Three-in-One: Dye-Volatile Cocrystals Exhibiting Intensity-Dependent Photochromic, Photomechanical, and Photocarving Response.

Cocrystallization of a cis-azobenzene dye with volatile molecules, such as pyrazine and dioxane, leads to materials that exhibit at least three different light-intensity-dependent responses upon irradiation with low-power visible light. The halogen-bond-driven assembly of the dye cis-(p-iodoperfluorophenyl)azobenzene with volatile halogen bond acceptors produces cocrystals whose light-induced behavior varies significantly depending on the intensity of the light applied. Low-intensity (<1 mW·cm-2) light irradiation leads to a color change associated with low levels of cis → trans isomerization. Irradiation at higher intensities (150 mW·mm-2) produces photomechanical bending, caused by more extensive isomerization of the dye. At still higher irradiation intensities (2.25 W·mm-2) the cocrystals undergo cold photocarving; i.e., they can be cut and written on with micrometer precision using laser light without a major thermal effect. Real-time Raman spectroscopy shows that this novel photochemical behavior differs from what would be expected from thermal energy input alone. Overall, this work introduces a rational blueprint, based on supramolecular chemistry in the solid state, for new types of crystalline light-responsive materials, which not only respond to being exposed to light but also change their response based on the light intensity.

Halogen bonding to the azulene π-system: cocrystal design of pleochroism.

We describe the rational development of a design for pleochroic molecular solids, by using C-Iπ halogen bonds to pre-organise different chromophores in a cocrystal. Using the structure of a known naphthalene cocrystal as a blueprint, we demonstrate how a highly robust C-Iπ motif permits the systematic exchange of original cocrystal components with azobenzene and azulene, resulting in optically interesting dichroic or pleochroic materials.

A Surface Reconstruction Route to High Productivity and Selectivity in CO2 Electroreduction toward C2+ Hydrocarbons.

Electrochemical carbon dioxide reduction (CO2 ) is a promising technology to use renewable electricity to convert CO2 into valuable carbon-based products. For commercial-scale applications, however, the productivity and selectivity toward multi-carbon products must be enhanced. A facile surface reconstruction approach that enables tuning of CO2 -reduction selectivity toward C2+ products on a copper-chloride (CuCl)-derived catalyst is reported here. Using a novel wet-oxidation process, both the oxidation state and morphology of Cu surface are controlled, providing uniformity of the electrode morphology and abundant surface active sites. The Cu surface is partially oxidized to form an initial Cu (I) chloride layer which is subsequently converted to a Cu (I) oxide surface. High C2+ selectivity on these catalysts are demonstrated in an H-cell configuration, in which 73% Faradaic efficiency (FE) for C2+ products is reached with 56% FE for ethylene (C2 H4 ) and overall current density of 17 mA cm-2 . Thereafter, the method into a flow-cell configuration is translated, which allows operation in a highly alkaline medium for complete suppression of CH4 production. A record C2+ FE of ≈84% and a half-cell power conversion efficiency of 50% at a partial current density of 336 mA cm-2 using the reconstructed Cu catalyst are reported.

Metal-Organic Frameworks Mediate Cu Coordination for Selective CO2 Electroreduction.

The electrochemical carbon dioxide reduction reaction (CO2RR) produces diverse chemical species. Cu clusters with a judiciously controlled surface coordination number (CN) provide active sites that simultaneously optimize selectivity, activity, and efficiency for CO2RR. Here we report a strategy involving metal-organic framework (MOF)-regulated Cu cluster formation that shifts CO2 electroreduction toward multiple-carbon product generation. Specifically, we promoted undercoordinated sites during the formation of Cu clusters by controlling the structure of the Cu dimer, the precursor for Cu clusters. We distorted the symmetric paddle-wheel Cu dimer secondary building block of HKUST-1 to an asymmetric motif by separating adjacent benzene tricarboxylate moieties using thermal treatment. By varying materials processing conditions, we modulated the asymmetric local atomic structure, oxidation state and bonding strain of Cu dimers. Using electron paramagnetic resonance (EPR) and in situ X-ray absorption spectroscopy (XAS) experiments, we observed the formation of Cu clusters with low CN from distorted Cu dimers in HKUST-1 during CO2 electroreduction. These exhibited 45% C2H4 faradaic efficiency (FE), a record for MOF-derived Cu cluster catalysts. A structure-activity relationship was established wherein the tuning of the Cu-Cu CN in Cu clusters determines the CO2RR selectivity.

2D Metal Oxyhalide-Derived Catalysts for Efficient CO2 Electroreduction.

