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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How does LiNO3 Make Lithium–Sulfur Batteries Long-Lasting?

Lithium–sulfur (Li–S) batteries are rechargeable batteries with elemental sulfur and metallic lithium as the cathode and anode, respectively. These batteries are promising electrochemical energy storage devices because their energy densities are three to five times higher than those of Li-ion batteries. Unfortunately, the practicality of Li–S batteries is hindered by their short lifetimes due to two processes that occur on the Li anode surface: the growth of Li dendrites and the irreversible polysulfide reduction. Adding LiNO3 into battery electrolytes has proven to be useful to prolong battery lifetimes, but the underlying mechanism is uncertain.

In Chemical Communications (doi: 10.1039/c9cc06504k), Sawangphruk and coworkers from Vidyasirimedhi Institute of Science and Technology, Thailand have offered valuable insights to settle the dispute over the effects of LiNO3. The researchers performed theoretical reactive molecular dynamics simulations and elucidated two roles of LiNO3 in Li–S batteries.

The first discovery was that LiNO3 promoted the formation of smooth, double-layered solid electrolyte interfaces (SEIs) on the Li surface. SEIs are thin layers composed of electrolyte-decomposition products, including Li-containing organic compounds and inorganic salts. By simulating the charge distribution near a Li metal surface, the authors mapped the Li-Li radial pair distribution profiles in three phases (Fig. 1a). The similarity between the profiles of the dense phase (the Li metal) and the nest phase evidenced the presence of an amorphous, Li-containing layer atop the Li metal surface. Beyond this amorphous layer was a liquid-like film with Li element distributed homogenously. This double-layered SEI altered the kinetics of Li deposition onto the Li surface upon charging, resulting in smooth and dense SEIs (Figs. 1b and c) that avoided Li dendrite formation.

Figure 1. (a) Li-Li radial pair distribution functions of the dense phase (Li metal), nest phase (the layer atop Li), and disperse phase (the outermost layer). (b and c) Top-view scanning electron microscopy images of the Li metal surface in (b) LiNO3-free and (c) LiNO3-containing electrolytes. Both electrolytes had lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) as a solute, and 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME) as solvents.

Another effect of LiNO3 was to capture polysulfide compounds. Through their simulations, the authors deduced the reaction pathways involving the electrolyte molecules, LiNO3 or LiClO4 additives, and lithium polysulfide compounds (Fig. 2a). The concentration of LixNOy, the reduction products of LiNO3 when contacted Li metal, in the LiNO3-containing electrolyte was much higher than those in the additive-free and LiClO4-containing electrolytes. First-principle calculations proved that the highly electro-negative N and O atoms in LixNOy could capture lithium polysulfides via dipole-dipole interactions. This process reduced the likelihood of polysulfide reduction on Li that passivated anodes.

Figure 2. (a) A scheme of the reaction pathways involving the electrolyte, additive, and polysulfide molecules. (b) Product distributions in electrolytes without additives and with LiNO3 or LiClO4.

LiNO3 elongates the lifetimes of Li–S batteries by forming smooth SEIs to impede Li dendrite formation, while maintaining the reactivity of Li anodes by capturing lithium polysulfides.

 

To find out more, please read:

Insight into the Effect of Additives Widely Used in Lithium–Sulfur Batteries

Salatan Duangdangchote, Atiweena Krittayavathananon, Nutthaphon Phattharasupakun, Nattanon Joraleechanchai, and Montree Sawangphruk

Chem. Commun., 2019, 55, 13951-13954

Tianyu Liu acknowledges John Elliott of Virginia Tech, the 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. Commun. and Chem. Sci. More information about him can be found at http://liutianyuresearch.weebly.com/.

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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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The Cram Lehn Pedersen Prize in Supramolecular Chemistry


 

The International Committee of the International Symposium on Macrocyclic and Supramolecular Chemistry is pleased to invite nominations for the Cram Lehn Pedersen Prize for young supramolecular chemists.

The Cram Lehn Pedersen Prize, named in honor of the winners of the 1987 Nobel Prize in Chemistry, will recognise significant original and independent work in supramolecular chemistry.

Those who were awarded their PhD on or after 1st January 2009 (or who have an award of PhD date together with allowable career interruptions* that would be commensurate with award of their PhD on or after 1st January 2009) are eligible for the 2020 award. The winner will receive a prize of £2000 and free registration for the ISMSC meeting in Sydney, Australia. In addition to giving a lecture at ISMSC, a short lecture tour will be organized after the meeting in consultation with the Editor of Chemical Communications, the sponsor of the award.

Nomination Details

You may nominate yourself, but a nomination letter is recommended. Nomination materials should include: CV, list of publications (divided into publications from your PhD and post-doc, and those from your independent work), and be sent to Prof. Roger Harrison (ISMSC Secretary) at roger_harrison@byu.edu by 31st December 2019.

