Spatially resolved analysis of short-range structure perturbations in a plastically bent molecular crystal

Halide ion recognition via chalcogen bonding in the solid state and in solution. Directionality and linearity

Benzylic selenocyanates are versatile anion receptors which operate in solution and in the solid state via chalcogen bonding interactions.

Stacking of cyclopentadienyl organometallic sandwich and half-sandwich compounds. Strong interactions of sandwiches at large offsets

73% of stacking of Cp sandwiches in the CSD is at large offsets, since these interactions are relatively strong. Much weaker stacking between Cp half-sandwiches is surprising 60% of all stacks, due to more simultaneous interactions.

Ferroelectricity and Piezoelectricity in Free-Standing Polycrystalline Films of Plastic Crystals

Plastic crystals represent a unique compound class that is often encountered in molecules with globular structures. The highly symmetric cubic crystal structure of plastic crystals endows these materials with multiaxial ferroelectricity that allows a three-dimensional realignment of the polarization axes of the crystals, which cannot be achieved using conventional molecular ferroelectric crystals with low crystal symmetry. In this work, we focused our attention on malleability as another characteristic feature of plastic crystals. We have synthesized the new plastic/ferroelectric ionic crystals tetramethylammonium tetrachloroferrate(III) and tetramethylammonium bromotrichloroferrate(III), and discovered that free-standing translucent films can be easily prepared by pressing powdered samples of these compounds. The thus obtained polycrystalline films exhibit ferroelectric polarization switching and a relatively large piezoelectric response at room temperature. The ready availability of functional films demonstrates the practical utility of such plastic/ferroelectric crystals, and considering the vast variety of possible constituent cations and anions, a wide range of applications should be expected for these unique and attractive functional materials.

Structural aspects of displacive transformations: what can optical microscopy contribute? Dehydration of Sm2(C2O4)3·10H2O as a case study

For martensitic transformations the macroscopic crystal strain is directly related to the corresponding structural rearrangement at the microscopic level. In situ optical microscopy observations of the interface migration and the change in crystal shape during a displacive single crystal to single crystal transformation can contribute significantly to understanding the mechanism of the process at the atomic scale. This is illustrated for the dehydration of samarium oxalate decahydrate in a study combining optical microscopy and single-crystal X-ray diffraction.

Atomic resolution of structural changes in elastic crystals of copper(II) acetylacetonate

Single crystals are typically brittle, inelastic materials. Such mechanical responses limit their use in practical applications, particularly in flexible electronics and optical devices. Here we describe single crystals of a well-known coordination compound-copper(II) acetylacetonate-that are flexible enough to be reversibly tied into a knot. Mechanical measurements indicate that the crystals exhibit an elasticity similar to that of soft materials such as nylon, and thus display properties normally associated with both hard and soft matter. Using microfocused synchrotron radiation, we mapped the changes in crystal structure that occur on bending, and determined the mechanism that allows this flexibility with atomic precision. We show that, under strain, the molecules in the crystal reversibly rotate, and thus reorganize to allow the mechanical compression and expansion required for elasticity and still maintain the integrity of the crystal structure.

Fluorescent organic single crystals with elastic bending flexibility: 1,4-bis(thien-2-yl)-2,3,5,6-tetrafluorobenzene derivatives

Organic single crystals with elastic bending flexibility are rare because they are generally brittle. We report here fluorescent organic single crystals based on thiophene-tetrafluorobenzene-thiophene derivatives, mainly 1,4-bis(thien-2-yl)-2,3,5,6-tetrafluorobenzene. Three derivatives were synthesized by Pd-catalyzed cross-coupling reactions (Stille or direct arylation pathways). The crystallization of the derivatives gave large (mm- or cm-scale) crystals. Two crystals of 1,4-bis(thien-2-yl)-2,3,5,6-tetrafluorobenzene, 1, and 1,4-bis(4-methylthien-2-yl)-2,3,5,6-tetrafluorobenzene, 3, bent under applied stress and quickly recovered its original shape upon relaxation. The other crystal of 1,4-bis(5-methylthien-2-yl)-2,3,5,6-tetrafluorobenzene, 2, showed brittle breakage under applied stress (normal behavior). Fibril lamella crystal structure based on criss-cross packed slip-stacked molecular wires and its structural integrity are important factors for the design and production of next generation crystal materials with elastic bending flexibility. Furthermore, mechanical bending-relaxation resulted in reversible change of the morphology and fluorescence (mechanofluorochromism). Such bendable crystals would lead to the next generation solid-state fluorescent and/or semiconducting materials.

