The Rise of the Dynamic Crystals

Elastic Organic Crystals Based on Barbituric Derivative: Multi-faceted Bending and Flexible Optical Waveguide

Elastic organic single crystals with light-emitting and multi-faceted bending properties are extremely rare. They have potential application in optical materials and have attracted the extensive attention of researchers. In this paper, we reported a structurally simple barbituric derivative DBDT, which was easily crystallized and gained long needle-like crystals (centimeter-scale) in DCM/CH₃ OH (v/v=2/8). Upon applying or removing the mechanical force, both the (100) and (040) faces of the needle-like crystal showed reversible bending behaviour, showing the nature of multi-faceted bending. The average hardness (H) and elastic modulus (E) were 0.28±0.01 GPa and 4.56±0.03 GPa for the (040) plane, respectively. Through the analysis of the single crystal data, it could be seen that the van der waals (C-H⋅⋅⋅π and C-H⋅⋅⋅C), H-bond (C-H⋅⋅⋅O) and π⋅⋅⋅π interactions between molecules were responsible for the generation of the crystal elasticity. Interestingly, elastic crystals exhibited optical waveguide characteristics in straight or bent state. The optical loss coefficients measured at 627 nm were 0.7 dBmm^-1 (straight state) and 0.9 dBmm^-1 (bent state), while the optical loss coefficient (α) were 1.5 dBmm^-1 (straight state) and 1.8 dBmm^-1 (bent state) at 567 nm. Notably, the elastic organic molecular crystal based on barbituric derivative could be used as the candidate for flexible optical devices.

Pressure-induced superelastic behaviour of isonicotinamide

Dynamic organic crystals have come to the fore as potential lightweight alternatives to inorganic actuators providing high weight-to-force ratios. We have observed pressure-induced superelastic behaviour in Form I of isonicotinamide. The reversible single-crystal to single-crystal transformation exhibited by the system is an important component for functioning actuators. Crucially, our observations have enabled us to propose a mechanism for the molecular movement supported by Pixel energy calculations, that may pave the way for the future design and development of functioning dynamic crystals.

Secondary Structure Tuning of a Pseudoprotein Between β-Meander and α-Helical Forms in the Solid-State

Tuning the secondary structure of a protein or polymer in the solid-state is challenging. Here we report the topochemical synthesis of a pseudoprotein and its secondary structure tuning in the solid-state. We designed the dipeptide monomer N₃ -Leu-Ala-NH-CH₂ -C≡CH (1) for topochemical azide-alkyne cycloaddition (TAAC) polymerization. Dipeptide 1 adopts an anti-parallel β-sheet-like stacked arrangement in its crystals. Upon heating, the dipeptide undergoes quantitative TAAC polymerization in a crystal-to-crystal fashion yielding large polymers. The reaction occurs between the adjacent monomers in the H-bonded anti-parallel stack, yielding pseudoprotein having a β-meander structure. When dissolved in methanol, this pseudoprotein changes its secondary structure from β-meander to α-helical form and it retains the new secondary structure upon desolvation. This work demonstrates a novel paradigm for tuning the secondary structure of a polymer in the solid-state.

Stimuli-responsive granular crystals assembled by dipolar and multipolar interactions

This work describes granular crystals held together by unusual, multipolar interactions and, under the application of an external bias, undergoing reversible structural transitions between closed and open forms. The system comprises two types of polymeric beads agitated on one or between two conductive plates and gradually acquiring charges by contact electrification. The charges thus developed induce a series of electrostatic images in the conductive supports and, in effect, the beads interact via dipolar or multipolar interactions, enabling the stabilization of non-electroneutral crystals. Furthermore, under an applied bias, the beads become polarized and their complex interactions (due to the series of image charges as well as series of image dipoles) result in open-pore crystals which return to compact forms upon bias removal. These effects are rationalized by analytical calculations, and the crystal structures observed in the experiments are reproduced by molecular dynamics simulations.

