The thermosalient effect is an extremely rare propensity of certain crystalline solids for self-actuation by elastic deformation or by a ballistic event. Here we present direct evidence for the driving force behind this impressive crystal motility. Crystals of a prototypical thermosalient material, (phenylazophenyl)palladium hexafluoroacetylacetonate, can switch between five crystal structures (a-e) that are related by four phase transitions including one thermosalient transition (a2g). The mechanical effect is driven by a uniaxial negative expansion that is compensated by unusually large positive axial expansion (260 Â 10 -6 K -1 ) with volumetric expansion coefficients (E250 Â 10 -6 K -1 ) that are among the highest values reported in molecular solids thus far. The habit plane advances at B10 4 times the rate observed with non-thermosalient transitions. This rapid expansion of the crystal following the phase switching is the driving force for occurrence of the thermosalient effect.
Crystals on the move: If they are subjected to a strong light stimulus, crystals of the cobalt coordination compound [Co(NH3)5(NO2)]Cl(NO3) undergo sudden jumps and leap over distances 10(2)-10(5) times their own size to release the strain that accumulates in their interior. The first quantitative kinematic analysis of this phenomenon is reported. The observed effect could be employed for actuation on the macroscopic scale.
While self-actuation and motility are habitual for humans and nonsessile animals, they are hardly intuitive for simple, lifeless, homogeneous objects. Among mechanically responsive materials, the few accidentally discovered examples of crystals that when heated suddenly jump, propelling themselves to distances that can reach thousands of times their own size in less than 1 ms, provide the most impressive display of the conversion of heat into mechanical work. Such thermosalient crystals are biomimetic, nonpolymeric self-actuators par excellence. Yet, due to the exclusivity and incongruity of the phenomenon, as well as because of the unavailability of ready analytical methodology for its characterization, the reasons behind this colossal self-actuation remain unexplained. Aimed at unraveling the mechanistic aspects of the related processes, herein we establish the first systematic assessment of the interplay among the thermodynamic, kinematic, structural, and macroscopic factors driving the thermosalient phenomenon. The collective results are consistent with a latent but very rapid anisotropic unit cell deformation in a two-stage process that ultimately results in crystal explosion, separation of debris, or crystal reshaping. The structural perturbations point to a mechanism similar to phase transitions of the martensitic family.
Four new homochiral metal-organic framework (MOF) isomers, [Zn(l-L(Cl))(Cl)](H(2)O)(2) (1), [Zn(l-L(Br))(Br)](H(2)O)(2) (2), [Zn(d-L(Cl))(Cl)](H(2)O)(2) (3), and [Zn(d-L(Br))(Br)](H(2)O)(2) (4) [L = 3-methyl-2-(pyridin-4-ylmethylamino)butanoic acid], have been synthesized by using a derivative of L-/D-valine and Zn(CH(3)COO)(2)·2H(2)O. A three-periodic lattice with a parallel 1D helical channel was formed along the crystallographic c-axis. Molecular rearrangement results in an unprecedented zeolitic unh-topology in 1-4. In each case, two lattice water molecules (one H-bonded to halogen atoms) form a secondary helical continuous water chain inside the molecular helix. MOFs 1 and 2 shows different water adsorption properties and hence different water affinity. The arrangement of water molecules inside the channel was monitored by variable-temperature single-crystal X-ray diffraction, which indicated that MOF 1 has a higher water holding capacity than MOF 2. In MOF 1, water escapes at 80 °C, while in 2 the same happens at a much lower temperature (∼40 °C). All the MOFs reported here shows reversible crystallization by readily reabsorbing moisture. In MOFs 1 and 2, the frameworks are stable after solvent removal, which is confirmed by a single-crystal to single-crystal transformation. MOFs 1 and 3 show high proton conductivity of 4.45 × 10(-5) and 4.42 × 10(-5) S cm(-1), respectively, while 2 and 4 shows zero proton conductivity. The above result is attributed to the fact that MOF 1 has a higher water holding capacity than MOF 2.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.