Electrochemical reduction of CO2 is a compelling route to store renewable electricity in the form of carbon-based fuels. Efficient electrochemical reduction of CO2 requires catalysts that combine high activity, high selectivity, and low overpotential. Extensive surface reconstruction of metal catalysts under high productivity operating conditions (high current densities, reducing potentials, and variable pH) renders the realization of tailored catalysts that maximize the exposure of the most favorable facets, the number of active sites, and the oxidation state all the more challenging. Earth-abundant transition metals such as tin, bismuth, and lead have been proven stable and product-specific, but exhibit limited partial current densities. Here, a strategy that employs bismuth oxyhalides as a template from which 2D bismuth-based catalysts are derived is reported. The BiOBr-templated catalyst exhibits a preferential exposure of highly active Bi ( 11¯0 ) facets. Thereby, the CO2 reduction reaction selectivity is increased to over 90% Faradaic efficiency and simultaneously stable current densities of up to 200 mA cm-2 are achieved-more than a twofold increase in the production of the energy-storage liquid formic acid compared to previous best Bi catalysts.

CO2 electroreduction to ethylene via hydroxide-mediated copper catalysis at an abrupt interface.

Carbon dioxide (CO2) electroreduction could provide a useful source of ethylene, but low conversion efficiency, low production rates, and low catalyst stability limit current systems. Here we report that a copper electrocatalyst at an abrupt reaction interface in an alkaline electrolyte reduces CO2 to ethylene with 70% faradaic efficiency at a potential of -0.55 volts versus a reversible hydrogen electrode (RHE). Hydroxide ions on or near the copper surface lower the CO2 reduction and carbon monoxide (CO)-CO coupling activation energy barriers; as a result, onset of ethylene evolution at -0.165 volts versus an RHE in 10 molar potassium hydroxide occurs almost simultaneously with CO production. Operational stability was enhanced via the introduction of a polymer-based gas diffusion layer that sandwiches the reaction interface between separate hydrophobic and conductive supports, providing constant ethylene selectivity for an initial 150 operating hours.

Hydronium-Induced Switching between CO2 Electroreduction Pathways.

Over a broad range of operating conditions, many CO2 electroreduction catalysts can maintain selectivity toward certain reduction products, leading to materials and surfaces being categorized according to their products; here we ask, is product selectivity truly a property of the catalyst? Silver is among the best electrocatalysts for CO in aqueous electrolytes, where it reaches near-unity selectivity. We consider the hydrogenations of the oxygen and carbon atoms via the two proton-coupled-electron-transfer processes as chief determinants of product selectivity; and find using density functional theory (DFT) that the hydronium (H3O+) intermediate plays a key role in the first oxygen hydrogenation step and lowers the activation energy barrier for CO formation. When this hydronium influence is removed, the activation energy barrier for oxygen hydrogenation increases significantly, and the barrier for carbon hydrogenation is reduced. These effects make the formate reaction pathway more favorable than CO. Experimentally, we then carry out CO2 reduction in highly concentrated potassium hydroxide (KOH), limiting the hydronium concentration in the aqueous electrolyte. The product selectivity of a silver catalyst switches from entirely CO under neutral conditions to over 50% formate in the alkaline environment. The simulated and experimentally observed selectivity shift provides new insights into the role of hydronium on CO2 electroreduction processes and the ability for electrolyte manipulation to directly influence transition state (TS) kinetics, altering favored CO2 reaction pathways. We argue that selectivity should be considered less of an intrinsic catalyst property, and rather a combined product of the catalyst and reaction environment.

Shape-Shifting Azo Dye Polymers: Towards Sunlight-Driven Molecular Devices.

The development of stimuli-responsive polymers is among the key goals of modern materials science. The structure and properties of such switchable materials can be designed to be controlled via various stimuli, among which light is frequently the most powerful trigger. Light is a gentle energy source that can target materials remotely, and with extremely high spatial and temporal resolution easily and cheaply. Reversible light-control over molecular mechanical properties in particular has in recent years attracted great interest due to potential applications as optical-to-mechanical conversion actuators and 'devices', enabling 'molecular robotic machines'. In this review, some recent examples and emerging trends in this exciting field of research are highlighted, covering a wide variety of polymer hosts that contain azobenzene photo-reversible switches. It is hoped that this review will help stimulate more interest towards the development of light-reversible materials for energy harvesting and conversion, and their successful incorporation into a wide variety of current and future high-tech applications in devices.

Assembly and dichroism of a four-component halogen-bonded metal-organic cocrystal salt solvate involving dicyanoaurate(I) acceptors.

We describe the use of dicyanoaurate ions as linear ditopic metal-organic acceptors for the halogen bond-driven assembly of a dichroic metal-organic cocrystal based on azobenzene chromophores. Structural analysis by single crystal X-ray diffraction revealed that the material is a four-component solid, consisting of anticipated anionic metal-organic halogen-bonded chains based on dicyanoaurate ions, as well as complex potassium-based cations and discrete molecules of the crown ether 15-crown-5. Importantly, the structural analysis revealed the parallel alignment of the halogen-bonded chains required for dichroic behaviour, confirming that crystal engineering principles developed for the design of halogen-bonded dichroic organic cocrystals are also applicable to metal-based structures. In the broader context of crystal engineering, the structure of the herein reported dichroic material is additionally interesting as the presence of an ion pair, a neutral azobenzene and a molecule of a room-temperature liquid make it an example of a solid that simultaneously conforms to definitions of a salt, a cocrystal, and a solvate.

