Driving Molecular Motors with Visible Light

While nanobots are still a thing of science fiction, the development of the fundamental scientific concepts that could lead to highly complex molecular-scale machines has been an active area of research for over 2 decades. Molecular motors rely on isomerization through metastable intermediate states that leads to unidirectional rotation via a rotor portion of the molecule. Initial work utilized molecules that convert high-energy UV-light into this directional motion. However, these systems are limited in their future utility, particularly in biological applications, if they need UV-light for activation. Recent work focuses on driving molecular motors with visible light via a range of approaches, including altering the absorbance profile of the motor molecule itself. Unfortunately, changing the electronics of the molecule has previously substantially decreased the quantum yield while resulting in only slight red shifts.

Researchers in The Netherlands recently developed 2nd generation molecular motors featuring a mixture of electron-donating and electron-withdrawing groups that exhibit substantial red-shifts. Density functional theory (DFT) calculations predicted that adding cyano- and methoxy- groups to opposite halves of the motor would shift the absorbance past 410 nm. The new molecules were synthesized via a general procedure shown in Figure 1, with yields ranging from 3 – 10%.

Figure 1. General synthetic route to cyano- and methoxy- substituted molecular motors and photoisomerization reaction that results in unidirectional motion.

The addition of the cyano- and methoxy- groups shifted the absorbance maxima corresponding to the HOMO-LUMO transitions to from 422 – 453 nm, an increase of over 60 nm from the parent molecule. When irradiated, the molecules transitioned from their stable ground state to the metastable isomers. These were characterized by UV-Vis spectroscopy, with the emergence of further red-shifted features (Figure 2), and 1H NMR.  The 1H NMR spectra were obtained with in-situ irradiation (which sounds like a sweet experimental setup) at various wavelengths and the ratios of several specific protons on the rotor. The derivative 2 with a methoxy- group on the rotor and cyano- groups on the stator demonstrated activity with irradiation at wavelengths up to 530 nm.

Figure 2. UV-Vis absorption spectra of stable (solid) and metastable (dashed) isomers of the molecular motors.

The energy of activation of the rotation for all three derivatives was determined by Eyring analysis and corroborated by DFT calculations. All were around 90 kJ/mol, with 2 requiring the most energy to elongate the central alkene bond and isomerize. The quantum yields of the motors for the forward reaction range from 5.8 – 11.5%, comparable to state-of-the-art UV absorbing motors. The quantum yields for the back reactions were calculated to be significantly lower than those for the forward reaction, which corresponds to the excess of metastable isomers observed under active irradiation. These motors also exhibit high photostability, with no significant change in the ground state absorbance after irradiation and cycling. This is promising for smart materials applications where stability is crucial. This work pushes forward the design of molecular motor systems that utilize visible rather than UV light.

To find out more please read:

Photoefficient 2nd generation molecular motors responsive to visible light

Lukas Pfeifer, Maximilian Scherübl, Maximilian Fellert, Wojciech Danowski, Jinling Cheng, Jasper Pol and Ben L. Feringa

Chem. Sci., 2019, 10, 8768-8773.

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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Can Al3+ Indeed Intercalate Layered Metal Oxides in Aqueous Electrolytes?

Intercalation of multivalent ions, e.g., Al3+, has received increasing attention as an energy-boosting strategy for rechargeable batteries. The charge storage process of the wide-spread Li-ion batteries relies on Li+ intercalating electrodes with layered structures. Multivalent-ion batteries could accommodate more charges than Li-ion batteries because their ions carry more charge than Li+. This concept, however, has now been challenged by a research team led by BenoÎt Limoges and Véronique Balland of Université de Paris, France. Their mechanistic investigation, published in Chemical Science, revealed that Al3+ ions were unable to intercalate electrodes in aqueous electrolytes.

The authors selected TiO2 arrays as the object of their study. These ~1 µm-high arrays were grown using the glancing angle deposition technique. By applying negative potentials to the TiO2 arrays, cations such as Al3+ could diffuse through the inter-array slits and interact with TiO2 (Figure 1).

Figure 1. Structure of the TiO2 arrays. (left) Scanning electron micrographs and (right) a cartoon illustrating ion diffusion pathways.

