Crystal engineering: from molecule to crystal

RP-HPLC METHOD DEVELOPMENT, VALIDATION, AND ITS PHARMACOKINETIC APPLICABILITY IN PRECLINICAL EVALUATION OF RHEIN TREATED WITH NOVEL DIACEREIN EUTECTICS

The current study represents the bio-analytical method for the estimation of Rhein (Rh, an active metabolite of Diacerein (DIA)) in rats treated with novel DIA eutectics to investigate the pharmacokinetics of DIA. A simple protein precipitation technique was used to extract Rh and internal standard (IS), p-aminobenzoic acid, and injected into a Phenomenex Gemini C18 column. The separation was achieved by gradient elution comprising of ammonium acetate (10 mM; pH 3.0) and acetonitrile in 18 min of run time at a flow rate of 0.8 mL/min with the retention time of 11.8 min (Rh) and 5.9 min (IS). The results revealed that the proposed method was linear over the range of 200-20,000 ng/mL (r² > 0.9988) of Rh and proved to be precise and accurate. The method was fully validated as per the USFDA guideline and the pharmacokinetic study in rats was performed for Rh following oral administration of the pure DIA and newly developed eutectics. Therefore, the present method could be used to estimate DIA to illustrate the comparative pharmacokinetic analysis. This can be also applied to its related multi-component formulations for future study.

Internal Electric Field-Induced Formation of Exotic Linear Acetonitrile Chains

The application of external electric and magnetic fields is a powerful tool for aligning molecules in a controlled way, if the thermal fluctuations are small. Here we demonstrate that the same holds for internal electric fields in a molecular cluster. The electric field of a single molecular dipole, HCl, is used to manipulate the aggregation mechanism of subsequently added acetonitrile molecules. As a result, we could form exotic linear acetonitrile (CH₃CN) chains at 0.37 K, as confirmed by infrared spectroscopy in superfluid helium nanodroplets. These linear chains are not observed in the absence of HCl and can be observed only when the internal electric field created by an HCl molecule is present. The accompanying simulations provide mechanistic insights into steric control, explain the selectivity of the process, and show that non-additive electronic polarization effects systematically enhance the dipole moment of these linear chains. Thus, adding more CH₃CN monomers even supports further quasi-linear chain growth.

Synthesis of a Pyridone-Based Phthalimide Fleximer and Its Characterization and Supramolecular Property Evaluation

In this study, a novel pyridone-based phthalimide fleximer, that is, ethyl 5-cyano-6-(3-(1,3-dioxoisoindolin-2-yl)propoxy)-4-(3-methoxyphenyl)-2-methylnicotinate, was synthesized, and its structure was established by the single-crystal X-ray diffraction method. The supramolecular self-assembly of the titled compound through noncovalent interactions was then investigated thoroughly. The titled compound crystallized with two symmetry-independent molecules (A and B, Z' = 2). In agreement with experimental observations, our density functional theory calculations also showed that the titled compound has a flexible motif and can occur in various conformations, including molecules A and B. The investigation of the supramolecular framework revealed that the molecules are notably bound by the nonclassical C-H···O and C-H···N hydrogen bonds and C-H···π interactions. Hirshfeld surface analysis was carried out to quantify the various intermolecular interactions. The dual anti-inflammatory activity of the tilted compound was also explored by molecular docking in the active sites of 5-LOX and COX-2 receptors, which revealed good binding affinities of -9.0 and -8.6 kcal/mol, respectively.

Structural and Biofunctional Insights into the Cyclo(Pro-Pro-Phe-Phe-) Scaffold from Experimental and In Silico Studies: Melanoma and Beyond

Short peptides have great potential as safe and effective anticancer drug leads. Herein, the influence of short cyclic peptides containing the Pro-Pro-Phe-Phe sequence on patient-derived melanoma cells was investigated. Cyclic peptides such as cyclo(Leu-Ile-Ile-Leu-Val-Pro-Pro-Phe-Phe-), called CLA, and cyclo(Pro-homoPro-β3homoPhe-Phe-), called P11, exert the cytotoxic and the cytostatic effects in melanoma cells, respectively. CLA was the most active peptide as it reduced the viability of melanoma cells to 50% of control at about 10 µM, whereas P11 at about 40 µM after 48 h incubation. Interestingly, a linear derivative of P11 did not induce any effect in melanoma cells confirming previous studies showing that cyclic peptides exert better biological activity compared to their linear counterparts. According to in silico predictions, cyclic tetrapeptides show a better pharmacokinetic and toxic profile to humans than CLA. Notably, the spatial structure of those peptides containing synthetic amino acids has not been explored yet. In the Cambridge Structural Database, there is only one such cyclic tetrapeptide, cyclo((R)-β2homoPhe-D-Pro-Lys-Phe-), while in the Protein Data Bank—none. Therefore, we report the first crystal structure of cyclo(Pro-Pro-β3homoPhe-Phe-), denoted as 4B8M, a close analog of P11, which is crucial for drug discovery. Comparative molecular and supramolecular analysis of both structures was performed. The DFT findings revealed that 4B8M is well interpreted in the water solution. The results of complex Hirshfeld surface investigations on the cooperativity of interatomic contacts in terms of electrostatic and energetic features are provided. In short, the enrichment ratio revealed O…H/H…O and C…H/H…C as privileged intercontacts in the crystals in relation to basic and large supramolecular H-bonding synthon patterns. Furthermore, the ability of self-assemble 4B8M leading to a nanotubular structure is also discussed.

