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Thermosalient materials are the ones that during heating/cooling undergo an energetic phase transition which is so sudden and abrupt that the crystals are ballistically projected to heights of several hundred times larger than their own dimensions. Apart from providing visually extremely attractive phenomenon, these materials have a tremendous technological potential as the future self-actuation device (nanoswitches, thermal sensors, artificial muscles, etc.)1. In situ X-ray analysis, often referred to as analysis at non- ambient conditions allow to study and investigate any macroscopic property of a material that is directly related to its structural property (e.g. crystallographic symmetry, crystallite size, vacancies, size and shape of nanoparticles or pores). Temperature, pressure, gas atmosphere and mechanical stress trigger phase transformation, chemical reactions, crystal proprieties and so on. Here we present a systematic experimental study of the thermosalient effect in Oxitropium bromide and Scopolamine methyl bromide with in-situ non- ambient x-ray diffraction measurements (in the range from 100K to 500K at low vacuum conditions), which enable us to elucidate the thermosalient phenomenon in this system. Oxitropium bromide and methylscopolamine bromide have very similar molecular structures, the only difference being that one ethyl group is replaced by methyl group in the case of methylscopolamine bromide. Both compounds have medical uses, oxitropium bromide is used as a bronchodilator, whereas methylscopolamine bromide used to prevent nausea and vomiting caused by motion sickness. They also both exhibit thermosalient effect – unexpected and abrupt jumping of the crystals during heating and cooling. Both systems are characterized by uniaxial negative thermal expansion, but there is an abrupt change of the unit cell parameters during the phase transition for the oxitropium bromide whereas the parameters are changing perfectly linearly in the whole temperature range of existence for methylscopolamine bromide. As in most of the thermosalient systems, immense negative thermal expansion seems to be the most likely candidate for the driving force behind this phenomenon. Therefore, the question remains – what is causing the jumping in methylscopolamine bromide – a system so close to oxitropium bromide but exhibiting totally different kind of thermosalient effect.

Molecular crystals that exhibit thermally activated motion are currently hot topic in the materials science. Such thermosalient materials undergo an energetic phase transition during heating/cooling which is so swift that the crystals are ballistically projected to heights of several hundred times larger than their own dimensions. These locomotive materials possess tremendous technological potential as the future self- actuating devices (nanoswitches, thermal sensors, artificial muscles, etc.) [1]. The majority of thermosalient crystals are organic compounds and several cases have been studied in great detail. Crystals of 1, 2, 4, 5- tetrabromobenzene (TeBB) jump several centimetres during the phase transition from the low- temperature β-form to the high- temperature γ- form. Hopping of the crystals happens at 45.0 °C for single crystals and at 45.5 °C for twinned crystals [2]. We explored the thermosalient effect in in 1, 2, 4, 5- tetrabromobenzene and 1, 2, 4, 5- tetrachlorobenzene (TeCB) organic alloys and the impact of the variation of composition of the alloys on the effect. Samples ranging from 0-70 wt% TeCB were prepared by crystallization from a solution of the two substances dissolved in ether, after which the solvent was evaporated under controlled conditions. Alloys with the amount of TeCB up to 8 wt% exhibit similar structural characteristics at room temperature ; they all crystallize in β form, and upon heating convert to γ. The phase transition temperature changes with alloy composition and coincides with the temperature of jumping. We have observed a linear correlation between temperature of jumping and alloy composition, with the presence of a threshold concentration for the activation of thermosalient effect. Alloys with the wt% of TeCB higher than 8 % do not show thermosalient behaviour. X-ray diffraction has also revealed the mechanism of formation of mixed crystals showing how the Br an Cl atoms are integrated in the alloy structure.

