IS NEGATIVE THERMAL EXPANSION GOVERNING FORCE FOR THERMOSALIENT EFFECT?

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.