Mechanical properties and peculiarities of molecular crystals.

In the last century, molecular crystals functioned predominantly as a means for determining the molecular structures via X-ray diffraction, albeit as the century came to a close the response of molecular crystals to electric, magnetic, and light fields revealed that the physical properties of molecular crystals were as rich as the diversity of molecules themselves. In this century, the mechanical properties of molecular crystals have continued to enhance our understanding of the colligative responses of weakly bound molecules to internal frustration and applied forces. Here, the authors review the main themes of research that have developed in recent decades, prefaced by an overview of the particular considerations that distinguish molecular crystals from traditional materials such as metals and ceramics. Many molecular crystals will deform themselves as they grow under some conditions. Whether they respond to intrinsic stress or external forces or interactions among the fields of growing crystals remains an open question. Photoreactivity in single crystals has been a leading theme in organic solid-state chemistry; however, the focus of research has been traditionally on reaction stereo- and regio-specificity. However, as light-induced chemistry builds stress in crystals anisotropically, all types of motions can be actuated. The correlation between photochemistry and the responses of single crystals-jumping, twisting, fracturing, delaminating, rocking, and rolling-has become a well-defined field of research in its own right: photomechanics. The advancement of our understanding requires theoretical and high-performance computations. Computational crystallography not only supports interpretations of mechanical responses, but predicts the responses itself. This requires the engagement of classical force-field based molecular dynamics simulations, density functional theory-based approaches, and the use of machine learning to divine patterns to which algorithms can be better suited than people. The integration of mechanics with the transport of electrons and photons is considered for practical applications in flexible organic electronics and photonics. Dynamic crystals that respond rapidly and reversibly to heat and light can function as switches and actuators. Progress in identifying efficient shape-shifting crystals is also discussed. Finally, the importance of mechanical properties to milling and tableting of pharmaceuticals in an industry still dominated by active ingredients composed of small molecule crystals is reviewed. A dearth of data on the strength, hardness, Young's modulus, and fracture toughness of molecular crystals underscores the need for refinement of measurement techniques and conceptual tools. The need for benchmark data is emphasized throughout.

Cooperative Photochemical Reaction Kinetics in Organic Molecular Crystals.

Photoreactive molecular crystals have been intensively investigated as next-generation functional materials. Changes in physicochemical properties are usually interpreted in terms of static pre- and post-reaction molecular structures and packings determined by X-ray structure analysis. However, to elucidate the dynamic properties, it is necessary to understand the dynamic nature of photochemical kinetics in crystals. Reaction dynamics in the crystal phase can be dramatically different from those in dilute solution because the local molecular environment evolves as the surrounding reactant molecules are transformed into products. In this Review article, we summarize multiple examples of photochemical reactions in the crystalline phase that do not follow classical kinetic behavior. We also discuss different theoretical methods that can be used to describe this behavior. This Review article should help provide a foundation for future workers to understand and analyze photochemical reaction kinetics in crystals.

Spatial distribution of single guest molecules along thickness of thin films of poly(2-hydroxyethyl acrylate).

We have investigated three-dimensional distribution and diffusion behaviors of single guest dyes in 1-µm thick films of poly(2-hydroxyethyl acrylate) (PHEA) by using astigmatism imaging method. Perylene diimide derivative (BP-PDI) in the PHEA films localized along the Z-axis at ca. Z = 600-700 nm distant from the interface (Z = 0) between PHEA and glass substrate. This Z-localization was not observed in different polymer films of poly(methyl methacrylate) (PMMA), poly(methyl acrylate) (PMA), and polystyrene (PSt). Because the glass transition temperature of the PHEA is lower than the room temperature, BP-PDI in the PHEA films exhibited Brownian motion, normal diffusion on the XY plane and confined motion along the Z-direction. For elucidating the mechanism of the peculiar localization of the guest dyes along film thickness in the PHEA films, we measured diffusion behaviors of different dyes, R6G and Atto 488, in 1-µm thick PHEA films, obtaining result that the Z-distributions of the dyes were overall similar to that of BP-PDI. The result indicates that the Z-localization of the guest dyes should be ascribed not to the interaction between glass surface and guest dye but mainly to the Z-dependent property of the PHEA film. Indeed, the lateral diffusion coefficients of the guest dyes depended on their Z-positions.

