Crystallization of Organic Molecules: Nonclassical Mechanism Revealed by Direct Imaging

From Ions to Crystals: A Comprehensive View of the Non-Classical Nucleation of Calcium Sulfate

The early stages of mineralization continue to be in the focus of intensive research due to their inherent importance for natural and engineered environments. While numerous observations have been reported for single steps in the pathways of various crystallizing phases in previous studies, the complexity of the underlying processes and their elusive character have left central questions unanswered in most cases. In the present work, we provide a detailed view on the nucleation of calcium sulfate mineralization-an abundant mineral with broad use in construction industry-in aqueous systems at ambient conditions. As experimental basis, a co-titration procedure with potentiometric, turbidimetric and conductometric detection was developed, allowing solution speciation and the formation of crystallization precursors to be monitored quantitatively as the level of nominal (super)saturation gradually increases. The nature and spatiotemporal evolution of these precursors was further elucidated by time-resolved small-angle X-ray scattering (SAXS) and analytical ultracentrifugation (AUC) experiments, complemented by cryogenic transmission electron microscopy (cryo-TEM) as a direct imaging technique. The results reveal how ions associate into nanometric primary species, which subsequently aggregate and develop anisotropic order by intrinsic structural reorganization. Our observations challenge the common understanding of fundamental notions such as the nucleation barrier or the meaning of supersaturation, with broad implications for mineralization phenomena in general and the formation of calcium sulfate in geochemical settings and industrial applications in particular.

Complex Pathways Drive Pluripotent Fmoc-Leucine Self-Assemblies

Nature uses complex self-assembly pathways to access distinct functional non-equilibrium self-assemblies. This remarkable ability to steer same set of biomolecules into different self-assembly states is done by avoiding thermodynamic pit. In synthetic systems, on demand control over 'Pathway Complexity' to access self-assemblies different from equilibrium structures remains challenging. Here we show versatile non-equilibrium assemblies of the same monomer via alternate assembly pathways. The assemblies nucleate using non-classical or classical nucleation routes into distinct metastable (transient hydrogels), kinetic (stable hydrogels) and thermodynamic structures [(poly)-crystals and 2D sheets]. Initial chemical and thermal inputs force the monomers to follow different assembly pathways and form soft-materials with distinct molecular arrangements than at equilibrium. In many cases, equilibrium structures act as thermodynamic sink which consume monomers from metastable structures giving transiently formed materials. This dynamics can be tuned chemically or thermally to slow down the dissolution of transient hydrogel, or skip the intermediate hydrogel altogether to reach final equilibrium assemblies. If required this metastable state can be kinetically trapped to give strong hydrogel stable over days. This method to control different self-assembly states can find potential use in similar biomimetic systems to access new materials for various applications.

A dual growth mode unique for organic crystals relies on mesoscopic liquid precursors

Organic solvents host the synthesis of high-value crystals used as pharmaceuticals and optical devices, among other applications. A knowledge gap persists on how replacing the hydrogen bonds and polar attraction that dominate aqueous environments with the weaker van der Waals forces affects the growth mechanism, including its defining feature, whether crystals grow classically or nonclassically. Here we demonstrate a rare dual growth mode of etioporphyrin I crystals, enabled by liquid precursors that associate with crystal surfaces to generate stacks of layers, which then grow laterally by incorporating solute molecules. Our findings reveal the precursors as mesoscopic solute-rich clusters, a unique phase favored by weak bonds such as those between organic solutes. The lateral spreading of the precursor-initiated stacks of layers crucially relies on abundant solute supply directly from the solution, bypassing diffusion along the crystal surface; the direct incorporation pathway may, again, be unique to organic solvents. Clusters that evolve to amorphous particles do not seamlessly integrate into crystal lattices. Crystals growing fast and mostly nonclassically at high supersaturations are not excessively strained. Our findings demonstrate that the weak interactions typical of organic systems promote nonclassical growth modes by supporting liquid precursors and enabling the spreading of multilayer stacks.

Amorphous aggregates with a very wide size distribution play a central role in crystal nucleation

There is mounting evidence that crystal nucleation from supersaturated solution involves the formation and reorganization of prenucleation clusters, contradicting classical nucleation theory. One of the key unresolved issues pertains to the origin, composition, and structure of these clusters. Here, a range of amino acids and peptides is investigated using light scattering, mass spectrometry, and in situ terahertz Raman spectroscopy, showing that the presence of amorphous aggregates is a general phenomenon in supersaturated solutions. Significantly, these aggregates are found on a vast range of length scales from dimers to 30-mers to the nanometre and even micrometre scale, implying a continuous distribution throughout this range. Larger amorphous aggregates are sites of spontaneous crystal nucleation and act as intermediates for laser-induced crystal nucleation. These results are shown to be consistent with a nonclassical nucleation model in which barrierless (homogeneous) nucleation of amorphous aggregates is followed by the nucleation of crystals from solute-enriched aggregates. This provides a novel perspective on crystal nucleation and the role of nonclassical pathways.

