Dynamic molecular crystals are an emerging class of crystalline materials that can respond to mechanical stress by dissipating internal strain in a number of ways. Given the serendipitous nature of the discovery of such crystals, progress in the field requires advances in computational methods for the accurate and high-throughput computation of the nanomechanical properties of crystals on specific facets which are exposed to mechanical stress. Here, we develop and apply a new atomistic model for computing the surface elastic moduli of crystals on any set of facets of interest using dispersion-corrected density functional theory (DFT-D) methods. The model was benchmarked against a total of 24 reported nanoindentation measurements from a diverse set of molecular crystals and was found to be generally reliable. Using only the experimental crystal structure of the dietary supplement, L-Aspartic acid, the model was subsequently applied under blind test conditions, to correctly predict the growth morphology, facet and nanomechanical properties of L-Aspartic acid to within the accuracy of the measured elastic stiffness of the crystal, 24.53 ± 0.56 GPa. This work paves the way for the computational design and experimental realization of other functional molecular crystals with tailor-made mechanical properties.
In line with current research goals involving water splitting for hydrogen production, this work aims to develop a noble-metal-free electrocatalyst for a superior hydrogen evolution reaction (HER). A single-step interfacial activation of Ti3C2Tx MXene layers was employed by uniformly growing embedded WS2 two-dimensional (2D) nanopetal-like sheets through a facile solvothermal method. We exploited the interactions between WS2 nanopetals and Ti3C2Tx nanolayers to enhance HER performance. A much safer method was adopted to synthesize the base material, Ti3C2Tx MXene, by etching its MAX phase through mild in situ HF formation. Consequently, WS2 nanopetals were grown between the MXene layers and on edges in a one-step solvothermal method, resulting in a 2D-2D nanocomposite with enhanced interactions between WS2 and Ti3C2Tx MXene. The resulting 2D-2D nanocomposite was thoroughly characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman, Fourier transform infrared (FTIR), and X-ray photoelectron spectroscopy (XPS) analyses before being utilized as working electrodes for HER application. Among various loadings of WS2 into MXene, the 5% WS2-Ti3C2Tx MXene sample exhibited the best activity toward HER, with a low overpotential value of 66.0 mV at a current density of -10 mA cm-2 in a 1 M KOH electrolyte and a remarkable Tafel slope of 46.7 mV·dec-1. The intercalation of 2D WS2 nanopetals enhances active sites for hydrogen adsorption, promotes charge transfer, and helps attain an electrochemical stability of 50 h, boosting HER reduction potential. Furthermore, theoretical calculations confirmed that 2D-2D interactions between 1T/2H-WS2 and Ti3C2Tx MXene realign the active centers for HER, thereby reducing the overpotential barrier.
Profound knowledge of the molecular structure and supramolecular organization of organic molecules is essential to understand their structure-property relationships. Herein we demonstrate the packing arrangement of partially disordered nitro-perylenediimide (NO2-PDI), revealing that the perylenediimide units exhibit an X-shaped packing pattern. The packing of NO2-PDI is derived using a complementary approach that utilises solid-state NMR (ssNMR) and 3D electron diffraction (3D ED) techniques. Perylenediimide (PDI) molecules are captivating due to their high luminescence efficiency and optoelectronic properties, which are related to supramolecular self-assembly. Increasing the alkyl chain length on the imide substituent poses a more significant challenge in crystallizing the resulting molecule. In addition to the alkyl tails, other functional groups, like the nitro group attached as a bay substituent, can also cause disorder. Such heterogeneity could lead to diffuse scattering, which then complicates the interpretation of diffraction experiment data, where perfect periodicity is expected. As a result, there is an unmet need to develop a methodology for solving the structures of difficult-to-crystallize materials. A synergistic approach is utilised in this manuscript to understand the packing arrangement of the disordered material NO2-PDI by making use of 3D ED, ssNMR and density functional theory calculations (DFT). The combination of these experimental and theoretical approaches provides great promise in enabling the structural investigation of novel materials with customized properties across various applications, which are, due to the internal disorder, very difficult to study by diffraction techniques. By effectively addressing these challenges, our methodology opens up new avenues for material characterization, thereby driving exciting advancements in the field.
