenomena.” In their experiment they elevated the temperature of the system to 49.0 oC and observed how long it took to recover to either 24.3 oC (well within the sustained regime) or 20.9 oC (near the behavioral boundary). They saw the expected dramatic increase in time to return to sustained oscillation, with the system recovering in 10.7 hrs at 24.3 oC, but taking three times as long, 32.8 hrs, when the final temperature was 20.9 oC (Figure 2). The experimental results were further validated by theoretical modeling, where experimentally challenging positions at the very edge of the boundary could be explored and showed the same general behavior.

Figure 2. Top: active monitoring experiments at two different temperatures showing the differences in time needed to recover sustained oscillations. Bottom: comparison of modeled and experimental recovery times.

The theoretical modeling was further used to create a map of passive monitoring space, looking at the variations in FWHM. They saw that as the system moves away from the center of the oscillation regime the FWHM increases as the peaks broaden. This becomes more pronounced as the conditions approach the behavioral boundary. This provides a baseline knowledge with multiple types of measurements to compare the stability of the simple system over a range of conditions. Additionally, this system and others like it can serve as the base for modeling more complex systems of which they are a part.

To find out more, please read:

Early warning signals in chemical reaction networks

Chem. Commun., 2020, Advance Article

Oliver R. Maguire, Albert S. Y. Wong, Jan Harm Westerdiep and Wilhelm T. S. Huck

About the blogger:

Dr. Beth Mundy recently received her PhD in chemistry from the Cossairt lab at the University of Washington in Seattle, Washington. Her research focused 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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Outstanding Reviewers for Chemical Communications in 2019

We would like to highlight the Outstanding Reviewers for Chemical Communications in 2019, as selected by the editorial team, for their significant contribution to the journal. The reviewers have been chosen based on the number, timeliness and quality of the reports completed over the last 12 months.

We would like to say a big thank you to those individuals listed here as well as to all of the reviewers that have supported the journal. Each Outstanding Reviewer will receive a certificate to give recognition for their significant contribution.

Dr Sai Bi, Qingdao University, ORCID: 0000-0002-7305-8233

Dr Torsten Brezesinski, Karlsruhe Institute of Technology, ORCID: 0000-0002-4336-263X

Professor Rodney Fernandes, Indian Institute of Technology Bombay, ORCID: 0000-0001-8888-0927

Dr Richard M Kellogg, Syncom BV, ORCID: 0000-0002-8409-829X

Dr Anabel Estella Lanterna, University of Ottawa, ORCID: 0000-0002-6743-0940

Dr Hao Li, University of Texas, ORCID: 0000-0002-7577-1366

Dr Xinhui Lou, Capital Normal University, ORCID: 0000-0001-6906-713X

Dr Arpad Molnar, University of Szeged, ORCID: 0000-0001-9191-450X

Dr Kyungsoo Oh, Chung-Ang University, ORCID: 0000-0002-4566-6573

Dr Yong Qin, Changzhou University, ORCID: 0000-0003-4563-8828

We would also like to thank the Chemical Communications board and the General chemistry community for their continued support of the journal, as authors, reviewers and readers.

 

If you would like to become a reviewer for our journal, just email us with details of your research interests and an up-to-date CV or résumé.  You can find more details in our author and reviewer resource centre

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Congratulations to the 2020 Cram Lehn Pedersen Prize winner: Chenfeng Ke

We are proud to announce that Dr. Chenfeng Ke, at Dartmouth College, is the recipient of this year’s Cram Lehn Pedersen Prize in Supramolecular Chemistry! This prize, sponsored by ChemComm, is named in honour of the winners of the 1987 Nobel Prize in Chemistry and recognises significant original and independent work in supramolecular chemistry. Our warmest congratulations to Chenfeng, a well-deserved winner!