Mechanical Motion in Crystals Triggered by Solid State Photochemical [2+2] Cycloaddition Reaction

Some special crystals respond to light by jumping, scattering or bursting just like popping of popcorn kernels on a hot surface. This rare phenomenon is called the photosalient (PS) effect. Molecular level control over the arrangement of light-responsive molecules in microscopic crystals for macroscale deformation or mechanical motion offers the possibility of using light to control smart material structures across the length scales. Photochemical [2+2] cycloaddition has recently emerged as a promising route to obtain photoswitchable structures and a wide variety of frameworks, but such reaction in crystals leading to macroscopic mechanical motion is relatively less explored. Study of chemistry of such novel soft crystals for the generation of smart materials is an imperative task. This minireview highlights recent advances in solid-state [2+2] cycloaddition in crystals to induce macroscale mechanical motion and thereby transduction of light into kinetic energy.

Bending for Better: Flexible Organic Single Crystals with Controllable Curvature and Curvature-Related Conductivity for Customized Electronic Devices

Electronic microdevices of self-bending coronene crystals are developed to reveal an unexplored link between mechanical deformation and crystal function. First, a facile approach towards length/width/curvature-controllable micro-crystals through bottom-up solution crystallization was proposed for high processability and stability. The bending crystal devices show a significant increase beyond seven orders of magnitude in conductivity than the straight ones, providing the first example of deformation-induced function enhancement in crystal materials. Besides, double effects caused by bending, including the change of π electron level as well as the enhancement of carrier mobility, were determined, respectively by the X-ray photoelectric spectroscopy and X-ray crystallography to coexist, contributing to the conductivity improvement. Our findings will promote future creation of flexible organic crystal systems with deformation-enhanced functional features towards customized smart devices.

Stepwise Stress-Induced Transformations of Metal-Organic Polyhedral Cluster-Based Assemblies: Where Conformational and Supramolecular Features Meet

Understanding the factors governing the formation of supramolecular structures and phase transitions between various forms of molecular crystals is pivotal for developing dynamic, stimuli-responsive materials and polymorph-controlled syntheses. Here, we investigate the pressure-induced dynamic of both the intrinsic molecular structure and the supramolecular network of a predesigned polyhedral oxo-centered zinc cluster incorporating monoanionic N,N'-diphenylformamidinate and featuring N-bonded phenyl groups in close proximity to the primary coordination sphere. We demonstrate that the model oxo cluster is prone to undergoing pressure-induced conformational transformations of the secondary coordination sphere and simultaneous stepwise (initially every second polyhedral molecule undergoes the conformational transformations) and reversible transitions from an ambient phase α to high-pressure phases β and γ, as single-crystal-to-single-crystal events. The observed phase transitions illustrate the key role of an interplay between the low-energy conformation perturbations and cooperative intra- and intermolecular noncovalent interactions.

Effect of Surface Properties on the Photo-Induced Crawling Motion of Azobenzene Crystals on Glass Surfaces

Photo-induced crawling motion of a crystal of 3,3′-dimethylazobenzene (DMAB) on a glass substrate having different surface properties was studied. When exposed to UV and visible lights simultaneously from different directions, crystals crawl continuously on a glass surface. On a hydrophilic surface, the crystals crawled faster than those on other surfaces but crystals showed spreading while they moved. On hydrophobic surfaces, on the other hand, the crystals showed little shape change and slower crawling motion. The contact angles of the liquid phase of DMAB on surface-modified glass substrates showed positive correlation with the water contact angles. The interaction of melted azobenzene with glass surfaces plays an important role for the crawling motion. We proposed models to explain the asymmetric condition that leads to the directional motion. Specifically by considering the penetration length of UV and visible light sources, it was successfully shown that the depth of light penetration is different at the position of a crystal. This creates a nonequilibrium condition where melting and crystallization are predominant in the same crystal.

