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Come for the colour changing crystals, stay for the science

31 May 2018

Synthesis of copper bimetallic complexes from imidazolyl ligands, and the solvatochromic materials formed upon crystallization and solvent guest-exchange. The solvatochromic behaviour was quantified with visible-region diffuse reflectance spectra.

During the first inorganic chemistry course I took during my undergraduate degree, our professor started the class by passing around some mineral samples, promising us that if we pursued the chemistry of metals we could work with beautifully coloured crystals every day. At the time, colour seemed like such a trite detail amongst the complexity of the subject. Why would you choose a field of study based on something so simple? Well, after a PhD dominated by pale yellow oils, I think I get it now.

Nikolayenko and Barbour at the University of Stellenbosch in South Africa bring us colour! The authors synthesised organometallic copper complexes, which crystallise to form porous single crystals that drastically change colour upon absorption of various solvents. The authors investigated the solvatochromic mechanism using X-ray crystallography, EPR, UV-visible spectroscopy and DFT calculations. Solvatochromic materials are not just made to look pretty; they have potential to be used as sensitive, selective and recyclable sensors to detect solvent vapours with useful applications in industrial process risk management, chemical threat detection and environmental monitoring.

The researchers synthesised a series of complexes comprised of a bidentate ligand with 2-methylimidazolyl groups coordinated to copper(II) ions. The complexes stack to form channels in the crystal, capable of trapping solvent molecules to give different coloured crystals: DMSO and THF-containing crystals are green (λ_max = 574 nm and 540 nm, respectively), those containing acetonitrile are red (λ_max = 624 nm), and crystals trapping acetone, ether and pentane are yellow (λ_max = 588), orange (λ_max = 598 nm) and red/brown (λ_max = 592 nm), respectively.

The authors revealed a correlation between the size of the solvent guest, coordination geometry of the copper complex, and the ligand field splitting. Small guests such as acetonitrile minimally perturb the metallocyclic framework, preserving a rhombic ligand field geometry (large δ_xy of g values in the EPR spectrum), small ligand d-orbital splitting and red-shifted optical spectra. Large guests such as THF have the opposite effect, giving ligand field geometries approaching tetragonal (small δ_xy), large ligand field d-orbital splitting and blue-shifted optical spectra.

By delving into the complexity beneath a seemingly simple phenomenon, Nikolayenko, Barbour and their co-workers have shown using a series of single-crystal complexes that there is nothing simple about colour (and nothing trite about detail).

To find out more please read:

Supramolecular solvatochromism: mechanistic insight from crystallography, spectroscopy, and theory

Varvara I. Nikolayenko, Lisa M. van Wyk, Orde Q. Munro, Leonard J. Barbour.
Chem. Commun., 2018, Advance Article
DOI: 10.1039/c8cc02197j

About the author

Zoë Hearne is a PhD candidate in chemistry at McGill University in Montréal, Canada, under the supervision of Professor Chao-Jun Li. She hails from Canberra, Australia, where she completed her undergraduate degree. Her current research focuses on transition metal catalysis to effect novel transformations, and out of the lab she is an enthusiastic chemistry tutor and science communicator.

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Pt3Ni-Coated Palladium Nano-branches Outperformed Pt in Catalyzing Ethanol Oxidation

30 May 2018

Researchers in China recently developed a new Pd-based catalyst that outperformed Pt, the benchmark catalyst for electrochemical oxidation of ethanol. This catalyst, synthesized by a one-pot chemical reduction method, consists of branched Pd nanocrystals coated with thin Pt₃Ni shells.

The ethanol oxidation reaction (EOR) is a typical anode reaction that drives the energy output from fuel cells. Due to its intrinsically slow kinetics, the reaction requires proper EOR catalysts to facilitate the oxidation. Pt-based materials are highly active in promoting EOR, but the scarcity of Pt leads to high costs and demands efficient methods to recycle these materials. In addition, the instable catalytic activity of Pt significantly reduces the lifetime of EOR catalysts containing Pt. Clearly, developing inexpensive EOR catalysts with comparable performance to Pt is meaningful for the affordability and durability of fuel cells.

