Photomechanically Induced Magnetic Field Response by Controlling Molecular Orientation in 9-Methylanthracene Microcrystals

Photo/Mechanical/Acidic Multi-Stimuli Responses and Information Encryption Design of Acylhydrazone Derivative

Stimuli-responsive crystalline materials have received much attention for being potential candidates of smart materials. However, the occurrence of polymorphism-driven stimuli responses in crystalline materials remains interesting but rare. Herein, three polymorphs of an acylhydrazone derivative, N'-[(E)-(1-benzofuran-2-yl) methylidene] pyridine -4-carbohydrazide (BFMP) were prepared. Form-1 undergoes a photomechanical response via E→Z photoisomerization under UV irradiation, accompanied by a decrease in fluorescence intensity and a change from colorless to yellow. Two types of Z→E thermal isomerization mechanisms with significant differences in conversion rate were observed at different temperatures in form-1. The solid-melt-solid transition has a faster conversion rate compared to the solid-solid transition due to freedom from lattice confinement. The transition from form-2 to form-3 can be achieved under grinding, coupled with a significant decrease in fluorescence intensity. The similar molecular stacking pattern of form-2 and form-3 provides a structural basis for the grinding-induced crystalline transition behavior. In addition, the presence of the pyridine moiety imparts an acidochromic property. The combination of photochromism and acidochromism explores the possible applications of acylhydrazone derivatives in information encryption.

Solid-State [4+4] Cycloaddition and Cycloreversion with Use of Unpaired Hydrogen-Bond Donors to Achieve Solvatomorphism and Stabilization

The crystal structure of a commercially available anthracene derivative, anthracene-9-thiocarboxamide, is reported here for the first time. The compound undergoes a [4+4] cycloaddition in the solid state to afford facile synthesis of the cycloadduct (CA). The cycloaddition is also reversible in the solid state using heat or mechanical force. Due to the presence of unpaired, strong hydrogen-bond donor atoms on the CA, significant solvatomorphism is achieved, and components of the solvatomorphs self-assemble into four different classes of supramolecular structures. The CA readily crystallizes with a variety of structurally-diverse solvents including those containing oxygen-, nitrogen-, or pi-acceptors. Some of the solvents the CA crystallized with include thiophene, benzene, and the three xylene isomers; thus, the CA was employed in industrially-relevant solvent separation. However, in competition studies, the CA did not exhibit selectivity. Lastly, it is demonstrated that the CA crystallizes with vinyl-containing monomers and is currently the only compound that crystallizes with both widely used monomers 4-vinylpyridine and styrene. Solid-state complexation of the CA with the monomers affords over a 50 °C increase in the monomer's thermal stabilities. The strategy of designing molecules with unused donors can be applied to achieve separations or volatile liquid stabilization.

Photo-actuators via epitaxial growth of microcrystal arrays in polymer membranes

Photomechanical crystals composed of three-dimensionally ordered and densely packed photochromes hold promise for high-performance photochemical actuators. However, bulk crystals with high structural ordering are severely limited in their flexibility, resulting in poor processibility and a tendency to fragment upon light exposure, while previous nano- or microcrystalline composites have lacked global alignment. Here we demonstrate a photon-fuelled macroscopic actuator consisting of diarylethene microcrystals in a polyethylene terephthalate host matrix. These microcrystals survive large deformations and show a high degree of three-dimensional ordering dictated by the anisotropic polyethylene terephthalate, which critically also has a similar stiffness. Overall, these ordered and compliant composites exhibit rapid response times, sustain a performance of over at least hundreds of cycles and generate work densities exceeding those of single crystals. Our composites represent the state-of-the-art for photochemical actuators and enable properties unattainable by single crystals, such as controllable, reversible and abrupt jumping (photosalient behaviour).