Understanding the Molecular Origin of the Collective Movement in a Diarylethene-based Photo-Responsive Actuator

Remotely controlling macroscopic movement is one of the key elements to realize intelligent materials for applications ranging from sensing to robotics. Over the last few years, a number of photomechanical materials based on diarylethene derivatives have been developed. However, a detailed picture of the structural evolution within these soft actuators is often missing. In this work, an atomistic investigation uncovers how the photo-induced molecular dynamics propagates to large-scale motion and results in macroscopic deformation of the crystal. By correlating the intramolecular rearrangement within the photo-responsive switching unit with the intermolecular packing, the molecular mechanism for the photomechanical phenomena is deciphered, which is fundamental for a rational development of photo-responsive actuators.

Constructing multicomponent cooperative functional systems using metal complexes of short flexible peptides

The construction of cooperative systems comprising several units is an essential challenge for artificial systems toward the development of sophisticated functions comparable to those found in biological systems. Flexible frameworks possessing various functional groups that can form weak intra/intermolecular interactions similar to those observed in biological systems have promising design features for artificial systems used to control cooperative systems. However, it is difficult to construct multiple component systems >1 nm using these flexible units by controlling the arrangement of functional units, beginning with the precise control of the cooperative switching of multiple units. In general, it is difficult for oligopeptides to form stable conformations by themselves, although they have designability and structural features suitable for the development of cooperative systems. Increasing the number of coordination bonds in peptides, which are stronger than hydrogen bonds, can be used to control the assembled peptide structures and stabilize their structures owing to the variety of coordination bonds and selective binding affinity. Thus, metal complexes of artificial short peptides have great potential for the development of multicomponent cooperative systems. Based on this concept, we have developed a series of novel metal complexes of flexible peptides and have achieved, to date, cooperative systems, the formation of giant structures, and precise control over the functional units that are the essential bases for designable multifunctional systems that can be regarded as artificial enzymes. In this feature article, we summarize these results and discuss the principal/essential design of artificial systems.

Dynamics of proton, ion, molecule, and crystal lattice in functional molecular assemblies

Dynamic molecular processes, such as short- or long-range proton (H⁺) and ion (M⁺) motions, and molecular rotations in electrical conducting and magnetic molecular assemblies enable the fabrication of electron-H⁺ (or M⁺) coupling systems, while crystal lattice dynamics and molecular conformation changes in hydrogen-bonded molecular crystals have been utilised in external stimuli responsive reversible gas-induced gate opening and molecular adsorption/desorption behavior. These dynamics of the polar structural units are responsible for the dielectric measurements. The H⁺ dynamics are formed from ferroelectrics and H⁺ conductors, while the dynamic M⁺ motions of Li⁺ and Na⁺ involve ionic conductors and coupling to the conduction electrons. In n-type organic semiconductors, the crystal lattices are modulated by replacing M⁺ cations, with cations such as Li⁺, Na⁺, K⁺, Rb⁺, and Cs⁺. The use of polar rotator or inversion structures such as alkyl amides, m-fluoroanilinium cations, and bowl-shaped trithiasumanene π-cores enables the formation of ferroelectric molecular assemblies. The host-guest molecular systems of ESIPT fluorescent chromic molecules showed interesting molecular sensing properties using various bases, where the dynamic transformation of the crystal lattice and the molecular conformational change were coupled to each other.

(9-Isocyanoanthracene)gold(I) Complexes Exhibiting Two Modes of Crystal Jumps by Different Structure Change Mechanisms

The first examples of single crystals exhibiting salient effects by different structure change mechanisms are reported. The crystals of newly prepared aryl(9-isocyanoanthracene)gold(I) complexes jump in response to two different external stimuli: ultraviolet (UV) irradiation and cooling. The photosalient effect is triggered by photodimerization reaction of the anthracene moieties under photoirradiation. By contrast, the thermosalient effect is caused by anisotropic thermal contraction upon cooling without a chemical structure change. By taking advantage of the multiple-jump feature, we also show sequential jumps of crystals by cooling and then UV irradiation for demonstration of the programmed motion of molecular crystals.

