Mechanically reconfigurable molecular crystals—ordered materials that can adapt to variable operating and environmental conditions by deformation, whereby they attain motility or perform work—are ...quickly shaping a new research direction in materials science, crystal adaptronics. Properties such as elasticity, superelasticity, and ferroelasticity, which are normally related to inorganic materials, and phenomena such as shape‐memory and self‐healing effects, which are well‐established for soft materials, are increasingly being reported for molecular crystals, yet their mechanism, quantification, and relation to the crystal structure of organic crystals are not immediately apparent. This Minireview provides a condensed topical overview of elastic, superelastic, and ferroelastic molecular crystals, new classes of materials that bridge the gap between soft matter and inorganic materials. The occurrence and detection of these unconventional properties, and the underlying structural features of the related molecular materials are discussed and highlighted with selected prominent recent examples.
Stretching the rules: The occurrence, detection, measurement, and structural origin of the elasticity, superelasticity, and ferroelasticity of molecular crystals are summarized within the context of crystal adaptronics, an evolving research direction in materials science. These exotic properties are rooted in the intermolecular interactions in molecular solids, and bridge the gap between shape‐memory alloys and shape‐memory polymers.
A smart acidochromic agarose‐based film with 1,4‐bis(para‐hydroxystyryl)benzene as the pH‐responsive fluorophore was prepared. This film can simultaneously harness the chemical potential of light and ...aerial humidity gradients to convert them into mechanical work. The strong reversible hygroscopicity of the agarose matrix induces swift locomotion by mechanical deformation owing to exchange of water with the surroundings. Driven by humidity, a 20 mg composite film coupled to a piezoelectric bending transducer sensor generates a peak output of approximately 80 mV, which corresponds to a power density of 25 μW kg−1. Excitation with UV light triggers isomerization of the chromophore, which appears as reshaping by spiraling, bending, or twisting of the film. The material also responds to changes in the pH value by reversible protonation of the fluorophore with rapid changes in color and fluorescence. The threefold sensing capability of this smart material could be utilized for the fabrication of multiresponsive actuating dynamic elements in biomedicine and soft robotics.
A composite self‐actuated material is capable of a rapid response to pH variations with color changes from colorless to yellow and with a drastic shift in the fluorescence emission from blue to yellow. Furthermore, this smart acidochromic agarose‐based film can harness the potentials of aerial‐humidity gradients and light energy to convert them into mechanical work.
Is a Bent Crystal Still a Single Crystal? Commins, Patrick; Karothu, Durga Prasad; Naumov, Panče
Angewandte Chemie International Edition,
July 22, 2019, Letnik:
58, Številka:
30
Journal Article
Recenzirano
The mention of the word “crystal” invokes images of minerals, gems, and rocks, all of which are inevitably solid, hard, and durable entities with well‐defined smooth faces and straight edges. With ...the discovery in the first half of the 20th century that many molecular crystals are soft and can be deformed in a similar way as rubber or plastic, this perception is changing, and both the concept and formal definition of what a crystal is may require reinterpretation. The seemingly naïve question posed in the title of this Minireview does not have a simple answer. Here, we discuss how the effects of the elastic and plastic deformation of molecular crystals on the diffraction signature give primary evidence of their degree of crystallinity. In most cases, the definition of a crystal holds for both elastically and plastically deformed crystals and, unless there is significant or complete physical separation of the crystal during the deformation, they can safely be considered (deformed) single crystals with a high concentration of defects.
Crystal clear? The ambiguity related to the single‐crystal nature of elastically and plastically deformable crystals as well as the correct use of the terms “single crystal/crystalline” and “polycrystal/crystalline” are clarified in this Minireview with selected examples of recently reported bending molecular crystals.
Materials that respond rapidly and reversibly to external stimuli currently stand among the top choices as actuators for real‐world applications. Here, a series of programmable actuators fabricated ...as single‐ or bilayer elements is described that can reversibly respond to minute concentrations of acetone vapors. By using templates, microchannel structures are replicated onto the surface of two highly elastic polymers, polyvinylidene fluoride (PVDF) and polyvinyl alcohol, to induce chiral coiling upon exposure to acetone vapors. The vapomechanical coiling is reversible and can be conducted repeatedly over 100 times without apparent fatigue. If they are immersed in liquid acetone, the actuators are saturated with the solvent and temporarily lose their motility but regain their shape and activity within seconds after the solvent evaporates. The desorption of acetone from the PVDF layer is four times faster than its adsorption, and the actuator composed of a single PVDF layer maintains its ability to move over an acetone‐soaked filter paper even after several days. The controllable and reproducible sensing capability of this smart material can be utilized for actuating dynamic elements in soft robotics.