“The CLP prize was envisioned to recognize young investigators in the field of supramolecular chemistry. In his short career, Professor Chenfeng Ke has shown outstanding creativity in the development of 3D-printed mechanically interlocked monoliths. He has also discovered transformations of fluorescent supramolecular networks and their guest-induced expansion. These and other innovations show that Professor Ke is a premier supramolecular chemist.” –  Roger Harrison, Secretary of the ISMSC International Committee

Chenfeng received his PhD in Supramolecular Chemistry from Nankai University in 2009. The Ke Functional Materials Group focuses on developing smart materials for 3D/4D printing applications, elastic crystalline porous organic materials for energy and environmental related applications, and carbohydrate receptors for biological applications. The research scheme overlaps organic synthesis, crystal engineering, polymer synthesis, materials characterization, and 3D printing, with an emphasis on the design of supramolecular materials that are noncovalently assembled.

The award will be presented at the 15th International Symposium on Macrocyclic and Supramolecular Chemistry held in Sydney, 12 – 16th July 2020!
This annual conference consists of sessions of invited lectures that focus upon a single topic area, award lectures and poster sessions. The conference will also feature emerging investigator talks.

You can register here.

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Upgrading Methanol Using Zinc-Indium-Sulfide and Solar Light

Based on the chemical formulae, can you figure out how to convert methanol (CH3OH) into ethylene glycol (HOCH2CH2OH)? If you have constantly practiced your organic chemistry, you might have already found the answer: combining two methanol molecules and eliminating one hydrogen molecule (H2). Indeed, this methanol-coupling reaction is a promising, low-cost chemical route to upgrade methanol to chemicals with more carbon atoms. The feasibility of this route, however, is low under mild conditions without catalysts to drive the reaction.

A group of scientists led by Shunji Xie and Ye Wang, both at Xiamen University, China, has developed an environmentally friendly catalyst, Zn2In2S5, for room-temperature methanol coupling to produce ethylene glycol using solar light. This work has been published in Chemical Communications (DOI: 10.1039/c9cc09205f).

The synthesized Zn2In2S5 catalyst is comprised of 1-3 layers of nanosheets. Through a hydrothermal reaction, the researchers first synthesized multi-layer Zn2In2S5 stacks in an aqueous solution (Fig. 1a). Subsequent ultrasonication exfoliated the stacks into few-layer Zn2In2S5­ nanosheets that were confirmed by transmission electron microscopy (Fig. 1b). Zn2In2S5 is a semiconductor and its valence band (VB) resides below the redox potential of ethylene glycol/methanol (Fig. 1c). The band alignment enables Zn2In2S5 to catalyze the oxidation of methanol to ethylene glycol.

Figure 1. (a) A scheme of the synthesis procedures of few-layer ZnmIn2Sm+3 (m=1-3) nanosheets. (b) Transmission electron microscopy images of few-layer Zn2In2S5 nanosheets. (c) Positions of the valence bands (VBs) and conduction bands (CBs) of different metal-sulfide semiconductors.

Zn2In2S5 and its composite exhibited high catalytic activity. Upon irradiation with visible light and solar light (AM 1.5), photo-induced electrons and holes generated in Zn2In2S5 (Fig. 2a). The electrons reduced protons in electrolytes and liberated hydrogen gas, while the holes moved to the Zn2In2S5 surface and split the C—H bond of methanol, forming ·CH2OH radicals. These radicals then dimerized into ethylene glycol. Through depositing a hydrogen evolution co-catalyst, cobalt monophosphide (CoP), and illuminating using AM 1.5 solar light, the authors observed that the CoP/Zn2In2S5 catalyst achieved a rapid formation rate (18.9 mmol gcat-1 h-1) and high selectivity (~90%) of ethylene glycol (Fig. 2b). The yield of ethylene glycol after 12 h of reaction was 4.5%.

Figure 2. (a) A scheme of the formation mechanisms of ethylene (from methanol) and 2,3-butanediol (from ethanol) on Zn2In2S5. EG: ethylene glycol. 2,3-BD: 2,3-butanediol. (b) Formation rates and ethylene glycol selectivity of Zn2In2S5 and CoP/Zn2In2S5 under two illumination conditions. AM 1.5: air mass 1.5 solar irradiance. HCHO is a byproduct.