Molecular crystals that exhibit thermally activated motion are currently hot topic in the materials science. Such thermosalient materials undergo an energetic phase transition during heating/cooling which is so swift that the crystals are ballistically projected to heights of several hundred times larger than their own dimensions. These locomotive materials possess tremendous technological potential as the future self- actuating devices (nanoswitches, thermal sensors, artificial muscles, etc.) [1]. The majority of thermosalient crystals are organic compounds and several cases have been studied in great detail. Crystals of 1, 2, 4, 5- tetrabromobenzene (TeBB) jump several centimetres during the phase transition from the low- temperature β-form to the high- temperature γ- form. Hopping of the crystals happens at 45.0 °C for single crystals and at 45.5 °C for twinned crystals [2]. We explored the thermosalient effect in in 1, 2, 4, 5- tetrabromobenzene and 1, 2, 4, 5- tetrachlorobenzene (TeCB) organic alloys and the impact of the variation of composition of the alloys on the effect. Samples ranging from 0-70 wt% TeCB were prepared by crystallization from a solution of the two substances dissolved in ether, after which the solvent was evaporated under controlled conditions. Alloys with the amount of TeCB up to 8 wt% exhibit similar structural characteristics at room temperature ; they all crystallize in β form, and upon heating convert to γ. The phase transition temperature changes with alloy composition and coincides with the temperature of jumping. We have observed a linear correlation between temperature of jumping and alloy composition, with the presence of a threshold concentration for the activation of thermosalient effect. Alloys with the wt% of TeCB higher than 8 % do not show thermosalient behaviour. X-ray diffraction has also revealed the mechanism of formation of mixed crystals showing how the Br an Cl atoms are integrated in the alloy structure.

Routine measurements, such as evaluation of the crystals’ habit under hot-stage microscope, can become very thrilling if the crystals suddenly start to jump around as if dancing to some inaudible music. This is actually a real effect called thermosalient effect. Thermosalient materials, or more colloquially called jumping crystals, are the ones that during heating/cooling undergo an energetic polymorphic phase transition which is so sudden and abrupt that the crystals are ballistically projected to heights of several hundred times larger than their own dimensions. Such single crystals provide an impressive display of the conversion of thermal energy into mechanical work and are thus excellent candidates for the production of active machines, such as “smart” medical devices or implants, artificial muscles, biomimetic kinetic devices, electromechanical devices, actuators, materials for electronics and heat- sensitive sensors [1- 2]. Our latest study shows a new kind of thermosalient effect in scopolamine bromide. As in most thermosalient systems, uniaxial negative thermal expansion seems to be the most likely candidate for the driving force behind this phenomenon. But one thing in which scopolamine bromide differs from other reported thermosalient materials is the absence of phase transition. In all the other thermosalient systems, thermosalient effect is explained by the accumulation of stress in the crystal lattice which is explosively released during the phase transition (which usually takes place on the timescale of less than a millisecond) thus causing the crystal locomotion. As a comparison, results of the study of thermosalient behavior of oxitropium bromide are presented as well. These two systems have very similar molecular structures, the only difference being that one ethyl group is replaced by methyl group in the case of scopolamine bromide. Both compounds have medical uses, oxitropium bromide is used as a bronchodilator, whereas scopolamine bromide used to prevent nausea and vomiting caused by motion sickness. They also both exhibit thermosalient effect. And this is where the difference stops. As already mentioned, In the case of oxitropium bromide thermosalient effect is caused by the topotactic phase transition during which unit cell drastically changes. On the other hand, surprisingly, scopolamine bromide does not seem to show any phase transition at all, but yet, its crystals are also joyfully jumping around during the heating or cooling of sample. Thermosalient effect is definitely propelled by different mechanisms in these two systems, thus demonstrating and confirming the full complexity and mystery of this phenomenon.

Molecular crystals, capable of fast and reversible change of shape in the form of jumping, twisting, curling, bursting and bending are quickly emerging as perspective actuators on the nano/microscale. These thermosalient materials (or more colloquially known as jumping crystals) can by the collective motion of their atoms use external energy provided as heat and transfer it into mechanical motion – work. In contrast to their polymer counterparts, in single crystals this process happens instantaneously. With the main aim of getting rapid and reversible motion of such materials, triggered by external heat, is at the frontier of the material science research. Here we present our results on three thermosalient systems: oxitropium bromide, methylscopolamine bromide and N’-2- propylidene-4- hydroxybenzohydrazide. Oxitropium bromide and methylscopolamine bromide have very similar molecular structures, the only difference being that one ethyl group is replaced by methyl group in the case of methylscopolamine bromide. Both compounds have medical uses, oxitropium bromide is used as a bronchodilator, whereas methylscopolamine bromide used to prevent nausea and vomiting caused by motion sickness. They also both exhibit thermosalient effect – unexpected and abrupt jumping of the crystals during heating and cooling. This is where the difference stops. In the case of oxitropium bromide thermosalient effect is caused, as in most other thermosalient compounds, by the topotactic phase transition during which unit cell changes drastically thus causing the crystals to jump to heights several times larger than their dimensions. On the other hand, surprisingly, methylscopolamine bromide does not seem to show any phase transition at all, but yet, its crystals are also joyfully jumping around during the heating or cooling of sample. In the third system, N’-2-propylidene- 4- hydroxybenzohydrazide, there is not one, but two thermosalient phase transitins. Our theoretical calculations prove that in this case thermosalient effect is caused by yet another unusual property – negative compressibility, which causes softening of low- energy phonons. These three systems which all exhibit thermosalient behavior, but which is caused by different mechanisms, demonstrate the complexity and mystery of this phenomenon.

