engleski

Thermosalient mystery

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.

molecular crystals, thermosalient materials, Oxitropium bromide, jumping crystals, X-ray difraction, scopolamine

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