*Allowable career interruptions include primary caregiver’s responsibilities, illness, disability or parental leave and must be outlined in a cover letter with supporting documentation. See  https://www.chem.byu.edu/faculty-and-staff/resources/international-symposium-on-macrocyclic-and-supramolecular-chemistry/awards/ for specific details.

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Strengthening Li+-Coordination Decelerates Li-Dendrite Growth in Li-Metal Batteries

Lithium-metal batteries are a family of rechargeable batteries with higher charge-storage capacities than those of lithium-ion batteries. The boosted charge-storage performance of lithium-metal batteries is rooted in its anode material – Li metal, as it possesses an ultrahigh theoretical capacity (3860 mAh/g). However, the growth of dendrites on Li surfaces during charging could short-circuit batteries, cause combustion, and trigger explosions.

A research group led by Feng Li at the Institute of Metal Research, Chinese Academy of Sciences, recently devised a strategy to suppress the notorious Li dendrite growth in lithium-metal batteries. By tuning the composition of the electrolytes, the authors strengthened the coordination between Li+ and electrolyte solvents, which slowed the growth of Li dendrites. This work has been published in Chemical Communications (doi: 10.1039/C9CC07092C).

The researchers introduced an electrolyte additive, tetraethylene glycol dimethyl ether (TEGDME), as a coordination ligand to Li+. Compared to other components in the electrolyte, i.e., 1,2-dimethoxyethane (DME) and 1,3-dioxolane (DOL), TEGDME contains more oxygen atoms that can form multiple, robust coordination bonds with Li+. Specifically, density functional theory calculations showed that the binding energy between Li+ and electrolyte molecules increased by 0.31 eV after introducing TEGDME, reaching an absolute value of 4.93 eV. The enhanced binding force made the separation of Li+ from TEGDME (a prerequisite for Li-dendrite growth) energetically consuming and kinetically sluggish (Figure 1). These characteristics could decelerate Li-dendrite formation and elongate battery lifetimes.

Figure 1. Lithium-dendrite growth in different electrolytes: (a) weak coordination with Li+ promotes fast dendrite growth while (b) strong coordination with Li+ decelerates dendrite formation.

To confirm the above idea, the authors assembled lithium batteries with TEGDME+DME+DOL or DME+DOL electrolytes. Cycling stability tests demonstrated that the battery with the TEGDME-added electrolyte survived 60 charge-discharge cycles at a current density of 1C, whereas the capacity of the battery without TEGDME rapidly decayed beyond 30 cycles under identical testing conditions (Figure 2a). Scanning electron microscopy images revealed that the number of rod-shaped Li dendrites on the anode in the TEGDME-added electrolyte (Figure 2c) was less than that in the TEGDME-free electrolyte (Figure 2b), further confirming that the enhanced cycling stability resulted from the Li-dendrite suppressing effect of TEGDME.

Figure 2. (a) Cycling stability performance of lithium-metal batteries with two different electrolytes. The cathode material in both batteries was lithium iron phosphate (LFP). (b and c) SEM images of the Li anode surface after charging in (b) DME+DOL and (c) DME+DOL+TEGDME electrolytes.

This work highlights the importance of tailoring the electrolyte composition for preserving the stability and safety of lithium-metal batteries.

 

To find out more, please read:

Suppressing Lithium Dendrite Formation by Slowing Its Desolvation Kinetics

Huicong Yang, Lichang Yin, Huifa Shi, Kuang He, Hui-Ming Cheng, and Feng Li

Chem. Commun., 2019, doi: 10.1039/C9CC07092C

Tianyu Liu acknowledges Xiaozhou Yang of Virginia Tech, the 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 as a way to introduce cutting-edge research to broad audiences. He is a blog writer for Chem. Commun. and Chem. Sci. More information about him can be found at http://liutianyuresearch.weebly.com/.

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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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Chemical Communications: Editor’s Choice

Be sure to read our latest Editor’s Choice article as chosen by Associate Editor Jean-Louis Reymond!

This article is free-to-access until 8th November and can be found alongside our previously chosen articles in our online Editor’s Choice web-collection!

Heptylmannose-functionalized cellulose for the binding and specific detection of pathogenic E. coli” by Jean-Louis Reymond:

In their communication “Heptylmannose-functionalized cellulose for the binding and specific detection of pathogenic E. coli” Madeleine Cauwel et al. exploited the well-known FimH lectin system to devise a selective detection system for adherent-invasive E. coli (AIEC) involved in the pathogenesis of Crohn’s disease (CD). FimH is well known to bind mannosyl glycosides and to occur in AIEC. The trick here was to prepare a modified cellulose (as nanofiber or paper) using click chemistry, profile its lectin binding with state-of-the art chip analysis, verify its ability to block binding of AIEC from a CD patient to intestinal epithelial cells and to decrease AIEC levels in gut microbiota in a murine model, and finally to show that the modified paper binds selectively to pathogenic AIEC but not to benign E. coli.