Light-driven flagella-like motion of coordination compound single crystals

Single crystals of coordination complexes that show mechanical motion under the influence of external stimuli are of great interest due to their applications in photoactuators, sensors and probes. The solid-state [2+2] cycloaddition reaction has been one of the most prominent chemical reactions for photoresponsive materials in recent years. However, a relatively limited number of compounds have been reported, and most of these compounds have only shown destructive photosalient effects. Here, we report two photoreactive Zn(II) metal complexes with a thiophene-based photoreactive linker, 2tpy (4-(2-(thiophen-2-yl)vinyl)pyridine). In addition, under photoirradiation these complexes showed flagella-like bending, first towards and subsequently away from the excitation light source. This is the first report of metal-complexes and the solid-state [2+2] cycloaddition reaction that presents flagella-like motion in single crystals.

Photo-controllable heterostructured crystals of metal-organic frameworks via reversible photocycloaddition

Metal-organic framework (MOF)-based heterostructures are attractive because they can provide versatile platforms for various applications but are limited by complex liquid epitaxial growth methods. Here, we employ photolithography to fabricate and control MOF-based heterostructured crystals via [4 + 4] photocycloaddition. A layered dysprosium-dianthracene framework, [Dy(NO3)3(depma2)1.5]·(depma2)0.5 (2) [depma2 = pre-photodimerized 9-diethylphosphonomethylanthracene (depma)] underwent a single-crystal-to-single-crystal transition at 140 °C to form [Dy(NO3)3(depma)(depma2)]·(depma2)0.5 (3). The dissociated anthracene moieties are face-to-face π-π interacted allowing a reversible photocycloaddition between 2 and 3. This structural transformation causes a luminescence switch between blue and yellow-green and thus can be used to fabricate erasable 2 + 3 heterostructured crystals for rewritable photonic barcodes. The internal strain at the heterostructure interface leads to photobending and straightening of the crystal, a photomechanical response that is fast, reversible and durable, even operating at 140 °C, making it promising for photoactuation. This work may inspire the development of intelligent MOF-based heterostructures for photonic applications.

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.

Engineering Photomechanical Molecular Crystals to Achieve Extraordinary Expansion Based on Solid-State [2 + 2] Photocycloaddition

Photomechanical molecular crystals are promising candidates for photoactuators and can potentially be implemented as smart materials in various fields. Here, we synthesized a new molecular crystal, (E)-3-(naphthalen-1-yl)acrylaldehyde malononitrile ((E)-NAAM), that can undergo a solid-state [2 + 2] photocycloaddition reaction under visible light (≥400 nm) illumination. (E)-NAAM microcrystals containing symmetric twinned sealed cavities were prepared using a surfactant-mediated crystal seeded growth method. When exposed to light, the hollow microcrystals exhibited robust photomechanical motions, including bending and dramatic directional expansion of up to 43.1% elongation of the original crystal length before fragmentation due to the photosalient effect. The sealed cavities inside the microcrystals could store different aqueous dye solutions for approximately one month and release the solutions instantly upon light irradiation. A unique slow-fast-slow crystal elongation kinematic process was observed, suggesting significant molecular rearrangements during the illumination period, leading to an average anisotropic crystal elongation of 37.0% (±3.8%). The significant molecular structure and geometry changes accompanying the photocycloaddition reaction, which propels photochemistry to nearly 100% completion, also facilitate photomechanical crystal expansion. Our results provide a possible way to rationally design molecular structures and engineer crystal morphologies to promote more interesting photomechanical behaviors.

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.

Correlating Reaction Dynamics and Size Change during the Photomechanical Transformation of 9-Methylanthracene Single Crystals

Photomechanical molecular crystals that expand under illumination could potentially be used as photon-powered actuators. In this study, we find that the use of high-quality single crystals of 9-methylanthracene (9MA) leads to more homogeneous reaction kinetics than that previously seen for polycrystalline samples, presumably due to a lower concentration of defects. Furthermore, simultaneous observation of absorbance and shape changes in single crystals revealed that the dimensional change mirrors the reaction progress, resulting in a smooth expansion of 7 % along the c-axis that is linearly correlated with reaction progress. The same expansion dynamics are highly reproducible across different single crystal samples. Organic single crystals exhibit well-defined linear expansions during 100 % photoconversion, suggesting that this class of solid-state phase change material could be used for actuation.

