Conspectus Mechanical properties of organic molecular crystals have been noted and studied over the years but the complexity of the subject and its relationship with diverse fields such as ...mechanochemistry, phase transformations, polymorphism, and chemical, mechanical, and materials engineering have slowed understanding. Any such understanding also needs conceptual advancessophisticated instrumentation, computational modeling, and chemical insightlack of such synergy has surely hindered progress in this important field. This Account describes our efforts at focusing down into this interesting subject from the viewpoint of crystal engineering, which is the synthesis and design of functional molecular solids. Mechanical properties of soft molecular crystals imply molecular movement within the solid; the type of property depends on the likelihood of such movement in relation to the applied stress, including the ability of molecules to restore themselves to their original positions when the stress is removed. Therefore, one is interested in properties such as elasticity, plasticity, and brittleness, which are linked to structural anisotropy and the degree to which a structure veers toward isotropic character. However, these matters are still by no means settled and are system dependent. While elasticity and brittleness are probably displayed by all molecular solids, the window of plasticity is perhaps the one that is most amenable to crystal engineering strategies and methods. In all this, one needs to note that mechanical properties have a kinetic component: a crystal that is elastic under slow stress application may become plastic or brittle if the same stress is applied quickly. In this context, nanoindentation studies have shown themselves to be of invaluable importance in understanding structural anisotropy. Several problems in solid state chemistry, including classical ones, such as the melting point alternation in aliphatic straight chain dicarboxylic acids and hardness modulation in solid solutions, have been understood more clearly with this technique. The way may even be open to picoindentation studies and the observation of molecular level movements. As in all types of crystal engineering, an understanding of the intermolecular interactions can lead to property oriented crystal design, and we present examples where complex properties may be deliberately turned on or off in organic crystals: one essentially fine-tunes the degree of isotropy/anisotropy by modulating interactions such as hydrogen bonding, halogen bonding, π···π interactions, and C–H···π interactions. The field is now wide open as is attested by the activities of several research groups working in the area. It is set to take off into the domains of smart materials, soft crystals, and superelasticity and a full understanding of solid state reactivity.
We report here the first elastically bendable single-component pharmaceutical crystal, celecoxib. Interlocked molecular packing without the slip plane and the presence of an isotropic hydrogen bond ...network are major structural features responsible for both the exceptional elastic flexibility and high stiffness of the celecoxib crystal as revealed by bending and nanomechanical studies. The molecular model of the exceptional elasticity is rationalized by the inhomogeneous spatial separations of molecules in the bent crystal, which is further confirmed by micro-Raman spectroscopy. The celecoxib crystal, exhibiting both therapeutic effects and elastic mechanical behavior, could be used to manufacture functional microdevices with novel medical applications.
We report here the first pharmaceutical twistable hydrogen-bonded two-dimensional plastic hydrate crystal of a well-known psychoactive drug, caffeine (CAH). The availability of two pairs of ...plastically bendable faces in orthogonal directions allows for twisting of the CAH crystals to obtain the helical morphology. We further demonstrate the first pharmaceutical application of such twistable crystals, i.e., exceptional tabletability. The plasticity of CAH is comparable to a commonly used highly plastic tablet diluent, microcrystalline cellulose. This example suggests that the strategies for designing twistable crystals, among other potential applications, can be used to effectively overcome compression problems of certain drugs and enable the development of high-quality drug tablets.
Two conformational polymorphs of 5-methyl-2-(2-nitrophenyl)amino-3-thiophenecarbonitrile (ROY) exhibited similar morphology but strikingly different mechanical flexibility, where the yellow needle ...(YN) is exceptionally elastic while orange needle (ON) is highly brittle. Similar to color differentiation, the distinct bending behaviors of YN and ON polymorphs are linked to different molecular conformations. Aided by crystal structural analysis, interaction energies, topology, nanoindentation, and elastic constant calculation, this study reveals new structural insights that explain the strikingly different mechanical behaviors of the two polymorphs.
A simple strategy to fabricate carbon dots (CDs) incorporated organically modified silica (ORMOSIL) films exhibiting tunable tricolor emission has been accomplished. First, the green-emitting CDs ...with excitation-independent nature and high quantum yield were prepared from o-phenylenediamine in ethanol by solvothermal method. These CDs after purification were dispersed in ORMOSIL sol, and their photoluminescence wavelengths were tuned to three intense luminescent colors (orange, yellow, and green) by adjusting the pH of the sol. It was observed that with pH tuning the functional groups residing on surface-passivated CDs are undergoing chemical modifications, and accordingly the PL emission of CDs in ORMOSIL sol systematically changes to orange, yellow, and green emissions, respectively. Interestingly, the structure of such surface-modified CDs can be well-preserved in the ORMOSIL film-matrix with substantial concentration to obtain the above tricolor luminescent films on glass. A systematic X-ray photoelectron spectroscopy study revealed that the blue shifting in triluminous films (from 560 to 510 nm) with pH increment was originated due to the deprotonation of the surface groups (−CONH– → −CO(N–)–; −NH3 + → −NH2) associated with the CDs. The plausible mechanism behind the rationalization of pH-triggered engineering of surface-passivated CD-ORMOSIL sols and their confinement in films has been explored.
