Mechanical Effects in Molecular Crystals - A Revival

"The classical perception of single crystals of molecular materials as rigid and brittle entities has downsized the research interest in mechanical effects that had been initiated and was active back in the 1980s. More recently, the modern analytical techniques for mechanical, electron-microscopic, structural, spectroscopic and kinematic characterization have contributed to accumulate compelling evidence that under certain circumstances, even some seemingly rigid single crystals can deform, bend, twist, hop, wiggle or perform other ""acrobatics"" that are atypical for non-soft matter. These examples contribute to a paradigm shift in our understanding of the elasticity of molecular crystals and also provide direct mechanistic insight into the structural perturbations at the limits of the susceptibility of ordered matter to internal and external mechanical force. As the relevance of motility and reshaping of molecular crystals is being recognized by the crystal research community as a demonstration of a very basic concept-conversion of thermal or light energy into work-a new and exciting crystal chemistry around mechanically responsive single crystals rapidly unfolds."

Single Crystal in Motion: An Insight into the Mechanism of Actuation

Dynamic materials that can rapidly transform one form of energy into another have recently attracted attention because they could be utilized as platform for actuation from the nanoscale to the macroscale. This rapidly expanding field has brought up an increasing number of examples of oftentimes serendipitous observations of macro-, milli- and nano-sized single crystals that can hop, bend, curl or twist when exposed to light, heat or external pressure and have the capability to induce motion of other objects. Among these biomimetic crystalline actuators, the so-called thermosalient (TS) crystals, when heated or cooled, exhibit spectacular macroscopic motility as a result of fast coupling of thermal energy with mechanical actuation (Figure 1). Some of these crystals are exceptionally robust and undergo mechanical actuation for several cycles without disintegration. Achieving concurrently fast and reversible actuation of molecular crystals remains a great challenge since mechanical reconfiguration of single crystals is generally accompanied by loss of integrity (cracking, fracturing, explosion, etc.), a serious pitfall that limits their compatibility with the basic requirements for applications as dynamic modules. Despite the potential importance of these biomimetic crystalline actuators as smart materials, the detailed mechanism of actuation and shape change is not understood well. Here we report systematic investigation of the mechanism of mechanical response of these crystalline materials with the aid of single crystal X-ray diffraction, powder X-ray diffraction using synchrotron radiation, and other advanced instrumental techniques.