Electrochemistry tests elucidated that the charge-storage process of TiO2 in an Al3+-containing aqueous electrolyte correlated to proton intercalation. This conclusion was mainly based on the nearly identical cyclic voltammograms (Figure 2, top) and capacity vs. potential curves (Figure 2, bottom) of the TiO2 arrays in both AlCl3 and acetic acid aqueous electrolytes. Since acetic acid solution had no Al3+, the observed charge-storage activity could not be attributed to Al3+ intercalation. Instead, the authors argued that protons dissociated either from hydrated Al3+ cations, [Al(H2O)6]3+ or acetic acid must intercalate TiO2 and result in the observed charge-storage capacities.

Figure 2. (top) Cyclic voltammograms and (bottom) capacity vs. potential curves of TiO2 in (left) Al3+-containing and (right) acetic acid aqueous electrolytes. The electrolytes are 0.3 M KCl with different concentrations of AlCl3 or acetic acid: 0 M (black), 25 mM (blue), 50 mM (purple), 100 mM (magenta), and 250 mM (red).

The authors believe that the misconception of Al3+ intercalation is due to the overlooking of Al3+ hydration, which is inevitable when Al3+ is present in aqueous electrolytes. Removing the water shell (a prerequisite for ion intercalation) is energy costly for Al3+ because of the strong binding force between water molecules and Al3+. Additionally, even if Al3+ ions intercalate TiO2, their movement is strongly hindered by Coulombic interactions within the TiO2 lattice. The immobilized intercalated ions would then block other ions from entering the TiO2 lattice. Together, both factors prevent Al3+ from intercalating into TiO2.

In summary, this work demonstrates that the charge-storage capacity of TiO2 in Al3+-containing aqueous electrolytes is most probably due to proton intercalation. This conclusion also applies to other multivalent cations, including Zn2+ and Mn2+, as shown in this work.

 

To find out more, please read:

On the Unsuspected Role of Multivalent Metal Ions on the Charge Storage of A Metal Oxide Electrode in Mild Aqueous Electrolytes

Yee-Seul Kim, Kenneth D. Harris, BenoÎt Limoges, and Véronique Balland

Chem. Sci., 2019, doi: 10.1039/c9sc02397f

Tianyu Liu acknowledges Zachary L. Croft of Virginia Tech, the U.S., for his constructive comments on this post.

About the blogger:

Tianyu Liu obtained his Ph.D. (2017) in Chemistry from University of California, Santa Cruz in the United States. He is passionate about the communication of scientific endeavors and cutting-edge research to both the general public and other 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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10th National Conference on Inorganic Chemistry of the Chinese Chemical Society

Chemical Science was pleased to support the 10th National Conference on Inorganic Chemistry of the Chinese Chemical Society which took place at Shandong University last month. Poster prizes were given on behalf of Chemical Science as well as Inorganic Chemistry Frontiers, Materials Chemistry Frontiers, Catalysis Science & Technology, Physical Chemistry Chemical Physics, Green Chemistry, Dalton Transactions, RSC Advances, Nanoscale, Nanoscale Advances, Materials Horizons and Journal of Materials Chemistry A, B and C. Energy & Environmental Science and Sustainable Energy & Fuels also provided a joint prize. The winners are:

Poster prize winners of the 10th National Conference on Inorganic Chemistry of the Chinese Chemical Society

Yuan Xiong 熊昱安 东南大学
Southeast University
Lingling Xu 徐令令 西安交通大学
Xi’an Jiaotong University
Fan Guo 郭帆 南京大学
Nanjing University
Wenbin Wang 王文彬 华中科技大学
Huazhong University of Science and Technology
Mengfei Li 李梦菲 中国石油大学(华东)
China University Of Petroleum
Jing Dong 董婧 北京理工大学
Beijing Institute Of Technology
Bingqi Han 韩冰琪 吉林大学
Jilin University
Wenzhu Yu 于文竹 山东大学
Shandong University
Liang Zhou 周亮 北京大学
Peking University
Genfeng Feng 冯根锋 南京大学
Nanjing University
Peipei Cui 崔培培 德州学院
Dezhou University
Xiaoting Liu 刘晓婷 南开大学
Nankai University
Dong Li 李冬 厦门大学
Xiamen University
Zhi Wang 王芝 山东大学
Shandong University
Shuang Liu 刘爽 东北师范大学
Northeast Normal University

Congratulations to all the winners!

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Breaking C-H bonds with water, iron, and electricity

As we transition to an energy future composed primarily of intermittent, renewable technologies, finding ways to store the excess generated charges will be critical. While the general public typically thinks of batteries, another area of massive interest is storing the electrons in a chemical bond. This can be accomplished by creating electrocatalysts that can couple an electric current with chemicals like water and oxygen to generate stable new bonds in molecules for later use as fuels. These renewably generated fuels would then be available whenever needed, like at night. However, finding efficient electrocatalysts composed of earth-abundant materials has proven challenging. Many researchers have turned to nature for inspiration by designing molecules that mimic the active sites of enzymes.