In the last century, molecular crystals functioned predominantly as a means for determining the molecular structures via X-ray diffraction, albeit as the century came to a close the response of molecular crystals to electric, magnetic, and light fields revealed that the physical properties of molecular crystals were as rich as the diversity of molecules themselves. In this century, the mechanical properties of molecular crystals have continued to enhance our understanding of the colligative responses of weakly bound molecules to internal frustration and applied forces. Here, the authors review the main themes of research that have developed in recent decades, prefaced by an overview of the particular considerations that distinguish molecular crystals from traditional materials such as metals and ceramics. Many molecular crystals will deform themselves as they grow under some conditions. Whether they respond to intrinsic stress or external forces or interactions among the fields of growing crystals remains an open question. Photoreactivity in single crystals has been a leading theme in organic solid-state chemistry; however, the focus of research has been traditionally on reaction stereo- and regio-specificity. However, as light-induced chemistry builds stress in crystals anisotropically, all types of motions can be actuated. The correlation between photochemistry and the responses of single crystals-jumping, twisting, fracturing, delaminating, rocking, and rolling-has become a well-defined field of research in its own right: photomechanics. The advancement of our understanding requires theoretical and high-performance computations. Computational crystallography not only supports interpretations of mechanical responses, but predicts the responses itself. This requires the engagement of classical force-field based molecular dynamics simulations, density functional theory-based approaches, and the use of machine learning to divine patterns to which algorithms can be better suited than people. The integration of mechanics with the transport of electrons and photons is considered for practical applications in flexible organic electronics and photonics. Dynamic crystals that respond rapidly and reversibly to heat and light can function as switches and actuators. Progress in identifying efficient shape-shifting crystals is also discussed. Finally, the importance of mechanical properties to milling and tableting of pharmaceuticals in an industry still dominated by active ingredients composed of small molecule crystals is reviewed. A dearth of data on the strength, hardness, Young's modulus, and fracture toughness of molecular crystals underscores the need for refinement of measurement techniques and conceptual tools. The need for benchmark data is emphasized throughout.
Although many examples of mechanically flexible crystals are currently known, their utility in all-flexible devices is not yet adequately demonstrated, despite their immense potential for fabricating high performance flexible devices. Here, we report two alkylated diketopyrrolopyrrole (DPP) semiconducting single crystals, one of which displays impressive elastic mechanical flexibility whilst the other is brittle. Using the single crystal structures and density functional theory (DFT) calculations, we show that the methylated diketopyrrolopyrrole (DPP-diMe) crystals, with dominant π-stacking interactions and large contributions from dispersive interactions, are superior in terms of their stress tolerance and field-effect mobility (µ FET) when compared to the brittle crystals of the ethylated diketopyrrolopyrrole derivative (DPP-diEt). Periodic dispersion-corrected DFT calculations revealed that upon the application of 3% uniaxial strain along the crystal growth (a)-axis, the elastically flexible DPP-diMe crystal displays a soft energy barrier of only 0.23 kJ mol-1 while the brittle DPP-diEt crystal displays a significantly larger energy barrier of 3.42 kJ mol-1, in both cases relative to the energy of the strain-free crystal. Such energy-structure-function correlations are currently lacking in the growing literature on mechanically compliant molecular crystals and have the potential to support a deeper understanding of the mechanism of mechanical bending. The field effect transistors (FETs) made of flexible substrates using elastic microcrystals of DPP-diMe retained µ FET (from 0.019 cm2 V-1 s-1 to 0.014 cm2 V-1 s-1) more efficiently even after 40 bending cycles when compared to the brittle microcrystals of DPP-diEt which showed a significant drop in µ FET just after 10 bending cycles. Our results not only provide valuable insights into the bending mechanism, but also demonstrate the untapped potential of mechanically flexible semiconducting crystals for designing all flexible durable field-effect transistor devices.
