Archive for the ‘Hot Articles’ Category

Making single-atom nickel sites from MOF/polymer composites

A single-atom catalyst (SAC) is pretty much what it says on the tin; individual metal atoms that act as active sites to speed up a reaction, dispersed on a supporting material. SACs are desirable in heterogeneous catalysis as they make use of every metal atom, leading to greater possible catalytic efficiencies and activities. Metal-organic frameworks (MOFs), porous and crystalline materials that are useful in their own right, have been identified as useful precursors for pyrolysis to form nanostructures and single-atom catalysts. Researchers in Switzerland and China have now demonstrated this strategy for the formation of single-atom nickel species for electrocatalysis, where their MOF-derived material showed excellent activity, efficiency and durability for the electrochemical CO2 reduction reaction (CO2RR).

Whilst MOF-derived nanostructures exist, single-atom sites are typically harder to achieve by pyrolysis. Most organic linkers within MOFs typically contain oxygen coordination sites to bind the metal, but this oxygen content is lost in pyrolysis due to CO2 formation. Without other Lewis-basic coordination sites within the MOF, pyrolysis can often lead to metal aggregation rather than dispersed single-atom sites for catalysis. The researchers overcame this challenge by adding both a polymer and a secondary nitrogen-rich compound to the MOF before pyrolysis, aiding in the formation of dispersed and stable single-atom metal sites.

Scheme showing the preparation of nickel-containing nitrogen-doped carbon catalysts, starting from the MOF on the left, adding the polymer in the middle, followed by pyrolysis to create the material on the right.

Figure 1: The preparation of the nickel-containing nitrogen-doped carbon catalysts

The researchers selected Ni2(NDISA) as the MOF to study, with nitrogen-containing naphthalene diimide salicylic acid (NDISA) linkers. They introduced a polydopamine (PDA) polymer into the porous channels of the MOF structure to create Ni2(NDISA)-PDA, and then further subjected the MOF/polymer composite to melamine as an additional nitrogen source. The researchers then subjected the MOF, the MOF/polymer composite and the MOF/polymer composite with melamine to pyrolysis, followed by etching with acid to remove any unbound nickel particles, to form the nickel-containing nitrogen-doped-carbon catalysts Ni/NC, Ni/NC-D and Ni/N-CNT, respectively (Figure 1). The MOF/polymer composite with melamine went on to form carbon nanotubes (CNTs) after pyrolysis as an effect of decomposition of the melamine.

The researchers used a range of techniques to characterise the materials. They found that the Ni/N-CNT material had the highest nickel loading and therefore the greatest number of dispersed single-atom nickel sites, owing to both the addition of the polymer that prevented aggregation of the metal and the addition of the nitrogen-rich melamine to aid nickel binding to the surface. All three materials were tested for electrochemical CO2 reduction, an important carbon neutral cycle. The three materials all showed selective production of CO and H2, with the Ni/N-CNT material showing the greatest faradaic efficiency and stability owing to the greater amount of nickel active sites. Overall, this simple strategy of combining a MOF precursor with a polymer and a nitrogen-rich source successfully enhanced the performance of the MOF-derived material with single-atom nickel sites, and has future potential in a wider variety of electrochemical applications using a range of MOF and polymer building blocks.

 

To find out more, please read:

A metal–organic framework/polymer derived catalyst containing single-atom nickel species for electrocatalysis

Shuliang Yang, Jie Zhang, Li Peng, Mehrdad Asgari, Dragos Stoian, Ilia Kochetygov, Wen Luo, Emad Oveisi, Olga Trukhina, Adam H. Clark, Daniel T. Sun and Wendy L. Queen

Chem. Sci., 2020, 11, 10991-10997

 

About the blogger:

Photograph of the author, Samantha AppsDr. Samantha Apps recently finished her post as a Postdoctoral Research Associate in the Lu Lab at the University of Minnesota, USA, and obtained her PhD in 2019 from Imperial College London, UK. She has spent the last few years, both in her PhD and postdoc, researching synthetic nitrogen fixation and transition metal complexes that can activate and functionalise dinitrogen. Outside of the lab, you’ll likely find her baking at home, where her years of synthetic lab training has sparked a passion in kitchen chemistry too.

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Osmium complex for photochemotherapy: a new way to win with hypoxic tumors

By Damayanti Bagchi, Guest Web Writer

Hypoxia, an event of inadequate oxygen supply to solid tumor, is related to aggressive propagation of malignancy by altering cancer cell metabolism. It also induces resistance to standard cancer chemotherapeutics and requires ‘out of the box’ thinking to deal with, often termed as alternative strategies.

Photodynamic therapy (PDT) uses a light activated chemical, termed photosensitizer (PS), which can behave like a drug only in the presence of specific light. The photon energy could excite the PS molecule that can produce reactive oxygen species (ROS) which in turn destroys cancer cells. PDT requires an oxygen-filled environment to be effective, which restricts its application in hypoxic tumors. Alternatively, photochemotherapy (PCT) follows a similar mechanism to PDT, and differs only by exploiting an oxygen-independent reaction pathway and hence could be used under hypoxic conditions.