Recent Development of Photodeformable Crystals: From Materials to Mechanisms

Photodeformable materials are a class of molecules that can convert photon energy into mechanical energy, which have attracted tremendous attention in the last few decades. Owing to their unique photoinduced deformable properties, including fast light-response and diverse mechanical behaviors, photodeformable materials have exhibited great potential in many practical applications such as actuators, photoswitches, artificial muscles, and bioimaging. In this review, we sort out the current state of photodeformable crystals and classify them into six categories by molecular structures: diarylethenes, azobenzenes, anthracenes, olefins, triarylethylenes, and other systems. Three distinct light-responsive mechanisms, photocyclization, trans-cis isomerization, and photodimerization, are revealed to play significant roles in the molecular photodeformation. Their corresponding photodeformable behaviors such as twisting, bending, hopping, bursting, and curling, as well as the potential applications, are also discussed. Furthermore, the challenges and prospective development directions of photodeformable crystals are highlighted.

Light-Powered Autonomous Flagella-Like Motion of Molecular Crystal Microwires

The ability to exhibit life-like oscillatory motion fueled by light represents a new capability for stimuli-responsive materials. Although this capability has been demonstrated in soft materials like polymers, it has never been observed in molecular crystals, which are not generally regarded as dynamic objects. In this work, it is shown that molecular crystalline microwires composed of (Z)-2-(3-(anthracen-9-yl)allylidene)malononitrile ((Z)-DVAM) can be continuously actuated when exposed to a combination of ultraviolet and visible light. The photo-induced motion mimics the oscillatory behavior of biological flagella and enables propagation of microwires across a surface and through liquids, with translational speeds up to 7 μm s^-1 . This is the first example of molecular crystals that show complex oscillatory behavior under continuous irradiation. A model that relates the rotation of the transition dipole moment between reversible E→Z photoisomerization to the microscopic torque can qualitatively reproduce how the rotational frequency depends on light intensity and polarization.

Bridging photochemistry and photomechanics with NMR crystallography: the molecular basis for the macroscopic expansion of an anthracene ester nanorod

Crystals composed of photoreactive molecules represent a new class of photomechanical materials with the potential to generate large forces on fast timescales. An example is the photodimerization of 9-tert-butyl-anthracene ester (9TBAE) in molecular crystal nanorods that leads to an average elongation of 8%. Previous work showed that this expansion results from the formation of a metastable crystalline product. In this article, it is shown how a novel combination of ensemble oriented-crystal solid-state NMR, X-ray diffraction, and first principles computational modeling can be used to establish the absolute unit cell orientations relative to the shape change, revealing the atomic-resolution mechanism for the photomechanical response and enabling the construction of a model that predicts an elongation of 7.4%, in good agreement with the experimental value. According to this model, the nanorod expansion does not result from an overall change in the volume of the unit cell, but rather from an anisotropic rearrangement of the molecular contents. The ability to understand quantitatively how molecular-level photochemistry generates mechanical displacements allows us to predict that the expansion could be tuned from +9% to -9.5% by controlling the initial orientation of the unit cell with respect to the nanorod axis. This application of NMR-assisted crystallography provides a new tool capable of tying the atomic-level structural rearrangement of the reacting molecular species to the mechanical response of a nanostructured sample.

Symmetry Breaking and Photomechanical Behavior of Photochromic Organic Crystals

Photomechanical materials exhibit mechanical motion in response to light as an external stimulus. They have attracted much attention because they can convert light energy directly to mechanical energy, and their motions can be controlled without any physical contact. This review paper introduces the photomechanical motions of photoresponsive molecular crystals, especially bending and twisting behaviors, from the viewpoint of symmetry breaking. The bending (right–left symmetry breaking) and twisting (chiral symmetry breaking) of photomechanical crystals are based on both intrinsic and extrinsic factors like molecular orientation in the crystal and illumination conditions. The ability to design and control this symmetry breaking will be vital for generating new science and new technological applications for organic crystalline materials.

