engleski

Acrobatics on the nanoscale

Molecular crystals that provide, when thermo- and/or photo-stimulated, a mechanical response on macroscopic scale are representing an ideal platform for the design of actuators, artificial muscles, biomimetic kinematic devices, heat and light sensors.The capability of collective molecular response to the external excitation results in various displacements within the crystal lattice. Such deformations of the crystal which are manifested, on the macroscopic scale, by the di-verse acrobatic forms such as jumping, twisting, curling, bend-ing and rotating.2 If considered for application as actuators, main advantage of crystalline materials, compared to the soft materials, lies in the rapidness of the response – mechanical actuations usually take place on the time scale of less than a millisecond. In this talk we will present results on novel thermosalinet material N’-2- propylidene-4-hydroxybenzohydrazide. The temperature-induced reversible phase transition of N’-2-propylidene-4-hydroxybenzohydrazide from polymorphic Form II to Form III, and vice versa, is accompanied by the dramatic change of the macroscopic dimensions of the crystal which resulted in the pronounced mechanical motion during the phase transition. Prior to the phase transition an extremely large uniaxial negative thermal expansion along one crystal axis (b- axis) was observed, together with positive thermal expansions along the other two crystal axis. Form III of N’-2-propylidene-4- hydroxybenzohydrazide exhibits the thermal expansion α_c= 360 x 10-6 K-1 which is the largest value ever noticed in any organic or metal-organic crystals. From the structural point of view, a thermosalient effect is escorted by the spring-like behavior of the zig-zag molecular assemblies along the c-axis. First principles electronic structure calculations show that negative thermal expansion arises from the elastic properties of the crystal which show uniaxial negative compressibilities. Elastic properties are also the reason for the reversibility of Form II to Form III transition in contrast to the irreversible Form I to Form II transition. Low energy spring-like phonons are easily thermally excited and can assist in the overcoming of the energy barrier between the two phases that precedes thermosalient transition.

thermosalient materials ; jumping crystals ; DFT ; negative thermal expansion ; HT-XRPD

nije evidentirano