The search for new effective PCT agents needs optimization in a lot of parameters given the stability of the compound, and sensitivity to different physical (light, heat) and chemical (pH, tumor micro-environment) conditions. The replication of hypoxic tumor conditions in a lab-based cell culture set up is quite challenging. Despite all of these tricks, scientists from the US, Canada and Italy came up with a new class of heavy-metal based photo-chemo agents, in a cross-country collaborative scientific investigation.

The researchers synthesized a range of Os(II)-oligothienyl-appended metal-ligand complexes that can produce highly effective cytotoxicity in hypoxia, through light- induced electron transfer reactions via triplet intra-ligand charge transfer excited states. The novel complex is defined as [Os(phen)2(IP-nT)]Cl2, where phen =1,10- phenanthroline, IP = imidazo[4,5-f][1,10]phenanthroline, and nT = thiophenes of varying chain lengths n (n = 0–4). Scheme 1 shows details of the structural variety.

Scheme 1: Molecular structures of reference compound [Os(phen)3]2+ and Os-0T–Os-4T.

Rigorous computational investigation using quantum mechanical calculations indicates all the Os(II) complexes have the requisite triplet state energies to sensitize ROS production by Type II energy transfer pathway but the actual effect in cells would likely depend on the local concentrations of these complexes and the redox environment. The detailed experimental photophysical studies reveal more on the structure-activity relationship of the complexes. All these complexes show broad absorption in the visible light region (400-550 nm) and weaker absorption in the near-infra red (NIR 700 nm) range. The excited state absorption spectra clearly show the difference in the long-lived triplet state with increasing conjugation with the addition of ligands (n=1-4). Figure 1 depicts the different photophysical pathways present in the complexes. The extended conjugation in Os-4T provides the longest triplet excited state lifetimes (3–4ms and 14–16ms), which suggests it’s superior ROS generation capability over the other complexes.

Figure 1: Jablonski diagrams depicting the proposed decay pathways in (a) Os-1T, (b) Os-3T, and (c) Os-4T. UV-vis spectra of Os-nT complexes at room temperature in acetonitrile (d). Transient absorption profiles for (e) Os-0T–Os-2T, and (f) Os-3T and Os-4T.

The complex Os-4T provides remarkable photocytotoxicity in human melanoma cells and the activity is dependent on the wavelength of the light source used. Authors reported an exceedingly high phototherapeutic index (PI) of 6500 (a value of more than 100 is highly active) under red light irradiation. The significant activity attained using NIR light expands the potential activity of the complex Os-4T via implication of the NIR biological window I. However, the light dependency of cytotoxicity changes in hypoxic conditions in which PI is found to be 90. The potential toxicity of the heavy metal-based complex is also investigated, and the complex is found to be safe up to 200 mg/kg doses in mice, which is pretty high compare to other PCT agents. The study gives the first insight into Os-based photo-chemo agents with remarkably high activity in both oxygen-rich normoxic and oxygen-deficient hypoxic conditions. Researchers are currently expanding this work and validating the effects of Os-4T in mice models and hope to proceed with clinical human trials for the eradication of the long-standing invincible problem of cancer.

To find out more, please read:

Breaking the barrier: an osmium photosensitizer with unprecedented hypoxic phototoxicity for real world photodynamic therapy

John A. Roque, III, Patrick C. Barrett, Houston D. Cole, Liubov M. Lifshits, Ge Shi, Susan Monro, David von Dohlen, Susy Kim, Nino Russo, Gagan Deep, Colin G. Cameron,* Marta E. Alberto* and Sherri A. McFarland*

Chem. Sci., 2020, 11, 9784-9806

About the blogger:

Dr Damayanti Bagchi is a postdoctoral researcher in Irene Chen’s lab at University of California, Los Angeles, United States. She obtained her PhD in Physical Chemistry from Satyendra Nath Bose National Centre for Basic Sciences, India. Her research is focused on spectroscopic studies of nano-biomaterials. She is interested in exploring light enabled therapeutics. She enjoys food and experimenting with various cuisines, which she found resembles products/ side products of chemical reactions!

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Chemical Science HOT Articles: October

We are pleased to share a selection of our referee-recommended HOT articles for October. We hope you enjoy reading these articles and congratulations to all the authors whose articles are featured! As always, Chemical Science is free to read & download. You can find our full 2020 HOT article collection here.