Symmetry assisted tuning of bending and brittle multi-component forms of probenecid

The mechanical flexibility known in the antihyperuricemia drug probenecid has been extended into multi-component systems using co-formers with two donor or acceptor sites, in contrast to systems with a single H-bond acceptor that exhibit brittle behaviour. The piperazinium salt demonstrates that GRAS co-formers can be used to maintain mechanical flexibility with drug molecules.

Elasticity in Macrophage-Synthesized Biocrystals

Supramolecular crystalline assembly constitutes a rational approach to bioengineer intracellular structures. Here, biocrystals of clofazimine (CFZ) that form in vivo within macrophages were measured to have marked curvature. Isolated crystals, however, showed reduced curvature suggesting that intracellular forces bend these drug crystals. Consistent with the ability of biocrystals to elastically deform, the inherent crystal structure of the principal molecular component of the biocrystals-the hydrochloride salt of CFZ (CFZ-HCl)-has a corrugated packing along the (001) face and weak dispersive bonding in multiple directions. These characteristics were previously found to be linked to the elasticity of other organic crystals. Internal stress in bent CFZ-HCl led to photoelastic effects on the azimuthal orientation of polarized light transmittance. We propose that elastic, intracellular crystals can serve as templates to construct functional microdevices with different applications.

A new multitalented azine ligand: elastic bending, single-crystal-to-single-crystal transformation and a fluorescence turn-on Al(iii) sensor

A multifunctional azine ligand with elastic bending with full phase retention, photoinduced SCSC transformation and sensitive Al³⁺ detection.

Curved crystal morphology, photoreactivity and photosalient behaviour of mononuclear Zn(II) complexes

A dramatic effect of crystal morphology, photoreactivity and photosalient property is observed in a zinc(II) complex due to solvent effects and fluorine substitution at the backbone of the ligand. Of the two crystal forms with a 3-fluoro derivative, one yielded a curved morphology of single crystals and the second form shows photoreactivity in the solid state, whereas crystals of the 2-fluoro derivative pop during the [2 + 2] photocycloaddition reaction. This is the first report documenting curved single crystals of metal complexes obtained naturally during crystallization, although such bent crystals have been observed in extended solids naturally, or bent by mechanical force or by UV irradiation.

Crystal phase transition of urea: what governs the reaction kinetics in molecular crystal phase transitions

The predominant type-I hydrogen-bonding network in urea crystals facilitates the solid-to-solid transformation between major crystal forms.

Structural analysis of elastically bent organic crystals using in situ indentation and micro-Raman spectroscopy

The structural dynamics of two elastically bendable, halogenated N-benzylideneaniline organic crystals were studied using an in situ three-point bending test and Raman spectroscopy.

Intermolecular interactions in molecular crystals: what’s in a name?

Structure–property relationships are the key to modern crystal engineering, and for molecular crystals this requires both a thorough understanding of intermolecular interactions, and the subsequent use of this to create solids with desired properties. There has been a rapid increase in publications aimed at furthering this understanding, especially the importance of non-canonical interactions such as halogen, chalcogen, pnicogen, and tetrel bonds. Here we show how all of these interactions – and hydrogen bonds – can be readily understood through their common origin in the redistribution of electron density that results from chemical bonding. This redistribution is directly linked to the molecular electrostatic potential, to qualitative concepts such as electrostatic complementarity, and to the calculation of quantitative intermolecular interaction energies. Visualization of these energies, along with their electrostatic and dispersion components, sheds light on the architecture of molecular crystals, in turn providing a link to actual crystal properties.

Flexible ferroelectric organic crystals

Flexible organic materials possessing useful electrical properties, such as ferroelectricity, are of crucial importance in the engineering of electronic devices. Up until now, however, only ferroelectric polymers have intrinsically met this flexibility requirement, leaving small-molecule organic ferroelectrics with room for improvement. Since both flexibility and ferroelectricity are rare properties on their own, combining them in one crystalline organic material is challenging. Herein, we report that trisubstituted haloimidazoles not only display ferroelectricity and piezoelectricity-the properties that originate from their non-centrosymmetric crystal lattice-but also lend their crystalline mechanical properties to fine-tuning in a controllable manner by disrupting the weak halogen bonds between the molecules. This element of control makes it possible to deliver another unique and highly desirable property, namely crystal flexibility. Moreover, the electrical properties are maintained in the flexible crystals.

