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.
Flexible organic single crystals are evolving as new materials for optical waveguides that can be used for transfer of information in organic optoelectronic microcircuits. Integration in ...microelectronics of such crystalline waveguides requires downsizing and precise spatial control over their shape and size at the microscale, however 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, resize and relocate single‐crystal microwaveguides in order to attain spatial control over their light output. Using an AFM cantilever tip, mechanically compliant acicular microcrystals of three N‐benzylideneanilines were bent to an arbitrary angle, sliced out from a bundle into individual crystals, cut into shorter crystals of arbitrary length, and moved across and above a solid surface. When excited by using laser light, such bent microcrystals act as active optical microwaveguides that transduce their fluorescence, with the total intensity of transduced light being dependent on the optical path length. This micromanipulation of the crystal waveguides using AFM is non‐invasive, and 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.
Atomic force microscopy was used to reshape, resize and relocate single‐crystal microwaveguides in order to attain spatial control over their light output. Using an AFM cantilever tip, acicular microcrystals of three N‐benzylideneanilines were bent to an arbitrary angle, sliced out from a bundle into individual crystals, cut into shorter crystals of arbitrary length, and moved across and above a solid surface.
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.
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.
Flexible organic crystals (elastic and plastic) are important materials for optical waveguides, tunable optoelectronic devices, and photonic integrated circuits. Here, we present highly elastic ...organic crystals of a Schiff base, 1‐((E)‐(2,5‐dichlorophenylimino)methyl)naphthalen‐2‐ol (1), and an azine molecule, 2,4‐dibromo‐6‐((E)‐((E)‐(2,6‐dichlorobenzylidene)hydrazono)methyl)phenol (2). These microcrystals are highly flexible under external mechanical force, both in the macroscopic and the microscopic regimes. The mechanical flexibility of these crystals arises as a result of weak and dispersive C−H⋅⋅⋅Cl, Cl⋅⋅⋅Cl, Br⋅⋅⋅Br, and π⋅⋅⋅π stacking interactions. Singly and doubly‐bent geometries were achieved from their straight shape by a micromechanical approach using the AFM cantilever tip. Crystals of molecules 1 and 2 display a bright‐green and red fluorescence (FL), respectively, and selective reabsorption of a part of their FL band. Crystals 1 and 2 exhibit optical‐path‐dependent low loss emissions at the termini of crystal in their straight and even in extremely bent geometries. Interestingly, the excitation position‐dependent optical modes appear in both linear and bent waveguides of crystals 1 and 2, confirming their light‐trapping ability.
Bent waveguides, especially, singly and doubly bent waveguides are essential for the advancement of photonic technologies. Here, two chemically and optically different flexible organic crystals were fabricated into singly and doubly‐bent shapes from straight geometry using micromechanical manipulation technique to demonstrate optical waveguiding behavior which is required for photonic circuit applications. (Atomic force microscopy cantilever tip abbreviated as AFM cantilever)
Although weak interactions, such as C–H···O and π-stacking, are generally considered to be insignificant, it is their reorganization that holds the key for many a solid-state phenomenon, such as ...phase transitions, plastic deformation, elastic flexibility, and mechanochromic luminescence in solid-state fluorophores. Despite this, the role of weak interactions in these dynamic phenomena is poorly understood. In this study, we investigate two co-crystal polymorphs of caffeine:4-chloro-3-nitrobenzoic acid, which have close structural similarity (2D layered structures), but surprisingly show distinct mechanical behavior. Form I is brittle, but shows shear-induced phase instability and, upon grinding, converts to Form II, which is soft and plastically shearable. This observation is in contrast to those reported in earlier studies on aspirin, wherein the metastable drug forms are softer and convert to stable and harder forms upon stressing. To establish a molecular-level understanding, we have investigated the two co-crystal polymorphs I and II by single-crystal X-ray diffraction, nanoindentation to quantify mechanical properties, and theoretical calculations. The lower hardness (from nanoindentation) and smooth potential surfaces (from theoretical studies) for shearing of layers in Form II allowed us to rationalize the role of stronger intralayer (sp2)C–H···O and nonspecific interlayer π-stacking interactions in the structure of II. Although the Form I also possesses the same type of interactions, its strength is clearly opposite, that is, weaker intralayer (sp3)C–H···O and specific interlayer π-stacking interactions. Hence, Form I is harder than Form II. Theoretical calculations and indentation on (111) of Form I suggested the low resistance of this face to mechanical stress; thus, Form I converts to II upon mechanical action. Hence, our approach demonstrates the usefulness of multiple techniques for establishing the role of weak noncovalent interactions in solid-state dynamic phenomena, such as stress-induced phase transformation, and hence is important in the context of solid-state pharmaceutical chemistry and crystal engineering.
Chikungunya virus (CHIKV) epidemics around the world have created public health concern with the unavailability of effective drugs and vaccines. This emphasizes the need for molecular understanding ...of host-virus interactions for developing effective targeted antivirals. Microarray analysis was carried out using CHIKV strain (Prototype and Indian) infected Vero cells and two host isozymes, MAPK activated protein kinase 2 (MK2) and MAPK activated protein kinase 3 (MK3) were selected for further analysis. The substrate spectrum of both enzymes is indistinguishable and covers proteins involved in cytokines production, endocytosis, reorganization of the cytoskeleton, cell migration, cell cycle control, chromatin remodeling and transcriptional regulation. Gene silencing and drug treatment were performed in vitro and in vivo to unravel the role of MK2/MK3 in CHIKV infection. Gene silencing of MK2 and MK3 abrogated around 58% CHIKV progeny release from the host cell and a MK2 activation inhibitor (CMPD1) treatment demonstrated 68% inhibition of viral infection suggesting a major role of MAPKAPKs during late CHIKV infection in vitro. Further, it was observed that the inhibition in viral infection is primarily due to the abrogation of lamellipodium formation through modulation of factors involved in the actin cytoskeleton remodeling pathway. Moreover, CHIKV-infected C57BL/6 mice demonstrated reduction in the viral copy number, lessened disease score and better survivability after CMPD1 treatment. In addition, reduction in expression of key pro-inflammatory mediators such as CXCL13, RAGE, FGF, MMP9 and increase in HGF (a CHIKV infection recovery marker) was observed indicating the effectiveness of the drug against CHIKV. Taken together it can be proposed that MK2 and MK3 are crucial host factors for CHIKV infection and can be considered as important target for developing effective anti-CHIKV strategies.
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