Crystal-to-Gel Transformation Stimulated by a Solid-State E→Z Photoisomerization

The molecule (E)-(5-(3-anthracen-9-yl-allylidene)-2,2-dimethyl-[1,3] dioxane-4,6-dione) (E-AYAD) undergoes E→Z photoisomerization. In the solid state, this photoisomerization process can initiate a physical transformation of the crystal that is accompanied by a large volume expansion (ca. 10 times), loss of crystallinity, and growth of large pores. This physical change requires approximately 10 % conversion of the E isomer to the Z isomer and results in a gel-like solid with decreased stiffness that still retains its mechanical integrity. The induced porosity allows the expanding gel to engulf superparamagnetic nanoparticles from the surrounding liquid. The trapped superparamagnetic nanoparticles impart a magnetic susceptibility to the gel, allowing it to be moved by a magnetic field. The photoinduced phase transition, starting with a compact crystalline solid instead of a dilute solution, provides a new route for in situ production of functional porous materials.

Interplay of anthracene luminescence and dysprosium magnetism by steric control of photodimerization

Systematic control of the intermolecular pair-wise [4 + 4] photocycloaddition of a series of dysprosium phosphonates through fine-tuning of two different phosphonate ligands, one with a bidentate blocker and one with an anthracene antenna, both with alkyl substituents, reveals a size dependent rate. With bulky isopropyl on the diphosphonate blocker little response to UV light is observed. In contrast, compounds with ethyl which has less steric hindrance exhibit almost complete photocycloaddition. Interestingly, the alkyl substituents attached to anthracene monophosphonate have no evident effect on the reaction rate. Although no direct relationship can be found between the substitutions and the observed differences in field-induced single molecule magnetism, remarkable changes in magnetic dynamics are observed for complexes before and after the complete photocycloaddition reactions.

Fabrication of Photoactuators: Macroscopic Photomechanical Responses of Metal-Organic Frameworks to Irradiation by UV Light

Photoreactive olefinic species are incorporated into a metal-organic framework (MOF), [Zn(bdc)(3-F-spy)] (1). Single crystals of 1 are shown to undergo three types of photomechanical macroscopic deformation upon illumination by UV light. To demonstrate the practical potential of this system, the inclusion of 1 in a PVA (polyvinyl alcohol) composite membrane, by exploiting hydrogen-bonding interactions, is presented. Using this composite membrane, the amplification of mechanical stress to achieve macroscopic actuation behavior is demonstrated. These results pave the way for the generation of MOF-based soft photoactuators that produce clearly defined mechanical responses upon irradiation with light. Such systems are anticipated to have considerable potential in photomechanical energy harvesting and conversion systems.

Photomechanochromic vs. mechanochromic fluorescence of a unichromophoric bimodal molecular solid: multicolour fluorescence patterning

The multicolour fluorescence responses to external mechanical forces vs. internal photomechanical stresses of a molecular solid are demonstrated and compared.

Mechanofluorochromism (MFC) of molecular solids generally results from the variation of intermolecular interactions induced by external mechanical forces. However, the use of internal photomechanical forces to perturb intermolecular interactions for multicolour fluorescence responses has yet to be demonstrated. Herein we report a unichromophoric anthracene–pentiptycene derivative (1) that displays both MFC and photomechanofluorochromism (PMFC), which lead to various fluorescence colours including red-green-blue (RGB) and near-pure white-light emission. Compound 1 crystallizes in two polymorphs, the yellow (Y) and green (G) emissive forms, in which the pairwise stacked anthracene groups undergo [4 + 4] photodimerization to form the UV (black) emissive photodimer 2 and meanwhile exert photomechanical stresses on the neighbouring molecules. While the photomechanical stresses cause an excimer-to-monomer switching that results in a blue fluorescent state for the Y form, a red-emissive “super dimer” is photomechanically produced for the G form. The recovery of the Y form demands heating, but the G form could be restored by selective photoexcitation of the super dimer. X-ray crystal structures of the Y and G forms and the photodimer 2 generated through single-crystal-to-single-crystal transformation provide a clue to the origins of polymorph-dependent PMFC. The corresponding MFC and mechano-activated vapofluorochromism (VFC) of 1 also shed light on the structure–property relationship. The ability to spatially and temporally control the fluorochromicity of 1 is demonstrated by a series of multicolour fluorescence patterning of “angelfishes”.