Edge-to-Center Propagation of Photochemical Reaction during Single-Crystal-to-Single-Crystal Photomechanical Transformation of 2,5-Distyrylpyrazine Crystals.

Photomechanical molecular crystals are promising materials for photon-powered artificial actuators. To interpret the photomechanical responses, the spatiotemporal distribution of photoproducts in crystals could be an important role in addition to molecular structures, molecular packings, illumination conditions, crystal morphology, crystal size, and so on. In this study, we have found that single crystals of 2,5-distyrylpyrazine show a smooth single-crystal-to-single-crystal photomechanical expansion, and the photochemical reaction propagates from the edge to the center of the single crystal. We revealed that the surface effect (special reactivity at the crystal surface) in addition to the cooperative effect (the reaction is facilitated by neighboring molecules) is essential for the edge-to-center propagation of the photochemical reaction. Our results would provide a foundation for future studies of the photochemical reaction dynamics in photomechanical molecular crystals.

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.

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.

Improving photosensitivity without changing thermal reactivity in photochromic diarylbenzenes based on accurate prediction by DFT calculations.

1,2-Diarylbenzenes (DABs) have been developed as a new family of fast T-type photochromic switches. However, the molecular design strategy for DABs with desired optical and thermal properties is not established. In this work, we explored the best functional in quantum chemical calculations to predict the properties of DABs. Furthermore, we newly designed and synthesized DABs based on the calculation using the best functional, resulting in the improvement of the photosensitivity in the UV-A region (i.e. a shift of absorption to lower energies and an increase in the absorption coefficient) without changing the thermal back-reaction rate.

Photoreversible Birefringence Change of Diarylethene Single Crystals as Revealed by Change in Molecular Polarizability Anisotropy.

Stimuli-responsive organic crystals represent a new frontier of material chemistry. Recently, we have reported photoreversible interference color change to multicolor in single crystals composed of a photochromic diarylethene derivative, 1,2-bis(2-ethyl-5-phenyl-3-thienyl)perfluorocyclopentene (1a), accompanied by the photochromic reaction. The origin of the interference color change is due to the photoinduced birefringence change in the photoisomerization of diarylethenes. In this study, we newly found that single crystals composed of 1,2-bis(2,5-dimethyl-3-thienyl)perfluorocyclopentene (2a) also exhibit a photoreversible interference color change. The birefringence value for crystal 2a increased with the photocyclization conversion, while that for crystal 1a decreased. The relationship between the photoinduced birefringence changes for crystals 1a and 2a and their molecular structures was discussed based on the change in the molecular polarizability anisotropy accompanied by the photochromic reaction. These results would provide not only new opportunities for the application of photochromic crystals but also useful strategies for the design of crystalline materials that exhibit the desired birefringence change.

Cyclization reaction dynamics of an inverse type diarylethene derivative as revealed by time-resolved absorption and fluorescence spectroscopies.

Photocyclization reaction dynamics of an inverse type diarylethene derivative was investigated in alkane solutions by means of ultrafast laser spectroscopies. Femtosecond transient absorption spectroscopy showed that the Franck-Condon state formed by photoexcitation is geometrically relaxed to a transient species within 100 fs and subsequently the cyclization process takes place with a time constant of 36 ps. This time constant is much longer than those in normal type derivatives. Steady-state and time-resolved fluorescence measurements with the aid of quantum chemical calculations revealed that there exist three kinds of conformers, one parallel and two anti-parallel forms, in the ground state. One of the anti-parallel conformers undergoes the cyclization reaction, while the other two conformers are nonreactive species and their major relaxation processes are radiative decay and intersystem crossing into the triplet states. The triplet states thus formed no longer undergo the cyclization reaction in the late time region.

Control of Photomechanical Crystal Twisting by Illumination Direction.