Spectroscopic Tracking of Salicylideneaniline Photocolored Crystals: An Attempt to Quantify Polymorph-Dependent Features toward Precise Structure-Function Correlation Analysis

Three polymorphs of salicylideneaniline (SA) were prepared, and their photochromic behavior was examined using a recently developed single-crystal microscopic UV-vis spectroscopy approach. This system enabled us to acquire absorption data during the bleaching process as a function of temperature and visible light intensity. First, we demonstrated that, in contrast to the generally accepted assumption, the bleaching curves were notably influenced by the degree of photosaturation at the initial stage. By modifying our kinetic model to include the term representing the initial degree of photosaturation, we successfully obtained the kinetic parameters intrinsic to each crystal structure. Second, we further analyzed the kinetic parameters to show that the bleaching process was accelerated by visible light irradiation to a significantly higher degree than by thermal relaxation. The two bleaching-prompting effects were quantitatively compared between two photochromic polymorphs, α₁ and α₂; the long life of the photoproduct in α₂ was attributed to efficient self-shielding from visible light irradiation enabled by its structural features. These results prompted us to reexamine the simple dualistic photochemical and thermal classification of photochromic systems and will provide a foundation for the precise structure-function analysis of crystalline materials, including SAs.

The Relevance of Experimental Charge Density Analysis in Unraveling Noncovalent Interactions in Molecular Crystals

The work carried out by our research group over the last couple of decades in the context of quantitative crystal engineering involves the analysis of intermolecular interactions such as carbon (tetrel) bonding, pnicogen bonding, chalcogen bonding, and halogen bonding using experimental charge density methodology is reviewed. The focus is to extract electron density distribution in the intermolecular space and to obtain guidelines to evaluate the strength and directionality of such interactions towards the design of molecular crystals with desired properties. Following the early studies on halogen bonding interactions, several "sigma-hole" interaction types with similar electrostatic origins have been explored in recent times for their strength, origin, and structural consequences. These include interactions such as carbon (tetrel) bonding, pnicogen bonding, chalcogen bonding, and halogen bonding. Experimental X-ray charge density analysis has proved to be a powerful tool in unraveling the strength and electronic origin of such interactions, providing insights beyond the theoretical estimates from gas-phase molecular dimer calculations. In this mini-review, we outline some selected contributions from the X-ray charge density studies to the field of non-covalent interactions (NCIs) involving elements of the groups 14-17 of the periodic table. Quantitative insights into the nature of these interactions obtained from the experimental electron density distribution and subsequent topological analysis by the quantum theory of atoms in molecules (QTAIM) have been discussed. A few notable examples of weak interactions have been presented in terms of their experimental charge density features. These examples reveal not only the strength and beauty of X-ray charge density multipole modeling as an advanced structural chemistry tool but also its utility in providing experimental benchmarks for the theoretical studies of weak interactions in crystals.

Influence of Multiple Binding Sites on the Supramolecular Assembly of N-[(3-pyridinylamino) Thioxomethyl] Carbamates

In this study, we investigated how the presence of multiple intermolecular interaction sites influences the heteromeric supramolecular assembly of N-[(3-pyridinylamino) thioxomethyl] carbamates with fluoroiodobenzenes. Three targets—R-N-[(3-pyridinylamino) thioxomethyl] carbamate (R = methyl, ethyl, and isobutyl)—were selected and crystallized, resulting in three parent structures, five co-crystals, and one co-crystal solvate. Three hydrogen-bonded parent crystal structures were stabilized by N-H···N hydrogen bonding and assembled into layers that stacked on top of one another. Molecular electrostatic potential surfaces were employed to rank binding sites (Npyr > C=S > C=O) in order to predict the dominant interactions. The N-H⋯H hydrogen bond was replaced by I⋯Npyr in 3/6 cases, I⋯C=S in 4/6 cases, and I⋯O=C in 1 case. Interestingly, the I⋯C=S halogen bond coexisted twice with I⋯Npyr and I⋯O=C. Overall, the MEPs were fairly reliable for predicting co-crystallization outcomes; however, it is crucial to also consider factors such as molecular flexibility. Finally, halogen-bond donors are capable of competing for acceptor sites, even in the presence of strong hydrogen-bond donors.