 

A metal–organic framework/polymer derived catalyst containing single-atom nickel species for electrocatalysis
Shuliang Yang, Jie Zhang, Li Peng, Mehrdad Asgari, Dragos Stoian, Ilia Kochetygov, Wen Luo, Emad Oveisi, Olga Trukhina, Adam H. Clark, Daniel T. Sun and Wendy L. Queen
Chem. Sci., 2020, 11, 10991-10997
DOI: 10.1039/D0SC04512H, Edge Article

Effect of curvature and placement of donor and acceptor units in cycloparaphenylenes: a computational study
Terri C. Lovell, Kaylin G. Fosnacht, Curtis E. Colwell and Ramesh Jasti
Chem. Sci., 2020, Advance Article
DOI: 10.1039/D0SC03923C, Edge Article

Total syntheses of spiroviolene and spirograterpene A: a structural reassignment with biosynthetic implications
Hyung Min Chi, Charles J. F. Cole, Pengfei Hu, Cooper A. Taylor and Scott A. Snyder
Chem. Sci., 2020, 11, 10939-10944
DOI: 10.1039/D0SC04686H, Edge Article

Coligand role in the NHC nickel catalyzed C–F bond activation: investigations on the insertion of bis(NHC) nickel into the C–F bond of hexafluorobenzene
Maximilian W. Kuntze-Fechner, Hendrik Verplancke, Lukas Tendera, Martin Diefenbach, Ivo Krummenacher, Holger Braunschweig, Todd B. Marder, Max C. Holthausen and Udo Radius
Chem. Sci., 2020, 11, 11009-11023
DOI: 10.1039/D0SC04237D, Edge Article

Multimerized self-assembled caged two-in-one siRNA nanoparticles for photomodulation of RNAi-induced gene silencing
Changmai Chen, Nannan Jing, Zhongyu Wang, Yu Zhang, Wei Chen and Xinjing Tang
Chem. Sci., 2020, Advance Article
DOI: 10.1039/D0SC03562A, Edge Article

Selective inhibition of the K+ efflux sensitive NLRP3 pathway by Cl channel modulation
Tessa Swanton, James A. Beswick, Halah Hammadi, Lucy Morris, Daniel Williams, Stephane de Cesco, Lina El-Sharkawy, Shi Yu, Jack Green, John B. Davis, Catherine B. Lawrence, David Brough and Sally Freeman
Chem. Sci., 2020, Advance Article
DOI: 10.1039/D0SC03828H, Edge Article

Controlling ultralong room temperature phosphorescence in organic compounds with sulfur oxidation state
Zhen Xu, Clàudia Climent, Christopher M. Brown, Duane Hean, Christopher J. Bardeen, David Casanova and Michael O. Wolf
Chem. Sci., 2020, Advance Article
DOI: 10.1039/D0SC04715E, Edge Article

Cation recognition on a fullerene-based macrocycle
Yoshifumi Hashikawa and Yasujiro Murata
Chem. Sci., 2020, Advance Article
DOI: 10.1039/D0SC05280A, Edge Article

A critical analysis of electrospray techniques for the determination of accelerated rates and mechanisms of chemical reactions in droplets
Grazia Rovelli, Michael I. Jacobs, Megan D. Willis, Rebecca J. Rapf, Alexander M. Prophet and Kevin R. Wilson
Chem. Sci., 2020, Advance Article
DOI: 10.1039/D0SC04611F, Edge Article

 

Chemical Science, Royal Society of Chemistry

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Ferrying electrons with ferrocene to enhance nickel electrochemistry

Redox-active transition metal complexes, those that can undergo multiple oxidation and reduction events, are ideal candidates for electrochemical energy storage and fuel technologies. A significant caveat to employing these complexes for electrochemical processes is that their solubilities can drastically change across redox states, creating insoluble oxidation or reduction products that precipitate out of solution. Even when the insoluble redox product is still chemically-intact (as in a reversible electrochemical reaction), it can often be difficult to electrochemically convert it back to its original, soluble redox state. Researchers in the US have now come up with a new technique to overcome this, using ferrocene as a redox mediator to assist with electron-transfer between the insoluble materials and electrodes.

The researchers studied the redox-active nickel complex, [Ni(PPh2NPh2)2(CH3CN)]2+, which is often used as a catalyst for electrochemical hydrogen evolution. The electrochemistry of the nickel complex was explored under non-catalytic conditions, where the absence of a proton source was previously unexplored. Cyclic voltammetry experiments of [Ni(PPh2NPh2)2]2+ in acetonitrile indicated two, one-electron reduction events that correspond to the NiII/Iand NiI/0 redox couples, both of which were electrochemically and chemically reversible at low concentrations (Figure 1). The researchers noted a concentration dependence, where the reversibility is increasingly lost at higher concentrations of the complex (blue scan, Figure 1A). This was attributed to the formation of the two-electron reduced product, [Ni(PPh2NPh2)2], which proved insoluble in acetonitrile, precipitating out of solution upon its electrochemical formation and depositing on the electrode surface.

Cyclic voltammograms of [Ni(PPh2NPh2)2]2+ in acetonitrile

Figure 1. Cyclic voltammograms of [Ni(PPh2NPh2)2]2+, showing the two redox events with the two, separate peaks. A) Concentration dependence, whereby reversibility decreases upon increasing concentration and B) Scan-rate dependence, whereby reversibility is regained at higher scan-rates.