Mechanical property design of molecular solids Mishra, Manish Kumar; Ramamurty, Upadrasta; Desiraju, Gautam R.
Current opinion in solid state & materials science,
December 2016, 2016-12-00, Volume:
20, Issue:
6
Journal Article
Peer reviewed
Display omitted
•Crystal engineering principles applied to design and modulation of mechanical properties of molecular solids.•Elastic crystals obtained by designing of isotropic weak interactions ...and interlocked crystal packing.•Solid-solution strengthening effectively utilized to engineer the hardness of organic solids.•Cocrystallization approach used to alter the mechanical properties of active pharmaceutical ingredients.•Nanoindentation technique establishes structure-mechanical properties correlations in molecular solids.
The current emphasis of crystal engineering, which has evolved over the past three decades through crystal packing analysis and identification of crystal design strategies, has shifted from structure to properties, i.e., design of molecular solids with targeted combination of properties. Amongst the panoply of chemical, physical, and biological properties that these materials exhibit, a comprehensive understanding of the mechanical properties is perhaps the most challenging as it involves connecting molecular level structural features to macroscopic mechanical behavior. However, the adoption of the nanoindentation technique, with which it is possible to measure—both quantitatively and accurately—the mechanical response of even small single crystals, in crystal engineering, has paved the way for substantial progress in the recent past. In this review, we summarize some recent results with an emphasis as to how one can design and control properties of molecular solids such as elastic modulus and hardness. This review closes with an enumeration of the key challenges that lie ahead. Such studies show a big scope for studying mechanical properties of organic crystals as a function of crystal structure, and in turn to understand their structure-property relationship for designing future smart materials. This emerging research field has prospects and a potential to play an important role in the future development of crystal engineering.
► One step process for synthesizing pure and mixed phase TiO2 nanoparticles. ► The crystallite size of prepared TiO2 at different temperature is in the range of 19–68nm. ► The tensile strain has been ...observed below 600°C and converted into compressive strain at 700°C. ► Photoluminescence spectroscopy (PL) exhibits the change in PL intensity with phase change. ► Different trends have been observed in emission edges.
Pure and mixed phase TiO2 have been prepared by sol–gel method; calcinated at four different temperatures. The influence of calcination temperature on crystallite size, morphology, band gap and luminescence properties of resultant material have been investigated. Different trends were observed in the phase transformation, particle growth, shift in energy band gap and in luminescence with the change in tensile strain to compressive strain of the prepared TiO2 nanomaterial. X-ray diffraction (XRD) showed that prepared nanocrystals have pure anatase and anatase-rutile mixed structures. The prepared samples having crystallite size between 19nm to 68nm were observed at different calcination temperatures. Williamson-Hall plot results indicate the presence of tensile strain at 400, 500 and 600°C while compressive strain at 700°C. Scanning electron microscopy (SEM) shows that the particles are non-uniform. Ultraviolet-Visible spectroscopy (UV-Vis) is used to calculate the energy band gap of materials and it has been observed that the band gap decreases with increase in temperature. Fourier transform infrared spectroscopy (FTIR) describes local environment around TiO2 nanoparticles. Photoluminescence spectroscopy (PL) exhibits the change in PL intensity with phase change and different trends have been observed in emission edges.
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.
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.
Slinky springs: Elastic organic crystals may be designed by selecting molecules that are likely to adopt isotropic packing with weak and only moderately polar interactions. The restorative ability of halogen bonds upon application of stress is particularly relevant in this context and is used in this family of Schiff bases.
Cumin is an annual, aromatic, herbaceous, medicinal, spice plant, most widely used as a food additive and flavoring agent in different cuisines. The study is intended to comprehensively analyse ...physiological parameters, biochemical composition and metabolites under salinity stress. Seed germination index, rate of seed emergence, rate of seed germination, mean germination time, plant biomass, total chlorophyll and carotenoid contents decreased concomitantly with salinity. In contrast, total antioxidant activity, H2O2, proline and MDA contents increased concurrently with stress treatments. Total phenolic and flavonoid contents were decreased initially about 1.4-fold at 50 mM, and thereafter increased about 1.2-fold at 100 mM NaCl stress. Relative water content remained unchanged up to 50 mM NaCl stress, and thereafter decreased significantly. About 2.8-fold electrolyte leakage was found in 50 mM, which increases further 4-fold at 100 mM NaCl stress. Saturated fatty acids (FAs) increased gradually with salinity, whereas unsaturation index and degree of unsaturation change arbitrarily along with the percent quantity of unsaturated FAs. Total lipid and fatty acid composition were significantly influenced by salinity stress. A total of 45 differentially expressed metabolites were identified, including luteolin, salvianolic acid, kaempferol and quercetin, which are phenolic, flavonoid or alkaloids in nature and contain antioxidant activities. Additionally, metabolites with bioactivity such as anticancerous (docetaxel) and antimicrobial (megalomicin) properties were also identified. The study evidenced that plant shoots are a rich source of metabolites, essential amino acids, phenolic compounds and fatty acids, which unveil the medicinal potential of this plant, and also provide useful insight about metabolic responses under salinity stress.
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