Shape-Memory and Self-Healing Effects in Mechanosalient Molecular Crystals

The thermosalient crystals of terephthalic acid are extraordinarily mechanically compliant and reversibly shape-shift between two forms with different crystal habits. While the transition of form II to form I is spontaneous, the transition of form I to form II is latent and can be triggered by applying local mechanical stress, whereby crystals leap several centimeters in air. This mechanosalient effect (mechanically stimulated motility) is due to sudden release of strain that has accrued in the crystal of form I, which is a metastable structure at ambient conditions. High-speed optical analysis and serial scanning electron microscopy reveal that the mechanical effect is due to rapid reshaping of crystal domains on a millisecond time scale triggered by mechanical stimulation. Mechanically pre-deformed crystals taken over the thermal phase transition exhibit memory effects and partially regain their shape, while cracked, sliced, or otherwise damaged crystals tend to recover their macroscopic integrity by restorative action of intermolecular π-π interactions in a manner which resembles the behavior of shape-memory and self-healing polymers. These observations provide additional evidence that the thermo-/photo-/mechanosalient effects are macroscopic manifestations of martensitic-type transitions in molecular solids.

Azo⋯phenyl stacking: a persistent self-assembly motif guides the assembly of fluorinated cis-azobenzenes into photo-mechanical needle crystals

We describe a novel, persistent motif of molecular assembly in photo-mechanical crystals and cocrystals of fluorinated cis-azobenzenes.

The role of intermolecular interactions involving halogens in the supramolecular architecture of a series of Mn(ii) coordination compounds

This study reveals the role of halogen-involving interactions in structural changes of supramolecular assemblies of manganese(ii) complexes including N-(4-halo)phenyl picolinamide ligands.

Photo-induced motion of azo dyes in organized media: from single and liquid crystals, to MOFs and machines

Incorporation of photo-reversible azo dyes into molecular assemblies provides a new family of advanced optical and photo-mechanical materials that enable the direct transformation of light energy into mechanical motion.

Bioinspired Reductionistic Peptide Engineering for Exceptional Mechanical Properties

A simple solution-processing and self-assembly approach that exploits the synergistic interactions between multiple hydrogen bonded networks and aromatic interactions was utilized to synthesize molecular crystals of cyclic dipeptides (CDPs), whose molecular weights (~0.2 kDa) are nearly three orders of magnitude smaller than that of natural structural proteins (50–300 kDa). Mechanical properties of these materials, measured using the nanoindentation technique, indicate that the stiffness and strength are comparable and sometimes better than those of natural fibres. The measured mechanical responses were rationalized by recourse to the crystallographic structural analysis and intermolecular interactions in the self-assembled single crystals. With this work we highlight the significance of developing small molecule based bioinspired design strategies to emulate biomechanical properties. A particular advantage of the successfully demonstrated reductionistic strategy of the present work is its amenability for realistic industrial scale manufacturing of designer biomaterials with desired mechanical properties.

Structure-mechanical property correlations in mechanochromic luminescent crystals of boron difluoride dibenzoylmethane derivatives

The structure and mechanical properties of crystalline materials of three boron difluoride dibenzoylmethane (BF2dbm) derivatives were investigated to examine the correlation, if any, among mechanochromic luminescence (ML) behaviour, solid-state structure, and the mechanical behaviour of single crystals. Qualitative mechanical deformation tests show that the crystals of BF2dbm( (t) Bu)2 can be bent permanently, whereas those of BF2dbm(OMe)2 exhibit an inhomogeneous shearing mode of deformation, and finally BF2dbmOMe crystals are brittle. Quantitative mechanical analysis by nano-indentation on the major facets of the crystals shows that BF2dbm( (t) Bu)2 is soft and compliant with low values of elastic modulus, E, and hardness, H, confirming its superior suceptibility for plastic deformation, which is attributed to the presence of a multitude of slip systems in the crystal structure. In contrast, both BF2dbm(OMe)2 and BF2dbmOMe are considerably stiffer and harder with comparable E and H, which are rationalized through analysis of the structural attributes such as the intermolecular interactions, slip systems and their relative orientation with respect to the indentation direction. As expected from the qualitative mechanical behaviour, prominent ML was observed in BF2dbm( (t) Bu)2, whereas BF2dbm(OMe)2 exhibits only a moderate ML and BF2dbmOMe shows no detectable ML, all examined under identical conditions. These results confirm that the extent of ML in crystalline organic solid-state fluorophore materials can be correlated positively with the extent of plasticity (low recovery). In turn, they offer opportunities to design new and improved efficient ML materials using crystal engineering principles.