In Situ Observation of the Structure of Crystallizing Magnesium Sulfate Heptahydrate Solutions with Terahertz Transmission Spectroscopy

Terahertz time-domain spectroscopy in a transmission geometry combined with visual analysis was used to investigate the crystallization process of MgSO₄ solution. Careful spectral analysis of both a feature at 1.6 THz and the overall magnitude of absorption allowed the extraction of information about the liquid phase before and during crystallization, aiding the investigation of solvation dynamics and the behavior of molecular species at phase boundaries. The method was reproducibly applied to a number of measurements on a series of solutions of three chosen concentrations at different temperatures. When increasing temperature at the end of the measurement, the dissolution of crystals was observed as well. The temperature-dependent absorption data of the semicrystalline systems were converted to the solvent concentrations using a recently developed method. Solutions of a series of concentrations were also investigated in the temperature range of 4-25 °C. The results were compared to the theoretical calculated values, and the consistent differences proved the existence of a hydration shell around the salt ions whose behavior is different from bulk water. Future work will focus on triggering nucleation at specific positions in order to study the very beginning of the crystallization process. MgSO₄ heptahydrate is used as a model system in this study, while the concept and the setup can be applied to other systems.

Theoretical Prediction of Structures and Properties of 2,4,6-Trinitro-1,3,5-Triazine (TNTA) Green Energetic Materials from DFT and ReaxFF Molecular Modeling

Nitryl cyanide, O2NCN, as a new high-energy molecule, has not yet been successfully synthesized. It has prompted us to conduct a theoretical study of its possible space structures and properties. The RESP charges and the most stable spatial structures demonstrate that crystal morphology is affected by both the main nonbonded interactions and the molecular arrangement. The crystal structure prediction indicated that there are seven structures, namely P1, P21, P212121, P21/c, Pna21, Pbca, and C2/c. The most stable space structure is likely to be Pna21 and the corresponding cell parameters are Z = 4, a = 8.69 Å, b = 9.07 Å, c = 9.65 Å, and α = β = γ = 90.0°. To further study the intermolecular interactions of TNTA, a series of theoretical analyses were employed, including Hirshfeld surface analysis and fingerprint plots. The pyrolysis mechanism and properties show that high temperatures can promote decomposition. The systematic search approach can be a new strategy to identify structures effectively and has the potential to provide systematic theoretical guidance for the synthesis of TNTA.

Hexamethylenetetramine–tridecanedioic acid (1/1): temperature-induced order–disorder structural phase transitions with phase co-existence

The cocrystal hexamethylenetetramine–tridecanedioic acid (1/1) (HMT–C13), C6H12N4·C13H24O4, was investigated by single-crystal X-ray diffraction techniques at several temperatures during cooling and heating processes. Our results show the formation of two crystalline phases, separated by a large temperature phase co-existence between 290 and 340 K. Phase I, stable above 341 K, presents an orthorhombic structure described in the space group Bmmb, with one N4(CH2)6·C13H22O4 adduct in its asymmetric unit. Phase II, stable below 290 K, presents a monoclinic symmetry described by the space group P21/c, with two N4(CH2)6·C13H22O4 adducts in its asymmetric unit. The phase co-existence is observed both upon cooling and heating, and seems to be related to a complex domain-growth dynamic within the crystal.

Solid-State and Particle Size Control of Pharmaceutical Cocrystals using Atomization-Based Techniques

Poor bioavailability and aqueous solubility represent a major constraint during the development of new API molecules and can influence the impact of new medicines or halt their approval to the market. Cocrystals offer a novel and competitive advantage over other conventional methods with respect towards the substantial improvement in solubility profiles relative to the single-API crystals. Furthermore, the production of such cocrystals through atomization-based methods allow for greater control, with respect to particle size reduction, to further increase the solubility of the API. Such atomization-based methods include supercritical fluid methods, conventional spray drying and electrohydrodynamic atomization/electrospraying. The influence of process parameters such as solution flow rates, pressure and solution concentration, in controlling the solid-state and final particle size are discussed in this review with respect to atomization-based methods. For the last decade, literature has been attempting to catch-up with new regulatory rulings regarding the classification of cocrystals, due in part to data sparsity. In recent years, there has been an increase in cocrystal publications, specifically employing atomization-based methods. This review considers the benefits to employing atomization-based methods for the generation of pharmaceutical cocrystals, examines the most recent regulatory changes regarding cocrystals and provides an outlook towards the future of this field.

Evaluation of Piperine as Natural Coformer for Eutectics Preparation of Drugs Used in the Treatment of Cardiovascular Diseases

Piperine (PIP) was evaluated as a natural coformer in the preparation of multicomponent organic materials for enhancing solubility and dissolution rate of the poorly water-soluble drugs: curcumin (CUR), lovastatin (LOV), and irbesartan (IBS). A screening based on liquid assisted grinding technique was performed using 1:1 drug-PIP molar ratio mixtures, followed by differential scanning calorimetry (DSC) and powder X-ray diffraction (PXRD) analyses. Three eutectic mixtures (EMs) composed of CUR-PIP, LOV-PIP, and IBS-PIP were obtained. Therefore, binary phase and Tamman's diagrams were constructed for each system to obtain the exact eutectic composition, which was 0.41:0.59, 0.29:0.71, and 0.31:0.69 for CUR-PIP, LOV-PIP, and IBS-PIP, respectively. Further, bulk materials of each system were prepared to characterize them through DSC, PXRD fully, Fourier transform infrared spectroscopy (FT-IR), and solution-state nuclear magnetic resonance (NMR) spectroscopy. In addition, the contact angle, solubility, and dissolution rate of each system were evaluated. The preserved characteristic in the PXRD patterns and FT-IR spectra of the bulk material of each system confirmed the formation of EM mixture without molecular interaction in solid-state. The formation of EM resulted in improved aqueous solubility and dissolution rate associated with the increased wettability observed by the decrease in contact angle. In addition, solution NMR analyses of CUR-PIP, LOV-PIP, and IBS-PIP suggested no significant intermolecular interactions in solution between the components of the EM. Hence, this study concludes that PIP could be an effective coformer to improve the solubility and dissolution rate of CUR, LOV, and IBS.

