Regulating thermosalient behaviour in three polymorphs

Abstract

Dynamic molecular crystals can be defined as crystalline materials that respond to external stimuli with mechanical responses on a macro-, micro-or nanoscopic scale (Naumov et al., 2015). The motility of dynamic crystals is usually triggered by a phase transition or chemical reaction without gaseous products, subsequently creating local stresses in the densely packed crystal lattice of the respective material (Commins et al., 2016). The amplification of such strain via cooperative processes can lead to reversible or irreversible phase deformation, instantaneous propulsion and even disintegration. Crystalline materials, which are classically defined as rigid and brittle entities, thus display a reversible mechanical response in the form of shape change (curling, bending, twisting) or locomotion (jumping, flipping, rotation, etc.) (Nath et al., 2014). A class of dynamic single crystals, known as thermosalient crystals, can propel themselves over distances hundreds of times larger than their own size when they are exposed to heat. The earliest known thermosalient crystal, (phenylazophenyl)palladium hexafluoroacetylacetone, was reported more than 30 years ago (Etter & Siedle, 1983). Only in recent years, however, have researchers begun to fully investigate thermosalient phase transitions on a structural, kinematic and mechanistic basis. The observed conversion of heat energy into mechanical motion by a densely packed crystal lattice represents a phenomenon that could be integrated into a host of new applications, e.g. actuators, sensors and pressure-sensitive applications (Commins et al., 2016). In general, a limited yet distinct anisotropic expansion of the crystal unit cell is accompanied by the nucleation and propagation of a new phase within the bulk crystal (phase transition) (Nath et al., 2014). The resulting build-up of strain at the phase interface, if larger than the cohesive interactions present within the crystal lattice, instigates a mechanical force (self-actuation). Phenomena such as molecular dimerization, intermolecular charge transfer and intermolecular proton transfer depend on the way the molecules are assembled in a crystal. Similarly, the dynamic properties of crystals, which occur in response to external stimuli, are also controlled by the intermolecular interactions. Theinteractions, hydrogen bonds, halogen bonds and coordination bonds contribute decisively to the cooperative behaviour of the dynamic switching process. In the case of thermoresponsive materials, the dissipation of thermal energy is influenced by the crystal packing (Naumov et al., 2015). The study of a dichloro derivative of N-salicylideneaniline (see Fig. 1) by Mittapalli et al., reported in this issue of IUCrJ, revealed four polymorphs, three of which exhibit mechanical responses such as jumping (forms I and III) and exploding (form II) during phase transition at high temperature (Mittapalli et al., 2017). This is a rare example of a combination of mechanophysical responses in polymorphs of the same compound (Steiner et al., 1...

Energy Conversion in Single‐Crystal‐to‐Single‐Crystal Phase Transition Materials

“…[49] These exciting results demonstrated the potential of organic thermosalient crystals to actually work as energy converters, actuators, or other dynamic components, [50] which have been further clarified by a number of recently reported examples on thermosalient organic crystals. [51][52][53][54][55][56][57][58][59][60][61][62][63][64][65][66][67] Crystals of organic macrocycle compounds have been also explored as thermomechanical SCSC phase transition materials. [68][69][70][71] Very recently, Nishihara and co-workers reported a disilanyl macrocycle compound C4 that exhibited a mechanical motion in a reversible thermal-induced SCSC phase transition.…”

“…[ 49 ] These exciting results demonstrated the potential of organic thermosalient crystals to actually work as energy converters, actuators, or other dynamic components, [ 50 ] which have been further clarified by a number of recently reported examples on thermosalient organic crystals. [ 51–67 ]…”

(9-Isocyanoanthracene)gold(I) Complexes Exhibiting Two Modes of Crystal Jumps by Different Structure Change Mechanisms

“…The strain is then released in a short time, resulting in crystal jumping . Thermosalient effects are caused by a packing structure change with ,− or without a thermal phase transition upon the temperature change. This structural alternation generates strain within the crystal.…”

Pressure and Temperature Induced Dual Responsive Molecular Crystals: Effect of Polymorphism

“…Mittapalli et al reported polymorphs of the halogensubstituted N-salicylideneaniline derivatives which show different thermosalient responses such as jumping (Forms I and III) and exploding (Form II) in its three polymorphs during phase transition at high temperature. 49,50 This was a unique example of a combination of mechanophysical responses in a polymorphic compound. The structural dynamics of the thermosalient behavior in different polymorphs was correlated with the packing structure and weak C−Cl•••O halogen bond interactions.…”

“…This study demonstrated the utility of hydrogen and halogen bonds in exhibiting the thermosalient effect of molecular crystals. 49,50 Recently, Matsumoto et al reported two polymorphs of a propylated diketopyrrolopyrrole dye where one polymorph (yellow form) showed thermosalient behavior upon heating. 51 The molecular conformation and the disordered propyl groups in the yellow form lead the heatstimulated mechanical response.…”