A research team led by Shuifen Xie at Huaqiao University and Shenzhen Research Institute of Xiamen University in China, have demonstrated a one-pot chemical reduction method of a novel EOR catalyst. This catalyst is made of Pt₃Ni-coated Pd nanocrystals as shown in Figure 1. There are three main advantages for this catalyst over the benchmark Pt: Firstly, the core material Pd is more affordable. Secondly, the ultrathin Pt-alloy coating, Pt₃Ni, contains relatively less Pt and is reported to exhibit high EOR catalytic activity. Thirdly, the little lattice mismatch between Pt and Pd allows seamless integration of the two metals that is beneficial to preserve the structural integrity and ensure excellent durability.

Figure 1. (a) The schematic illustration showing the key steps of the one-pot chemical reduction method. The catalyst is formed via consecutive reduction of Pd²⁺, Pt²⁺ and Ni²⁺ to Pd nano-branches, Pt nanoparticles and Pt₃Ni coatings, respectively. (b) A TEM image of a representative morphology of a branched nanocrystal. (c) Elemental mappings depict that Pt and Ni elements exist mainly in the shell while Pd is in the core.

Electrochemical characterizations revealed that the catalytic performance of the Pt₃Ni-coated Pd nanocrystals outperformed those of two commercial catalysts: Pt/C (Pt particles supported on activated carbon) and Pd black (a fine powder elemental Pd). Figure 2a compares the linear-sweep voltammograms of the three samples. The synthesized catalyst showed appreciably enhanced oxidation current at potentials beyond 0.4 V vs. RHE. The histograms in Figure 2b clearly display that the mass activity and the specific activity of the synthesized nanocrystals are the highest. The authors ascribed the superior performance to the high surface area (42.50 m² g^-1), the ultrathin Pt₃Ni coating with its {111} crystal planes exposed, and the core-shell configuration.

Figure 2. (a) Linear-sweep voltammograms of the synthesized catalyst (Pd@Pt₃Ni/C), Pt/C and Pd black. (b) The comparison of mass activity (i.e. oxidation current normalized to the masses of the catalysts) and specific activity (i.e. oxidation current normalized to the areas of the catalysts) of Pd@Pt₃Ni/C, Pt/C and Pd black.

This work signifies the feasibility of Pd-based nano-catalysts as alternatives to Pt towards catalyzing EOR. It is also expected to encourage the effort in developing a diverse array of inexpensive and high-performance catalysts for other reactions pertaining to fuel cells, including but not limited to oxidation of fuels other than ethanol and oxygen reduction reactions.

To find out more please read:

One-Pot Synthesis of Pd@Pt₃Ni Core-Shell Nanobranches with Ultrathin Pt₃Ni{111} Skins for Efficient Ethanol Electrooxidation

Yuanyuan Wang, Wei Wang, Fei Xue, Yong Cheng, Kai Liu, Qiaobao Zhang, Maochang Liu and Shuifen Xie

Chem. Commun., 2018, DOI: 10.1039/c8cc02816h

About the blogger:

Tianyu Liu obtained his Ph.D. (2017) in Physical Chemistry from University of California, Santa Cruz in the United States. He is passionate about scientific communication to introduce cutting-edge research to both the general public and scientists with diverse research expertise. 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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The CO2-Capturing Mechanism of Quaternary Nitrogen-Containing Polymers Revealed Experimentally

21 May 2018

A group of scientists from Washington University at St. Louis, USA have disclosed experimentally how CO₂ is captured by polymers with quaternary nitrogen cations. Using solid-state nuclear magnetic resonance (NMR), the authors established that CO₂ molecules were absorbed as bicarbonate anions (HCO₃^–).

The increasing amount of CO₂ has posed a number of concerning environmental issues such as climate change, rising sea level and ocean acidification. Capturing CO₂ from the atmosphere is an effective way to lower the CO₂ concentration. Recently, a family of polymer absorbents containing quaternary nitrogen functional groups, termed humidity-swing polymers, have been identified as promising absorbents to absorb CO₂ directly from air. However, the limited understanding of the chemical mechanism related to their CO₂-capturing capability hindered the development of these promising absorbents.

In ChemComm, Yang et al. used solid-state ¹³C NMR to explore how CO₂ molecules were captured and released. Figure 1a presents the NMR spectra of a humidity-swing polymer absorbent itself (top), upon contacting with CO₂ (middle) and after releasing CO₂ (bottom). The most striking feature is the appearance of an additional sharp peak at a chemical shift of 161 ppm in the middle spectrum, which did not show up in the other two spectra. The authors further studied the shape evolution of the additional peak, with respect to temperature, and concluded that the peak was due to HCO₃^– anions. Additionally, the authors also identified the presence of hydroxide anions in the absorbent after CO₂ was released.