[(Histamine)(18-crown-6)2][BF4]2 is a high-temperature piezoelectric

We discovered that [(histamine)(18-crown-6)₂][BF₄]₂ is a high-temperature host-guest inclusion complex that presents decent piezoelectric properties (d₃₃ = 5 pC/N), undergoes a phase transition at 406 K, and also possesses potential ferroelectricity. This work provides a new idea for constructing host-guest inclusion piezoelectrics.

The role of solvation in proton transfer reactions: implications for predicting salt/co-crystal formation using the ΔpKa rule

The ΔpK_a rule is commonly applied by chemists and crystal engineers as a guideline for the rational design of molecular salts and co-crystals. For multi-component crystals containing acid and base constituents, empirical evidence has shown that ΔpK_a > 4 almost always leads to salts, ΔpK_a < -1 almost always leads to co-crystals and ΔpK_a between -1 and 4 can be either. This paper reviews the theoretical background of the ΔpK_a rule and highlights the crucial role of solvation in determining the outcome of the potential proton transfer from acid to base. New data on the frequency of the occurrence of co-crystals and salts in multi-component crystal structures containing acid and base constituents show that the relationship between ΔpK_a and the frequency of salt/co-crystal formation is influenced by the composition of the crystal. For unsolvated co-crystals/salts, containing only the principal acid and base components, the point of 50% probability for salt/co-crystal formation occurs at ΔpK_a ≈ 1.4, while for hydrates of co-crystals and salts, this point is shifted to ΔpK_a ≈ -0.5. For acid-base crystals with the possibility for two proton transfers, the overall frequency of occurrence of any salt (monovalent or divalent) versus a co-crystal is comparable to that of the whole data set, but the point of 50% probability for observing a monovalent salt vs. a divalent salt lies at ΔpK_a,II ≈ -4.5. Hence, where two proton transfers are possible, the balance is between co-crystals and divalent salts, with monovalent salts being far less common. Finally, the overall role played by the "crystal" solvation is illustrated by the fact that acid-base complexes in the intermediate region of ΔpK_a tip towards salt formation if ancillary hydrogen bonds can exist. Thus, the solvation strength of the lattice plays a key role in the stabilisation of the ions.

Electrolyte-Gated Organic Field-Effect Transistors for Quantitative Monitoring of the Molecular Dynamics of Crystallization at the Solid-Liquid Interface

Quantitative measurements of molecular dynamics at the solid-liquid interface are of crucial importance in a wide range of fields, such as heterogeneous catalysis, energy storage, nanofluidics, biosensing, and crystallization. In particular, the molecular dynamics associated with nucleation and crystal growth is very challenging to study because of the poor sensitivity or limited spatial/temporal resolution of the most widely used analytical techniques. We demonstrate that electrolyte-gated organic field-effect transistors (EGOFETs) are able to monitor in real-time the crystallization process in an evaporating droplet. The high sensitivity of these devices at the solid-liquid interface, through the electrical double layer and signal amplification, enables the quantification of changes in solute concentration over time and the transport rate of molecules at the solid-liquid interface during crystallization. Our results show that EGOFETs offer a highly sensitive and powerful, yet simple approach to investigate the molecular dynamics of compounds crystallizing from water.

Studying the impact of the pre-exponential factor on templated nucleation

Traditionally, the enhancement of nucleation rates in the presence of heterogeneous surfaces in crystallisation processes has been attributed to the modification of the interfacial energy of the system according to the classical nucleation theory. However, recent developments have shown that heterogeneous surfaces instead alter the pre-exponential factor of nucleation. In this work, the nucleation kinetics of glycine and diglycine in aqueous solutions have been explored in the presence and absence of a heterogeneous surface. Results from induction time experiments show that the presence of a heterogeneous surface increases the pre-exponential factor by 2-fold or more for both glycine and diglycine, while the interfacial energy remains unchanged for both species. This study suggests that the heterogeneous surface enhances the nucleation rate via hydrogen bond formation with both glycine and diglycine. This is verified by hydrogen bond propensity calculations, molecular functionality analysis, and calculation of the time taken for a solute molecule to attach to the growing nucleus, which is an order of magnitude shorter than the estimated lifetime of the hydrogen bond. The effect of the heterosurface is of greater magnitude for diglycine than for glycine, which may be due to the heightened molecular complementarity between the hydrogen bond donor and acceptor sites on diglycine and the heterosurface.