Once the researchers established the electrochemically-driven solubility changes for the nickel complex, they looked at enhancing the overall reversibility of this reaction. Whilst the two-electron reduction to form insoluble [Ni(PPh2NPh2)2] proceeded smoothly, regenerating [Ni(PPh2NPh2)2]2+ by oxidation of this insoluble product was slow and inefficient, due to poor electron transfer between the deposited material and the electrode. The researchers therefore added ferrocene as a freely diffusing redox mediator to the electrochemical reaction, to essentially shuttle electrons from the insoluble reduction product to the electrode. This proved successful, with subsequent electrochemical experiments of [Ni(PPh2NPh2)2]2+ in the presence of ferrocene showing faster and catalytic regeneration of the original nickel complex.

Redox cycle scheme for [Ni(PPh2NPh2)2]2+

Figure 2. A scheme showing the redox cycle of [Ni(PPh2NPh2)2]2+, with annotations to describe the experimental kinetics observed.

In addition to the experimental studies, the researchers also turned to mathematical modelling to gain more understanding of electrochemically-driven solubility cycling in electrochemical reactions. Two models were presented showing the effect of the deposited materials on the electrochemical response, either with or without possible electrode inhibition effects. Overall, the researchers have presented a unique strategy for improving the reversibility of redox reactions that are limited by insoluble redox products, which is beneficial for systems where both materials deposit on electrodes or are suspended in solution.

 

To find out more, please read:

Redox mediators accelerate electrochemically-driven solubility cycling of molecular transition metal complexes

Katherine J. Lee, Kunal M. Lodaya, Cole T. Gruninger, Eric S. Rountree and Jillian L. Dempsey

Chem. Sci., 2020, 11, 9836-9851

 

About the blogger:

Dr. Samantha Apps just finished her post as a Postdoctoral Research Associate in the Lu Lab at the University of Minnesota, USA, and obtained her PhD in 2019 from Imperial College London, UK. She has spent the last few years, both in her PhD and postdoc, researching synthetic nitrogen fixation and transition metal complexes that can activate and functionalise dinitrogen. Outside of the lab, you’ll likely find her baking at home, where her years of synthetic lab training has sparked a passion in kitchen chemistry too.

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Chemical Science HOT Articles: September

We are pleased to share a selection of our referee-recommended HOT articles for September. We hope you enjoy reading these articles and congratulations to all the authors whose articles are featured! As always, Chemical Science is free to read & download. You can find our full 2020 HOT article collection here.

 

Nonadiabatic dynamics in multidimensional complex potential energy surfaces
Fábris Kossoski and Mario Barbatti
Chem. Sci., 2020, 11, 9827-9835
DOI: 10.1039/D0SC04197A, Edge Article

Rhodium-catalysed tetradehydro-Diels–Alder reactions of enediynes via a rhodium-stabilized cyclic allene
Srinivas Thadkapally, Kaveh Farshadfar, Melanie A. Drew, Christopher Richardson, Alireza Ariafard, Stephen G. Pyne and Christopher J. T. Hyland
Chem. Sci., 2020, Advance Article
DOI: 10.1039/D0SC04390G, Edge Article

‘Sacrificial’ supramolecular assembly and pressure-induced polymerization: toward sequence-defined functionalized nanothreads
Margaret C. Gerthoffer, Sikai Wu, Bo Chen, Tao Wang, Steven Huss, Shalisa M. Oburn, Vincent H. Crespi, John V. Badding and Elizabeth Elacqua
Chem. Sci., 2020, Advance Article
DOI: 10.1039/D0SC03904G, Edge Article

Redox mediators accelerate electrochemically-driven solubility cycling of molecular transition metal complexes
Katherine J. Lee, Kunal M. Lodaya, Cole T. Gruninger, Eric S. Rountreea and Jillian L. Dempsey
Chem. Sci., 2020, 11, 9836-9851
DOI: 10.1039/D0SC02592E, Edge Article

Chiral Fe(ii) complex catalyzed enantioselective [1,3] O-to-C rearrangement of alkyl vinyl ethers and synthesis of chromanols and beyond
Lifeng Wang, Pengfei Zhou, Qianchi Lin, Shunxi Dong, Xiaohua Liu and Xiaoming Feng
Chem. Sci., 2020, 11, 10101-10106
DOI: 10.1039/D0SC04340K, Edge Article

Proteomimetic surface fragments distinguish targets by function
Attila Tököli, Beáta Mag, Éva Bartus, Edit Wéber, Gerda Szakonyi, Márton A. Simon, Ágnes Czibula, Éva Monostori, László Nyitray and Tamás A. Martinek
Chem. Sci., 2020, Advance Article
DOI: 10.1039/D0SC03525D, Edge Article

Enhancing the photodynamic therapy efficacy of black phosphorus nanosheets by covalently grafting fullerene C60
Yajuan Liu, Daoming Zhu, Xianjun Zhu, Gaoke Cai, Jianhua Wu, Muqing Chen, Pingwu Du, Yongshun Chen, Wei Liu and Shangfeng Yang
Chem. Sci., 2020, Advance Article
DOI: 10.1039/D0SC03349A, Edge Article

Acid–base chemistry at the single ion limit
Vignesh Sundaresan and Paul W. Bohn
Chem. Sci., 2020, Advance Article
DOI: 10.1039/D0SC03756G, Edge Article