Figure 1. (a) The solid-state ¹³C NMR spectra of the humidity-swing polymeric absorbent (structure shown in the inset of the middle spectrum) itself (top), upon contacting with CO₂ (middle) and after releasing CO₂ (bottom). (b) The proposed pathways of how CO₂ molecules interact with the quaternary-N anions of the absorbent.

The researchers then proposed the CO₂ adsorption-desorption mechanism (illustrated in Figure 1b) based on the experimental results. The storage and release of CO₂ depend on the humidity level of the surroundings: When the humidity is low, the polymer absorbs CO₂ and forms HCO₃^– anions; the negative charge of HCO₃^– is counter-balanced by the neighboring quaternary N cations. When the humidity is increased, HCO₃^– anions combine with water and decompose to CO₂ and hydroxide anions. This proposed pathway does not involve CO₃^2- anions, which differs from the previously-reported mechanisms derived from theoretical simulations.

The published results represent the first set of experimental evidence elucidating how CO₂ molecules interact with humidity-swing polymeric absorbents. The acquired mechanistic insight could provide valuable guidelines for the design of CO₂ absorbents with ultrahigh absorption capacity.

To find out more please read:

Humidity-Swing Mechanism for CO₂ Capture from Ambient Air
Hao Yang, Manmilan Singh and Jacob Schaefer
Chem. Commun., 2018, DOI: 10.1039/c8cc02109k

About the blogger:

Tianyu Liu obtained his Ph.D. (2017) in Physical Chemistry from University of California, Santa Cruz in United States. He is passionate about scientific communication to introduce cutting-edge research to both the general public and scientists with diverse research expertise. 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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Ruthenium Currency for a Hydrogen Fuel Economy

02 May 2018

A group of researchers at the Chinese Academy of Sciences and Southwest University want us to kick the fossil fuels habit. Their research comes to us from China, a country using roughly one quarter of the world’s yearly energy consumption, and where the finite nature of fossil fuels is a very real threat to energy supply security. Leading in energy use, China also leads the world in electricity production from renewable sources and investment in clean energy projects.

Hydrogen is considered a viable alternative to fossil fuels as it is energy rich, more so than petrol or ethanol at 39 kWh/kg (petrol: 13 kWh/kg, ethanol: 8.2 kWh/kg), and upon combustion emits only water vapour. However, hydrogen is often obtained from fossil fuels, and it will only be a practical option for the world’s future energy needs if it can be produced from a renewable source.

Preparation of the Ru2P/reduced graphene oxide electrocatalyst for the hydrogen evolution reaction

Preparation of the Ru₂P/reduced graphene oxide catalyst

To this end, water splitting offers a solution. In a water electrolysis cell, hydrogen is produced at the cathode via the hydrogen evolution reaction (HER, 2H⁺ + 2e^– –> H₂), and molecular oxygen is produced at the anode (2H₂O –> O₂ + 4H⁺ + 4e^–). It is ideal in theory, but high energy efficiencies are required to make water splitting viable, and this relies on the development of catalytic electrodes to minimize overpotentials required to drive the reaction. Currently, state of the art HER electrocatalysts use platinum, which is expensive and rare. Furthermore, platinum catalysts are most efficient in an acidic electrolyte and proceed 2-3 times slower in alkaline solutions. On the other hand, the best oxygen evolution catalysts perform better in alkaline environments. Using an alkaline electrolyte has overall advantages as it is less corrosive, thus increasing the stability and lifetime of the electrolytic cell.

The authors have developed a HER catalyst, using ruthenium, with overpotentials and current densities superior to Pt/C in both alkaline and acidic conditions.

DFT calculation to probe the hydrogen adsorption energies on the active catalytic surface of the Ru2P on reduced graphene oxide catalyst.

DFT calculation to probe the hydrogen adsorption energies on the active catalytic surface of the Ru₂P catalyst. a) and b) front and side views of the calculated Ru₂P/reduced graphene oxide surface. c) free energy diagram for the HER with different catalysts.

