Mechanically responsive crystalline materials

Fundamental mechanical properties are universally relevant to all types of materials and they have always been topics of interest. The mechanical properties of elemental metals, minerals, ceramics and polymers have long been researched extensively, employing various tools. In the case of molecular crystals, high-pressure and powder compaction techniques have been in use for a long time and continue to be popular today. Studies on structure–property relationships in molecular crystals, in the context of different types of anisotropic stresses caused by different stimuli, such as mechanical strain, heat, light and solvents, have recently gained momentum (Fig. 1) due to the increased accessibility of a range of new characterization techniques. The emerging deeper insights from these studies have begun to make an impact on our ability to design useful materials or provide solutions to the real-life problems in certain areas. A continuum of understanding from molecule to crystal to bulk properties is fast emerging in crystal engineering.

Fig. 1 Number of published papers retrieved from the Web of Science Core Collection (© 2021 Clarivate) using key words of “mechanical properties” and “molecular crystals” and citations from 1997 through to 2020.

We are delighted to be able to bring out this themed issue at a time when the topic is being fast integrated into several traditional areas of materials science, such as pharmaceutical solid-state chemistry, materials engineering, optoelectronics, computational chemistry, mechanochemistry and other applied areas where crystal engineering has a reach. The research groups working on mechanical properties or mechanically responsive molecular crystals are spread out across different geographical locations of the world and come from different scientific backgrounds. This themed issue has contributions from Australia, China, Denmark, Finland, Germany, India, Ireland, Japan, Mexico, Singapore, Spain, the United Kingdom and the United States of America, covering the topics of nanoindentation, photomechanical actuation, the computational prediction of mechanical properties, photochemical reactions, diffraction tools, MOFs, cocrystals, and so on. They are summarized below.

Kobatake and coworkers (DOI: 10.1039/D0CE01705A) investigated the photomechanical bending of twinned 1,2-bis(2-methyl-5-(4-undecyloxyphenyl)-3-thienyl)-perfluorocyclopentene crystals. The bending behaviors were explained based on the effects of the twin components and the absorption anisotropy of diarylethene molecules on crystal twisting.

Asahi and coworkers (DOI: 10.1039/D1CE00208B) presented a machine learning-based regression approach for modeling the deflection and force of a photo-bending crystal, which varied with light intensity and crystal size. The statistical model developed can be used to optimize the performance of photo-responsive crystals as actuators.

Hasija and Chopra (DOI: 10.1039/D1CE00173F) reviewed recent advances in understanding the crystal structure and mechanical flexibility of molecular crystals. Strategies of engineering mechanical properties through forming polymorphs, modifying molecular structures and forming multi-component crystals were discussed. Both the potential and challenges of mechanically flexible molecular crystals were highlighted.

Lawrence, Ramamurty and coworkers (DOI: 10.1039/D0CE01723J) described the brittle and plastic mechanical behaviors of two epimeric series of substituted menthyloxy-2′-esters. The plastic and brittle crystals corresponded to the structure features of slip planes and interdigitated planes, respectively.

Hayashi (DOI: 10.1039/D1CE00400J) highlighted several useful properties of mechanically deformable crystals of functional molecules, including optical properties, electric conductivity, optical waveguide and magnetic properties. Outstanding issues facing the characterization of the mechanical properties of such crystals were also pointed out.

Lu and coworkers (DOI: 10.1039/D1CE00086A) found that a larger number of fluorine atoms on chalcones led to a higher reactivity of photo-induced [2 + 2] cycloaddition reactions in crystals and different extents of motion of the crystals. Distinct mechanical behaviors upon cycloaddition reactions were explained by the change in unit cell dimensions and the contraction or expansion of the phototropic surface.

Rather and Saha (DOI: 10.1039/D1CE00467K) first correlated the thermal expansion behavior with the elastic bending of Cu(acac)2. This correlation was then used to derive a possible bending mechanism in a 9,10-anthraquinone crystal from its thermal expansion behavior. If validated, this approach could complement other less accessible techniques for probing the structural basis of the elastic flexibility of molecular crystals.

Takamizawa and coworkers (DOI: 10.1039/D1CE00359C) reported a photoluminescent organosuperelastic crystal of 7-amino-4-methylcoumarin. This is the first example of superelastic organic crystals simultaneously exhibiting photoluminescence. The twinning based superelastic crystal maintained a constant color during the course of its deformation.

Hayashi and co workers (DOI: 10.1039/D1CE00388G) investigated the bending-induced photoluminescence (PL) and PL anisotropy in an elastic organic crystal by spatially resolved μ-PL spectroscopy. The results confirmed that the molecular packing and its directional arrangement changed with the crystal deformation, where the contraction on the inner side of the crystal leads to a blue shift of the PL spectrum.

Zhang and coworkers (DOI: 10.1039/D1CE00464F) prepared and characterized two rare isomeric zeolite-like MOFs. One of them showed mechanically responsive luminescence emission after being ground. This crystal could also selectively adsorb CO₂ over N₂.

Zhang and coworkers (DOI: 10.1039/D1CE00192B) reported two polymorphs of a green emitting organic compound and characterized their mechanical properties and light emitting properties. In contrast to the brittle orange polymorph, the red emitting polymorph could elastically bend while not losing the ability to transmit light. Hence, it would be more suitable as an active optical waveguiding medium.

Panda and coworkers (DOI: 10.1039/D1CE00460C) reported a new green fluorescent molecular crystal that displayed various macroscopic actuation and shape changes by selectively illuminating the crystal from different directions and on different parts of the crystal.