Simple but effective chemistry, thorough experiments with relevant samples, impressive results. Chemical biology at its best.

 

 

Find our full Editor’s Choice collection online!

Keep up-to-date with our latest journal news on Twitter @ChemCommun or via our blog!

Learn more about ChemComm online! Submit your latest high impact research here!

 

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ChemComm Emerging Investigator Lectureship – nominations now open!

Know an outstanding emerging scientist who deserves recognition? Nominate now for the 2020 ChemComm Emerging Investigator Lectureship

We are pleased to welcome nominations for the 2020 Emerging Investigator Lectureship for ChemComm.

All nominations must be received by Friday 29th November 2019.

ChemComm Emerging Investigator LectureshipChemComm Banner
• Recognises emerging scientists in the early stages of their independent academic career.
• Eligible nominees should have completed their PhD in 2012 or later.
Appropriate consideration will be given to those who have taken a career break or followed a different study path.

Lectureship details
• The recipient of the lectureship will be invited to present a lecture at three different locations over a 12-month period, with at least one of these events taking place at an international conference.
• The recipient will receive a contribution of £1500 towards travel and accommodation costs for their lectures, as well as a certificate.
• The recipient will be asked to contribute a review article for the journal.

How to nominate
Self-nomination is not permitted. Nominators must send the following to the editorial team via 
chemcomm-rsc@rsc.org by Friday 29th November 2019.
• Recommendation letter, including the name, contact details and website URL of the nominee.
• A one-page CV for the nominee, including a summary of their education, dates of key career achievements, a list of up to five of their top independent publications, total numbers of publications and patents, and other indicators of esteem, together with evidence of career independence.
• A copy of the candidate’s best publication to date (as judged by the nominator).
• Two supporting letters of recommendation from two independent referees. These should not be someone from the same institution or the candidate’s post doc or PhD supervisor.
• The nominator and independent referees should comment on the candidate’s presenting skills.

Incomplete nominations or those not adhering to the above requirements will not be considered, and nominees will not be contacted regarding any missing or incorrect documents.

Selection procedure
• The editorial team will screen each nomination for eligibility and draw up a shortlist of candidates based on the nomination documents provided.
• The recipient of the lectureship will then be selected and endorsed by a selection panel composed of members of the ChemComm Editorial Board. The winner will be announced in the first half of 2020.

NB: Please note that members of the selection panel from the ChemComm Editorial Board are not eligible to nominate, or provide references, for this lectureship.

For any queries, please contact the editorial team at chemcomm-rsc@rsc.org.

 

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Chemical Communications: Editor’s Choice

Be sure to read our Editor’s Choice articles as chosen by Associate Editors Prof. Penny Brothers & Prof. Manfred Scheer!

Both articles are free-to-access until 4th October and can be found alongside our previously chosen articles in our online Editor’s Choice web-collection!

 

NO sorption, in-crystal nitrite and nitrate production with arylamine oxidation in gas–solid single crystal to single crystal reactions” by et al., as chosen by Penny Brothers:

This year marks 100 years since Alfred Werner’s death in 1919, and it is over a century since he won the 2013 Nobel prize for developing the conceptual framework that we now understand as coordination chemistry. Studies on cobalt complexes formed the cornerstone of Werner’s work, and this paper shows they are still relevant and important well into the 21st century, although with some surprising twists.  Single crystals of tetranuclear Co(II) and Co(III) complexes chemisorb nitric oxide (NO) which, after exposure to O2 physisorbed from air, is transformed to nitrite, nitrate and an aryl nitro group in remarkable single crystal to single crystal reactions.  The medical and biological significance of NO and the solventless redox chemistry all occurring in the crystalline phase suggest exciting possibilities for its highly selective capture and conversion.

 

 

Imidazolium-benzimidazolates as convenient sources of donor-functionalised normal and abnormal N-heterocyclic carbenes” by et al., as chosen by Manfred Scheer:

Mesomeric betaines are related to N-heterocyclic carbenes because of their interconversion by tautomerisation and therefore can act as “instant carbenes”. The authors established now imidazolium-benzimidazolates as a new and highly versatile “instant carbene” system. Depending on the steric demand of the imidazole N-substituent, normal but also abnormal NHC carbene coordination is observed. Thus, unstable but nevertheless highly interesting species are available starting from stable betainic precursors. Therefore, this paper contributes substantially to the chemistry of normal and abnormal N-heterocyclic carbenes.

 

 

 

Find our full Editor’s Choice collection online!

Keep up-to-date with our latest journal news on Twitter @ChemCommun or via our blog!

Learn more about ChemComm online! Submit your latest high impact research here!

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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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