Oleksandr S. Bushuyev

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.

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.

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.

Photo-mechanical azobenzene cocrystals and in situ X-ray diffraction monitoring of their optically-induced crystal-to-crystal isomerisation

We demonstrate the first supramolecular cocrystallisation strategy to generate crystalline azobenzene materials with a range of photo-mechanical and thermochemical properties: from those that exhibit isomerisation without any change in crystal shape to those that undergo a crystal-to-crystal cis–trans isomerisation accompanied by large scale bending. The latter permitted the use of variable temperature single crystal X-ray diffraction for the first in situ monitoring of structural changes behind the cis–trans isomerisation in the solid-state, which revealed a topotactic process mediated by an amorphous phase. While the design of photo-mechanical azobenzene solids has so far focused on polymer matrices and gels, the herein presented cocrystallisation approach represents the first methodology for generating new photo-mechanical azobenzene crystals from a limited number of photo-active building blocks, opening a route to potential bio-mimetic and light-harvesting materials based on crystalline solids.

Fluorinated azobenzenes with highly strained geometries for halogen bond-driven self-assembly in the solid state

Attempted cocrystallisation of brominated and iodinated octafluoroazobenzene derivatives with morpholine led to the exhaustive replacement of fluorine substituents that are in ortho-positions to the azobenzene with sterically demanding morpholine groups. The resulting molecules exhibit a highly unusual strained conformation of the azobenzene unit, in which the terminal phenyl rings adopt a mutually nearly completely orthogonal orientation. Substitution of ortho-fluorine groups with N-morpholine fragments provides molecules with active halogen bond donor and acceptor sites that guide the molecular self-assembly in the solid state towards the formation of polymeric halogen-bonded chains.

Photo-induced motion of azo dyes in organized media: from single and liquid crystals, to MOFs and machines

The field of photo-mechanical effects is a burgeoning branch of materials science dealing with the direct transformation of light energy into mechanical motion. While the research in the field has historically focused on polymeric materials, the past decades have seen the rapid emergence of crystalline, photo-mechanically active materials capable not only of converting light into mechanical motion, but also coupling such behaviour with other materials properties, e.g. microporosity in metal–organic frameworks (MOFs). This Highlight article focuses on the rapidly emerging, new area of photo-mechanical materials based on crystalline azobenzenes. The discovery of photo-mechanical motion in a needle-shaped crystal of an azobenzene led to an explosion of research and new developments in less than 5 years, revealing new types of photo-mechanical behaviour, crystal engineering routes to easily create libraries of crystalline photo-mechanical materials, in situ and real-time studies of structural changes during photo-mechanical effect using X-ray diffraction, and the discovery of new supramolecular interactions enabling the engineering of photo-mechanical azobenzene crystals in terms of molecular stacking, as well as crystal morphology.