Substantial Confinement of Crystal Growth of Organic Crystalline Materials in Metal-Organic Membrane Microshells

This study proposes a robust microshell encapsulation system in which a metal-organic membrane (MOM), consisting of phytic acids (PAs) and metal ions, intrinsically prevents the molecular crystal growth of organic crystalline materials (OCMs). To develop this system, OCM-containing oil-in-water (O/W) Pickering emulsions were enveloped with the MOM, in which anionic pulp cellulose nanofiber (PCNF) primers electrostatically captured zinc ions at the O/W interface and chelated with PA, thus producing the MOM with a controlled shell thickness at the micron scale. We ascertained that the MOM formation fills and covers ∼75% of the surface pore size of PCNF films, which enhances the interfacial modulus by 2 orders of magnitude compared to that when treated with bare PCNFs. Through a feasibility test using a series of common OCMs, including ethylhexyl triazone, avobenzone, and ceramide, we demonstrated the excellent ability of our MOM microshell system to stably encapsulate OCMs while retaining their original molecular structures over time. These findings indicate that our MOM-reinforced microshell technology can be applied as a platform to substantially confine the crystal growth of various types of OCMs.

Mesoscale Clusters in the Crystallisation of Organic Molecules

The early stages of the molecular self-assembly pathway leading to crystal nucleation have a significant influence on the properties and purity of organic materials. This mini review collates the work on organic mesoscale clusters and discusses their importance in nucleation processes, with a particular focus on their critical properties and susceptibility to sample treatment parameters. This is accomplished by a review of detection methods, including dynamic light scattering, nanoparticle tracking analysis, small angle X-ray scattering, and transmission electron microscopy. Considering the challenges associated with crystallisation of flexible and large-molecule active pharmaceutical ingredients, the dynamic nature of mesoscale clusters has the potential to expand the discovery of novel crystal forms. By collating literature on mesoscale clusters for organic molecules, a more comprehensive understanding of their role in nucleation will evolve and can guide further research efforts.

Hybrid and composite materials of organic crystals

Organic molecular crystals have historically been viewed as delicate and fragile materials. However, recent studies have revealed that many organic crystals, especially those with high aspect ratios, can display significant flexibility, elasticity, and shape adaptability. The discovery of mechanical compliance in organic crystals has recently enabled their integration with responsive polymers and other components to create novel hybrid and composite materials. These hybrids exhibit unique structure-property relationships and synergistic effects that not only combine, but occasionally also enhance the advantages of the constituent crystals and polymers. Such organic crystal composites rapidly emerge as a promising new class of materials for diverse applications in optics, electronics, sensing, soft robotics, and beyond. While specific, mostly practical challenges remain regarding scalability and manufacturability, being endowed with both structurally ordered and disordered components, the crystal-polymer composite materials set a hitherto unexplored yet very promising platform for the next-generation adaptive devices. This Perspective provides an in-depth analysis of the state-of-the-art in design strategies, dynamic properties and applications of hybrid and composite materials centered on organic crystals. It addresses the current challenges and provides a future outlook on this emerging class of multifunctional, stimuli-responsive, and mechanically robust class of materials.

Nonclassical Crystal Growth of Supramolecular Polymers in Aqueous Medium

A mechanistic understanding of the principles governing the hierarchical organization of supramolecular polymers offers a paradigm for tailoring synthetic molecular architectures at the nano to micrometric scales. Herein, the unconventional crystal growth mechanism of a supramolecular polymer of superbenzene(coronene)-diphenylalanine conjugate (Cr-FFOEt ) is demonstrated. 3D electron diffraction (3D ED), a technique underexplored in supramolecular chemistry, is effectively utilized to gain a molecular-level understanding of the gradual growth of the initially formed poorly crystalline hairy, fibril-like supramolecular polymers into the ribbon-like crystallites. The further evolution of these nanosized flat ribbons into microcrystals by oriented attachment and lateral fusion is probed by time-resolved microscopy and electron diffraction. The gradual morphological and structural changes reveal the nonclassical crystal growth pathway, where the balance of strong and weak intermolecular interactions led to a structure beyond the nanoscale. The role of distinct π-stacking and H-bonding interactions that drive the nonclassical crystallization process of Cr-FFOEt supramolecular polymers is analyzed in comparison to analogous molecules, Py-FFOEt and Cr-FF forming helical and twisted fibers, respectively. Furthermore, the Cr-FFOEt crystals formed through nonclassical crystallization are found to improve the functional properties.

