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Crystalline spring: Single crystals formed from caffeine, 4‐chloro‐3‐nitrobenzoic acid, and methanol (1:1:<1) have an interlocked structure with weak interactions in the three perpendicular directions, as well as solvent channels, and display reversible elastic bending. Excellent conservation of long‐range order even after many bending cycles is observed, thus demonstrating the opportunities for flexible organic materials.

The intermolecular interactions and structural features in crystals of seven halogenated N-benzylideneanilines (Schiff bases), all of which exhibit remarkable flexibility, were examined to identify the common packing features that are the raison d'être for the observed elasticity. The following two features, in part related, were identified as essential to obtain elastic organic crystals: 1) A multitude of weak and dispersive interactions, including halogen bonds, which may act as structural buffers for deformation through easy rupture and reformation during bending; and 2) corrugated packing patterns that would get interlocked and, in the process, prevent long-range sliding of molecular planes.

The exceptional mechanical flexibility observed with certain organic crystals defies the common perception of single crystals as brittle objects. Here, we describe the morphostructural consequences of plastic deformation in crystals of hexachlorobenzene that can be bent mechanically at multiple locations to 360° with retention of macroscopic integrity. This extraordinary plasticity proceeds by segregation of the bent section into flexible layers that slide on top of each other, thereby generating domains with slightly different lattice orientations. Microscopic, spectroscopic and diffraction analyses of the bent crystal showed that the preservation of crystal integrity when stress is applied on the (001) face requires sliding of layers by breaking and re-formation of halogen-halogen interactions. Application of stress on the (100) face, in the direction where π···π interactions dominate the packing, leads to immediate crystal disintegration. Within a broader perspective, this study highlights the yet unrecognized extraordinary malleability of molecular crystals with strongly anisotropic supramolecular interactions.

Image reproduced with permission from Abbie TrewinOther articles published in this issue include:Dipyrrin based homo-and hetero-metallic infinite architectures Stéphane A. Baudron, CrystEngComm, 2010,We present an overview of very recent advances in the understanding of structure-mechanical property correlations in molecular crystals. After the introductory part on some classical twodimensional structures from the literature, we survey recent reports (mostly since 2005) pertinent to the mechanical properties of molecular crystals studied by application of external stress using a range of techniques. This includes both qualitative (shearing, bending and brittle crystals) and quantitative (nanoindentation, powder compaction and high-pressure) studies on establishing the correlation of anisotropic mechanical behaviour with the underlying crystal structure. Section 9, emphasizes on the usefulness of crystal engineering approach to improve the mechanical properties of molecular crystals, particularly the active pharmaceutical ingredients for their better tabletability properties. The parallels of the phenomena in other class of well studied materials are also appropriately drawn and discussed in the context of structure-mechanical property relationship. In the final part we comment on the prospects and ramifications of this emerging field.

An elastic organic crystal, 2,6-dichlorobenzylidine-4-fluoro-3-nitroaniline (DFNA), which also shows thermosalient behavior, is studied. The presence of these two distinct properties in the same crystal is unusual and unprecedented because they follow respectively from isotropy and anisotropy in the crystal packing. Therefore, while both properties lead from the crystal structure, the mechanisms for bending and thermosalience are quite independent of one another. Crystals of the low-temperature (α) form of the title compound are bent easily without any signs of fracture with the application of deforming stress, and this bending is within the elastic limit. The crystal structure of the α-form was determined (P21/c, Z = 4, a = 3.927(7) Å, b = 21.98(4) Å, c = 15.32(3) Å). There is an irreversible phase transition at 138 °C of this form to the high-temperature β-form followed by melting at 140 °C. Variable-temperature X-ray powder diffraction was used to investigate the structural changes across the phase transition and, along with an FTIR study, establishes the structure of the β-form. A possible rationale for strain build-up is given. Thermosalient behavior arises from anisotropic changes in the three unit cell parameters across the phase transition, notably an increase in the b axis parameter from 21.98 to 22.30 Å. A rationale is provided for the existence of both elasticity and thermosalience in the same crystal. FTIR studies across the phase transition reveal important mechanistic insights: (i) increased π···π repulsions along [100] lead to expansion along the a axis; (ii) change in alignment of C-Cl and NO2 groups result from density changes; and (iii) competition between short-range repulsive (π···π) interactions and long-range attractive dipolar interactions (C-Cl and NO2) could lie at the origin of the existence of two distinctive properties.

Ten new co-crystals of an antibacterial drug sulfamethazine (SFZ) with various carboxylic acid and amide co-formers have been synthesized. These new forms are characterized by single crystal X-ray diffraction, infrared spectroscopy, differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). Crystal structures with 4-hydroxybenzoic acid (HBA), 2,4-dihydroxybenzoic acid (DHB), 3,4-dichlorobenzoic acid (DCB), sorbic acid (SOR), fumaric acid (FUM), 1-hydroxy-2-naphthoic acid (1HNA), benzamide (BEN), picolinamide (PIC), 4-hydroxybenzamide (HBEN), and 3-hydroxy-2-naphthoic acid (3HNA) are determined. The SFZ molecule displays co-former assisted amidine to imidine tautomerism in the co-crystals in that the sulfonamide NH proton moves to one of the pyrimidine N atoms. In all the cases, the SFZ forms a robust hydrogen bonded synthon with a carboxylic acid (amidine(SFZ)···acid/imidine(SFZ)···acid) or amide (imidine(SFZ)···amide) group from the co-former. The SFZ molecule, in all the carboxylic amide and carboxylic acids, HBA and 3HNA co-crystals, exists in the imidine tautomeric form while it exists in amidine tautomeric form in the rest of the acid co-crystals. Density functional theory (DFT) calculations revealed that the amidine tautomer in free SFZ is much more stable than its imidine tautomeric form, while when it is hydrogen bonded to the co-formers via acid or amide groups, the difference is greatly minimized. But the synthon formation between the stable amidine(SFZ) and amide co-former is sterically hindered; hence the SFZ tautomerizes itself to the imidine(SFZ) form to facilitate the formation of a robust imidine(SFZ)···amide synthon in all the amide based co-crystals in this study. Solubility properties of some of the new co-crystal forms are also studied. The crystal structures are analyzed in the context of hydrogen bond competition between various acceptors and donors, in the presence of other competing functional groups, in the active pharmaceutical ingredient (API) co-crystals.

Flexible organic single crystals are evolving as new materials for optical waveguides that can be used for transfer of information in organic optoelectronic microcircuits.I ntegration in microelectronics of such crystalline waveguides requires downsizing and precise spatial control over their shape and sizea tt he microscale,h owever that currently is not possible due to difficulties with manipulation of these small, brittle objects that are prone to cracking and disintegration. Here we demonstrate that atomic force microscopy(AFM) can be used to reshape,r esizea nd relocate single-crystal microwaveguides in order to attain spatial control over their light output. Using an AFM cantilever tip,m echanically compliant acicular microcrystals of three N-benzylideneanilines were bent to an arbitrary angle,s liced out from ab undle into individual crystals,cut into shorter crystals of arbitrary length, and moved across and aboveasolid surface.W hen excited by using laser light, such bent microcrystals act as active optical microwaveguides that transduce their fluorescence,w ith the total intensity of transduced light being dependent on the optical path length. This micromanipulation of the crystal waveguides using AFM is non-invasive,a nd after bending their emissive spectral output remains unaltered. The approach reported here effectively overcomes the difficulties that are commonly encountered with reshaping and positioning of small delicate objects (the "thick fingers" problem), and can be applied to mechanically reconfigure organic optical waveguides in order to attain spatial control over their output in two and three dimensions in optical microcircuits.