The electrocatalyst is comprised of small, uniform Ru₂P nanoparticles (~2-4 nm) evenly distributed on reduced graphene oxide sheets. The activity of the prepared catalyst (1.0 mg cm^-2) for the HER was measured in an acidic medium (0.5 M H₂SO₄) and the overpotential to achieve a current density of -10 mA cm^-2 was -22 mV, superior to Pt/C (-27 mV). In an alkaline environment (1.0 M KOH) catalyst performance was enhanced, with an overpotential of -13 mV (29 mV lower than Pt/C). High Faradaic efficiencies of more than 98% were measured in both acidic and alkaline solutions. Additionally, analysis was undertaken to further understand how the structure and composition of the catalyst influences its activity. Double layer capacitance measurements gave clues about the catalyst surface, while theoretical DFT calculations were used to study H-adsorption energies.

There is no way to avoid the reality that ruthenium is also a rare and costly metal, and for this reason may not hold the key to solving our energy woes. However, of real value are the insights gained from probing the structure function relationship of this highly active catalyst, which may guide the synthesis of rationally-designed catalysts using inexpensive and abundant materials.

To find out more please read:

Ultrasmall Ru₂P nanoparticles on graphene: a highly efficient hydrogen evolution reaction electrocatalyst in both acidic and alkaline media

Tingting Liu, Shuo Wang, Qiuju Zhang, Liang Chen, Weihua Hu, Chang Ming Li.
Chem. Commun., 2018, 54, 3343-3346
DOI: 10.1039/c8cc01166d

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In-Situ Electron Paramagnetic Resonance Spectroscopy Revealed the Charge Storage Behavior of Activated Carbon

19 Apr 2018

Recently in Chem. Commun., Wang et al. from University of Manchester and Liverpool John Moores University, U.K. demonstrated that in-situ electron paramagnetic resonance (EPR) spectroscopy was a powerful tool to study the charge storage mechanism of activated carbon.

Activated carbon is a type of microporous carbon used for electrodes of supercapacitors (a family of charge-storage devices similar to batteries). Conventional electrochemical testing techniques (e.g. cyclic voltammetry) are able to evaluate the overall performance of electrode materials but are unable to reveal the charge storage mechanism at the atomic level. Understanding the charge storage mechanism is crucial to guide the design and synthesis of electrode materials with improved performance. During the past decade, the development of numerous in-situ probing techniques has allowed materials researchers to explore the microscopic charge-discharge behaviour of supercapacitor electrodes.

In the published paper, in-situ EPR spectroscopy was used to study the electrochemical properties of activated carbon under different external potentials. EPR is very sensitive to electron spins originating from unpaired electrons that are generated upon charging or discharging electrode materials. This characteristic makes EPR a suitable technique for in-situ studies. To carry out the experiments, the authors designed and constructed a capillary three-electrode testing cell (Figure 1a). This cell was placed in an EPR spectrometer and its activated carbon electrode was connected to an external power source (to apply external potentials to the activated carbon electrode). The authors collected the spectra of the activated carbon electrode at selected applied potentials, an example of which is shown in Figure 1b.

Analysis of the obtained spectra offered important information about how the surface of activated carbon changed at different potentials. Specifically, the authors deconvoluted the signal into two components: the narrow signal and the broad signal corresponding to the blue and red curves in Figure 1b, respectively. The peak intensity of the narrow signal increased drastically when charging the electrode, but remained almost unchanged when altering the testing temperature. This observation suggests that the origin of the narrow peaks was the surface-localized electrons. These localized electrons were likely from the oxidized products (i.e. radicals) of carboxylate and alkoxide groups on the surface of the activated carbon, evolved during the charging process. The broad signal was ascribed to electrons located on aromatic units (e.g. graphene domains) and its intensity was found to be proportional to the number of ions electrically adsorbed on the activated carbon surface.

Figure 1. (a) The structure of the self-built capillary three-electrode cell: CE – counter electrode (Pt wire); RE – reference electrode (Ag/AgCl); WE – working electrode (activated carbon). (b) A typical EPR signal (black) that can be deconvoluted into narrow peaks (blue) and broad peaks (red).

This work highlights EPR spectroscopy as a novel tool for in-situ investigation of the charge-storage mechanism of carbon-based supercapacitor electrodes, and could be potentially extended to study other types of materials. The availability of diverse in-situ techniques is expected to provide more in-depth fundamental understanding that will guide researchers to rationally develop electrodes with optimized performance.