Preparation of Nanosized Clusters of Irinotecan Hydrochloride Trihydrate for Injection Concentrate to Reduce Carbon Footprint

The surprisingly stable irinotecan hydrochloride trihydrate injection concentrate having a supersaturated concentration of 20 mg/mL at 25 °C was due to the frustration of 150-nm sized liquid-like nanosized clusters formed by the aggregation of dimers of 1.5 nm in an aqueous phase, evidenced by the non-linearity of van't Hoff plot and dynamic light scattering measurement. The adoption of this stable supersaturated solution at 20 mg/mL by manufacturers as the commercial concentration was beneficial due to the less volume being involved throughout the manufacturing, handling, storage and transportation of the commercial product, while also enabling a versatile on-site concentration adjustment by dilution prior to intravenous administration. Regarding the physical characteristic of the solid state of irinotecan hydrochloride trihydrate, it was found to exist as a channel hydrate as evidenced by single-crystal, and high-temperature X-ray diffraction experiments. Dehydration takes place at approximately 35 °C as demonstrated by thermogravimetric analysis. Because of its non-stoichiometric nature under various RH values revealed by dynamic vapor sorption, the irinotecan hydrate salt raw material must be kept at 25 °C or below, and under the relative humidity of 40% to 60% to maintain the original stoichiometric ratio of the raw material.

Insight on the Intracellular Supramolecular Assembly of DTTO: A Peculiar Example of Cell-Driven Polymorphism

The assembly of supramolecular structures within living systems is an innovative approach for introducing artificial constructs and developing biomaterials capable of influencing and/or regulating the biological responses of living organisms. By integrating chemical, photophysical, morphological, and structural characterizations, it is shown that the cell-driven assembly of 2,6-diphenyl-3,5-dimethyl-dithieno[3,2-b:2',3'-d]thiophene-4,4-dioxide (DTTO) molecules into fibers results in the formation of a "biologically assisted" polymorphic form, hence the term bio-polymorph. Indeed, X-ray diffraction reveals that cell-grown DTTO fibers present a unique molecular packing leading to specific morphological, optical, and electrical properties. Monitoring the process of fiber formation in cells with time-resolved photoluminescence, it is established that cellular machinery is necessary for fiber production and a non-classical nucleation mechanism for their growth is postulated. These biomaterials may have disruptive applications in the stimulation and sense of living cells, but more crucially, the study of their genesis and properties broadens the understanding of life beyond the native components of cells.

Sequential Sol-Gel Self-Assembly and Nonclassical Gel-Crystal Transformation of the Metal-Organic Framework Gel

Metal-organic framework (MOF) gel, an emerging subtype of MOF structure, is unique in formation and function; however, its evolutionary process remains elusive. Here, the evolution of a model gel-based MOF, UiO-66(Zr) gel, is explored by demonstrating its sequential sol-gel self-assembly and nonclassical gel-crystal transformation. The control of the sol-gel process enables the observation and characterization of structures in each assembly stage (phase-separation, polycondensation, and hindered-crystallization) and facilitates the preparation of hierarchical materials with giant mesopores. The gelation mechanism is tentatively attributed to the formation of zirconium oligomers. By further utilizing the pre-synthesized gel, the nonclassical gel-crystal transformation is achieved by the modulation in an unconventional manner, which sheds light on crystal intermediates and distinct crystallization motions ("growth and splitting" and "aggregation and fusion"). The overall sol-gel and gel-crystal evolutions of UiO-66(Zr) enrich self-assembly and crystallization domains, inspire the design of functional structures, and demand more in-depth research on the intermediates in the future.