Crystal chemistry and photomechanical behavior of 3,4-dimethoxycinnamic acid: correlation between maximum yield in the solid-state topochemical reaction and cooperative molecular motion

A new monoclinic polymorph, form II (P21/c, Z = 4), has been isolated for 3,4-dimethoxycinnamic acid (DMCA). Its solid-state 2 + 2 photoreaction to the corresponding α-truxillic acid is different from that of the first polymorph, the triclinic form I ([Formula: see text], Z = 4) that was reported in 1984. The crystal structures of the two forms are rather different. The two polymorphs also exhibit different photomechanical properties. Form I exhibits photosalient behavior but this effect is absent in form II. These properties can be explained on the basis of the crystal packing in the two forms. The nanoindentation technique is used to shed further insights into these structure-property relationships. A faster photoreaction in form I and a higher yield in form II are rationalized on the basis of the mechanical properties of the individual crystal forms. It is suggested that both Schmidt-type and Kaupp-type topochemistry are applicable for the solid-state trans-cinnamic acid photodimerization reaction. Form I of DMCA is more plastic and seems to react under Kaupp-type conditions with maximum molecular movements. Form II is more brittle, and its interlocked structure seems to favor Schmidt-type topochemistry with minimum molecular movement.

Unusually Large Young's Moduli of Amino Acid Molecular Crystals

Young's moduli of selected amino acid molecular crystals were studied both experimentally and computationally using nanoindentation and dispersion-corrected density functional theory. The Young modulus is found to be strongly facet-dependent, with some facets exhibiting exceptionally high values (as large as 44 GPa). The magnitude of Young's modulus is strongly correlated with the relative orientation between the underlying hydrogen-bonding network and the measured facet. Furthermore, we show computationally that the Young modulus can be as large as 70-90 GPa if facets perpendicular to the primary direction of the hydrogen-bonding network can be stabilized. This value is remarkably high for a molecular solid and suggests the design of hydrogen-bond networks as a route for rational design of ultra-stiff molecular solids.

Hardness Alternation in α,ω-Alkanedicarboxylic Acids

The variation of hardness as a function of the number of carbon atoms in α,ω-alkanedicarboxylic acids, C(N)H(2N-2)O4 (4≤N≤9), was examined by recourse to nanoindentation on the major faces of single crystals. Hardness exhibits odd-even alternation, with the odd acids being softer and the even ones harder; the differences decrease with increasing chain length. These variations are similar to those seen for other mechanical, physical, and thermal properties of these diacids. The softness of odd acids is rationalized due to strained molecular conformations in them, which facilitate easier plastic deformation. Relationships between structural features, such as interplanar spacing, interlayer separation distance, molecular chain length, and signatures of the nanoindentation responses, namely, discrete displacement bursts, were also examined. Shear sliding of molecular layers past each other during indentation is key to the mechanism for plastic deformation in these organic crystals.

Highly bent crystals formed by restrained π-stacked columns connected via alkylene linkers with variable conformations

A reproducible formation of strongly bent crystals was accomplished by structurally restraining macrocyclic π-conjugated molecules. The model π-units consist of two 9,10-bis(2-thienylethynyl)anthracenes with a strong propensity for stacking, which are connected in a macrocyclic fashion via two alkylene linkers. The correlation between the crystalline morphology and the macrocyclic structures restrained by a variety of flexible alkylene linker combinations was systematically studied. Bent crystals were obtained only with specific alkylene linkers of appropriate chain length. The alkylene linkers can adopt different conformations in the crystal packing, so as to fill voids within the macrocycle. The ability to form several similar molecular structures with different alkylene conformations gives rise to contaminations of different crystalline phases within a single crystal, and it is these phase contaminations which are responsible for the bending of the crystals.

Growth modulation of bent micro crystals to single crystals in a one-dimensional coordination framework

Di-n-butyltinbis(O-methylmethylphosphonate), 1 exhibits the formation of slender micro crystallites with unique bending features and iodine-induced modulation of crystal morphology to cubic shaped single crystals.

Anisotropy in the mechanical properties of organic crystals: temperature dependence

Nanoindentation technique was employed on organic molecular crystals to investigate how the anisotropy in the mechanical behavior changes within the temperature range of 283 to 343 K.