To find out more please read:

In-Situ Electrochemical Electron Paramagnetic Resonance Spectroscopy as A Tool to Probe Electric Double Layer Capacitance

Bin Wang, Alistair J. Fielding and Robert A. W. Dryfe

Chem. Commun. 2018, DOI: 10.1039/c8cc00450a

About the blogger:

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

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Synthesis of Maleimide Dyes with Colourful Fluorescent Emissions

18 Apr 2018

A group of researchers based at universities spanning the UK, China and Spain have synthesised a diverse library of fluorescent maleimide dyes with the aim of developing a structure-function relationship, relating substituent effects to the optical properties of such molecules. This work is not only important to build upon fundamental understanding of the fluorescence mechanism, but to develop knowledge that may be used to guide the synthesis of organic fluorophores which demand particular optical properties.

Organic fluorescent molecules are used as tools in many areas such as forensics, genetic analysis, DNA sequencing and biotechnology. Maleimides are commonly used as fluorescent labels for proteins, as they can couple with the thiol groups of cysteine residues. They are suited to this purpose as they are stable, easily functionalised, give strong emissions and do not perturb the protein structure to a large extent.

Molecules fluoresce upon absorption of UV or visible light, elevating an electron from a ground state orbital to a higher-energy orbital and resulting in a singlet excited state. Relaxation to the ground state occurs rapidly (~ 10 ns) with concomitant emission of a photon – this is what we observe as ‘fluorescence’. The emitted photon almost always has a longer wavelength than the absorbed light, a phenomenon known as the ‘Stokes shift’.

Structures of selected amino-halo-maleimides and alkoxy-halo-maleimides synthesised for the study

With three dihalomaleimide precursers in hand (Cl, Br and I) the researchers assembled a library of amino-halo-maleimides, amino-alkoxy-maleimides, and amino-thio-maleimides. They varied the R groups bound to the N, O and S heteroatoms to include aliphatic, phenyl and benzyl examples.

The optical properties of the amino-halo-maleimides in diethyl ether were examined and the emission wavelengths were measured to be 461-487 nm, giving green-blue fluorescence. The fluorescence quantum yields, a measure of the quantity of emitted photons compared to absorbed photons and an indication of emission brightness, decreased with the electronegativity of the halide (Cl: 37%, Br: 30%, I: 8%). Like many fluorescent molecules in solution the compounds exhibited solvafluorochromism: when the polarity of the solvent alters the optical properties. In protic solvents (methanol and water) the fluorescence quantum yields decreased to below 1% and the emission wavelengths increased by 73-109 nm. On the other hand, in non-polar solvents (cyclohexane) the fluorescence quantum yield increased, up to 56% for the chloro analogue.

a) The UV and emission spectra of fluorescent maleimides bearing amino (2a-c) and alkoxy (3a, 3b) substituents. b) The quantum yields of selected amino and alkoxymaleimides. c) The solvafluorochromism effect for three maleimides (2a-c) in various solvents.

Compared to their amino-substituted counterparts, alkoxy-halo-maleimides have lower quantum yields (reduction of 20-25%), indicating the increased electron-donating capacity of the amine substituent is important for fluorescence intensity. Furthermore, the slight decrease in the emission wavelengths of alkoxy-halo-maleimides (458-465 nm) gives them blue fluorescent emissions. Amino-thio-maleimides, with greater electron-donating capacity than both the amino and alkoxy analogues, have increased emission wavelengths (526-564 nm), thus yellow fluorescent emissions.

This study is a worthwhile read for anyone who uses fluorescent molecules in their work, those wishing to understand a little more about the practical principles of fluorescence and all those curious minds who like to form their own hypotheses.

To find out more please read:

Rational design of substituted maleimide dyes with tunable fluorescence and solvafluorochromism

Yujie Xie, Jonathan T. Husband, Miquel Torrent-Sucarrat, Huan Yang, Weisheng Liu, Rachel K. O’Reilly.
Chem. Commun., 2018, 54, 3339 – 3342
DOI: 10.1039/C8CC00772A

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Synthesis of Oxazolines with a Triple Activation Twist

04 Apr 2018

A group of researchers based in Valencia, Spain, have developed an enantio- and diastereoselective method to synthesize oxazolines, using a combination of silver and an organocatalyst that is only one degree of separation away from a classic British cocktail (to get that one, you might have to keep reading). This method complements the current literature by employing unactivated ketone substrates to prepare products with two adjacent chiral centres, one of which is quaternary.