The Effect of Chain Length and Conformation on the Nucleation of Glycine Homopeptides during the Crystallization Process

To explore the effect of chain length and conformation on the nucleation of peptides, the primary nucleation induction time of glycine homopeptides in pure water at different supersaturation levels under various temperatures has been determined. Nucleation data suggest that longer chains will prolong the induction time, especially for chains longer than three, where nucleation will occur over several days. In contrast, the nucleation rate increased with an increase in the supersaturation for all homopeptides. Induction time and nucleation difficulty increase at lower temperatures. However, for triglycine, the dihydrate form was produced with an unfolded peptide conformation (pPII) at low temperature. The interfacial energy and activation Gibbs energy of this dihydrate form are both lower than those at high temperature, while the induction time is longer, indicating the classical nucleation theory is not suitable to explain the nucleation phenomenon of triglycine dihydrate. Moreover, gelation and liquid-liquid separation of longer chain glycine homopeptides were observed, which was normally classified to nonclassical nucleation theory. This work provides insight into how the nucleation process evolves with increasing chain length and variable conformation, thereby offering a fundamental understanding of the critical peptide chain length for the classical nucleation theory and complex nucleation process for peptides.

Nonclassical Crystallization and Core-Shell Structure Formation of Ibuprofen from Binary Solvent Solutions

Liquid-liquidphase separation (LLPS) or dense liquid intermediates during the crystallization of pharmaceutical molecules is common; however, their role in alternative nucleation mechanisms is less understood. Herein, we report the formation of a dense liquid intermediate followed by a core-shell structure of ibuprofen crystals via nonclassical crystallization. The Raman and SAXS results of the dense phase uncover the molecular structural ordering and its role in nucleation. In addition to the dimer formation of ibuprofen, which is commonly observed in the solution phase, methyl group vibrations in the Raman spectra show intermolecular interactions similar to those in the solid phase. The SAXS data validate the cluster size differences in the supersaturated solution and dense phase. The focused-ion beam cut image shows the attachment of nanoparticles, and we proposed a possible mechanism for the transformation from the dense phase into a core-shell structure. The unstable phase or polycrystalline core and its subsequent dissolution from inside to outside or recrystallization by reversed crystal growth produces the core-shell structure. The LLPS intermediate followed by the core-shell structure and its dissolution enhancement unfold a new perspective of ibuprofen crystallization.

Cinematographic Recording of a Metastable Floating Island in Two- and Three-Dimensional Crystal Growth

Many chemical reactions go through a cascade of events in which a series of metastable intermediates appear, and crystal nucleation is no exception. Although the consensus on the energetics of nucleation suggests the formation of metastable states preceding the crystal growth, little experimental evidence has been reported for their dynamics at an atomistic level. Operando imaging of two-dimensional nucleation on a defect-free NaCl nanocrystal in carbon nanotubes using a millisecond angstrom-resolution transmission electron microscope revealed the formation of a metastable "floating island" (FI) that migrates thermally on the (100) facet of NaCl as the first intermediate of epitaxy. The speed of the migration at 298 K is estimated to be larger than 0.3 nm ms^-1. When a crystal tumbles in a container, a space repeatedly forms between the crystal and the container wall that hosts the FI. Tumbling changes the surface energy repeatedly and promotes the conversion of the FI into a new epitaxial layer. We anticipate that this surface catalysis mechanism found on the nanoscale also operates in bulk heterogeneous nucleation where agitation and attrition accelerate crystallization.

Multistep Crystallization of Pharmaceutical Amorphous Nanoparticles via a Cognate Pathway of Oriented Attachment: Direct Evidence of Nonclassical Crystallization for Organic Molecules

Crystallization of organic molecules is important in a wide range of scientific disciplines. However, in contrast to maturely studied crystallization of inorganic materials, the crystallization mechanisms of organic molecules involving nucleation and crystal growth are still poorly understood. Here, we used time-resolved cryogenic transmission electron microscopy to directly map the morphological evolution of amorphous cyclosporin A (CyA) nanoparticles during CyA crystallization. We successfully observed its initial nucleation and found that the amorphous CyA nanoparticles crystallized via a pathway cognate with oriented attachment, which is the nonclassical crystallization mechanism usually reported for inorganic compounds. Crystalline mesostructured intermediates (mesocrystals) were formed during crystallization. This study revealed clear and direct evidence of mesocrystal formation and oriented attachment in organic pharmaceuticals, providing new insights into the crystallization of organic molecules and theories of nonclassical crystallization.

The Non-Classical Crystallization Mechanism of a Composite Biogenic Guanine Crystal

Spectacular colors and visual phenomena in animals are produced by light interference from highly reflective guanine crystals. Little is known about how organisms regulate crystal morphology to tune the optics of these systems. By following guanine crystal formation in developing spiders, a crystallization mechanism is elucidated. Guanine crystallization is a "non-classical," multistep process involving a progressive ordering of states. Crystallization begins with nucleation of partially ordered nanogranules from a disordered precursor phase. Growth proceeds by orientated attachment of the nanogranules into platelets which coalesce into single crystals, via progressive relaxation of structural defects. Despite their prismatic morphology, the platelet texture is retained in the final crystals, which are composites of crystal lamellae and interlamellar sheets. Interactions between the macromolecular sheets and the planar face of guanine appear to direct nucleation, favoring platelet formation. These findings provide insights on how organisms control the morphology and optical properties of molecular crystals.