High-pressure reversibility in a plastically flexible coordination polymer crystal

Single crystals which exhibit mechanical flexibility are promising materials for advanced technological applications. Before such materials can be used, a detailed understanding of the mechanisms of bending is needed. Using single crystal X-ray diffraction and microfocus Raman spectroscopy, we study in atomic detail the high-pressure response of the plastically flexible coordination polymer [Zn(μ-Cl)₂(3,5-dichloropyridine)₂]_n (1). Contradictory to three-point bending, quasi-hydrostatic compression of (1) is completely reversible, even following compression to over 9 GPa. A structural phase transition is observed at ca. 5 GPa. DFT calculations show this transition to result from the pressure-induced softening of low-frequency vibrations. This phase transition is not observed during three-point-bending. Microfocus synchrotron X-ray diffraction revealed that bending yields significant mosaicity, as opposed to compression. Hence, our studies indicate of overall disparate mechanical responses of bulk flexibility and quasi-hydrostatic compression within the same crystal lattice. We suspect this to be a general feature of plastically bendable materials.

Mechanically flexible single crystals are promising materials for advanced technological applications. Here, the authors study the high pressure response of a plastically flexible coordination polymer and provide indication of an overall disparate mechanical response of bulk flexibility and quasi-hydrostatic compression within the same crystal lattice.

Photothermally Driven High-Speed Crystal Actuation and Its Simulation

Mechanically responsive crystals have been increasingly explored, mainly based on photoisomerization. However, photoisomerization has some disadvantages for crystal actuation, such as a slow actuation speed, no actuation of thick crystals, and a narrow wavelength range. Here we report photothermally driven fast-bending actuation and simulation of a salicylideneaniline derivative crystal with an o-amino substituent in enol form. Under ultraviolet (UV) light irradiation, these thin (<20 μm) crystals bent but the thick (>40 μm) crystals did not due to photoisomerization; in contrast, thick crystals bent very quickly (in several milliseconds) due to the photothermal effect, even by visible light. Finally, 500 Hz high-frequency bending was achieved by pulsed UV laser irradiation. The generated photothermal energy was estimated based on the photodynamics using femtosecond transient absorption. Photothermal bending is caused by a nonsteady temperature gradient in the thickness direction due to the heat conduction of photothermal energy generated near the crystal surface. The temperature gradient was calculated based on the one-dimensional nonsteady heat conduction equation to simulate photothermally driven crystal bending successfully. Most crystals that absorb light have their own photothermal effects. It is expected that the creation and design of actuation of almost all crystals will be possible via the photothermal effect, which cannot be realized by photoisomerization, and the potential and versatility of crystals as actuation materials will expand in the near future.

Light-Driven Crystal-Polymer Hybrid Actuators

Recently, soft robots, which are made of soft and light organic materials, have attracted much attention because of improved safety for daily interactions with humans. Mechanically responsive materials that can move macroscopically by external stimuli, such as light and heat, have been studied extensively over the past two decades, and they are expected to be applicable to soft robots. Among them, mechanically responsive crystals are attractive in terms of a larger Young’s modulus and faster response speed compared with polymers and gels. However, it is impractical to use one piece of a single crystal as a crystal machine; it is difficult to control the size of crystals and obtain large crystals. Hybridization of crystals with polymers is one way to create actuators with more realistic movements. Herein, we report a hybrid crystal assembly in which plate-like salicylideneaniline crystals are aligned in polymer films by a “rubbing” technique, a new approach which is inexpensive, easy, and applicable to a wide range of crystals and polymers. The hybrid films bent reversibly upon alternate irradiation with ultraviolet and visible light. The hybrid films bent as fast as single crystals, even when larger than single-crystal size, showing great mechanical performance originating from the advantages of both molecular crystals (fast response time) and polymers (large size). This work enriches the development of light-driven hybrid actuators composed of molecular crystals and polymers.

