Search

The amplification of microscopic molecular motions so as to produce a controlled macroscopic body effect is the main challenge in the development of molecular mechanical devices. That amplification requires the coherent and ordered movement of each molecule of a whole macroscopic set, such as that taking place in a single-crystal-to-single-crystal transition. Actually, single-crystal-to-single-crystal transitions in molecular crystals can produce a variety of mechanical effects potentially useful in the development of smart materials. A challenging issue in these dynamic crystals, propedeutic to many possible applications in devices, is the gaining of a strict control over the mechanical effects associated with the transition. Here we report an example in which the control of the mechanical effects was successfully obtained. The compound studied undergoes a reversible single-crystal-to-single-crystal transition at 71 °C, from a planar stacked to a herringbone type packing. To this transition, a reversible macroscopic self-reshaping of the crystal is associated. Depending on the morphology, the crystal specimen undergoes a reversible longitudinal expansion of about 20% or a reversible transverse expansion of 20%, the other two dimensions of the crystal specimen being substantially unchanged. The amount of the macroscopic reshaping effect (20%) fully matches the relative variation of the sole unit cell parameter that changes during the transition (from 8.139 to 9.666 Å) in a sort of scale-invariant process. This represents striking evidence of controlled translation of sub-nanometer molecular motions up to the macroscopic scale of body motion.

Imines obtained by condensation of 4-hydroxybenzohydrazide with aliphatic ketones are a rare example of a class of compounds showing a remarkable tendency to crystallize in acentric polar space groups (Pna21 or Cc). In fact, all of the (seven) compounds studied up to now show at least one polar polymorph. In some cases, polymorphism was detected, and a nonpolar centrosymmetric phase was also identified (P21/c or P21/n space group). With the aim to disclose the conditions that can favor the formation of acentric structures in molecular crystals, we report, in this paper, a theoretical analysis (ab initio density functional theory with periodic boundary) of the lattice energy and density of all the packing modes observed in the whole set of imines. The computational analysis has been performed by optimizing each compound in its own experimental packings (actual crystal structures) and also in the packings of the other compounds of the class (virtual structures). The experimental crystallographic data and the theoretical analysis suggest that two conformers, basically differing for the orientation of the phenolic H atom in the plane of the phenyl ring, compete, in solution, for the formation of polar or centrosymmetric packings. The transitions between polar and centrosymmetric polymorphs are of diffusive type, and single crystals are not preserved, while the transitions between different polar polymorphs can be of single-crystal-to-single-crystal type.

Reaction of N-rich conjugated bis(3,4-diamino-1,2,4-triazole)s with dilute hydrochloric acid affords bis(3,4-diamino-1,2,4-triazol-2-ium)chlorides. These compounds form layered structures in which planar layers of molecules are parallelly stacked. The π-stacking distance of the layers is relatively short, as compared with all-carbon-containing aromatic compounds, ranging between 3.00 and 3.22 Å at 173 K, and several contacts shorter than the sum of van der Waals radii are observed in the crystal structures. The features of the crystal structures are discussed in terms of the high nitrogen content of the compounds and of the H-bonding patterns. Periodic ab initio theoretical calculations of the crystal structures have allowed decomposing the lattice energy into various contributions in order to put up the relevance of van der Waals interactions for the π-stacking. In particular, it is found that van der Waals interactions account for about 10% of the total lattice energy and about 50% of the stacking energy (interlayer energy).