Molecular Mechanism of Organic Crystal Nucleation: A Perspective of Solution Chemistry and Polymorphism

Crystal nucleation determining the formation and assembly pathway of first organic materials is the central science of various scientific disciplines such as chemical, geochemical, biological, and synthetic materials. However, our current understanding of the molecular mechanisms of nucleation remains limited. Over the past decades, the advancements of new experimental and computational techniques have renewed numerous interests in detailed molecular mechanisms of crystal nucleation, especially structure evolution and solution chemistry. These efforts bifurcate into two categories: (modified) classical nucleation theory (CNT) and non-classical nucleation mechanisms. In this review, we briefly introduce the two nucleation mechanisms and summarize current molecular understandings of crystal nucleation that are specifically applied in polymorphic crystallization systems of small organic molecules. Many important aspects of crystal nucleation including molecular association, solvation, aromatic interactions, and hierarchy in intermolecular interactions were examined and discussed for a series of organic molecular systems. The new understandings relating to molecular self-assembly in nucleating systems have suggested more complex multiple nucleation pathways that are associated with the formation and evolution of molecular aggregates in solution.

In situ optical spectroscopy of crystallization: One crystal nucleation at a time

While crystallization is a ubiquitous and an important process, the microscopic picture of crystal nucleation is yet to be established. Recent studies suggest that the nucleation process can be more complex than the view offered by the classical nucleation theory. Here, we implement single crystal nucleation spectroscopy (SCNS) by combining Raman microspectroscopy and optical trapping induced crystallization to spectroscopically investigate one crystal nucleation at a time. Raman spectral evolution during a single glycine crystal nucleation from water, measured by SCNS and analyzed by a nonsupervised spectral decomposition technique, uncovered the Raman spectrum of prenucleation aggregates and their critical role as an intermediate species in the dynamics. The agreement between the spectral feature of prenucleation aggregates and our simulation suggests that their structural order emerges through the dynamic formation of linear hydrogen-bonded networks. The present work provides a strong impetus for accelerating the investigation of crystal nucleation by optical spectroscopy.

An investigation of the kinetics and thermodynamics of NaCl nucleation through composite clusters

Having a good understanding of nucleation is critical for the control of many important processes, such as polymorph selection during crystallization. However, a complete picture of the molecular-level mechanisms of nucleation remains elusive. In this work, we take an in-depth look at the NaCl homogeneous nucleation mechanism through thermodynamics. Distinguished from the classical nucleation theory, we calculate the free energy of nucleation as a function of two nucleus size coordinates: crystalline and amorphous cluster sizes. The free energy surface reveals a thermodynamic preference for a nonclassical mechanism of nucleation through a composite cluster, where the crystalline nucleus is surrounded by an amorphous layer. The thickness of the amorphous layer increases with an increase in supersaturation. The computed free energy landscape agrees well with the composite cluster-free energy model, through which phase specific thermodynamic properties are evaluated. As the supersaturation increases, there is a change in stability of the amorphous phase relative to the solution phase, resulting in a change from one-step to two-step mechanism, seen clearly from the free energy profile along the minimum free energy path crossing the transition curve. By obtaining phase-specific diffusion coefficients, we construct the full mesoscopic model and present a clear roadmap for NaCl nucleation.

Mesoscale clusters of organic solutes in solution and their role in crystal nucleation

It is becoming evident that primary nucleation of crystals of organic molecules from solution is often anything but ‘classical’ in its complexity. It is also becoming increasingly clear that mesoscopic...

Adsorption-Inhibition of Clathrate Hydrates by Self-Assembled Nanostructures

The mechanism by which safranine O (SFO), an ice growth inhibitor, halts the growth of single crystal tetrahydrofuran (THF) clathrate hydrates was explored using microfluidics coupled with cold stages and fluorescence microscopy. THF hydrates grown in SFO solutions exhibited morphology changes and were shaped as truncated octahedrons or hexagons. Fluorescence microscopy and microfluidics demonstrated that SFO binds to the surface of THF hydrates on specific crystal planes. Cryo-TEM experiments of aqueous solutions containing millimolar concentrations of SFO exhibited the formation of bilayered lamellae with an average thickness of 4.2±0.2 nm covering several μm² . Altogether, these results indicate that SFO forms supramolecular lamellae in solution, which might bind to the surface of the hydrate and inhibit further growth. As an ice and hydrate inhibitor, SFO may bind to the surface of these crystals via ordered water molecules near its amine and methyl groups, similar to some antifreeze proteins.