Structure and dynamics of catalytically competent but labile paramagnetic metal-hydrides: the Ti(iii)-H in homogeneous olefin polymerization
Enrico Salvadori, Mario Chiesa, Antonio Buonerba and Alfonso Grassi
Chem. Sci., 2020, Advance Article
DOI: 10.1039/D0SC04967K, Edge Article

Catalytic asymmetric synthesis of quaternary trifluoromethyl α- to ε-amino acid derivatives via umpolung allylation/2-aza-Cope rearrangement
Xi-Shang Sun, Xing-Heng Wang, Hai-Yan Tao, Liang Wei and Chun-Jiang Wang
Chem. Sci., 2020, Advance Article
DOI: 10.1039/D0SC04685J, Edge Article

 

Chemical Science, Royal Society of Chemistry

Submit to Chemical Science today! Check out our author guidelines for information on our article types or find out more about the advantages of publishing in a Royal Society of Chemistry journal.

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A radical twist to halogenations using boron tribromide

Organoboranes are extremely useful reagents for chemical synthesis; their Lewis acidic nature makes them reactive towards nucleophilic species, and their ability to participate in free-radical processes widely expands their synthetic use. Trialkylboranes (BR3) are the most widely studied in terms of borane radical chemistry, whereby alkyl radicals (R) can be generated through homolytic substitution at the boron atom under oxygen conditions to then participate in various alkylation reactions, as shown in Scheme 1a. This extremely mild radical-initiation system, using just O2 instead of heat or light for radical generation, is highly desirable in chemical synthesis, particularly for the formation of thermally unstable products.

Scheme showing radical generation from organoboranes

Scheme 1: (A) Previously known radical chemistry with organoboranes and (B) radical reactivity using trihaloboranes.

Researchers in both China and the US have now applied this concept of radical generation using trihaloboranes for halogenation. Halogenation reactions are extremely important in chemical synthesis, since the resulting halogenated products are ideal precursors for installing a wide range of functional groups through substitution chemistry. Typically, halogenation of organic molecules using trihaloboranes has been attributed to their Lewis acidic nature, but the researchers have now shown that these reagents can also act as halogen radical donors (as shown in Scheme 1b).

The researchers selected boron tribromide (BBr3) as a bromide radical donor (Br), since the B-O bond that forms upon radical generation using O2 is much stronger than the B-Br bond that breaks, making the process thermodynamically favourable. They applied this approach to investigate the hydrobromination of cyclopropanes, for the novel and selective formation of the anti-Markovnikov haloalkane product. The researchers initially optimised the reaction of cyclopropylbenzene (1a) with BBr3/O2 and found that the addition of a proton source (e.g. H2O or alcohol) was sufficient to terminate the radical reaction and give the anti-Markovnikov product (2a) as the major species (Scheme 2). Using these conditions, the substrate scope could be expanded for the hydrobromination of a wide range of cyclopropanes, including typically challenging alcohol or amine-functionalised substrates.

Reaction optimisation scheme and table for hydrobromination of cyclopropane with BBr3

Scheme 2: Initial reaction optimisation of hydrobromination of cyclopropylbenzene (1a) to give the anti-Markovnikov product (2a) as the major species.

To establish that the hydrobromination reactivity was occurring via a radical process rather than a possible acid-mediated pathway, the researchers conducted a series of control experiments. The addition of radical scavengers resulted in only the formation of the Markovnikov product, suggesting the radical process is necessary for the anti-Markovnikov selectivity observed. The absence of oxygen also shut down the reactivity, which further indicates the radical pathway as shown in Scheme 1b. Additional computations modelled a possible pathway analogous to the established radical alkylation using trialkylboranes, showing an energetically favourable radical pathway for the hydrobromination of cyclopropylbenzene using BBr3/O2 (Figure 1).

Energy profile diagram for the radical hydrobromination of cyclopropanes

Figure 1: The calculated energy profile for the hydrobromination reaction

The results in this study demonstrate that trihaloboranes, like trialkylboranes, can act as radical donors for halogenation reactions, allowing for previously unreported anti-Markovnikov selectivity in the hydrobromination of cyclopropanes. This radical reactivity could be applied in the future for the halogenation of many different organic molecules, giving way to new methods to affect selectivity that cannot be achieved using traditional acid-mediated pathways.

 

To find out more, please read:

Boron tribromide as a reagent for anti-Markovnikov addition of HBr to cyclopropanes

Matthew H. Gieuw, Shuming Chen, Zhihai Ke, K. N. Houk and Ying-Yeung Yeung

Chem. Sci., 2020, 11, 9426-9433

 

About the blogger:

Dr. Samantha Apps just finished her post as a Postdoctoral Research Associate in the Lu Lab at the University of Minnesota, USA, and obtained her PhD in 2019 from Imperial College London, UK. She has spent the last few years, both in her PhD and postdoc, researching synthetic nitrogen fixation and transition metal complexes that can activate and functionalise dinitrogen. Outside of the lab, you’ll likely find her baking at home, where her years of synthetic lab training has sparked a passion in kitchen chemistry too.