Dynamic organic crystals are rapidly gaining traction as a new class of smart materials for energy conversion, however, they are only capable of very small strokes (<12%) and most of them operate through energetically cost-prohibitive processes at high temperatures. We report on the exceptional performance of an organic actuating material with exceedingly large stroke that can reversibly convert energy into work around room temperature. When transitioning at 295-305 K on heating and at 265-275 K on cooling the ferroelectric crystals of guanidinium nitrate exert a linear stroke of 51%, the highest value observed with a reversible operation of an organic single crystal actuator. Their maximum force density is higher than electric cylinders, ceramic piezoactuators, and electrostatic actuators, and their work capacity is close to that of thermal actuators. This work demonstrates the hitherto untapped potential of ionic organic crystals for applications such as light-weight capacitors, dielectrics, ferroelectric tunnel junctions, and thermistors.
The ability to rationally design and predictably construct crystalline solids has been the hallmark of crystal engineering research. To date, numerous examples of multicomponent crystals comprising organic molecules have been reported. However, the crystal engineering of cocrystals comprising both organic and inorganic chemical units is still poorly understood and mostly unexplored. Here, we report a new diverse set of higher-order cocrystals (HOCs) based on the structurally versatile-yet largely unexplored-phosph(V/V)azane heterosynthon building block. The novel ternary and quaternary cocrystals reported are held together by synergistic and orthogonal intermolecular interactions. Notably, the HOCs can be readily obtained either via sequential or one-pot mechanochemical methods. Computational modelling methods reveal that the HOCs are thermodynamically driven to form and that their mechanical properties strongly depend on the composition and intermolecular forces in the crystal, offering untapped potential for optimizing material properties.
Organic crystals are emerging as mechanically compliant, light-weight and chemically versatile alternatives to the commonly used silica and polymer waveguides. However, the previously reported organic crystals were shown to be able to transmit visible light, whereas actual implementation in telecommunication devices requires transparency in the near-infrared spectral range. Here we demonstrate that single crystals of the amino acid L-threonine could be used as optical waveguides and filters with high mechanical and thermal robustness for transduction of signals in the telecommunications range. On their (00[Formula: see text]) face, crystals of this material have an extraordinarily high Young's modulus (40.95 ± 1.03 GPa) and hardness (1.98 ± 0.11 GPa) for an organic crystal. First-principles density functional theory calculations, used in conjunction with analysis of the energy frameworks to correlate the structure with the anisotropy in the Young's modulus, showed that the high stiffness arises as a consequence of the strong charge-assisted hydrogen bonds between the zwitterions. The crystals have low optical loss in the O, E, S and C bands of the spectrum (1250-1600 nm), while they effectively block infrared light below 1200 nm. This property favors these and possibly other related organic crystals as all-organic fiber-optic waveguides and filters for transduction of information.
Amino Acids/chemistry , Communication , Fiber Optic Technology/methods , Transducers , Anisotropy , Crystallization , Crystallography, X-Ray , Elastic Modulus , Hardness , Hydrogen Bonding , Threonine/chemistry
The discovery of molecular ionic cocrystals (ICCs) of active pharmaceutical ingredients (APIs) widens the opportunities for optimizing the physicochemical properties of APIs whilst facilitating the delivery of multiple therapeutic agents. However, ICCs are often observed serendipitously in crystallization screens and the factors dictating their crystallization are poorly understood. We demonstrate here that mechanochemical ball milling is a versatile technique for the reproducible synthesis of ternary molecular ICCs in less than 30â min of grinding with or without solvent. Computational crystal structure prediction (CSP) calculations have been performed on ternary molecular ICCs for the first time and the observed crystal structures of all the ICCs were correctly predicted. Periodic dispersion-corrected DFT calculations revealed that all the ICCs are thermodynamically stable (mean stabilization energy=-2â kJ mol-1 ) relative to the crystallization of a physical mixture of the binary salt and acid. The results suggest that a combined mechanosynthesis and CSP approach could be used to target the synthesis of higher-order molecular ICCs with functional properties.
