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."

Control of a thermosalient phase transition by solid solutions

Thermosalient crystals that exhibit macro-scale motion upon phase transition could be useful as actuators that are capable of converting thermal energy into motion or mechanical work in macroscopic devices.[1] The application capability of these miniature actuators for energy conversion depends on the temperature range and dynamics of transition. While the thermo-mechanical performance cannot be systematically varied with a pure molecular crystal, solid solutions could present a way to intentionally tune both the dynamics and the temperature of the transition in a continuous manner (Figure 1). To verify this hypothesis, Zn(2,2'-bpy)Br₂,[2] was selected as a thermosalient material which could form solid solutions (or mixed complexes) with Zn(2,2'-bpy)Cl₂. Only one form (isomorphous to one of the two Zn(2,2'-bpy)Br₂ forms) has been reported for the chloride.[3] The results indicate that indeed, the two complexes form solid solutions in varying ratios. The mixed crystals undergo the same phase transformation as the pure Zn(2,2'-bpy)Br₂ at a Cl/Br-ratio-dependent temperature. The temperature and dynamics of the thermosalient phenomenon correlates with the Cl/Br-ratio.

Photosalient Effect: Dramatic conversion of light into mechanical motion

Materials showing mechanical response in presence of external stimuli are of relevance for the design of nanoscale actuating devices for a variety of small-scale applications including actuators, flexible electronics, artificial muscles, and others. In recent years, molecular actuators[1] (molecular rotor, elevator, etc.) and several macroscopic systems based on liquid-crystal elastomers, gels, and other polymers[2] have been developed. The most recent efforts are aimed at achieving rapid, reversible, maximum and fatigueless response with single crystals which display optimum coupling between light and the mechanical energy. When exposed to light, certain single crystals can jump up to thousands times their own size. The term "photosalient" was introduced recently to describe this phenomenon.[3] The photosalient effect in the motile crystals represents a direct and visually impressive demonstration of the conversion of light into mechanical motion through a photochemical reaction on a macroscopic scale, which sets the platform for the design of fast biomimetic and technomimetic actuating materials that can mimic animal motions, dynamics of macromolecules, or dynamic technical elements, in the future. In this presentation, we will describe the mechanical response from photosalient single crystals that undergo photoinduced linkage isomerization. To understand the mechanistic details, the mechanism of the process was studied by X-ray photodiffraction, kinematic analysis, IR spectroscopy and mechanical characterization. In contrast to many other solid-state transformations that involve nucleation and propagation of the reaction interface, in this system the reaction proceeds homogeneously whereupon solid solutions form without apparent phase separation.

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.

Thermosalient crystals - new materials for clean energy conversion

Thermosalient compounds, colloquially known as "jumping crystals", are promising materials for fabrication of actuators that are also being considered as materials for clean energy conversion because they are capable of direct conversion of thermal energy into mechanical motion. During heating and/or cooling, these materials undergo rapid phase transitions accompanied by large and anisotropic change in their unit-cell dimensions at relatively small volume change, causing the crystals to jump up to height of several centimeters. Although the list of about a dozen reported thermosalient materials has been expanded recently, this extraordinary phenomenon remains poorly understood. The main practical burden with the analysis of these crystals is their propensity to disintegrate during the transition. By using a combination of structural, microscopic, spectroscopic, and thermoanalytical techniques, we have investigated the thermosalient effect in a prototypal example of a thermosalient solid, the anticholinergic agent oxitropium bromide, and we proposed the mechanism responsible for the effect. We found that heating/cooling over the phase transition causes conformational changes in the oxitropium cation, which are related to increased separation between the ion pairs in the lattice. On heating, this change triggers rapid anisotropic expansion by 4% of the unit cell, whereby the b axis increases by 11% and the c axis decreases by 7%. The phase transition is reversible, and shows a thermal hysteresis of approximately 20 K. Additional interesting observations were that the high-temperature phase of this material can also be obtained by short exposure of the room temperature phase to UV light or with grinding.