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

Gas Release as an Efficient Strategy to Tune Mechanical Properties and Thermoresponsiveness of Dynamic Molecular Crystals

Dynamic molecular crystals combining multiple and finely tunable functionalities are attracting and an increasing attention due to their potential applications in a broad range of fields as efficient energy transducers and stimuli-responsive materials. In this context, a multicomponent organic salt, piperazinium trifluoroacetate (PZTFA), endowed with an unusual multidimensional responsive landscape is reported. Crystals of the salt undergo smooth plastic deformation under mechanical stress and thermo-induced jumping. Furthermore, via controlled crystal bending and release of trifluoroacetic acid from the lattice, which is anticipated from the design of the material, both the mechanical response and the thermoresponsive behavior are efficiently tuned while partially preserving the crystallinity of the system. In particular, mechanical deformation hampers guest release and hence the macroscopic jumping effect, while trifluoroacetic acid release stiffens the crystals. These complex adaptive responses establish a new crystal engineering strategy to gain further control over dynamic organic crystals.

Polymorph driven diversification of photosalient responses in a zinc(II) coordination complex

A novel crystallographic form of a Zn(II) coordination complex [Zn(4-ohbz)2(4-nvp)2] (1-Form-III) (H4-ohbz = 4-hydroxybenzoic acid and 4-nvp = (E)-4-(1-naphthylvinyl)pyridine), undergoes a solid-state photochemical [2+2] cycloaddition reaction accompanied by a moderate photosalient effect, whereby single-crystals show cracking and splitting. This UV-induced cycloaddition accompanies a single-crystal to single-crystal transformation, allowing for continuous monitoring of the unit cell parameters. The new polymorph represents an intermediate form of the two previously reported crystallographic forms of [Zn(4-ohbz)2(4-nvp)2], and provides novel insight into moderating the magnitude of photosalient responses across polymorphic materials.

Bending, Twisting, and Propulsion of Photoreactive Crystals by Controlled Gas Release

The rapid release of gas by a chemical reaction to generate momentum is one of the most fundamental ways to elicit motion that could be used to sustain and control the motility of objects. We report that hollow crystals of a three-dimensional supramolecular metal complex that releases gas by photolysis can propel themselves or other objects and advance in space when suspended in mother solution. In needle-like regular crystals, the reaction occurs mainly on the surface and results in the formation of cracks that evolve due to internal pressure; the expansion on the cracked surface of the crystal results in bending, twisting, or coiling of the crystal. In hollow crystals, gas accumulates inside their cavities and emanates preferentially from the recess at the crystal terminus, propelling the crystals to undergo directional photomechanical motion through the mother solution. The motility of the object which can be controlled externally to perform work delineates the concept of "crystal microbots", realized by photoreactive organic crystals capable of prolonged directional motion for actuation or delivery. Within the prospects, we envisage the development of a plethora of light-weight, efficient, autonomously operating robots based on organic crystals with high work capacity where motion over large distances can be attained due to the large volume of latent gas generated from a small volume of the crystalline solid.

Generating Stable Nitrogen Bubble Layers on Poly(methyl methacrylate) Films by Photolysis of 2-Azidoanthracene

2-Azidoanthracene (2N3-AN) can act as a photochemical source of N2 gas when dissolved in an optically transparent polymer such as poly(methyl methacrylate) (PMMA). Irradiation at 365 or 405 nm of a 150 μm-thick polymer film submerged in water causes the rapid appearance of a surface layer of bubbles. The rapid appearance of surface bubbles cannot be explained by normal diffusion of N2 through the polymer and likely results from internal gas pressure buildup during the reaction. For an azide concentration of 0.1 M and a light intensity of 140 mW/cm2, the yield of gas bubbles is calculated to be approximately 40%. The dynamics of bubble growth depend on the surface morphology, light intensity, and 2N3-AN concentration. A combination of nanoscale surface roughness, high azide concentration, and high light intensity is required to attain the threshold N2 gas density necessary for rapid, high-yield bubble formation. The N2 bubbles adhered to the PMMA surface and survived for days under water. The ability to generate stable gas bubbles "on demand" using light permits the demonstration of photoinduced flotation and patterned bubble arrays.

Unraveling the Solid-State Photoreactivity of Carbonylbis(4,1-Phenylene)dicarbonazidate with Laser Flash Photolysis

Solid-state photoreactions are generally controlled by the rigid and ordered nature of crystals. Herein, the solution and solid-state photoreactivities of carbonylbis(4,1-phenylene)dicarbonazidate (1) were investigated to elucidate the solid-state reaction mechanism. Irradiation of 1 in methanol yielded primarily the corresponding amine, whereas irradiation in the solid state gave a mixture of photoproducts. Laser flash photolysis in methanol showed the formation of the triplet ketone (TK) of 1 (τ ∼ 99 ns), which decayed to triplet nitrene 31N (τ ∼ 464 ns), as assigned by comparison to its calculated spectrum. Laser flash photolysis of a nanocrystalline suspension and diffuse reflectance laser flash photolysis also revealed the formation of TK of 1 (τ ∼ 106 ns) and 31N (τ ∼ 806 ns). Electron spin resonance spectroscopy and phosphorescence measurements further verified the formation of 31N and the TK of 1, respectively. In methanol, 31N decays by H atom abstraction. However, in the solid state, 31N is sufficiently long lived to thermally populate its singlet configuration (11N). Insertion of 11N into the phenyl ring to produce oxazolone competes with 31N cleavage to form a radical pair. Notably, 1 did not exhibit photodynamic behavior, likely because the photoreaction occurs only on the crystal surfaces.