The Molecular Ordering and Double-Channel Carrier Generation of Nonfullerene Photovoltaics within Multi-Length-Scale Morphology

The success of nonfullerene acceptor (NFA) solar cells lies in their unique physical properties beyond the extended absorption and suitable energy levels. The current study investigates the morphology and photophysical behavior of PBDB-T donor blending with ITIC, 4TIC, and 6TIC acceptors. Single-crystal study shows that the π-π stacking and side-chain interaction dictate molecular assembly, which can be carried to blended films, forming a multi-length-scale morphology. Spontaneous carrier generation is seen in ITIC, 4TIC, and 6TIC neat films and their blended thin films using the PBDB-T donor, providing a new avenue of zero-energy-loss carrier formation. The molecular packing associated with specific contacts and geometry is key in influencing the photophysics, as demonstrated by the charge transfer and carrier lifetime results. The 2D layer of 6TIC facilitates the exciton-to-polaron conversion, and the largest photogenerated polaron yield is obtained. The new mechanism, together with the highly efficient blending region carrier generation, has the prospect of the fundamental advantage for NFA solar cells, from molecular assembly to thin-film morphology.

Solid-State Assembly by Chelating Chalcogen Bonding in Quinodimethane Tetraesters Fused with a Chalcogenadiazole

In contrast to p-quinodimethane tetraesters, which undergo facile polymerization due to their diradical character, newly synthesized 1 and 2 consisting of a chalcogenadiazole fused to a p-naphthoquinodimethane tetraester are thermodynamically stable due to butterfly-shaped deformation. Such a folded molecular structure is also favorable for chalcogen bond (ChB) formation through intermolecular close contacts between a chalcogen atom (E: Se or S) and the oxygen atoms of ester groups in a crystal. The less-explored chelating-ChB through a C=O⋅⋅⋅E⋅⋅⋅O=C contact [Se⋅⋅⋅O: 2.94-3.37 Å] is the key supramolecular synthon for the formation of a one-dimensional rod-like assembly in a crystal, which is commonly observed in selenadiazole-tetraesters (1) with OMe, OEt, and OiPr groups. The formation of inclusion cavities between the rods shows that 1 could serve as solid-state host molecules for clathrate formation, as found in a hexane-solvated crystal. In contrast, thiadiazole-tetraesters (2) are less suitable for the formation of a rod-like assembly since the ChB involving S is less effective, and thus is overwhelmed by weak hydrogen bonds through C-H⋅⋅⋅O contacts.

Cold photo-carving of halogen-bonded co-crystals of a dye and a volatile co-former using visible light

The formation of co-crystals by the assembly of molecules with complementary molecular recognition functionalities is a popular strategy to design or improve a range of solid-state properties, including those relevant for pharmaceuticals, photo- or thermoresponsive materials and organic electronics. Here, we report halogen-bonded co-crystals of a fluorinated azobenzene derivative with a volatile component-either dioxane or pyrazine-that can be cut, carved or engraved with low-power visible light. This cold photo-carving process is enabled by the co-crystallization of a light-absorbing azo dye with a volatile component, which gives rise to materials that can be selectively disassembled with micrometre precision using low-power, non-burning laser irradiation or a commercial confocal microscope. The ability to shape co-crystals in three dimensions using laser powers of 0.5-20 mW-substantially lower than those used for metals, ceramics or polymers-is rationalized by photo-carving that targets the disruption of weak supramolecular interactions, rather than the covalent bonds or ionic structures targeted by conventional laser beam or focused ion beam machining processes.

A Dual Approach on Experimental, Theoretical Insight of Structural Elucidation, Hirshfeld Surface Analysis, Optical and Electrochemical Properties of Acyl Thiourea-Ethynyl Hybrid Derivatives

Hybrid moieties of ethynylated-thiourea, Th1 and Th2 have been synthesised via the addition reaction between ethynyl derivatives and 4-tert-butylbenzoyl isothiocyanate in acetone, and were characterised by selected spectroscopic methods (i.e., ¹H and ¹³C NMR, UV-visible, FT-IR) and elemental analysis. Thermogravimetric analysis indicated that Th1 and Th2 were relatively stable up to ca. 210 °C. Single-crystal X-ray diffraction was used to identify the crystal structure of Th2 in which the centre of 1-acyl thiourea moiety (-C(O)NHC(S)NH) exhibits S conformation. The Hirshfeld surface analysis has allowed visualizing the crystal packing, which is characterised by the prolonged intermolecular N-H⋯O = C and N-H⋯S = C hydrogen-bonding interactions within Th2 molecule. Electrochemical data of both compounds correspondingly exhibit irreversible redox potential processes. Besides, frontier molecular orbitals and Natural Bond Orbital population analysis were computed at the B3LYP/6-31G (d, p) level of approximation, suggesting strong delocalization of the electronic density through a conjugated π-system involving the ethynyl-phenyl and thiourea groups.

GRAPHICAL ABSTRACT

Figure of molecular structure for acyl thiourea-ethynyl derivative. Two derivatives of acyl thiourea-ethynyl were synthesised and characterised by selected spectroscopic methods such as 1H and 13C NMR, UV-visible, FT-IR, elemental, thermal, electrochemical, X-ray diffraction, and density functional theory (DFT) calculation for molecular orbitals and natural bond orbital population analysis.