Amorphous Solid Dispersions (ASDs): The Influence of Material Properties, Manufacturing Processes and Analytical Technologies in Drug Product Development

Poorly water-soluble drugs pose a significant challenge to developability due to poor oral absorption leading to poor bioavailability. Several approaches exist that improve the oral absorption of such compounds by enhancing the aqueous solubility and/or dissolution rate of the drug. These include chemical modifications such as salts, co-crystals or prodrugs and physical modifications such as complexation, nanocrystals or conversion to amorphous form. Among these formulation strategies, the conversion to amorphous form has been successfully deployed across the pharmaceutical industry, accounting for approximately 30% of the marketed products that require solubility enhancement and making it the most frequently used technology from 2000 to 2020. This article discusses the underlying scientific theory and influence of the active compound, the material properties and manufacturing processes on the selection and design of amorphous solid dispersion (ASD) products as marketed products. Recent advances in the analytical tools to characterize ASDs stability and ability to be processed into suitable, patient-centric dosage forms are also described. The unmet need and regulatory path for the development of novel ASD polymers is finally discussed, including a description of the experimental data that can be used to establish if a new polymer offers sufficient differentiation from the established polymers to warrant advancement.

Recent progress and future challenges in the supramolecular polymerization of metal-containing monomers

The self-assembly of discrete molecular entities into functional nanomaterials has become a major research area in the past decades. The library of investigated compounds has diversified significantly, while the field as a whole has matured. The incorporation of metal ions in the molecular design of the (supra-)molecular building blocks greatly expands the potential applications, while also offering a promising approach to control molecular recognition and attractive and/or repulsive intermolecular binding events. Hence, supramolecular polymerization of metal-containing monomers has emerged as a major research focus in the field. In this perspective article, we highlight recent significant advances in supramolecular polymerization of metal-containing monomers and discuss their implications for future research. Additionally, we also outline some major challenges that metallosupramolecular chemists (will) have to face to produce metallosupramolecular polymers (MSPs) with advanced applications and functionalities.

Real-Time Fluorescence Visualization of Nanoparticle Aggregation and the Polymorph-Transition Process of a Mechanofluorochromic Difluoroboron-β-Diketone Derivative

The use of organic nanomaterials in biomedical and optical devices has been widely studied. The key to improving the performance and stability of these devices is to control the fabrication process, which determines the phase stability and photophysical properties. In this study, fluorescence changes were observed during the reprecipitation process of mechanofluorochromic molecules of dibenzoyl(methanato)boron difluoride. The cyan-emission phase (C-phase) was first identified. The time evolution of the resolved fluorescence spectra revealed that the green-emission phase (G-phase) was formed from the amorphous phase with yellow emission via the C-phase, in addition to the direct formation of the G-phase. Combined with the results of the investigation into the thermal properties, the fluorescence changes clearly indicate a two-step nucleation process and Ostwald's rule of stages for polymorph transition, which enables us to not only provide guidance for controlling the fabrication process but also propose the ripening process for organic nanoparticle formation.

Continuum Crystallization Model Derived from Pharmaceutical Crystallization Mechanisms

The crystallization mechanisms of organic molecules in solution are not well-understood. The mechanistic scenarios where crystalline order evolves directly from the molecularly dissolved state ("classical") and from initially formed amorphous intermediates ("nonclassical") are suggested and debated. Here, we studied crystallization mechanisms of two widely used analgesics, ibuprofen (IbuH) and etoricoxib (ETO), using direct cryogenic transmission electron microscopy (cryo-TEM) imaging. In the IbuH case, parallel crystallization pathways involved diverse phases of high and low density, in which the instantaneous formation of final crystalline order was observed. ETO crystallization started from well-defined round-shaped amorphous intermediates that gradually evolved into crystals. This mechanistic diversity is rationalized by introducing a continuum crystallization paradigm: order evolution depends on ordering in the initially formed intermediates and efficiency of molecular rearrangements within them, and there is a continuum of states related to the initial order and rearrangement rates. This model provides a unified view of crystallization mechanisms, encompassing classical and nonclassical pictures.