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.

Photocrystallography of [Ru(bpy)2(dmso)2]2+ reveals an O-bonded metastable state

We report the first instance of observing the phototriggered isomerization of dmso ligands on a bis sulfoxide complex, [Ru(bpy)₂(dmso)₂], in the crystalline solid state. The solid-state UV-vis spectrum of the crystal demonstrates an increase in optical density around 550 nm after irradiation, which is consistent with the solution isomerization results. Digital images of the crystal before and after irradiation display a notable color change (pale orange to red) and cleavage occurs along planes, (1̄01) and (100), during irradiation. Single crystal X-ray diffraction data also confirms that isomerization is occurring throughout the lattice and a structure that contains a mix of the S,S and O,O/S,O isomer was attained from a crystal irradiated ex situ. In situ irradiation XRD studies reveal that the percentage of the O-bonded isomer increases as a function of 405 nm exposure time.

Exceptionally Long Lifetimes of Strongly Entangled Acyl-Trityl Radical Pairs Photochemically Generated in Crystalline Trityl Ketones

Triplet acyl-alkyl radical pairs generated by pulsed laser excitation within the constraints of their nanocrystalline ketone precursors were recently introduced as a potential platform for the robust and repeated instantiation of spin qubit pairs for applications in quantum information science. Here, we report the transient spectroscopy of a series of nanocrystalline trityl-alkyl and trityl-aryl ketones capable of generating correlated triplet radical pairs with persistent triphenylmethyl radicals forced to remain within bonding distances of highly reactive acyl radicals. Whereas triplet trityl-acyl radical pairs decay by competing product-forming decarbonylation and intersystem crossing, triplet trityl-benzoyl radical pairs have lifetimes of up to ca. 4 ms and exclusively regenerate the starting ketone. We propose that these long lifetimes are the result of the short inter-radical distances and the colinear orientation of the two singly occupied orbitals, which are expected to result in large singlet-triplet energy gaps, large zero-field splitting parameters, and a poor geometry for spin-obit coupling. Ketones generating trityl-benzoyl radical pairs demonstrate promising performance along multiple dimensions that are crucial for quantum information science.

Electrically conductive hybrid organic crystals as flexible optical waveguides

Hybrid materials capitalize on the properties of individual materials to attain a specific combination of performance assets that is not available with the individual components alone. We describe a straightforward approach to preparation of sandwich-type hybrid dynamic materials that combine metals as electrically conductive components and polymers as bending, momentum-inducing components with flexible organic crystals as mechanically compliant and optically transducive medium. The resulting hybrid materials are conductive to both electricity and light, while they also respond to changes in temperature by deformation. Depending on the metal, their conductivity ranges from 7.9 to 21.0 S µm^‒1. The elements respond rapidly to temperature by curling or uncurling in about 0.2 s, which in one typical case corresponds to exceedingly fast deformation and recovery rates of 2187.5° s^‒1 and 1458.3° s^‒1, respectively. In cyclic operation mode, their conductivity decreases less than 1% after 10,000 thermal cycles. The mechanothermal robustness and dual functionality favors these materials as candidates for a variety of applications in organic-based optics and electronics, and expands the prospects of application of organic crystals beyond the natural limits of their dynamic performance.

Surprising Chemistry of 6-Azidotetrazolo[5,1-a]phthalazine: What a Purported Natural Product Reveals about the Polymorphism of Explosives

6-Azidotetrazolo[5,1-a]phthalazine (ATPH) is a nitrogen-rich compound of surprisingly broad interest. It is purported to be a natural product, yet it is closely related to substances developed as explosives and is highly polymorphic despite having a nearly planar structure with little flexibility. Seven solid forms of ATPH have been characterized by single-crystal X-ray diffraction. The structures show diverse patterns of molecular organization, including both stacked sheets and herringbone packing. In all cases, N···N and C-H···N interactions play key roles in ensuring molecular cohesion. The high polymorphism of ATPH appears to arise in part from the ability of virtually every atom of nitrogen and hydrogen in the molecule to take part in close N···N and C-H···N contacts. As a result, adjacent molecules can adopt many different relative orientations that are energetically similar, thereby generating a polymorphic landscape with an unusually high density of potential structures. This landscape has been explored in detail by the computational prediction of crystal structures. Studying ATPH has provided insights into the field of energetic materials, where access to multiple polymorphs can be used to improve performance and clarify how it depends on molecular packing. In addition, our work with ATPH shows how valuable insights into molecular crystallization, often gleaned from statistical analyses of structural databases, can also come from in-depth empirical and theoretical studies of single compounds that show distinctive behavior.

