The Thermosalient Phenomenon. “Jumping Crystals” and Crystal Chemistry of the Anticholinergic Agent Oxitropium Bromide

Abstract

The anticholinergic agent oxitropium bromide possesses rich crystal chemistry, most remarkably exhibiting a strong thermosalient effect ("jumping crystal" effect), a mechanical property with potential applications in organic-based actuators. The thermosalient effect, manifested in forceful jumps of up to several centimeters, was investigated by a combination of structural, microscopic, spectroscopic, and thermoanalytical techniques, providing data on which to base a proposed mechanism for the phenomenon. Direct observation of the effect in a single crystal and structure determination of both phases revealed that the jumping of the crystals is a macroscopic manifestation of a highly anisotropic change in the cell volume. 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. At the molecular level, the cation acts as a molecular shuttle composed of two rigid parts (epoxy-aza-tricyclic-nonyl portion and phenyl ring) that are bridged by a flexible ester linkage. The structure of the rigid, inert aza-tricyclic portion remains practically unaffected by the temperature, suggesting a mechanism in which the large, thermally accumulated strain is transferred over the ester bridge to the phenyl ring, which rotates to trigger the phase transition. Mechanistic details of the higher temperature solid-state phenomena are also presented. The high-temperature phase can also be obtained by grinding or UV irradiation of the room-temperature phase. In addition, if it is irradiated with UV light in the presence of KBr, the high-temperature phase undergoes intramolecular photochemical rearrangement. Heating the high-temperature phase to slightly below the melting temperature results in an additional solid-state reaction that results in the conversion of the salt to a mixture of neutral compounds.

Direct visualization of a guest-triggered crystal deformation based on a flexible ultramicroporous framework

“…A unique, reversible thermal expansion behaviour was observed for the crystal included with N,N-dimethylformamide ([Mn(pba) 2 ] Á 0.75DMF or 1 . DMF), which exhibits a phase transition at 208-215 K. In this narrow temperature range, V changes þ 0.29% meanwhile, and more remarkably, the unit-cell parameters a, b and c change À 2.6, þ 3.0 and À 0.20%, respectively 46,47 , which in principle could be ARTICLE useful for thermomechanical actuators 22,48 . Above the phase transition temperature, 1 .…”

Colossal positive and negative thermal expansion and thermosalient effect in a pentamorphic organometallic martensite

“…This observation of crystal motility was to become the first documented report of the thermosalient (TS) effect (Hüpf-Effekt)-the propensity of certain crystals to hop and leap when heated over a phase transition 9,10,18,19 . Two polymorphs, yellow form (a) and red form (b), were characterized 17 .…”

“…The capability of rapid energy transfer through the dense and ordered packing of molecular single crystals evolves as a new platform for efficient actuation. Dynamic single crystals capable of bending, twisting, curling, rolling or locomotion [4][5][6][7][8][9][10] are prospective candidates for the design of dynamic elements, including artificial tissues and components in microfluidic devices. The fast energy transfer secured by efficient coupling between the thermal/light energy and mechanical energy stands as their main asset towards the design of fast actuators.…”

Insights into the Crystallisation Process from Anhydrous, Hydrated and Solvated Crystal Forms of Diatrizoic Acid

“…Upon further heating, the crystals start to move and jump whilst undergoing pseudomorphosis, indicating radical changes to the crystal structure. [31] A preparation of the dihydrate in paraffin oil shows bubble formation accompanying the pseudomorphosis verifying the release of water, which is complete at about 135 °C. Dehydration at such high temperatures is unusual for organic hydrates without cation coordination of the incorporated water and demonstrates the extraordinary stability of this crystal form.…”