A series of programmed actuators composed of single‐ or bilayer elements capable of mechanical actuation in response to minute acetone vapors is reported. The bilayer actuators having microchannel patterns on the surface respond to acetone vapors with chiral coiling and directional curling, while the single‐layer actuator maintains its ability for perpetual motion over an acetone‐soaked filter paper.
Molecular crystals can be bent elastically by expansion or plastically by delamination into slabs that glide along slip planes. Here we report that upon bending, terephthalic acid crystals can ...undergo a mechanically induced phase transition without delamination and their overall crystal integrity is retained. Such plastically bent crystals act as bimorphs and their phase uniformity can be recovered thermally by taking the crystal over the phase transition temperature. This recovers the original straight shape and the crystal can be bent by a reverse thermal treatment, resulting in shape memory effects akin of those observed with some metal alloys and polymers. We anticipate that similar memory and restorative effects are common for other molecular crystals having metastable polymorphs. The results demonstrate the advantage of using intermolecular interactions to accomplish mechanically adaptive properties with organic solids that bridge the gap between mesophasic and inorganic materials in the materials property space.
Hygroinduced motion is a fundamental process of energy conversion that is essential for applications that require contactless actuation in response to the day-night rhythm of atmospheric humidity. ...Here we demonstrate that mechanical bistability caused by rapid and anisotropic adsorption and desorption of water vapour by a flexible dynamic element that harnesses the chemical potential across very small humidity gradients for perpetual motion can be effectively modulated with light. A mechanically robust material capable of rapid exchange of water with the surroundings is prepared that undergoes swift locomotion in effect to periodic shape reconfiguration with turnover frequency of <150 min(-1). The element can lift objects ∼85 times heavier and can transport cargos ∼20 times heavier than itself. Having an azobenzene-containing conjugate as a photoactive dopant, this entirely humidity-driven self-actuation can be controlled remotely with ultraviolet light, thus setting a platform for next-generation smart biomimetic hybrids.
One of the most inevitable limitations of any material that is exposed to mechanical impact is that they are inexorably prone to mechanical damage, such as cracking, denting, gouging, or wearing. To ...confront this challenge, the field of polymers has developed materials that are capable of autonomous self‐healing and recover their macroscopic integrity similar to biological organisms. However, the study of this phenomenon has mostly remained within the soft materials community and has not been explored by solid‐state organic chemists. The first evidence of self‐healing in a molecular crystal is now presented using crystals of dipyrazolethiuram disulfide. The crystals were mildly compressed and the degree of healing was found to be 6.7 %. These findings show that the self‐healing properties can be extended beyond mesophasic materials and applied towards the realm of ordered solid‐state compounds.
Healing crystals: Single crystals of dipyrazolethiuram disulfide display self‐healing properties in air when using only moderate mechanical compression. The compound shows that the self‐healing phenomenon found in polymers can be extended to crystalline materials.
Dynamic molecular crystals have recently received ample attention as an emerging class of energy-transducing materials, yet have fallen short of developing into fully realized actuators. Through the
...-
surface isomerization of three crystalline azobenzene materials, here, we set out to extensively characterize the light-to-work energy conversion of photoinduced bending in molecular crystals. We distinguish the azobenzene single crystals from commonly used actuators through quantitative performance evaluation and specific performance indices. Bending molecular crystals have an operating range comparable to that of microactuators such as microelectromechanical systems and a work-generating capacity and dynamic performance that qualifies them to substitute micromotor drivers in mechanical positioning and microgripping tasks. Finite element modeling, applied to determine the surface photoisomerization parameters, allowed for predicting and optimizing the mechanical response of these materials. Utilizing mechanical characterization and numerical simulation tools proves essential in accelerating the introduction of dynamic molecular crystals into soft microrobotics applications.
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