The underlying molecular and crystal properties affecting the crystallisation of ionic cocrystals (ICCs) with the general formula A-B+N (A- = anion, B+ = cation and N = neutral acid molecule; 1 : 1 : 1 stoichiometry) are reported for a limited set of known crystal structures determined following the cocrystallisation of either 4-aminopyridine (which forms salts) or 4-dimethylaminopyridine (which forms salts and ICCs) with the same set of monoprotic acids with a single hydroxy or halogen substitution at the ortho or para position. Periodic density functional theory calculations (PBE + D2) on the energetic driving force for ICC crystallisation for a set of known crystal structures with well characterised acid, salt and ICC structures show that all but 1 of the 7 experimental ICC structures surveyed were more stable than the sum of their component salt and acid structures with 4 displaying relative stabilities (ΔEICC) ranging from 2.47-8.02 kJ mol-1. The majority of molecular ICCs that are more stable with respect to their component salt and acid structures display the formation of discrete intermolecular O-HacidOanion hydrogen bonds with the D11(2) graph set between the carboxylic acid OH donor and the carboxylate oxygen acceptor of the anion. Computed crystal form landscapes for model 1 : 1 salts derived from acid-base pairs (involving 4-dimethylaminopyridine) known to form molecular ICCs show that on average the most stable predicted polymorphs of the 1 : 1 salts have efficient packing of the ions with packing coefficients in the range 65-80% and this is comparable to the packing coefficients of the most stable predicted polymorphs of 1 : 1 salts (involving 4-aminopyridine) that have no ICCs reported. This suggests that the cocrystallisation of equimolar amounts of the 1 : 1 salt and the acid to form a 1 : 1 : 1 molecular ICC is a complicated phenomenon that cannot be explained on the basis of inefficiencies in the crystal packing of the salt ions.
Repeated attempts to crystallize 1-adamantane-methyl-amine hydro-chloride as an anhydrate failed but the salt was successfully crystallized as a solvate (2C11H20N+·2Cl-·0.5C4H8O2·H2O), with water and 1,4-dioxane playing a structural role in the crystal and engaging in hydrogen-bonding inter-actions with the cation and anion. Computational crystal-structure prediction was used to rationalize the solvent-inclusion behaviour of this salt by computing the solvent-accessible voids in the predicted low-energy structures for the anhydrate: the global lattice-energy minimum structure, which has the same packing of the ions as the solvate, has solvent-accessible voids that account for 3.71% of the total unit-cell volume and is 6â kJâ mol-1 more stable than the next most stable predicted structure.
The crystal energy landscapes of the salts of two rigid pharmaceutically active molecules reveal that the experimental structure of amantadine hydrochloride is the most stable structure with the majority of low-energy structures adopting a chain hydrogen-bond motif and packings that do not have solvent accessible voids. By contrast, memantine hydrochloride which differs in the substitution of two methyl groups on the adamantane ring has a crystal energy landscape where all structures within 10â kJâ mol(-1) of the global minimum have solvent-accessible voids ranging from 3 to 14% of the unit-cell volume including the lattice energy minimum that was calculated after removing water from the hydrated memantine hydrochloride salt structure. The success in using crystal structure prediction (CSP) to rationalize the different hydration propensities of these substituted adamantane hydrochloride salts allowed us to extend the model to predict under blind test conditions the experimental crystal structures of the previously uncharacterized 1-(methylamino)adamantane base and its corresponding hydrochloride salt. Although the crystal structure of 1-(methylamino)adamantane was correctly predicted as the second ranked structure on the static lattice energy landscape, the crystallization of a Z' = 3 structure of 1-(methylamino)adamantane hydrochloride reveals the limits of applying CSP when the contents of the crystallographic asymmetric unit are unknown.
Amantadine/analogs & derivatives , Amantadine/chemistry , Crystallization , Crystallography, X-Ray , Hydrogen Bonding , Models, Molecular , Salts/chemistry , Thermodynamics , Water/chemistry