Kawatsuki and coworkers (DOI: 10.1039/D1CE00456E) investigated the mechanoresponsive photoluminescence of a rod-like alkyltolane-terminated cyanostilbene luminophore. While crystals of this molecule did not respond to mechanical treatment, the precipitated solid exhibited different PL properties, which were affected by grinding.

Uchida and coworkers (DOI: 10.1039/D1CE00444A) described the topographic changes of a 7-methoxycoumarin single crystal upon exposure to deep UV light irradiation. When irradiated with UV lights with different wavelengths, distinct changes on the crystal surface were observed as a result of the different depths of photodimerization from the crystal surface. The different shapes of crystals obtained through sublimation exhibited versatile mechanical behaviors, e.g., bending, breaking and photosalient behavior, when exposed to the UV lights.

Chandrasekar and coworkers (DOI: 10.1039/D1CE00475A) reported a crystal of a D–π–A type phosphorescent molecule, which exhibited both mechanical flexibility and phosphorescence at room temperature.

Rodríguez-Molina and coworkers (DOI: 10.1039/D1CE00442E) reported six mechanofluorochromic crystals which changed emission upon amorphization by grinding. DFT calculations suggested that conformational changes could explain their sensitivity to mechanical grinding. The recrystallization of the amorphous phase upon exposure to dichloromethane made them possible candidates for bistable switches.

Kato and coworkers (DOI: 10.1039/D1CE00459J) reported elastically flexible solvated crystals of two polyhalogenated platinum(II) complexes. The mechanical flexibility and luminescence depended on the solvents in the crystals.

Liu, Yang and coworkers (DOI: 10.1039/D1CE00449B) reported crystals exhibiting multi-color fluorescence emission with tunable lifetimes and efficiency. These crystals also exhibited mechanochromic and solvent-induced luminescence switching behaviors. They presented a clear relationship between the crystal structure and fluorescence properties.

Ito and coworkers (DOI: 10.1039/D1CE00445J) investigated crystals of a thienyl-substituted benzothiadiazole derivative that exhibited mechanochromic luminescence. They showed that the emission color of these crystals could be determined by the dihedral angle between benzothiadiazole and the thiophene rings, as well as the mode of the packing structures.

Gómez-Lor and coworkers (DOI: 10.1039/D1CE00462J) synthesized a diacetylene-linked bis-benzothiadiazole (BTD) derivative and investigated its polymorphism and mechanochromic properties. Loosely bound molecules within the weakly interacting planes were required for the BTD crystal to exhibit mechanochromism.

Bhattacharya, List-Kratochvil, Emmerling and coworkers (DOI: 10.1039/D1CE00642H) reported the simultaneous tuning of mechanical flexibility and photoluminescence properties through forming polymorphs of CPMBP. Their observations suggest that mechanical and optical properties can possibly be independently tuned by pursuing different polymorphs of photoluminescent molecules.

Rath and Vittal (DOI: 10.1039/D1CE00441G) reviewed the stimuli-responsive properties of coordination polymers, including photomechanical, mechanical and thermosalient behaviors. They also outlined the challenges and potential applications of this class of materials.

Guerin and coworkers (DOI: 10.1039/D1CE00453K) reviewed concepts important to DFT, including dispersion correction, limitations, its applications in characterizing the elastic properties and elastic anisotropy of crystals, and the correction of thermal contributions for calculating elastic constants at room temperature. They rightfully envisioned the continuous improvement of the accuracy of DFT predictions of elastic constants.

Kikuchi and Matsumoto (DOI: 10.1039/D1CE00454A) reported a tetramorphic system which exhibited thermosalient effects during phase transitions.

Osakada and coworkers (DOI: 10.1039/D1CE00457C) provided a detailed characterization of pseudorotaxane crystals, including the phase transition, dielectric constant and the formation of a gel from a PhCl solution.

Rantanen and coworkers (DOI: 10.1039/D1CE00539A) investigated the effects of different dehydration pathways, thermally induced or solvent-induced, on the surface roughness, elastic moduli and work functions of dehydrated crystals using AFM. These single particle properties are also linked to the different bulk properties of the dehydrated crystals in terms of static charging.

McMurtrie, Clegg and coworkers (DOI: 10.1039/D1CE00401H) reviewed the micro-focus X-ray diffraction technique and its potential applications in understanding the structural changes accompanying the elastic deformation of molecular crystals.

Cruz-Cabeza and coworkers (DOI: 10.1039/D0CE01659D) studied the structural origin of the anisotropic mechanical properties of carbamazepine form III crystals revealed by nanoindentation. Out of the three large crystal faces, the most likely slip plane is parallel to the (020) face, which was supported by examining the features of the indents using AFM and MD-shearing simulation, among other techniques.

Stevens and coworkers (DOI: 10.1039/D1CE00542A) presented the use of powder Brillouin light scattering as a useful technique for studying interaction anisotropy in molecular crystals, which facilitates the better understanding of the different deformation responses of crystals to an applied mechanical stress.

Papers in this collection and previously published work mainly cover a range of topics around mechanical properties and mechanically responsive crystals. Reports related to the understanding of the dynamics involved in many of these solid-state processes remain scarce. We hope that future research will move in this direction to better address these challenges. In this context, high end, time resolved diffraction techniques and advanced computational tools could be extremely valuable.

Finally, we are grateful to Andrew Shore, Michelle Canning, Mike Andrews, Debora Giovanelli and Samuel Oldknow for their tremendous support in assembling this themed issue.

---