Unraveling Halogen Effects in Supramolecular Polymerization

Halogens play a crucial role in numerous natural processes and synthetic materials due to their unique physicochemical properties and the diverse interactions they can engage in. In the field of supramolecular polymerization, however, halogen effects remain poorly understood, and investigations have been restricted to halogen bonding or the inclusion of polyfluorinated side groups. Recent contributions from our group have revealed that chlorine ligands greatly influence molecular packing and pathway complexity phenomena of various metal complexes. These results prompted us to explore the role of the halogen nature on supramolecular polymerization, a phenomenon that has remained unexplored to date. To address this issue, we have designed a series of archetypal bispyridyldihalogen Pt^II complexes bearing chlorine (1), bromine (2), or iodine (3) and systematically compared their supramolecular polymerization in nonpolar media using various experimental methods and theory. Our studies reveal a remarkably different supramolecular polymerization for the three compounds, which can undergo two competing pathways with either slipped (kinetic) or parallel (thermodynamic) molecular packing. The halogen exerts an inverse effect on the energetic levels of the two self-assembled states, resulting in a single thermodynamic pathway for 3, a transient kinetic species for 2, and a hidden thermodynamic state for 1. This seesaw-like bias of the energy landscape can be traced back to the involvement of the halogens in weak N-H···X hydrogen-bonding interactions in the kinetic pathway, whereas in the thermodynamic pathway the halogens are not engaged in the stabilizing interaction motif but rather amplify solvophobic effects.

Enhanced Two-Photon Absorption in Two Triphenylamine-Based All-Organic Compounds

Two-photon absorption (TPA) enables the excitation of molecules by comparatively lower energy photons with longer penetration depth and higher spatial precision control, which advances the uses of organic molecules in various applications. In this work, we report two simple all-organic molecules C₄₂H₃₃N (compound 3) and C₁₃₈H₁₆₈N₄ (compound 14) with strong TPA and fluorescent emission activity. Density functional theory calculations show that the enhanced oscillator strengths could be responsible for improved TPA and emission activity for compound 14 compared to those for 3. The degradation of C₁₃₈H₁₆₈N₄ under focused laser illumination without circulation is almost negligible within 5 h, making it a candidate for laser dyes. Solid-state measurements confirm the presence of a direct band gap for electron transition that determines the channel to release the absorbed energy and functionality of the studied molecules. This work emphasizes that a high TPA cross-section and selectable energy relaxation (fluorescent emission or heat dissipation) are equally important to the design of advanced functional TPA molecules.

Three-step nucleation of metal-organic framework nanocrystals

Metal-organic frameworks (MOFs) are crystalline nanoporous materials with great potential for a wide range of industrial applications. Understanding the nucleation and early growth stages of these materials from a solution is critical for their design and synthesis. Despite their importance, the pathways through which MOFs nucleate are largely unknown. Using a combination of in situ liquid-phase and cryogenic transmission electron microscopy, we show that zeolitic imidazolate framework-8 MOF nanocrystals nucleate from precursor solution via three distinct steps: 1) liquid-liquid phase separation into solute-rich and solute-poor regions, followed by 2) direct condensation of the solute-rich region into an amorphous aggregate and 3) crystallization of the aggregate into a MOF. The three-step pathway for MOF nucleation shown here cannot be accounted for by conventional nucleation models and provides direct evidence for the nonclassical nucleation pathways in open-framework materials, suggesting that a solute-rich phase is a common precursor for crystallization from a solution.

Complex structures arising from the self-assembly of a simple organic salt

Molecular self-assembly is the spontaneous association of simple molecules into larger and ordered structures¹. It is the basis of several natural processes, such as the formation of colloids, crystals, proteins, viruses and double-helical DNA². Molecular self-assembly has inspired strategies for the rational design of materials with specific chemical and physical properties³, and is one of the most important concepts in supramolecular chemistry. Although molecular self-assembly has been extensively investigated, understanding the rules governing this phenomenon remains challenging. Here we report on a simple hydrochloride salt of fampridine that crystallizes as four different structures, two of which adopt unusual self-assemblies consisting of polyhedral clusters of chloride and pyridinium ions. These two structures represent Frank-Kasper (FK) phases of a small and rigid organic molecule. Although discovered in metal alloys^4,5 more than 60 years ago, FK phases have recently been observed in several classes of supramolecular soft matter^6-11 and in gold nanocrystal superlattices¹² and remain the object of recent discoveries¹³. In these systems, atoms or spherical assemblies of molecules are packed to form polyhedra with coordination numbers 12, 14, 15 or 16. The two FK structures reported here crystallize from a dense liquid phase and show a complexity that is generally not observed in small rigid organic molecules. Investigation of the precursor dense liquid phase by cryogenic electron microscopy reveals the presence of spherical aggregates with sizes ranging between 1.5 and 4.6 nanometres. These structures, together with the experimental procedure used for their preparation, invite interesting speculation about their formation and open different perspectives for the design of organic crystalline materials.