Oxazolines are a versatile functional group, found in synthetic precursors and functional molecules and materials. They are a common structural feature in many natural products and in medicinal chemistry they are incorporated into synthetic drug candidates as bioisosteres of other functional groups, such as thiazoles and imidazoles. Oxazolines can also be used to prepare poly(2-alkyl/aryl-oxazoline) polymers, which have utility in biomedical fields to prepare hydrogels, nanoparticles for drug delivery and imaging, and polymer-protein conjugates. In an organic synthesis laboratory oxazolines are encountered as ligands in asymmetric catalysis, such as the bis(oxazoline) ‘BOX’ family of compounds.

Synthesis of oxazolines from ketones and isocyanaoacetate esters via a formal [3+2] cycloaddition reaction.

The researchers optimised the reaction with acetophenone and tert-butyl isocyanoacetate, and found that Ag₂O (2.5 mol%) and a dihydroquinine squaramide organocatalyst (5 mol%) promoted the reaction to obtain a quantitative yield of the cis-oxazoline (80:20 cis/trans) in 24 hours with excellent enantiomeric excess (99/93% e.e.). The researchers tested the reaction scope and found that acetophenone derivatives substituted with electron-withdrawing and donating groups (NO₂, Cl, Br, Me) gave the cis-oxazoline products in reliable yields (60 – 99%), fair to good diastereoselectivity (56:44 – 95:5 cis/trans) and excellent enantioselectivities (91 – 99% e.e.). Other examples tested included cyclohexanone, 2-acetylthiophene, deoxybenzoin and acetone as well as aliphatic ketones with a combination of methyl, isopropyl and cyclopropyl groups (63-99% yield, 45:55 – 98:2 cis/trans, 56 – 98% e.e.).

Diagram showing the triple activation of the substrates achieved using a dihydroquinone squaramide organocatalyst combined with a silver lewis acid.

Diagram showing the triple activation of substrates using a dihydroquinine squaramide organocatalyst combined with a silver lewis acid.

The dihydroquinine squaramide organocatalyst used is a composite structure comprising a rigid cyclobutendione ‘squaramide’ and a reduced quinine analogue. The former makes strong hydrogen bonds with carbonyl derivatives while the latter confers chirality on the catalyst and has a Brønsted basic quinuclidine moiety. Quinine is an alkaloid isolated from the cinchona plant, and is the same stuff in anti-malarial tonic that is better washed down with a measure of gin. The reaction proceeds via silver-coordination to the terminal carbon of the isocyanate, accelerating deprotonation of the alpha protons by the quinuclidine base and forming a nucleophilic enoate. The electrophilicity of the ketone is enhanced by the formation of two hydrogen bonds with the N-H bonds of the squaramide, shaping a doubly activated system which rapidly undergoes a formal [3+2] cycloaddition to give the products.

At first glance this seems to be a simple and straightforward transformation, but in reality the authors have succeeded in optimising a reaction for unactivated substrates which has stereocontrol over two chiral centres, one of which is quaternary. Furthermore, the reaction uses a combination of two catalysts reacting via three modes of activation.

To find out more please read:

Enantioselective synthesis of chiral oxazolines from unactivated ketones and isocyanoacetate esters by synergistic silver/organocatalysis

Pablo Martínez-Pardo, Gonzalo Blay, M. Carmen Muñoz, José R. Pedro, Amparo Sanz-Marco and Carlos Vila.
Chem. Commun., 2018, 54, 2862 – 2865
DOI: 10.1039/c8cc00856f

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An Industrial Revolution on the Nanoscale

27 Mar 2018

“What are the possibilities of small but movable machines? They may or may not be useful, but they surely would be fun to make”. In December 1959 Richard Feynman addressed the annual meeting of the American Physical Society at Caltech with a talk entitled ‘There’s Plenty of Room at the Bottom’, imploring the scientific community to start thinking small, like ‘entire 24 volumes of the Encyclopaedia Britannica on the head of a pin’ kind of small. Many quote this lecture as when the notion of nanomachines first entered the scientific sphere – with talk of miniature cars, injectable molecular ‘surgeons’ and machines that place atoms side by side to synthesize any molecule imaginable. The lecture reads like the description of the futuristic setting in ‘Back to the Future’, an exploration of possibilities at a time when we fundamentally lacked the tools to make them a reality.

