engleski

Thermosalient crystals - acrobatics on the nanoscale

Targeted design of new actuating materials which are capable of fast, reversible and controllable mechanical motion in response to external stimuli (thermal or light) is at the frontier of the contemporary materials science research. Among these, the few serendipitously discovered examples of crystals that suddenly jump while heated, to distances several thousand times larger than their own dimension in less than 1 ms, provide the most impressive display of the conversion of thermal energy into mechanical work. These thermosalient crystals are biomimetic, nonpolymeric self-actuators par excellence. Yet, due to exclusivity and individuality of the phenomenon, but also due to the complexity of analytical methods for its characterization, reasons behind this colossal self-actuation remain unexplained. In order to fully elucidate the thermosalient phenomenon experimental methods solely are not adequate but rather have to be complemented by the state-of-the-art quantomechanical modeling and DFT calculations. In the scope of this lecture, an overview of the most important examples of the thermosalient systems reported in the literature so far will be given, with the special emphasis on two compounds: (i) oxitropium bromide and (ii) N'-2-propylidene-4- hydroxybenzohydrazide. In oxitropium bromide, the anticholinergic agent, thermosalient effect is a macroscopic manifestation of a highly anisotropic change in the cell volume (Skoko et al., 2010). The cell distortion is accompanied by a conformational change of the oxitropium cation, which triggers increased separation between the ion pairs in the lattice at nearly identical separation between the cation and the anion within each ion pair. The high temperature phase can also be obtained by grinding or UV irradiation of the room-temperature phase. It was reported (Centore et al., 2012) that N'-2-propylidene-4- hydroxybenzohydrazide exhibits three polymorphic modifications (I, II and III) with the phase transitions III being thermosalient. Our experiments showed that reversible III↔II phase transition is thermosalient as well. Detailed structural (high-temperature in-situ XRPD) and theoretical (DFT) study using nonemipirical vdW-DF-cx functional was performed in order to explain jumping phenomenon in this system.

Thermosalient effect ; topotactic phase transition ; crystal structure

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