Photomechanical molecular crystals have been investigated as mesoscopic photoactuators. Here, we report how the photomechanical twisting of 1,2-bis(2-methyl-5-phenyl-3-thienyl)perfluorocyclopentene (1a) crystals depends on illumination direction. The ribbon-like crystal of 1a could be successfully prepared by a sublimation method. The ribbon crystal exhibited reversible photomechanical crystal twisting upon alternating irradiation with ultraviolet (UV) and visible light. Moreover, changing the UV illumination direction with respect to the crystal resulted in different twisting modes, ranging from helicoid to cylindrical. Control of photomechanical crystal deformation by illumination direction provides a convenient and useful way to generate a variety of photomechanical motions from a single crystal.

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

A surfactant-assisted seeded-growth method is used to form single-crystal platelets composed of 9-methylanthracene with two different internal molecular orientations. The more stable form exhibits a photoinduced twisting, as observed previously for 9-methylanthracene microribbons grown by the floating drop method. However, the newly discovered elongated hexagonal platelets undergo a photoinduced rolling-up and unrolling. The ability of the rolled-up cylindrical shape to trap superparamagnetic nanoparticles enables it to be carried along in a magnetic field gradient. The new photoinduced shape change, made possible by a novel surfactant-assisted crystal growth method, opens up the possibility of using light to modulate the crystal translational motion.

Strategy for Molecular Design of Photochromic Diarylethenes Having Thermal Functionality.

Thermal reactivities of photochromic diarylethene closed-ring isomers can be controlled by the introduction of substituents at the reactive positions. Diarylethenes having bulky alkyl groups undergo thermal cycloreversion reactions. When bulky alkoxy groups are introduced, the diarylethenes have both thermal cycloreversion reactivities and low photocycloreversion quantum yields. Such photochromic compounds can be applied to thermally reusable photoresponsive-image recordings. The thermal cycloreversion reactivity of the closed-ring isomers can be evaluated using specific steric substituent constants and be correlated with the parameters. By introduction of trimethylsilyl or methoxymethyl groups at the reactive positions, the diarylethene closed-ring isomers undergo thermal irreversible reactions to produce by-products at high temperatures. These diarylethenes may be useful for secret-image recordings. Furthermore, thiophene-S,S-dioxidized diarylethenes having secondary alkyl groups at the reactive positions undergo thermal by-product formation reactions, in addition to the photostability of the colored closed-ring isomers. Such materials may be used for light-starting thermosensors. The thermal by-product formation reactivity can be evaluated by the specific substituent constants and theoretical calculations of quantum chemistry. These results supply the strategy for the molecular design of the photochromic diarylethenes having thermal functionality.

Optical properties and solvatofluorochromism of fluorene derivatives bearing S,S-dioxidized thiophene.

We synthesized fluorene derivatives having phenylthiophene (FPT) or benzothiophene (FBT), and their S,S-dioxidized compounds (FPTO2, FPTO4, FBTO2 and FBTO4), which are prepared by oxidation of the thiophene rings in FPT and FBT with m-chloroperoxybenzoic acid. FPT and FBT exhibited similar optical properties for absorption maximum wavelength, fluorescence maximum wavelength and fluorescence quantum yield. However, the absorption and fluorescence spectra of FPTO2 were largely shifted toward a longer wavelength in comparison with those of FPT, and their fluorescence quantum yields dramatically decreased with oxidation. In contrast, the absorption and fluorescence spectra and the fluorescence quantum yields of FBTO2 and FBTO4 were similar to those of FBT. Moreover, FPTO2 and FBTO2 showed strong solvatofluorochromism. Such solvent dependent properties are ascribed to the charge transfer character of the molecule.

Dependence of photoinduced bending behavior of diarylethene crystals on irradiation wavelength of ultraviolet light.