Photo-induced structural modification of silk gels containing azobenzene side groups.

Azobenzene modification of Bombyx mori silkworm silk creates a photo-responsive 'azosilk' biomaterial, allowing for 3D laser patterning. Written regions fluoresce, and become fluid-filled raised 'micro-blisters' with a 10-fold photo-softening effect of the modulus. Patterning is facile and versatile, with potential applications as soft tunable materials for dynamic cell guidance and microfluidics.

Azo···phenyl stacking: a persistent self-assembly motif guides the assembly of fluorinated cis-azobenzenes into photo-mechanical needle crystals.

We describe a novel, persistent motif of molecular assembly in photo-mechanical crystals and cocrystals of fluorinated cis-azobenzenes. The azo···phenyl stacking, preserved upon either chemical substitution or halogen-bonded cocrystallization, guides the assembly of fluorinated cis-azobenzenes into columnar stacks and drives the formation of crystals with needle-like morphologies optimal for photo-mechanical motion.

Controlling Motion at the Nanoscale: Rise of the Molecular Machines.

As our understanding and control of intra- and intermolecular interactions evolve, ever more complex molecular systems are synthesized and assembled that are capable of performing work or completing sophisticated tasks at the molecular scale. Commonly referred to as molecular machines, these dynamic systems comprise an astonishingly diverse class of motifs and are designed to respond to a plethora of actuation stimuli. In this Review, we outline the conditions that distinguish simple switches and rotors from machines and draw from a variety of fields to highlight some of the most exciting recent examples of opportunities for driven molecular mechanics. Emphasis is placed on the need for controllable and hierarchical assembly of these molecular components to display measurable effects at the micro-, meso-, and macroscales. As in Nature, this strategy will lead to dramatic amplification of the work performed via the collective action of many machines organized in linear chains, on functionalized surfaces, or in three-dimensional assemblies.

Shaping crystals with light: crystal-to-crystal isomerization and photomechanical effect in fluorinated azobenzenes.

Unusually long thermal half-lives of perhalogenated cis-azobenzenes enabled their structural characterization and the first evidence of a crystal-to-crystal cis → trans azobenzene isomerization. Irradiation with visible light transforms a perhalogenated cis-azobenzene single crystal into a polycrystalline aggregate of its trans-isomer in a photomechanical transformation that involves a significant, controllable, and thermally irreversible change of crystal shape. This is the first demonstration of permanent photomechanical modification of crystal shape in an azobenzene.

Fast, reversible, and general photomechanical motion in single crystals of various azo compounds using visible light.

Pseudostilbene-type single crystals exhibit ubiquitous, fast, and reversible photomechanical motion under visible-light irradiation. Push-pull substituents impart extremely rapid switching using just one wavelength of light by shortening the lifetime of the cis-form. This results in a bending motion in the microsecond regime. The influence of crystal density, thickness, and molecular orientation on optimization of the photomechanical effect is investigated.

Metal-organic frameworks (MOFs) as safer, structurally reinforced energetics.

Second-generation cobalt and zinc coordination architectures were obtained through efforts to stabilize extremely sensitive and energetic transition-metal hydrazine perchlorate ionic polymers. Partial ligand substitution by the tridentate hydrazinecarboxylate anion afforded polymeric 2D-sheet structures never before observed for energetic materials. Carefully balanced reaction conditions allowed the retention of the noncoordinating perchlorate anion in the presence of a strongly chelating hydrazinecarboxylate ligand. High-quality X-ray single-crystal structure determination revealed that the metal coordination preferences lead to different structural motifs and energetic properties, despite the nearly isoformulaic nature of the two compounds. Energetic tests indicate highly decreased sensitivity and DFT calculations suggest a high explosive performance for these remarkable structures.

Ionic polymers as a new structural motif for high-energy-density materials.

Energetic materials have been used for nearly two centuries in military affairs and to cut labor costs and expedite laborious processes in mining, tunneling, construction, demolition, and agriculture, making a tremendous contribution to the world economy. Yet there has been little advancement in the development of altogether new energetic motifs despite long-standing research efforts to develop superior materials. We report the discovery of new energetic compounds of exceptionally high energy content and novel polymeric structure which avoid the use of lead and mercury salts common in conventional primary explosives. Laboratory tests indicate the remarkable performance of these Ni- and Co-based energetic materials, while DFT calculations indicate that these are possibly the most powerful metal-based energetic materials known to date, with heats of detonation comparable with those of the most powerful organic-based high explosives currently in use.