As in ‘Back to the Future’, which predated yet predicted the emergence of mobile-banking technology, video calling and personal drones, Richard Feynman’s plea for scientists to prepare molecular-scale machines has also become a reality, and for their successes in this field Jean-Pierre Sauvage, Sir Fraser Stoddart and Ben Feringa were jointly awarded the Nobel Prize in Chemistry in 2016.

A group of researchers based in London and Singapore have written a feature article introducing both the foundational work in this field and state-of-the-art examples. Nanomachines are single molecules or molecular assemblies on the nanoscale (this review defines a 1 – 100 nm scope) that have the ability to perform ‘useful work’ upon application of an external energy source. To extract work (often in the form of controlled mechanical movement) molecular machines are designed to operate at a thermodynamically far-from-equilibrium state, maintained by an energy input, with movement occurring as the system relaxes towards equilibrium. At the synthetic level, molecules are designed with components which have restricted translational and rotational movements with respect to each other, and the ability to control these movements is key to obtaining the desired function.

A catalytically active rotaxane synthesised by Nolte and co-workers acts as a tiny epoxidising machine , moving along a polybutadiene polymer

The catalytically active rotaxane synthesised by Nolte and co-workers acts as a tiny epoxidising machine, moving along a polybutadiene polymer

One of the first advances towards the synthesis of nanomachines was by the research group of Jean-Pierre Sauvage, who achieved the templated synthesis of catenanes; structures with two circular molecules that are interlocked like two links in a chain. It was subsequently shown that a catenane motor could be prepared, with one ring rotating with respect to the other in a controlled manner. Fraser Stoddart further contributed to the field with ‘rotaxanes’, composite molecules comprising a ring threaded onto an axle. Nanomachines based on rotaxanes have been developed and include switches, shuttles and ‘molecular elevators’. A state-of-the-art example of a catalytically active rotaxane synthesised by Nolte and co-workers in 2003 demonstrates the potential of nanomachines to revolutionise organic synthesis. The rotaxane is constructed with a magnesium-bound porphyrin, which threads onto a polybutadiene polymer (300 kDa, 98% cis) and catalyses the epoxidation of the double bonds (turnover number: 140, cis/trans ratio of the polyepoxide: 1:4).

Ben Feringa’s electric nano-car, a single molecule with four fluorene ‘wheels’ capable of driving across a copper surface

In 2011 Ben Feringa and co-workers synthesized the worlds tiniest electric car using the same design principles they had used to create a spinning motor in 1999. The car is a single molecule with the ability to propel itself across a crystalline copper surface upon activation by a voltage pulse, with 10 pulses moving the car 6 nm across the surface. The car itself is comprised of a central diyne strut bonded at each end to carbazole ‘axles’. Each axle is bound through alkenes to two fluorene ‘wheels’. The key design elements are the alkenes and two chiral methyl substituents on each axle which forces each wheel to twist out of the plane. For one wheel rotation: an electronic excitation induces trans–cis isomerisation of the alkene causing a quarter turn of the wheel such that it sits adjacent to the methyl group. Next, a vibrational excitation induces helical inversion, allowing the wheel to push past the methyl group another quarter turn. Another isomerisation and helical inversion completes a full rotation. Research achievements like these demonstrate mechanical work on the nanoscale, with the vision of achieving movement on the macroscale via synchronised motion.

These examples represent a small subset of those discussed in the feature article review, which not only spans the current scope of molecular-scale machines, but reviews the design principles guiding their development and the possibilities nanomachines represent in the future of scientific research.

To find out more please read:

Artificial molecular and nanostructures for advanced nanomachinery

Elizabeth Ellis, Suresh Moorthy, Weng-I Katherine Chio and Tung-Chun Lee.
Chem. Commun., 2018, Advance Article
DOI: 10.1039/c7cc09133h

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Buckyball’s Hydrogen Spillover Effect at Ambient Temperature Observed Experimentally for the First Time

26 Mar 2018

A group of scientists from Tohoku University, Japan experimentally demonstrated the hydrogen spillover effect of buckyball (a.k.a. fullerene or C₆₀). They achieved this breakthrough using mass spectroscopy, and their findings were published recently in Chem. Commun.