Researchers in the United States used this approach, focusing on creating high-valent iron-oxo species, which others previously identified as the key catalytic intermediates in multiple enzymatic reactions. These types of species have traditionally been synthesized by reacting a reduced iron complex with an oxygen transfer reagent, but the researchers developed a system to generate highly reactive species using electricity as the reaction driving force and water as the oxygen source. The studied catalyst is a commercially available iron(III)-aquo complex with a tetraamido macrocyclic ligand (TAML) as the ancillary ligand (Figure 1B).

Figure 1. A. Cyclic voltammagram of (TAML)Fe in acetonitrile. B. Structure of (TAML)Fe and cyclic voltammagram showing increased current upon the addition of ethylbenzene.

When analyzed by cyclic voltammetry, an electrochemical technique where you cycle the voltage between set points and measure the current output, the (TAML)Fe shows two redox events at around 650 and 1250 mV that the researchers attributed to generating the FeIV-OH and FeV(O) species (Figure 1). Addition of ethylbenzene, which should react with the FeV species, increased the current at voltages of 1250 mV and higher, indicating (TAML)Fe turnover. However, isolating the Fev(O) species proved challenging as it reacts rapidly with the (TAML)Fe to form an FeIV dimer. This also limits the efficiency of the overall system by decreasing the amount of the most reactive species in solution.

Figure 2. A, B. Products generated by oxidation of various substrates screened with (TAML)Fe for electrocatalysis with isolated yield and calculated conversion in parenthesis. C. Substrates that did not react with the (TAML)Fe complex.

Both the FeIV dimer and FeV(O) species proved capable of oxidizing C-H bonds in ethylbenzene, but the FeV(O) is much more reactive and increases the oxidation rate at high electrochemical potentials. The researchers tested the scope of (TAML)Fe reactivity using a series of compounds with benzylic C-H bonds (Figure 2). They found that the (TAML)Fe performed well with electron-rich and electron-neutral derivatives, with an electron-deficient nitro-substituted derivative showing lower reactivity. Several substrates with non-benzylic C-H bonds showed high selectivity for oxidation at the benzylic C-H bond. (TAML)Fe also showed high electrocatalytic activity for oxidizing alcohols and converted substrates as simple as cyclohexanol and as complex as a steroid to ketones in high yields (up to 97%).

This study of an earth-abundant, stable, and commercially available electrocatalyst acts as a baseline for further studies with other similar metal complexes. Despite the efficiency limits attributed to dimerization, the high stability and selectivity of the (TAML)Fe could lead to its use with a broader range of substrates with varied functional groups.

To find out more please read:

Electrochemical C–H oxygenation and alcohol dehydrogenation involving Fe-oxo species using water as the oxygen source

Amit Das, Jordan E. Nutting and Shannon S. Stahl

Chem. Sci., 2019, 10, 7542-7548

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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International Conference on Energy Materials and Interfaces

Last month the North East Centre for Energy Materials (NECEM) held the International Conference on Energy Materials and Interfaces in Newcastle, UK, which was sponsored by Chemical Science. The conference covered topics including flexible photovoltaics, thermoelectric devices, computational simulations of interfaces in energy capture devices, applications of 2D materials in energy capture and storage, tailored interfaces in turbines and new conc‌epts in electrical and electrochemical energy storage.

Chemical Science sponsored a poster prize, which was awarded to Stephen Campbell from Northumbria University. RSC journals Energy & Environmental Science and Sustainable Energy & Fuels also awarded poster prizes to David Alejandro Palacios Gomez from Durham Unviersity and Wei-Hsiang Lin from National Tsing Hua University.

Energy & Environmental Science poster prize winner David Alejandro Palacios Gomez, from Durham University

Sustainable Energy & Fuels poster prize winner Wei-Hsiang Lin from National Tsing Hua University

Congratulations to the prize winners from everyone at Chemical Science!

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26th International Symposium: Synthesis in Organic Chemistry

The 26th International Symposium on Synthesis in Organic Chemistry was held in Cambridge last month, showcasing exciting work in this core area of organic chemistry – synthesis. Chemical Science, alongside Organic and Biomolecular Chemistry, was proud to sponsor the event, which included talks covering a variety of aspects of modern synthesis and provided a forum for emerging methodologies and strategies.