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Aggregation makes fluorescent probes better and brighter

Fluorescence, the phenomenon where a molecule re-emits light upon absorption of electromagnetic radiation, is used in biological imaging to visualise structures, processes and diseases. Emission of these fluorescent molecules, known as fluorophores, in the near-infrared region is particularly advantageous, allowing for enhanced tissue penetration and reduced photodamage. Near-infrared (NIR) fluorophores are therefore attractive probes for bioimaging but are currently limited with problems such as low brightness or quenching of the emission by aggregation.

To overcome this aggregation-caused quenching effect, researchers in China turned to fluorophores that have aggregation-induced emission (AIE) properties. Aggregation-induced emission (AIE) is a concept where molecules only fluoresce upon aggregation in concentrated solutions, and not in dilute solutions where they can freely rotate. The researchers therefore designed their fluorophore to contain the molecular rotor tetraphenylethene, that can induce AIE effects and therefore boost and brighten the fluorescence.

The researchers prepared a suite of fluorophores using a central donor-acceptor-donor core, with methoxy-tetraphenylethene (MTPE) as the donor and thieno[3,4,-b]pyrazine (TP) as the acceptor. Substituents on the TP acceptor were varied, and the effects on aggregation and the fluorescence were investigated. Density functional theory calculations gave the researchers insight into the molecular conformations of the fluorophores, as shown in Figure 1. The 3 variants all showed twisted geometries (top row, Figure 1), indicating high degrees of rotation, which could then be restricted through aggregation and give rise to the desired AIE effects. Additionally, the calculations measured electronic distributions, confirming high degrees of electron conjugation in the molecules (see the HOMO diagrams, Figure 1) that are essential for fluorescence.

DFT results of AIE fluorophores

Figure 1: Results from density functional theory calculations to show molecular geometries and electron conjugation within the suite of fluorophores

The fluorescence characteristics of the variants were measured by absorption and emission/photoluminescence spectra. The absorption spectra in DMSO (Figure 2a) shows absorptions between 518 and 543 nm, with the most red-shifted (longer wavelength) absorption displayed for the most conjugated variant (MTPE-TP3). The effect of aggregation on the fluorescence was measured by adding water (in which the fluorophores showed poor solubility) to the DMSO solutions, and the resulting photoluminescence intensities showed an increase with higher water fractions. This increase in brightness (i.e. intensity) is explained by the water affecting aggregation of the fluorophores and inducing the AIE effect (Figures 2b and c).

Fluorescence spectra and aggregation effects of AIE fluorophores

Figure 2: a) Absorption spectra of the fluorophore variants; b) photoluminescence spectra of the most conjugated variant, MTPE-TP3 with different water fractions; c) corresponding photoluminescence intensity plotted against water fractions for all three variants. d) to f) additionally indicate the effect of increased viscosity (and aggregation) upon glycerol addition to the fluorophores.

The researchers also formulated nanoparticles for each fluorophore variant to allow for better water solubility and therefore biocompatibility. They found that the absorption and emission of the nanoparticles became both brighter and more red-shifted and were now within the near-infrared range for favourable biological imaging. In vitro and in vivo testing of these nanoparticles in breast cancer cells and tumour-bearing mice verified that the AIE-nanoparticles are suitable for biological imaging, and indicate their potential to assist with tumour diagnosis in future clinical settings.

 

To find out more, please read:

Simultaneously boosting the conjugation, brightness and solubility of organic fluorophores by using AIEgens

Ji Qi, Xingchen Duan, Yuanjing Cai, Shaorui Jia, Chao Chen, Zheng Zhao, Ying Li, Hui-Qing Peng, Ryan T. K. Kwok, Jacky W. Y. Lam, Dan Ding  and  Ben Zhong Tang

Chem. Sci., 2020, 11, 8438-8447

 

About the blogger:

Dr. Samantha Apps is a Postdoctoral Research Associate in the Lu Lab at the University of Minnesota, USA, and obtained her PhD in 2019 from Imperial College London, UK. She has spent the last few years, both in her PhD and postdoc, researching synthetic nitrogen fixation and transition metal complexes that can activate and functionalise dinitrogen. Outside of the lab, you’ll likely find her baking at home, where her years of synthetic lab training has sparked a passion in kitchen chemistry too.

 

 

 

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HOT Articles: August

We are pleased to share a selection of our referee-recommended HOT articles for August. We hope you enjoy reading these articles and congratulations to all the authors whose articles are featured! As always, Chemical Science is free to read & download. You can find our full 2020 HOT article collection here.

