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Non-stoichiometric molecular mixed crystals have potential as functional materials, the properties of which can be tailored by rationally changing their composition. The guidelines for their preparation were summarized over thirty years ago by Alexander Kitaigorodsky. Here those principles are revised in light of new studies on the acridine/phenazine system, and solvent-assisted grinding is presented as a convenient synthetic procedure to afford a more homogeneous product than traditional solvent-evaporation methods. Finally, the proposed prerequisite of crystal isostructurality/isomorphicity for the pure compounds, which seems to be violated in the present case, is discussed.

A new polymorph of succinic acid is an unusually challenging test for metastable polymorph prediction.

The comparison of new structural data with crystal structures stored in the Cambridge Structural Database (CSD) is becoming ordinary when dealing with recognition and identification of new solid forms. Indeed,...

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Five novel co-crystals of thiophanate-ethyl (TE), an agrochemical active, with di(2-pyridyl)ketone (1), 2-benzoylpyridine (2), 3-benzoylpyridine (3), 4-phenylpyridine (4) and biphenyl (5) were found and crystal structures of four of them (TE1-TE3, TE5) solved by single crystal X-ray diffraction. Three of the cocrystals (TE1-TE3) form by way of a reliable pyridine-amine hydrogen bond synthon and one (TE5) because of close packing effects. The fifth co-crystal was identified by X-ray powder diffraction. The work demonstrates the usage of a reliable supramolecular synthon for crystal engineering, while concurrently reminds that the close packing of even very similar molecules cannot be fully predicted.

Thermosalient crystals are solids that exhibit motion at macroscale as a consequence of a thermally induced phase transition. They represent an interesting scientific phenomenon and could be useful as actuators for the conversion of thermal energy into motion or mechanical work. The potential utilization of these miniature transducers in real-world devices requires controllable phase transition (i.e. predetermined temperature). While it is difficult to control these thermomechanical performances with a singlecomponent molecular crystal, "tunable" properties could be accomplished by using solid solutions. To verify this hypothesis, the thermosalient materials [Zn(bpy)Br2] (bpy = 2,2′-bipyridine) was selected and its synthesis was performed in the presence of chloride ions. The resulting mixed crystals ([Zn(bpy)Br2(1-x)Cl2x]) show that the product undergoes the expected thermosalient phase transition, and temperature of the onset of the phase transition and the transition enthalpy depend on the Cl/Br-ratio.One of the goals of the materials science is to establish methods for the design and fabrication of functional materials, the performance of which can be easily optimized for the intended application(s). In molecular materials, the physicochemical properties can be controlled, at least in principle, at various levels of structural hierarchy. In some cases (e.g., pharmaceuticals and dyes) this optimization can be accomplished by adjusting the formulation of the final product (excipients, particle size and shape, etc.).[1] When this approach is not viable, a more demanding chemical modification of the constituents of the material is required. Alternatively, some material properties can be modified by changing the way molecules are ordered and interact in the solid state. To alter those properties, which are intrinsic to the solid structure, a change in molecular packing is required.[2] In single-component and stoichiometric multicomponent crystals (cocrystals, hydrates, etc.), this crystal engineering [3] (or supramolecular) approach has proved successful in many instances, [4] although generally it remains impossible to predict the properties of the modified material. On the contrary, in non-stoichiometric multicomponent crystals (solid solutions) the structure and properties vary regularly and can be "tuned" by controlling the composition.[5] Indeed, from the crystal engineering point of view, solid solutions of molecular systems have enabled, among other things, modulation of crystal structure and phase transition temperatures [6] (including melting point), [7] as well as optical, [8] electronic, [9] mechanical [10] and physisorption [11] properties.Here, we set as our goal to attain control over the mechanical response from thermosalient crystals, [12] an emerging class of exotic materials that exhibit swift motions at a macroscale as a result of very fast structural phase transitions. These materials -and their photochemical counterparts (photosalient crystals) [13] -are capable of conve...

A series of small, unsymmetrical pyridine-2,6-dicarboxylamide oligoamide foldamers with varying lengths and substituents at the end groups were synthetized to study their conformational properties and folding patterns. The @-type folding pattern resembled the oxyanion-hole motifs of enzymes, but several alternative folding patterns could also be characterized. Computational studies revealed several alternative conformers of nearly equal stability. These folding patterns differed from each other in their intramolecular hydrogen-bonding patterns and aryl-aryl interactions. In the solid state, the foldamers adopted either the globular @-type fold or the more extended S-type conformers, which were very similar to those foldamers obtained computationally. In some cases, the same foldamer molecule could even crystallize into two different folding patterns, thus confirming that the different folding patterns are very close in energy in spite of their completely different shapes. Finally, the best match for the observed NOE interactions in the liquid state was a conformation that matched the computationally characterized helix-type fold.