The symposium featured talks from researchers working at the cutting-edge of synthesis in organic chemistry, including Chemical Science Associate Editor Vy Dong, Frances Arnold, Shankar Balasubramanian, Tanja Gulder, Robert Knowles, Daniele Leonori, David Nicewicz, Robert Phipps, Tobias Ritter, Tomislav Rovis, Franziska Schoenebeck, Hiroaki Suga, Edward Tate, F Dean Toste, Matthew Tudge, and William Unsworth.

Chemical Science’s Executive Editor, May Copsey, was proud to present the Poster Prizes for the meeting, sponsored by Chemical Science and Organic and Biomolecular Chemistry. Congratulations to Roman Abrams (University of Bristol) and Claire Flitcroft (The University of Sydney) for winning the judge’s choice poster prizes, and to Tobias Wagener (Westfälische Wilhelms-Universität Münster) and Thomas Brouder (University College Cork) for winning the participant’s vote prizes. There was a fantastic array of posters – congratulations to all those who displayed work.

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Microscale Motion and Light

Chemical Science recently sponsored the successful Microscale Motion and Light conference which took place last month in Dresden, Germany. The event ran from the 22nd – 26th July and focussed on the below topics including discussion by invited speakers of the exciting new opportunities for smart materials and applications.:

Professor Tom Mallouk hands Linlin Wang her award for best poster (first prize)

  • Micromotion
  • Photochemistry
  • Catalysis
  • Active matter
  • Non-equilibrium-systems
  • Colloids
  • Water splitting
  • Electrochemistry
  • Light-responsive materials
  • Colloidal assembly

Chemical Science provided two best poster prizes which were handed out by the conference co-organiser, Professor Tom Mallouk (Penn State University, USA).

The prize winners were Linlin Wang who was awarded first prize and Tao Huang who received the second prize for best poster.

Professor Tom Mallouk (left) hands Tao Huang (right) his award for best poster (second prize)

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Bioorthogonal & Bioresponsive 2019

Chemical Science recently sponsored Bioorthogonal & Bioresponsive 2019 in Edinburgh, which brought together chemists and biologists interested in the latest advances in bioorthogonal and bioresponsive strategies.

The meeting featured stunning talks from a variety of international experts, including Jason Chin, Ben Davis, Karen Faulds, Sarah Heilshorn, Ludovic Jullien and Vince Rotello.

We finished the first day with a poster session and drinks reception on the top floor of the Institute of Genetics & Molecular Medicine, with a stunning view across the city. There was a fantastic display of posters, and Chemical Science along with Organic and Biomolecular Chemistry were delighted to sponsor the poster prizes. Congratulations to both of the prize winners!

Sally Vanden-Hehir (left) won the Chemical Science prize for best poster and flash presentation.
Sam Benson (right) won the Organic and Biomolecular Chemistry prize for best poster.

Organisers and hosts Marc Vendrell and Asier Unciti-Broceta, prize winners Sally Vanden-Hehir and Sam Benson, and Chemical Science representative Amelia Newman.

The beautiful view from the top of the Institute of Genetics & Molecular Medicine.

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Making Gyroid Polymer Films to Speed up Proton Conduction

Proton exchange membranes (PEMs) are essential to the functionality of fuel cells. They conduct protons in electrolytes and drive electricity generation by oxidizing fuels. Following the success of Nafion® –– a family of commercial proton-conductive fluoropolymers –– materials researchers around the globe are developing innovative PEMs with high proton conductivities and affordable prices.

A group of Japanese researchers has recently synthesized self-standing polymer films with a gyroid nanostructure. These films possess two unique characteristics that other PEMs rarely have: a high proton conductivity in the order of 10-1 S/cm and retention of the conductivity across a wide temperature range (20-120 °C). This finding has been published in Chem. Sci. (doi: 10.1039/C9SC00131J).

The authors used a tailor-made macromolecule, Diene-GZI (Figure 1a), as the building block. It had an amphipathic structure, with one end being a hydrophilic zwitterionic group and another end of a hydrophobic alkyl chain. When mixed with bis(trifluoromethanesulfonyl)imide and water, multiple Diene-GZI molecules could assemble together into a gyroid network –– an infinitely periodic minimal surface (Figure 1b). After the self-assembly, ultra-violet-irradiation-induced polymerization solidified the morphology of the gyroid nanostructure.

Figure 1. (a) The molecular structure of Diene-GZI. (b) Solidification of the self-assembled gyroid via polymerization.