Certain transition metal nanoparticles (e.g. Ru, Pt and Ni) can capture hydrogen molecules. The capture process generally involves three sequential steps. Firstly, hydrogen molecules split into hydrogen atoms on the metal surface. Secondly, the yielded hydrogen atoms migrate on the surface towards substrates under the metal nanoparticles and, finally, these atoms fix onto the substrates. The second step is termed the “spillover effect” (Figure 1a). Previous studies predicted that curved graphene sheets could enhance the hydrogen spillover effect at ambient temperatures, but solid experimental evidence has remained inadequate.

To gather evidence for this prediction, Nishihara et al. studied the material buckyball, a spherical carbon nanosphere that represents an extremely curved graphene sheet. The researchers selected ketjenblack (KB), a type of porous carbon sheet, as the substrate, and deposited Pt nanoparticles (1-3 nm in diameter) and buckyball molecules onto it (Figure 1b). They found that the Pt and buckyball-decorated KB stored a higher amount of hydrogen compared to the Pt-loaded KB. This observation indirectly confirmed the previous prediction, as hydrogen storage capacity may be improved by enhancing the spillover effect.

Figure 1. (a) A schematic illustration showing how a hydrogen molecule is split on Pt surface [process (1)] followed by the spillover effect [processes (2) and (2′)]. (b) A schematic illustration of the structure of Pt and buckyball-decorated KB. The inset panel displays two forms of hydrogen bound to the composite: the physically adsorbed di-hydrogen molecules, and the spillover hydrogen atoms anchored on the KB substrate and buckyballs.

The authors then sought time-of-flight mass spectroscopy to obtain more evidence. This spectroscopic technique is capable of identifying molecules with different mass to charge ratios (m/z). As shown in Figure 2, after treating the buckyball and Pt-loaded KB with deuterium molecules (D₂), the spectrum (red) exhibited two additional peaks with m/z of ~723.5 and ~724.5 (highlighted by arrows in the figure) compared to those of the buckyball reference (black) and the buckyball and Pt-loaded KB prior to D₂ dosage (blue). The authors ascribed these two new peaks to single D atom-adsorbed buckyballs with different amounts of carbon isotopes (¹²C and ¹³C). The presence of the two new peaks clearly showed that buckyballs could host hydrogen atoms to enhance the spillover effect. In addition, upon exposing the D-containing buckballs to air, both of the newly-merged peaks disappeared, suggesting that D atom adsorption was reversible.

Figure 2. The time-of-flight mass spectroscopy spectra of buckyball (black), Pt and buckyball-decorated KB before (blue) and after (red) exposure to D2, and after exposure to air (green). Pictures on the right show the molecular structure of a buckyball molecule and two deuterium-incorporated buckyball molecules (with different number of 13C isotope). Deuterium is used to avoid the interference from the 13C isotope.

This work could serve as a reference for future studies of the spillover effect induced by buckyballs interacting with other metal nanoparticles. The increasing availability of in-depth fundamental insight could refine our understanding of ambient-temperature hydrogen storage.

To find out more please read:

Enhanced Hydrogen Spillover to Fullerene at Ambient Temperature

Hirotomo Nishihara, Tomoya Simura and Takashi Kyotani

Chem. Commun. 2018, DOI: 10.1039/c8cc00265g

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Congratulations to the 2018 Cram Lehn Pedersen Prize winner: Rafal Klajn

06 Mar 2018

We are proud to announce that Dr. Rafal Klajn, at the Weizmann Institute of Science in Israel, as 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 Rafal, a well-deserved winner!

Dr. Rafal Klajn

Rafal is an Associate Professor at the Weizmann Institute of Science and will receive the award during the 2018 International Symposium on Macrocyclic and Supramolecular Chemistry (ISMSC).

This annual conference consists of sessions of invited lectures that focus upon a single topic area, award lectures and poster sessions. This year, the conference will also feature evening sessions on supramolecular chemistry with keynote speakers as well as an exciting series of Nobel Lectures on the final day!

Find out more and register here.

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Buckyball’s Hydrogen Spillover Effect at Ambient Temperature Observed Experimentally for the First Time

Congratulations to the 2018 Cram Lehn Pedersen Prize winner: Rafal Klajn

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