The dependence of the photoinduced bending behavior of diarylethene crystals on the ultraviolet light irradiation wavelength was investigated. When irradiated with 365 nm light, a crystal of 1,2-bis(5-methyl-2-phenyl-4-thiazolyl)perfluorocyclopentene (1a) bends toward the incident light. On the other hand, when irradiated with 380 nm light, the crystal of 1a first bends away from the light source and then bends toward the incident light. To explain this bending behavior, we propose a comprehensive mechanism based on the depth of the photochromic reaction from the crystal surface. This mechanism is successfully supported by the change of cell parameters associated with the photochromic reaction upon irradiation with 380 nm light, which was determined by in situ X-ray crystallographic analysis.

Crystal thickness dependence of the photoinduced crystal bending of 1-(5-methyl-2-(4-(p-vinylbenzoyloxymethyl)phenyl)-4-thiazolyl)-2-(5-methyl-2-phenyl-4-thiazolyl)perfluorocyclopentene.

Rod-like crystals of 1-(5-methyl-2-(4-(p-vinylbenzoyloxymethyl)phenyl)-4-thiazolyl)-2-(5-methyl-2-phenyl-4-thiazolyl)perfluorocyclopentene with lengths of over 1 mm showed photoreversible bending over 100 cycles upon irradiation with alternating ultraviolet (UV) and visible light. The crystals bent toward the incident light due to a contraction of the crystal length and a gradient of the crystal thickness, which depend on the extent of photoisomerization. It was observed that the bending speed depends on the crystal thickness, and the curvature change on changing the crystal thickness agrees well with Timoshenko's bimetal model, as well as with the observation that crystals of 1,2-bis(2-methyl-5-(4-(1-naphthoyloxymethyl)phenyl)-3-thienyl)perfluorocyclopentene bend away from the incident light due to an expansion of the crystal length and a gradient of the crystal thickness, which depend on the extent of photoisomerization. It was revealed that Timoshenko's bimetal model can be applied to photoinduced crystal bending behaviors caused by both contraction and expansion. These findings are very useful for evaluating and designing photomechanical actuators.

Diarylethene Photoswitches Undergoing 6π Azaelectrocyclic Reaction: Disrotatory Thermal Cycloreversion of the Closed-Ring Isomer.

Gaining insight into the dynamics of electrocyclic reactions is very important from both fundamental and application perspectives. In this study, we developed novel diarylethene photoswitches that undergo 6π azaelectrocyclic reaction. We found that they exhibit fast thermally reversible type (T-type) photochromism, in contrast to the fact that common diarylethenes exhibit photochemically reversible type (P-type) photochromism. The quantum chemical calculations revealed that the fast T-type photochromism originates from the unprecedented disrotatory thermal cycloreversion of the closed-ring isomer. Our results provide useful information not only for the dynamics of the 6π azaelectrocyclic reaction but also for the further development of diarylethene photoswitches utilizing the 6π azaelectrocyclic reaction.

Fabrication of Hyperbranched Photomechanical Crystals Composed of a Photochromic Diarylethene.

We report the fabrication of hyperbranched hollow crystals of 1,2-bis(2,5-dimethyl-3-thienyl)perfluorocyclopentene on a concave surface of the spherical glass substrate by sublimation and their practical photomechanical behaviors. The number of units of the branched structure of the hollow crystals composed of this compound is proportional to the substrate curvature of the substrate. Compared with the sublimation process of the same compound on the flat glass substrate, two kinds of the thin film domains are generated separately in the center and around the edge of the spherical glass substrate. Especially under the high relative humidity condition, the boundaries between these thin film domains move gradually around the edge through the center during as long as 6 h of sublimation time so that the hyperbranched hollow crystals are densely produced on the entire surface of the substrate. These hyperbranched hollow crystals can be prepared with the highly ordered molecular packing due to the very slow formation process of the crystalline walls of the hollow structures. Furthermore, the photo-induced bending behaviors in the few- and highly-branched hollow crystals have the practical roles in moving and bending the minute objects according to their characteristics of these branched shapes.

Patterning submicron photomechanical features into single diarylethene crystals using electron beam lithography.