The high proton conductivity of the polymer film originated from its three-dimensional gyroid structure. Since the gyroid surface was densely coated with the hydrophilic zwitterionic chains, the film could readily uptake as high as 15.6 wt.% of water at a relative humidity of 90%. The adsorbed water layers formed a three-dimensional continuous pathway along the gyroid surface, serving as proton-conduction expressways and resulting in a high conductivity in the order of 10-1 S/cm. Due to the strong binding force between water and the zwitterionic groups, heating the polymer film to 120 °C did not decrease the water content significantly, and thus, the proton conductivity remained high. Additionally, the control films with no gyroid structures were unable to compete with the gyroid film in terms of proton conductivities within the measured temperature range (Figure 2).

Figure 2. The dependence between proton conductivities and temperature. Legends: red solid circles – gyroid film; others – control samples without the gyroid nanostructure.

This work highlights the critical role of rational design of raw materials to augment the proton conductivities of PEMs. The advantage of the gyroid phase in speeding up ion diffusion could also inspire innovative materials in applications demanding ultrafast ion transport, e.g., supercapacitor electrodes.

 

To find out more, please read:

Gyroid Structured Aqua-Sheets with Sub-Nanometer Thickness Enabling 3D Fast Proton Relay Conduction

Tsubasa Kobayashi, Ya-xin Li, Ayaka Ono, Xiang-bing Zeng, and Takahiro Ichikawa

Chem. Sci., 2019, 10, 6245-6253

About the blogger:

Tianyu Liu obtained his Ph.D. (2017) in 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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Tryptophan, Featuring BN/CC Isosterism

Ever wanted to find a way to replace a carbon-carbon (CC) double bond with another bond that will change the physical and chemical properties of a molecule without significantly altering its sterics? Look no further than a boron-nitrogen bond! BN/CC isosterism involves substituting a CC double bond with a BN bond, which can substantially change the electronic properties of a molecule while keeping it the same size. This isosterism could be a powerful tool in biomedical studies of biologically relevant arene-containing organic molecules, which are plentiful. However, few studies report on the differences in functions cause by substituting a BN bond into an arene. Initial results suggest that the BN compounds can have similar or increased activity and availability when compared to the natural, all carbon molecules.

Figure 1. Image of naturally occuring tryptophan and the BN-tryptophan analogue.

Researchers in the United States synthesized a BN-analogue of tryptophan (Figure 1) for use as an unnatural amino acid (UAA) to study and intentionally alter the properties of proteins. Tryptophan, in addition to making Americans sleepy at Thanksgiving, is relatively rare, but participates in pi system interactions and is the primary source of native protein fluorescence. This makes it an important target for UAA research. The researchers synthesized the sodium salt of BN-tryptophan in a 6-step process, which can be modified to resolve the two enantiomers by chiral HPLC. The BN-tryptophan exhibits noticeably red-shifted absorbance and emission spectra, with the fluorescence maximum shifted by almost 40 nm in the BN compound.

In order to test whether the BN-tryptophan could be incorporated into proteins, researchers incorporated it into media without tryptophan and monitored whether E. coli cells that lacked the ability to produce tryptophan would grow. They found that the cells grew when in the presence of BN-tryptophan, but to a significantly lesser degree than with an equivalent quantity of natural tryptophan. However, cell growth increased when the media contained both BN-tryptophan and natural tryptophan. This suggests that cells will accept BN-tryptophan as a tryptophan analogue, but they don’t tolerate full replacement well.

Figure 2. Representation of the protein sequence, structures of other tryptophan analogues, and fluorescence plot for the studied substrates.

Further studies incorporated BN-tryptophan and three other previously utilized tryptophan analogues into a green fluorescent protein (GFP). For fluorescence to be detected, the analogue must be incorporated into the protein and then accurately read by cells. The BN-tryptophan performs as well or better than the established tryptophan analogues, proving its functionality (Figure 2). The proteins with BN-tryptophan also demonstrate several different properties than those containing natural tryptophan; their fluorescence is red shifted and they are more susceptible to oxidation by hydrogen peroxide. These alterations in activity could prove useful in future studies.

To find out more please read:

Synthesis and characterization of an unnatural boron and nitrogen-containing tryptophan analogue and its incorporation into proteins

Katherine Boknevitz, James S. Italia, Bo Li, Abhishek Chatterjee and Shih-Yuan Liu

Chem. Sci., 2019, 10, 4994–4998.

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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