Nanooptomechanical Transduction in a Single Crystal with 100% Photoconversion

Materials that exhibit nanooptomechanical transduction in their single-crystal form have prospective use in light-driven molecular machinery, nanotechnology, and quantum computing. Linkage photoisomerization is typically the source of such transduction in coordination complexes, although the isomers tend to undergo only partial photoconversion. We present a nanooptomechanical transducer, trans-[Ru(SO2)(NH3)4(3-bromopyridine)]tosylate2, whose S-bound η1-SO2 isomer fully converts into an O-bound η1-OSO photoisomer that is metastable while kept at 100 K. Its 100% photoconversion is confirmed structurally via photocrystallography, while single-crystal optical absorption and Raman spectroscopies reveal its metal-to-ligand charge-transfer and temperature-dependent characteristics. This perfect optical switching affords the material good prospects for nanooptomechanical transduction with single-photon control.

Mechanically compliant single crystals of a stable organic radical

Mechanically compliant organic crystals are the foundation of the development of future flexible, light-weight single-crystal electronics, and this requires reversibly deformable crystalline organic materials with permanent magnetism. Here, we report and characterize the first instance of a plastically bendable single crystal of a permanent organic radical, 4-(4'-cyano-2',3',4',5'-tetrafluorophenyl)-1,2,3,5-dithiadiazolyl. The weak interactions between the radicals render single crystals of the β phase of this material exceedingly soft, and the S-N interactions facilitate plastic bending. EPR imaging of a bent single crystal reveals the effect of deformation on the three-dimensional spin density of the crystal. The unusual mechanical compliance of this material opens prospects for exploration into flexible crystals of other stable organic radicals towards the development of flexible light-weight organic magnetoresistance devices based on weak, non-hydrogen-bonded interactions in molecular crystals.

Mechanically robust amino acid crystals as fiber-optic transducers and wide bandpass filters for optical communication in the near-infrared

Organic crystals are emerging as mechanically compliant, light-weight and chemically versatile alternatives to the commonly used silica and polymer waveguides. However, the previously reported organic crystals were shown to be able to transmit visible light, whereas actual implementation in telecommunication devices requires transparency in the near-infrared spectral range. Here we demonstrate that single crystals of the amino acid L-threonine could be used as optical waveguides and filters with high mechanical and thermal robustness for transduction of signals in the telecommunications range. On their (00\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\bar 1$$\end{document}1¯) face, crystals of this material have an extraordinarily high Young’s modulus (40.95 ± 1.03 GPa) and hardness (1.98 ± 0.11 GPa) for an organic crystal. First-principles density functional theory calculations, used in conjunction with analysis of the energy frameworks to correlate the structure with the anisotropy in the Young’s modulus, showed that the high stiffness arises as a consequence of the strong charge-assisted hydrogen bonds between the zwitterions. The crystals have low optical loss in the O, E, S and C bands of the spectrum (1250−1600 nm), while they effectively block infrared light below 1200 nm. This property favors these and possibly other related organic crystals as all-organic fiber-optic waveguides and filters for transduction of information.

Fiber-optics based on organic crystals could have potential for unique telecommunications applications but typically transmit visible wavelengths. Here the authors present mechanically robust organic crystals with favourable optical properties across the main telecommunication bands in the near-infrared.