Chemical Diversity for Tailoring Negative Thermal Expansion

Negative thermal expansion (NTE), referring to the lattice contraction upon heating, has been an attractive topic of solid-state chemistry and functional materials. The response of a lattice to the temperature field is deeply rooted in its structural features and is inseparable from the physical properties. For the past 30 years, great efforts have been made to search for NTE compounds and control NTE performance. The demands of different applications give rise to the prominent development of new NTE systems covering multifarious chemical substances and many preparation routes. Even so, the intelligent design of NTE structures and efficient tailoring for lattice thermal expansion are still challenging. However, the diverse chemical routes to synthesize target compounds with featured structures provide a large number of strategies to achieve the desirable NTE behaviors with related properties. The chemical diversity is reflected in the wide regulating scale, flexible ways of introduction, and abundant structure-function insights. It inspires the rapid growth of new functional NTE compounds and understanding of the physical origins. In this review, we provide a systematic overview of the recent progress of chemical diversity in the tailoring of NTE. The efficient control of lattice and deep structural deciphering are carefully discussed. This comprehensive summary and perspective for chemical diversity are helpful to promote the creation of functional zero-thermal-expansion (ZTE) compounds and the practical utilization of NTE.

Packing Preferences of Chalcones: A Model Conjugated Pharmaceutical Scaffold

We sought the crystal packing preferences of the chalcone scaffold by analyzing 232 single-component crystal structures of chalcones with a small (six or fewer non-hydrogen atoms) substituent on either or both rings, including the unsubstituted molecule. This covers 216 molecules, as some are polymorphic, and 277 independent molecular conformations, as 16% of the crystal structures have more than one symmetry independent molecule. Quantum mechanical conformational profiles of the unsubstituted molecule and the almost 5000 crystal structures within 20 kJ mol^-1 of the global minimum generated in a crystal structure prediction (CSP) study have been used to complement this analysis. Although π conjugation would be expected to favor a planar molecule, there are a significant number of crystal structures containing nonplanar molecules with an approximately 50° angle between the aromatic rings. The relative orientations of the molecules in the inversion-related dimers and translation-related dimers in the experimental crystal structures show the same trends as in the CSP-generated structures for the unsubstituted molecule, allowing for the substituent making the side-to-side distances larger. There is no type of dimer geometry associated with particularly favorable lattice energies for the chalcone core. Less than a third of the experimental structures show a face-to-face contact associated with π···π stacking. Analysis of the experimental crystal structures with XPac and Mercury finds various pairs of isostructural crystals, but the largest isostructural set had only 15 structures, with all substituents (mainly halogens) in the para position. The most common one-dimensional motif, found in half of the experimental crystal structures, is a translation-related side-to-side packing, which can be adopted by all the observed conformations. This close-packed motif can be adopted by chalcones with a particularly wide variety of substituents as the substituents are at the periphery. Thus, although the crystal structures of the substituted chalcones show thermodynamically plausible packings of the chalcone scaffold, there is little evidence for any crystal engineering principle of preferred chalcone scaffold packing beyond close packing of the specific molecule.

A new bioactive cocrystal of coumarin-3-carboxylic acid and thiourea: detailed structural features and biological activity studies

Cocrystallization is a phenomenon widely used to enhance the biological and physicochemical properties of active pharmaceutical ingredients (APIs). The present study deals with the synthesis of a cocrystal of coumarin-3-carboxylic acid (2-oxochromene-3-carboxylic acid, C₁₀H₆O₄), a synthetic analogue of the naturally occurring antioxidant coumarin, with thiourea (CH₄N₂S) using the neat grinding method. The purity and homogeneity of the coumarin-3-carboxylic acid-thiourea (1/1) cocrystal was confirmed by single-crystal X-ray diffraction, FT-IR analysis and thermal stability studies based on differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). Detailed geometry analysis via density functional theory (DFT) demonstrated that the 1:1 cocrystal stoichiometry is sustained by N-H...O hydrogen bonding between the amine (-NH₂) groups of thiourea and the carbonyl group of coumarin. The synthesized cocrystal exhibited potent antioxidant activity (IC₅₀ = 127.9 ± 5.95 µM) in a DPPH radical scavenger assay in vitro in comparison with the standard N-acetyl-L-cysteine (IC₅₀ = 111.6 ± 2.4 µM). The promising results of the present study highlight the significance of cocrystallization as a crystal engineering tool to improve the efficacy of pharmaceutical ingredients.

Structure-property relationship in thioxotriaza-spiro derivative: Crystal structure and molecular docking analysis against SARS-CoV-2 main protease

Detailed structural and non-covalent interactions in thioxotriaza-spiroderivative (DZ2) are investigated by single crystal structure anslysis and computational approaches. Its results were compared with the previously reported spiroderivative (DZ1). The crystal structure analysis revealed various C–H…O, N–H…O, C–H…N and N–H…S hydrogen bonds involved in constructing several dimeric motifs to stabilize the crystal packing. The differences and similarities in the relative contribution of non-covalent interactions in DZ1 and DZ2 compounds are compared using the Hirshfeld surface analysis and 2D fingerprint plots. The binding energies of specific molecular pairs and homodimers have been obtained using molecule–molecule interaction energy calculation. The hierarchy and topology of pair-wise intermolecular interactions are visualized through energy frameworks. The nature and strength of intra and intermolecular interactions were characterized using non-covalent interaction index analysis and the quantum theory of atoms in molecule approach. Further, molecular docking of compounds (DZ1 and DZ2) with SARS-CoV-2 main protease for COVID-19 is performed. And the superposition of these ligands and inhibitor N3, which is docked into the binding pocket of 7BQY, is presented. The binding affinity of −6.7 kcal/mol is observed, attributed to hydrogen bonding and hydrophobic interactions between the ligand and the amino acid residues of the receptor.