Engineering Shapes and Sizes of Molecular Crystals to Achieve Versatile Photomechanical Behaviors

Photomechanical molecular crystals, which can directly convert light energy to mechanical energy and do mechanical work at different scales, are promising for future photoactuators. However, one of the bottlenecks in this area is how to harness the crystal shapes and sizes to achieve desired photomechanical motions and behaviors for versatile functionalities. To date, numerous techniques and strategies have been explored and developed to overcome this obstacle. In this perspective, we will summarize the progress recently made on the crystal shape and size engineering platform. Then we briefly touch on possible applications of photomechanical molecular crystals by introducing some built photoresponsive implementations. Finally, we will identify some fundamental challenges and suggestions for future applications.

A Green Chemistry Approach toward the Stereospecific Synthesis of Densely Functionalized Cyclopropanes via the Solid-State Photodenitrogenation of Crystalline 1-Pyrazolines

The cyclopropane ring features prominently in active pharmaceuticals, and this has spurred the development of synthetic methodologies that effectively incorporate this highly strained motif into such molecules. As such, elegant solutions to prepare densely functionalized cyclopropanes, particularly ones embedded within the core of complex structures, have become increasingly sought-after. Here we report the stereospecific synthesis of a set of cyclopropanes with vicinal quaternary stereocenters via the solvent-free solid-state photodenitrogenation of crystalline 1-pyrazolines. Density functional theory calculations at the M062X/6-31+G(d,p) level of theory were used to determine the origin of regioselectivity for the synthesis of the 1-pyrazolines; favorable in-phase frontier molecular orbital interactions are responsible for the observation of a single pyrazoline regioisomer. It was also shown that the loss of N₂ may take place via a highly selective solid-state thermal reaction. Scalability of the solid-state photoreaction is enabled through aqueous nanocrystalline suspensions, making this method a "greener" alternative to effectively facilitate the construction of cyclopropane-containing molecular scaffolds.

Photomechanical crystalline materials: new developments, property tuning and applications

This highlight gives an overview of the mechanism development, property tuning and application exploration of photomechanical crystalline materials.

Programmable photoresponsive materials based on a single molecule via distinct topochemical reactions

Engineering the preorganization of photoactive units remains a big challenge in solid-state photochemistry research. It is of not only theoretical importance in the construction of topochemical reactions but also technological significance in the fabrication of advanced materials. Here, a cyanostilbene derivative, (Z)-2-(3,5-bis(trifluoromethyl)phenyl)-3-(naphthalen-2-yl) acrylonitrile (BNA), was crystallized into two polymorphs under different conditions. The two crystals, BNA-α and BNA-β, have totally different intra-π-dimer and inter-π-dimer hierarchical architectures on the basis of a very simple monomer, which provides them with distinct reactivities, functions and photoresponsive properties. Firstly, two different types of solid-state [2 + 2] photocycloaddition reaction: (i) a typical olefin-olefin cycloaddition reaction within the symmetric π-dimers of BNA-α and (ii) an unusual olefin-aromatic ring cycloaddition reaction within the offset π-dimers of BNA-β have been observed, respectively. Secondly, the crystal of BNA-α can be bent to 90° without any fracture, exhibiting outstanding flexibility upon UV irradiation, while the reversible photocycloaddition/thermal cleavage process (below 100 °C) accompanied by unique fluorescence changes can be achieved in the crystal of BNA-β. Finally, micro-scale photoactuators and light-writable anti-counterfeiting materials have been successfully fabricated. This work paves a simple way to construct smart materials through a bottom-up way that is realized by manipulating hierarchical architectures in the solid state.

Tribochemistry, Mechanical Alloying, Mechanochemistry: What is in a Name?

Over the decades, the application of mechanical force to influence chemical reactions has been called by various names: mechanochemistry, tribochemistry, mechanical alloying, to name but a few. The evolution of these terms has largely mirrored the understanding of the field. But what is meant by these terms, why have they evolved, and does it really matter how a process is called? Which parameters should be defined to describe unambiguously the experimental conditions such that others can reproduce the results, or to allow a meaningful comparison between processes explored under different conditions? Can the information on the process be encoded in a clear, concise, and self-explanatory way? We address these questions in this Opinion contribution, which we hope will spark timely and constructive discussion across the international mechanochemical community.

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.

In Crystallo Snapshots of Rh2-Catalyzed C-H Amination

While X-ray crystallography routinely provides structural characterization of kinetically stable pre-catalysts and intermediates, elucidation of the structures of transient reactive intermediates, which are intimately engaged in bond-breaking and -making during catalysis, is generally not possible. Here, we demonstrate in crystallo synthesis of Rh₂ nitrenoids that participate in catalytic C-H amination, and we characterize these transient intermediates as triplet adducts of Rh₂. Further, we observe the impact of coordinating substrate, which is present in excess during catalysis, on the structure of transient Rh₂ nitrenoids. By providing structural characterization of authentic C-H functionalization intermediates, and not kinetically stabilized model complexes, these experiments provide the opportunity to define critical structure-activity relationships.