Reversible Thermosalience in a One-Dimensional Coordination Polymer Preceded by Anisotropic Thermal Expansion and the Shape Memory Effect

Thermally responsive crystals hold great potential for their use as actuating materials by acting as energy transducers to convert heat energy to mechanical work. Control over defined phase transition temperature with rapid reconfiguration is of great advantage for actuation. The thermosalient (TS) effect is a rarely observed phenomenon in coordination polymers (CPs), let alone the reversibility of thermosalience in CPs. Herein, we report the reversible TS effect in a one-dimensional CP due to the martensitic phase transition during both heating and cooling cycles. The TS effect was preceded by anisotropic thermal expansion showing high expansion coefficients. In addition, the nonmolecular crystals show reversible contraction and recovery during multiple heating-cooling cycles due to the self-restorative shape memory effect. The reversible actuation of the CP could be repeated for 20 heating-cooling cycles in differential scanning calorimetry experiments, suggesting its great potential as a multicyclic actuator. Such thermal responsive behavior is unique in metal-organic materials.

Elastic Crystalline Fibers Composed of a Nickel(II) Complex

The generation of elastic crystalline fibers from a nonfibrous crystal of metal complex is demonstrated. Applying mechanical stimuli to a platelike crystal of Ni^II(salophen) [1; H₂salophen = N,N'-bis(salicylidene)-o-phenylenediamine] resulted in this complex being transformed into crystal fibers, which could be bent into a loop and demonstrated its high elasticity. Single-crystal X-ray diffraction analyses revealed that the transformation reflects the presence of molecular strands that are composed of a one-dimensional assembly of the slip-stacked arrangement by nearly planar Ni(salophen) molecules. The fiber flexibility was demonstrated to be lost upon the introduction of chloroform solvent molecules into the crystal lattice by recrystallization.

A Future Perspective on Phototriggered Isomerizations of Transition Metal Sulfoxides and Related Complexes

Photochromic molecules are examples of light-activated bistable molecules. We highlight the design criteria for a class of ruthenium and osmium sulfoxide complexes that undergo phototriggered isomerization of the bound sulfoxide. The mode of action in these complexes is an excited-state isomerization of the sulfoxide from S-bonded to O-bonded. We discuss the basic mechanism for this transformation and highlight specific examples that demonstrate the effectiveness and efficiency of the isomerization. We subsequently discuss future research directions within the field of phototriggered sulfoxide isomerizations on transition metal polypyridine complexes. These efforts involve new synthetic directions, including the choice of metal as well as new ambidentate ligands for isomerization.

Dynamic effects in crystalline coordination polymers

Dynamic macroscopic behaviour of single crystals of coordination polymers when subjected to light, heat, and mechanical force.

A plastically bendable and polar organic crystal

An organic crystal of the polar space group Pc that is capable of plastic bending is reported, and its high dielectric constant and strong second-order harmonic generation (SHG) effect have been demonstrated.

Understanding the elastic bending mechanism in a 9,10-anthraquinone crystal through thermal expansion study

Thermal expansion study has been used to understand the mechanism of elastic bending in 9,10-anthraquinone. Expansion along the bending axis due to bending is expected to resemble the thermal expansion along the same direction.

Exploring the diversity of elastic responses of crystalline cadmium(ii) coordination polymers: from elastic towards plastic and brittle responses

Noticeable differences in mechanically induced elastic responses were observed for isostructural crystalline coordination polymers, and their mechanical properties were examined through a highly integrated approach, using both theory and experiment.

Desolvation induced crystal jumping: reversible hydration and dehydration of a spironolactone–saccharin cocrystal with water as the jumping-mate

We present a spironolactone–saccharin cocrystal hydrate as the first example of a crystal that jumps without changes in either the lattice parameter or the molecular conformation to highlight the unique advantages of the jumping-mate strategy.

Multi-color mechanochromic luminescence of three polymorphic crystals of a donor–acceptor-type benzothiadiazole derivative

The three polymorphic crystals of a donor–acceptor dye exhibited different luminescence colors, which changed in response to mechanical grinding.

Light-fueled rapid macroscopic motion of a green fluorescent organic crystal

We report here a new green fluorescent organic crystal of an amide functionalized acrylonitrile derivative (E-ArF2) that displays various types of macroscopic response when illuminated with UV light (390 nm).