Imagine the surprise on one’s face when, during the microscopic evaluation of the crystals’ habits, these crystals start to literally jump and fly on the stage providing a vivid example of thermosalient effect. Thermosalient materials are the ones that during heating/cooling undergo an energetic phase transition which is so sudden and abrupt that the crystals are ballistically projected to heights of several hundred times larger than their own dimensions. Apart from providing visually extremely attractive phenomenon, these materials have a tremendous technological potential as the future self-actuation device (nanoswitches, thermal sensors, artificial muscles, etc.)1. Several exciting experimental studies on the origin of the thermosalient effect were published resulting in an astonishing wealth of information about the effect, but still we are far from the full understanding of the peculiarities and all the interplays and forces that govern this phenomenon. Here we present a systematic experimental study of the thermosalient effect in Scopolamine methyl bromide. As in most of the thermosalient systems, immense negative thermal expansion seems to be the most likely candidate for the driving force behind this phenomenon. This study presents combination of optical and electron microscopy, together with the x-ray diffraction measurements. Optical microscopy provides a direct insight into this visually extremely attractive phenomenon which is a manifestation of the macroscopic transformation of thermal energy into mechanical work. This analysis gives us a wealth of information about mechanical behaviour of thermosalient crystals – the macroscopic change of the crystal habit during the transformation, their trajectory, initial velocity and the dependence of the jumping height on the heating rate. On the other side, high resolution transmission electron microscopy gives us an insight into the fine details of the crystal structure and the stress field which is responsible for the thermosalient effect. These two microscopic techniques, coupled with the in- situ non-ambient x-ray powder diffraction measurements, enable us to fully elucidate the thermosalient phenomenon in this system.

Mechanically responsive molecular crystals, i.e. the ones that provide mechanical response to external stimuli, such as change of temperature or pressure or exposure to light, are rapidly getting attention and are at the forefront of the material science research. Main advantages, in comparison to their polymer counterparts, are the rapidness of the response, faster energy transfer and facile fabrication. Among these materials, one class certainly stands out – thermosalient materials or, more colloquially called, jumping crystals. These materials, when exposed to the change of temperature, undergo a single crystal to single crystal polymorphic phase transition which is accompanied with hopping of the crystals which literally jump off the stage to the heights of several hundred times bigger than their own dimensions. Apart from being visually extremely attractive phenomenon, thermosalient effect has tremendous technological potential since these materials could be used in the fabrication of future actuators, artificial muscles, biomimetic devices or heat sensors [1]. Oxitropium bromide (OXTB), an anticholinergic agent, is an example of a compound that exhibits strong thermosalient effect. This study is a natural continuation of previous investigations which revealed that the jumping of the crystals is a macroscopic manifestation of a highly anisotropic change in the unit-cell volume [1]. Here, an in situ high- temperature and low- temperature single-crystal study on OXTB was undertaken from RT to 400 K. The selected crystal was mounted in quartz vials and kept still with the use of quartz wool to avoid mechanical stress on its surfaces. Such mounting proved efficient in the study of jumping crystals, as quartz fibres are flexible enough to accommodate crystal movements at the transition. In this way, we were able to determine thermal behaviour before and after the topotactic thermosalient phase transition. As in most other cases of thermosalient materials, OXTB also exhibited uniaxial negative thermal expansion which, in our opinion, is a generating force for this fascinating phenomenon.