 

Nucleation mechanisms and speciation of metal oxide clusters
Enric Petrus, Mireia Segado and Carles Bo
Chem. Sci., 2020, 11, 8448-8456
DOI: 10.1039/D0SC03530K, Edge Article

Boron tribromide as a reagent for anti-Markovnikov addition of HBr to cyclopropanes
Matthew H. Gieuw, Shuming Chen, Zhihai Ke, K. N. Houk and Ying-Yeung Yeung
Chem. Sci., 2020, Advance Article
DOI: 10.1039/D0SC02567D, Edge Article

Free-standing metal–organic framework (MOF) monolayers by self-assembly of polymer-grafted nanoparticles
Kyle Barcus and Seth M. Cohen
Chem. Sci., 2020, 11, 8433-8437
DOI: 10.1039/D0SC03318A, Edge Article

Recent advances of group 14 dimetallenes and dimetallynes in bond activation and catalysis
Franziska Hanusch, Lisa Groll and Shigeyoshi Inoue
Chem. Sci., 2020, Advance Article
DOI: 10.1039/D0SC03192E, Minireview

Dissipative self-assembly, competition and inhibition in a self-reproducing protocell model
Elias A. J. Post and Stephen P. Fletcher
Chem. Sci., 2020, Advance Article
DOI: 10.1039/D0SC02768E, Edge Article

A bipedal DNA nanowalker fueled by catalytic assembly for imaging of base-excision repairing in living cells
Meng-Mei Lv, Jin-Wen Liu, Ru-Qin Yu and Jian-Hui Jiang
Chem. Sci., 2020, Advance Article
DOI: 10.1039/D0SC03698F, Edge Article

Exploring modular reengineering strategies to redesign the teicoplanin non-ribosomal peptide synthetase
Milda Kaniusaite, Robert J. A. Goode, Julien Tailhades, Ralf B. Schittenhelm and Max J. Cryle
Chem. Sci., 2020, Advance Article
DOI: 10.1039/D0SC03483E, Edge Article

Engineering micromechanics of soft porous crystals for negative gas adsorption
Simon Krause, Jack D. Evans, Volodymyr Bon, Irene Senkovska, Sebastian Ehrling, Paul Iacomic, Daniel M Többens, Philip L. Llewellyn, Dirk Wallacher, Manfred S. Weiss, Bin Zheng, Pascal G. Yot, Guillaume Maurin, François-Xavier Coudert and Stefan Kaskel
Chem. Sci., 2020, Accepted Manuscript
DOI: 10.1039/D0SC03727C, Edge Article

Simultaneous Manifestations of Metallic Conductivity and Single-Molecule Magnetism in a Layered Molecule-based Compound
Yongbing Shen, Masahiro Yamashita, Brian. K. Breedlove, Carmen Herrmann, Kaiji Uchida, Goulven Cosquer, Manabu Ishikawa, Akihiro Otsuka, Shinji K Yoshina, Takefumi Yoshida, Hideki Yamochi, Seiu Katagiri, Hiroshi Ito and Haitao Zhang
Chem. Sci., 2020, Accepted Manuscript
DOI: 10.1039/D0SC04040A, Edge Article

Chemical Science, Royal Society of Chemistry

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HOT Articles: July

We are pleased to share a selection of our referee-recommended HOT articles for July. We hope you enjoy reading these articles and congratulations to all the authors whose articles are featured! As always, Chemical Science is free to read & download. You can find our full 2020 HOT article collection here.

 

Exohedral functionalization vs. core expansion of siliconoids with Group 9 metals: catalytic activity in alkene isomerization
Nadine E. Poitiers, Luisa Giarrana, Volker Huch, Michael Zimmer and David Scheschkewitz
Chem. Sci., 2020, Advance Article
DOI: 10.1039/D0SC02861D

Deoxygenative α-alkylation and α-arylation of 1,2-dicarbonyls
Shengfei Jin, Hang T. Dang, Graham C. Haug, Viet D. Nguyen, Hadi D. Arman and Oleg V. Larionov
Chem. Sci., 2020, Advance Article
DOI: 10.1039/D0SC03118F

Cellular uptake and targeting of low dispersity, dual emissive, segmented block copolymer nanofibers
Steven T. G. Street, Yunxiang He, Xu-Hui Jin, Lorna Hodgson, Paul Verkade and Ian Manners
Chem. Sci., 2020, Advance Article
DOI: 10.1039/D0SC02593C

Mechanochemical synthesis of glycine oligomers in a virtual rotational diamond anvil cell
Brad A. Steele, Nir Goldman, I-Feng W. Kuo and Matthew P. Kroonblawd
Chem. Sci., 2020, Advance Article
DOI: 10.1039/D0SC00755B

Total synthesis of endiandric acid J and beilcyclone A from cyclooctatetraene
Oussama Yahiaoui, Adrian Almass and Thomas Fallon
Chem. Sci., 2020, Advance Article
DOI: 10.1039/D0SC03073B

Template effects of vesicles in dynamic covalent chemistry
Carlo Bravin and Christopher A. Hunter
Chem. Sci., 2020, Advance Article
DOI: 10.1039/D0SC03185B

Simultaneously boosting the conjugation, brightness and solubility of organic fluorophores by using AIEgens
Ji Qi, Xingchen Duan, Yuanjing Cai, Shaorui Jia, Chao Chen, Zheng Zhao, Ying Li, Hui-Qing Peng, Ryan T. K. Kwok, Jacky W. Y. Lam, Dan Ding and Ben Zhong Tang
Chem. Sci., 2020, Advance Article
DOI: 10.1039/D0SC03423A