Zn2In2S5 was demonstrated to effectively catalyze C—H cleavages and C—C couplings of different alcohols, e.g., from ethanol to 2,3-butanediol.

 

To find out more, please read:

C–H Activations of Methanol and Ethanol and C–C Couplings into Diols by Zinc–Indium–Sulfide Under Visible Light

Haikun Zhang, Shunji Xie, Jinyuan Hu, Xuejiao Wu, Qinghong Zhang, Jun Cheng, and Ye Wang

Chem. Commun., 2020, DOI: 10.1039/c9cc09205f

 

The blogger acknowledges Zac Croft at Virginia Tech, U.S., for his careful proofreading of this post.

 

About the blogger:

Tianyu Liu obtained his Ph.D. (2017) in Chemistry from the University of California, Santa Cruz, in the United States. He is passionate about the communication of scientific endeavors to both the general public and other scientists with diverse research expertise to introduce cutting-edge research to broad audiences. He is a blog writer for Chem. Comm. and Chem. Sci. More information about him can be found at http://liutianyuresearch.weebly.com/.

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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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What Does the New Carbon Allotrope Look Like, Theoretically?

A long-lasting dispute regarding the most stable structure of cyclo[18]carbon, a new carbon allotrope, has been settled. Cyclo[18]carbon is an all-carbon ring comprised of eighteen interconnected carbon atoms. It is proposed to have two possible structures: the cumulenic structure with only carbon-carbon double bonds (Figure 1a), and the polyynic structure having alternating carbon-carbon triple and single bonds (Figure 1b). Recent experiments have confirmed that the polyynic structure is the stable form, but theorists were still puzzled: Why can’t the various computational methods reach an agreement on the molecular structure of cyclo[18]carbon?

Figure 1. The (a) cumulenic and (b) polyynic structures of cyclo[18]carbon.

Anton J. Stasyuk and coworkers from the University of Girona, Spain, offered an answer in ChemComm (DOI: 10.1039/C9CC08399E). They found that the simulated structure strongly depended on the type of functionals used in density functional theory (DFT), which is a computational tool to derive energy-minimum molecular structures. The functionals used for DFT calculations are mathematical terms that can tune the simulation accuracy.

The authors discovered that the weight of the exact exchange term (HF% exchange) in the DFT functionals determined the most stable simulated structure of cyclo[18]carbon. The researchers compared 13 functionals with various percentages of HF% exchanges. They found that functionals with the HF% exchange higher than 50% predicted the appreciably different lengths of the neighboring bonds (quantified as the bond length alternation, the vertical axis of Figure 2), corresponding to the polyynic structure (Figure 2, red zone). This structure was recently observed experimentally. Functionals with lower HF% exchange either obtained the cumulenic structure (Figure 2, green zone) or the mixed cumulenic-polyynic structure (Figure 2, gray zone).

Figure 2. Variation in the HF% exchange of the B3LYP functional changed the predicted molecular structure of cyclo[18]carbon. BLA: Bond length alternation.

With the correct functionals identified, the authors revealed the electronic properties of cyclo[18]carbon. Calculations showed that cyclo[18]carbon was a strong electron acceptor, making it the smallest all-carbon electron acceptor reported so far.

 

To find out more, please read:

Cyclo[18]Carbon: Smallest All-Carbon Electron Acceptor

Anton J. Stasyuk, Olga A. Stasyuk, Miquel Solà, and Alexander Voityuk

Chem. Commun., 2019, DOI: 10.1039/C9CC08399E

Tianyu Liu acknowledges Zac Croft at Virginia Tech, U.S., for his careful proofreading of this post.

 

About the blogger:

Tianyu Liu obtained his Ph.D. (2017) in Chemistry from the University of California, Santa Cruz, in the United States. He is passionate about the communication of scientific endeavors to both the general public and other scientists with diverse research expertise to introduce cutting-edge research to broad audiences. He is a blog writer for Chem. Comm. and Chem. Sci. More information about him can be found at http://liutianyuresearch.weebly.com/.

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