The crystallization of decanoic acid/dopamine supramolecular self-assemblies in the presence of coacervates

HYPOTHESIS

Supramolecular self-assemblies involving non-covalent interactions play important roles in material science as well as living systems as they result in unique properties and/or functions. However, understanding of their self-assembly mechanism and crystallization has remained rudimentary.

EXPERIMENT

Here, we focus on biomolecular fatty acid and dopamine, which commonly exist in biological systems and closely related to neurodegenerative diseases, and investigate their self-assembly pathway by optical and fluorescence microscopy, DLS, SAXS, TEM, 2D-NMR, etc. FINDINGS: It is found that they could form the crystalline plates in solution or via a metastable liquid - liquid phase separation (LLPS). The nucleation and growth of crystalline plates observed occurs in solution or the dilute phase of LLPS, and not within the concentrated coacervate phase. This is because in coacervate, dopamine intercalates into fatty acid through hydrophobic and electrostatic interaction, which hinders the rearrangement of molecules and nucleation process, whereas in solution or dilute phase, they have the mobility to arrange into ordered structures to maximize electrostatic, hydrogen bonding and π-π interactions, leading to nucleation and crystallization. Moreover, the transitions between the coacervates and crystalline phase can be realized by adjusting the temperature. Our results shed light on the multistep nucleation in the presence of LLPS, as well as molecular mechanisms involved, thus further extending the nucleation-growth mechanisms.

Conservation of the Hydrogen-Bonded Pyridone Homosynthon in Halogen-Bonded Cocrystals

Seven cocrystals of pyridone and perfluorinated halocarbons have been prepared. In all cases pairs of pyridone molecules are connected into dimers by two N-H···O hydrogen bonds, forming the characteristic pyridone homosynthon of R₂ ²(8) topology. These dimers further act as acceptors of halogen bonds through the two pyridone oxygen atoms, forming two (in six cases) or three (in one case) halogen bonds with the donor molecules. The stoichiometry of the cocrystals obtained and the overall topology of the supramolecular architecture depend primarily on the topicity of the halogen bond donor, with the monotopic donor yielding a cocrystal of 1:1 stoichiometry comprising discrete supramolecular complexes, the ditopic donors cocrystals of 1:2 stoichiometry comprising chains, and the tritopic donor a cocrystal of 1:2 stoichiometry comprising hydrogen- and halogen-bonded layers. The results indicate that the pyridone homosynthon is a robust and reliable supramolecular synthon that is conserved in halogen-bonded cocrystals of pyridone.

Isostructural Halogen Exchange and Halogen Bonds: The Case of N-(4-Halogenobenzyl)-3-halogenopyridinium Halogenides

Six N-(4-halogenobenzyl)-3-halogenopyridinium cations were prepared by reacting meta-halogenopyridines (Cl, Br, and I) with (4-halogenobenzyl) bromides (Br and I) and were isolated as bromide salts, which were further used to obtain iodides and chlorides. Sixteen compounds (out of 18 possible cation/anion combinations) were obtained; two crystallized as hydrates and 14 as solvent free salts, 11 of which belonged to one isostructural series and 3 to another. All crystal structures comprise halogen-bonded chains, with the anion as an acceptor of two halogen bonds, with the pyridine and the benzyl halogen substituents of two neighboring cations. The halogen bonds with the pyridine halogen show a linear correlation between the relative halogen bond length and angle, which primarily depend on the donor halogen. The parameters of the other halogen bonds vary with all three halogens, indicating that the former halogen bond is the dominant interaction. This is also in accord with the calculated electrostatic potential in the σ-holes of the halogens and the thermal properties of the solids. The second isostructural group comprises combinations of the best halogen bond donors and acceptors, and features a more favorable halogen bond geometry of the dominant halogen bond, reaffirming its significance as the main factor in determining the structure.

Ultrafast energy transfer between π-stacked aromatic rings upon inner-valence ionization

Non-covalently bound aromatic systems are ubiquitous and govern the physicochemical properties of various organic materials. They are important to many phenomena of biological and technological relevance, such as protein folding, base-pair stacking in nucleic acids, molecular recognition and self-assembly, DNA-drug interactions, crystal engineering and organic electronics. Nevertheless, their molecular dynamics and chemical reactivity, particularly in electronic excited states, are not fully understood. Here, we observe intermolecular Coulombic decay in benzene dimers, (C₆H₆)₂-the simplest prototypes of noncovalent π-π interactions between aromatic systems. Intermolecular Coulombic decay is initiated by a carbon 2s vacancy state produced by electron-impact ionization and proceeds through ultrafast energy transfer between the benzene molecules. As a result, the dimer relaxes with the emission of a further low-energy electron (<10 eV) and a pair of C₆H₆⁺ cations undergoing Coulomb explosion. Coincident fragment-ion and electron momentum spectroscopy, accompanied by ab initio calculations, enables us to elucidate the dynamical details of this ultrafast relaxation process.