Enhanced enzymatic activity exerted by a packed assembly of a single type of enzyme
Huyen Dinh, Eiji Nakata, Kaori Mutsuda-Zapater, Masayuki Saimura, Masahiro Kinoshita and Takashi Morii
Chem. Sci., 2020, Advance Article
DOI: 10.1039/D0SC03498C

Structure-activity relationships in well-defined conjugated oligomer photocatalysts for hydrogen production from water
Catherine M. Aitchison, Michael Sachs, Marc Little, Liam Wilbraham, Nick J. Brownbill, Chris Kane, Frédéric Blanc, Martijn Zwijnenburg, James Durrant, Reiner Sebastian Sprick and Andrew Cooper
Chem. Sci., 2020, Accepted Manuscript
DOI: 10.1039/D0SC02675A
Chemical Science, Royal Society of Chemistry

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Biradical bismuth makes its debut

Low-valent compounds are attractive in chemical synthesis and catalysis due to their highly reactive nature. Carbenes are the archetypal example, where the carbon atom is divalent with two valence electrons, but group 15 analogues have also gained recent interest as reactive intermediates in fundamental transformations. These low-valent compounds (E-R), where the group 15 atom (E) has an oxidation state of +1 and is bound to just one additional atom, are extremely reactive and therefore challenging to isolate. The lighter congeners of nitrogen and phosphorus (as nitrenes, N-R, and phosphinidenes, P-R) have been isolated, but the heavier homologues are much more difficult to access and tend to undergo degradation. Stabilisation through adduct formation with Lewis bases had previously allowed for the formation of the heaviest group 15 bismuth homologue, and these stabilised bismuthinidenes showed potential in electrocatalytic and photophysical applications. Researchers in Germany and Switzerland have now reported for the first time the generation of a free and non-stabilised organometallic bismuthinidene compound, methylbismuth (BiMe), in the gas phase (Figure 1).

Low valent group 15 structures

Figure 1: Structures and examples of low-valent group 15 compounds, with their electronic ground state configuration

The researchers targeted the non-stabilised organometallic bismuthinidene using a top-down approach, by breaking the Bi-C bonds of the higher valent and well-defined BiMe3 precursor (Scheme 1). They achieved this by pyrolysis of BiMe3, with subsequent analysis by photoelectron-photoion coincidence spectroscopy (PEPICO), that allows the recording of photoionisation mass spectra to detect ions produced by the pyrolysis. As shown by the photoionisation mass spectra in Figure 2, pyrolysis resulted in methyl loss through Bi-C homolytic cleavage, with higher pyrolysis power (bottom trace) showing full conversion of BiMe3 with by of m/z = 254. Stepwise methyl loss down to atomic bismuth was observed with m/z = 209 for Bi+, but notably BiMe+ was observed at m/z = 224, indicating bismuthinidene formation.

BiMe generation

Scheme 1: Stepwise methyl abstraction from BiMe3 to generate bismuthinidene BiMe in the  gas phase by flash pyrolysis

Photoionisation mass spectra for BiMe

Figure 2: Photoionisation mass spectra showing methyl loss in the conversion of BiMe3 to BiMe by pyrolysis. Top trace = without pyrolysis, middle trace = low pyrolysis power, bottom trace = high pyrolysis power

The researchers further probed the electronic nature of the generated bismuthinidene by additional photoelectron spectroscopy and simulations. An ionisation energy of 7.88 eV was determined, and indicated the triplet (biradical) ground state (structure 3 in Scheme 1) as the lowest energy structure. This contrasts to the lighter N and P congeners, where the methylene species are the most energetically favoured (like 5 in Scheme 1). The researchers also conducted experiments to investigate the stepwise methyl abstraction via BiMe2, determining a bond dissociation energy of 210 kJ mol-1 for the first Bi-C homolytic cleavage and demonstrating that this methyl abstraction could also be achieved under moderate reaction conditions. Overall, this report indicates that non-stabilised bismuthinidenes can be generated, with the potential for future exploitation as reactive intermediates in synthetic chemistry.

 

To find out more, please read:

Methylbismuth: an organometallic bismuthinidene biradical

Deb Pratim Mukhopadhyay, Domenik Schleier, Sara Wirsing, Jacqueline Ramler, Dustin Kaiser, Engelbert Reusch, Patrick Hemberger, Tobias Preitschopf, Ivo Krummenacher, Bernd Engels,* Ingo Fischer* and Crispin Lichtenberg*

Chem. Sci., 2020, Advance Article

 

About the blogger:

Dr. Samantha Apps is a Postdoctoral Research Associate in the Lu Lab at the University of Minnesota, USA, and obtained her PhD in 2019 from Imperial College London, UK. She has spent the last few years, both in her PhD and postdoc, researching synthetic nitrogen fixation and transition metal complexes that can activate and functionalise dinitrogen. Outside of the lab, you’ll likely find her baking at home, where her years of synthetic lab training has sparked a passion in kitchen chemistry too.

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