Photoinduced topographical surface changes and photoresponse of the crystals of 7-methoxycoumarin

Photoinduced topographical changes, bending, and photosalient effect due to the dimerization reaction were observed on a single crystal of 7-methoxycoumarin, upon deep UV (254 nm) light irradiation.

Blue- and white-light-emitting 2D-coordination polymers and their solid-state photodimerization reaction

Two Pb(ii) coordination polymers (CPs) isolated and show 2D-sheet-like structures. CPs with the formate, undergoes solid-state photodimerization and displays blue emission. While 2D sheet with acetate is photostable and show white-light emission.

Single-Crystal-to-Single-Crystal [2 + 2] Photocycloaddition Reaction in a Photosalient One-Dimensional Coordination Polymer of Pb(II)

In a remarkable example, we report a one-dimensional coordination polymer (CP) of Pb(II) showing photosalient (PS) properties triggered by [2 + 2] cycloaddition of olefinic ligands, which is seldom observed in CPs. Macroscopic rod-shaped crystals show various photomechanical effects such as jumping, splitting, rolling, and breaking upon UV illumination. In this rare example, we could determine the solid-state structure of the 100% dimerized product and three intermediate structures, even after the shattering of crystals into small pieces. Detailed mechanistic investigation from the single-crystal data indicates that the strain generated in the unit cell due to anisotropic expansion played a bigger role for the PS effects. Nucleated growth of the photoproduct crystal created different domains inside the single crystal, which multiplied the already developed stress leading to the photomechanical movements. This example falls in the gray area of a clean single-crystal-to-single-crystal (SCSC) transformation and violent PS effect. Such photochemical behavior has never been reported before.

Scalable Synthesis of Vicinal Quaternary Stereocenters via the Solid-State Photodecarbonylation of a Crystalline Hexasubstituted Ketone

We report the synthesis and stereospecific solid-state photodecarbonylation of a hexasubstituted ketone featuring six distinct α-substituents. The photoproduct of the solid-state transformation features vicinal all-carbon quaternary stereocenters. While reactions carried out in bulk powders and aqueous crystalline suspensions were complicated by secondary photochemistry of the primary photoproduct, optimal conditions provided good yields and recyclable starting material. Subsequent transformations of the α-substituents having orthogonal chemical reactivity illustrate the potential of this transformation toward constructing complex architectures.

Cracking under Internal Pressure: Photodynamic Behavior of Vinyl Azide Crystals through N2 Release

When exposed to UV light, single crystals of the vinyl azides 3-azido-1-phenylpropenone (1a), 3-azido-1-(4-methoxyphenyl)propenone (1b), and 3-azido-1-(4-chlorophenyl)propenone (1c) exhibit dramatic mechanical effects by cracking or bending with the release of N₂. Mechanistic studies using laser flash photolysis, supported by quantum mechanical calculations, show that each of the vinyl azides degrades through a vinylnitrene intermediate. However, despite having very similar crystal packing motifs, the three compounds exhibit distinct photomechanical responses in bulk crystals. While the crystals of 1a delaminate and release gaseous N₂ indiscriminately under paraffin oil, the crystals of 1b and 1c visibly expand, bend, and fracture, mainly along specific crystallographic faces, before releasing N₂. The photochemical analysis suggests that the observed expansion is due to internal pressure exerted by the gaseous product in the crystal lattices of these materials. Lattice energy calculations, supported by nanoindentation experiments, show significant differences in the respective lattice energies. The calculations identify critical features in the crystal structures of 1b and 1c where elastic energy accumulates during gas release, which correspond to the direction of the observed cracks. This study highlights the hitherto untapped potential of photochemical gas release to elicit a photomechanical response and motility of photoreactive molecular crystals.

Tuning the photomechanical behavior and excellent elasticity of azobenzene via cocrystal engineering

Obtaining crystals with different photomechanical responses and excellent mechanical properties simultaneously through cocrystal engineering based on the same photoactive molecule.