Lithography methods are commonly used to create structures in inorganic semiconductors like silicon but have not been widely applied to organic crystals. In this work, electron beam lithography (EBL) is used to pattern structures into single organic photomechanical crystals composed of 1,2-bis(2-methyl-5-phenyl-3-thienyl)perfluorocyclopentene. The electron beam creates amorphous regions of decomposed molecules, while the reactivity of the unexposed crystal regions is preserved under a removable Au coating. Exposure of the patterned crystal to 365 nm light causes ridges of amorphous material to increase in height by 30-70%. This height increase can be reversed by visible light exposure and undergo multiple cycles. The reversible surface morphology changes are strong enough to rupture a sheet of graphene placed on top of the patterned crystal. Surprisingly, the change in dimensions of the EBL features is an order of magnitude larger than the changes in overall crystal dimensions as deduced from X-ray diffraction experiments and microscopy observations. A dynamic extrusion model is presented to explain how nanoscale features imprinted into single crystals can amplify molecular-level photomechanical changes. This work demonstrates the capability of EBL methods to produce sub-micron structural features on single photomechanical crystals, providing a new route to monolithic light-powered actuator devices.

Performance of Composite Glass-Diarylethene Crystal Photomechanical Actuator Membranes.

Hybrid organic-inorganic composites based on organic photochromic crystals embedded in inorganic templates provide a new approach to photomechanical materials. Diarylethene (DAE) nanowire crystals grown in Al2O3 membranes have exhibited reversible photoinduced bending and lifting [Dong, X., Chem. Mater. 2019, 31, 1016-1022]. In this paper, the hybrid approach is extended to porous SiO2 membranes. Despite the different template material (SiO2 instead of Al2O3) and much larger channels (5 µm diameter instead of 0.2 µm diameter), similar photomechanical behavior is observed for this new class of organic-inorganic hybrid actuators. The ability to reuse individual glass templates across different DAE filling cycles allows us to show that the DAE filling step is crucial for determining the mechanical work done by the bending template. The bending curvature also depends quadratically on the template thickness, in good agreement with theory. The light-induced bending can be repeated for up to 150 cycles without loss of performance, suggesting good fatigue resistance. The results in this paper demonstrate that the hybrid organic-inorganic approach can be extended to other host materials and template geometries. They also suggest that optimizing the organic filling and template thickness could improve the work output by an order of magnitude.

Control of surface wettability and photomicropatterning with a polymorphic diarylethene crystal upon photoirradiation.

A photochromic diarylethene crystal of 1,2-bis(2-methyl-6-nitro-1-benzothiophen-3-yl)perfluorocyclopentene (1 a) was found to undergo a thermodynamic phase transition at 180 °C to form a needle-like crystal, designated as 1 a-γ. The phase transition involves melting of the initial α-crystal and growth of the γ-crystal. The phase transition temperature decreased with the presence of the closed-ring isomer (1 b) in the crystal because of the decrease in the melting temperature. Upon irradiation with ultraviolet (UV) light, compound 1 a in the α-crystal was converted into 1 b to an extent of 20 %. Consequently, the α-crystal containing 20 % of 1 b underwent the phase transition accompanied by melting of the crystal and growth of the γ-crystal even at 170 °C. Photomicropatterning by the phase transition upon irradiation with UV light using a photomask, followed by heating at 170 °C, was successfully accomplished with a resolution in the microcrystalline pattern of about 20 µm. The contact angle with water on the γ-microcrystalline phase on a glass substrate was larger than that on the α-microcrystalline phase by 20°. This can be ascribed to a difference in the roughness of the surface. Furthermore, the γ-microcrystal was also found to be formed upon heating an amorphous film of 1 a in poly(methyl methacrylate) for 2 min at 130 °C. The crystallized area exhibited a higher water contact angle than the amorphous area. Upon irradiation of the amorphous film with UV light, such crystallization did not take place because of the impurity effect of 1 b in 1 a. Photomicropatterning by the crystallization in the polymer showed a pattern with a higher resolution of about 4 µm, which was much better than that of the neat crystal. This photopatterning process represents a useful tool for controlling the surface wettability in relevant applications.