Molecular Aggregation Dynamics via a Liquid-like Cluster Intermediate during Heterogeneous Evaporation as Revealed by Hyperspectral Camera Fluorescence Imaging

A hyperspectral camera (HSC) is a camera with great potential to obtain spectral information at each pixel, together with spatial imaging. HSC fluorescence imaging enables the molecular aggregation dynamics of the evaporative crystallization process to be followed in real-time. The key intermediate liquid-like cluster state for the two-step nucleation mechanism is visualized by the fluorescence color changes of mechanochromic luminescent dibenzoylmethanatoboron difluoride derivatives. Three types of emissive species (Crystal, BG-aggregates, and Amorphous) are generated from monomers in solution (low order and density) via liquid-like cluster (high density and low order) during solvent evaporation. These emissive species have partially different aggregated states based on fluorescence decay and fluorescence excitation spectral measurements. In terms of crystallization dynamics, our results indicate that it is important not only to generate supersaturated states but also to maintain the survival time of the liquid-like cluster. Moreover, we demonstrate that HSC fluorescence imaging can be a powerful tool for visualizing heterogeneous molecular aggregation processes.

Multicomponent Solids of DL-2-Hydroxy-2-phenylacetic Acid and Pyridinecarboxamides

We prepared cocrystals of DL-2-Hydroxy-2-phenylacetic acid (D, L-H2ma) with the pyridinecarboxamide isomers, picolinamide (pic) and isonicotinamide (inam). They were characterized by elemental analysis, single crystal and powder X-ray, IR spectroscopy and 1H and 13C NMR. The crystal and molecular structures of (pic)-(D-H2ma) (1), (nam)-(L-H2ma) (2) and (inam)-(L-H2ma) (3) were studied. The crystal packing is stabilized primarily by hydrogen bonding and in some cases through π-π stacking interactions. The analysis of crystal structures reveals the existence of the characteristic heterosynthons with the binding motif R22(8) (primary amide–carboxilic acid) between pyridinecarboxamide molecules and the acid. Other synthons involve hydrogen bonds such as O-H(carboxyl)···N(pyridine) and O-H(hydroxyl)···N(pyridine) depending on the isomer. The packing of 1 and 3 is formed by tetramers, for whose formation a crystallization mechanism based on two stages is proposed, involving an amide–acid (1) or amide–amide (3) molecular recognition in the first stage and the formation of others, and interdimeric hydrogen bonding interactions in the second. The thermal stability of the cocrystals was studied by differential scanning calorimetry and thermogravimetry. Further studies were conducted to evaluate other physicochemical properties of the cocrystals in comparison to the pure coformers. Density-functional theory (DFT) calculations (including NCIplot and QTAIM analyses) were performed to further characterize and rationalize the noncovalent interactions.

Recent advances in drug polymorphs: Aspects of pharmaceutical properties and selective crystallization

Drug polymorphism, an established term used to describe the phenomenon that a drug can exist in different crystalline phases, has attracted great interests in pharmaceutical field in consideration of its important role in affecting the pharmaceutical performance of oral formulations. This paper presents an overview of recent advances in the research on polymorphic drug systems including understandings on nucleation, crystal growth, dissolution, mechanical properties, polymorphic transformation, etc. Moreover, new strategies and mechanisms in the control of polymorphic forms are also highlighted in this review. Furthermore, challenges and trends in the development of polymorphic drugs are briefly discussed, aiming at developing effective and efficient pharmaceutical formulations containing the polymorphic drugs.

Polymorphism in the 1/1 Pterostilbene/Picolinic Acid Cocrystal

The crystal structures of two new polymorphs of the 1/1 pterostilbene/picolinic acid cocrystal have been analyzed by single-crystal X-ray diffraction and studied by means of DFT calculations and a set of computational tools (QTAIM, NCIplot, MEP). The observation of a new R2 2(10) synthon in each of the two polymorphs has been analyzed energetically, characterized using the topology of the electron density, and rationalized using the MEP surfaces. The exceptional bioavailability of the cocrystal is explained on the basis of BFDH morphology calculations, and the study is complemented by a deep analysis of the supramolecular synthons formed by both neutral and zwitterionic forms of picolinic acid, a versatile coformer for crystal engineering.

Design of coordination polymers based on combinations of 1,2-diphenylethane-1,2-diyl diisonicotinate with Cu(ii), Zn(ii), Cd(ii) and Co(ii)

Five new supramolecular coordination polymers of different dimensionalities (L-Cu(acac)2, L-Cu(hfac)2, L-ZnCl2, L-CdI2 and L-CoCl2) based on the use of the flexible organic ligand L (1,2-diphenylethane-1,2-diyl diisonicotinate) are reported.