Control over size, shape, and photonics of self-assembled organic nanocrystals

The facile fabrication of free-floating organic nanocrystals (ONCs) was achieved via the kinetically controlled self-assembly of simple perylene diimide building blocks in aqueous medium. The ONCs have a thin rectangular shape, with an aspect ratio that is controlled by the content of the organic cosolvent (THF). The nanocrystals were characterized in solution by cryogenic transmission electron microscopy (cryo-TEM) and small-angle X-ray scattering. The ONCs retain their structure upon drying, as was evidenced by TEM and atom force microscopy. Photophysical studies, including femtosecond transient absorption spectroscopy, revealed a distinct influence of the ONC morphology on their photonic properties (excitation energy transfer was observed only in the high-aspect ONCs). Convenient control over the structure and function of organic nanocrystals can enhance their utility in new and developed technologies.

Non-classical crystallisation pathway directly observed for a pharmaceutical crystal via liquid phase electron microscopy

Non-classical crystallisation (NCC) pathways are widely accepted, however there is conflicting evidence regarding the intermediate stages of crystallisation, how they manifest and further develop into crystals. Evidence from direct observations is especially lacking for small organic molecules, as distinguishing these low-electron dense entities from their similar liquid-phase surroundings presents signal-to-noise ratio and contrast challenges. Here, Liquid Phase Electron Microscopy (LPEM) captures the intermediate pre-crystalline stages of a small organic molecule, flufenamic acid (FFA), a common pharmaceutical. High temporospatial imaging of FFA in its native environment, an organic solvent, suggests that in this system a Pre-Nucleation Cluster (PNC) pathway is followed by features exhibiting two-step nucleation. This work adds to the growing body of evidence that suggests nucleation pathways are likely an amalgamation of multiple existing non-classical theories and highlights the need for the direct evidence presented by in situ techniques such as LPEM.

Photochemical Methods for the Real-Time Observation of Phase Transition Processes upon Crystallization

We have used the fluorescence detection of phase transformation dynamics of organic compounds by photochemical methods to observe a real-time symmetry breaking process. The organic fluorescent molecules vary the fluorescence spectra depending on molecular aggregated states, implying fluorescence spectroscopy can be applied to probe the evolution of the molecular-assembling process. As an example, the amorphous-to-crystal phase transformation and crystallization with symmetry breaking at droplet during the solvent evaporation of mechanofluorochromic molecules are represented in this review.

Observation of crystallisation dynamics by crystal-structure-sensitive room-temperature phosphorescence from Au(I) complexes

The aggregation behaviour of Au(I) complexes in condensed phases can affect their emission properties. Herein, aggregation-induced room-temperature phosphorescence (RTP) is observed from the crystals of trinuclear Au(I) complexes. The RTP is highly sensitive to the crystal structure, with a slight difference in the alkyl side chains causing not only a change in the crystal structure but also a shift in the RTP maximum. Furthermore, in nanocrystals, reversible RTP colour changes are induced by phase transitions between crystal polymorphs during crystal growth from solution or the pulverisation of bulk crystals. The colour change mechanism is discussed in terms of intermolecular interactions in the crystal structure of the luminescent aggregates. The results suggest that the behaviour in nanocrystals may differ from that in bulk crystals. These insights will advance the fundamental understanding of crystallisation mechanisms and may aid in the discovery of new materials properties for solids with nano- to micrometre sizes.

Molecular evolution pathways during nucleation of small organic molecules: solute-rich pre-nucleation species enable control over the nucleation process

Increasing evidence has shown that nucleation pathways involving disordered pre-nucleation species exist in the nucleation process of many types of solid state products, especially inorganic solid state products. Studying the thermodynamic and kinetic properties of these pre-nucleation species is crucial to understand and control the nucleation process of solid state products. In this work, the evolution pathway of molecular or supramolecular structures during the nucleation process was investigated by using 2-cyano-4'-methylbiphenyl (OTBN) as a model compound. In the resultant solutions, similar pre-nucleation clusters were analyzed and characterized by dynamic light scattering (DLS), small-angle X-ray scattering (SAXS) and nanoparticle tracking analysis (NTA). It was found that the clusters were disordered and liquid-like and did not represent any of the known OTBN condensed phases. They were of interest since they may be the key sites for the formation of new crystal nuclei of OTBN. It was demonstrated that the change in the solvation effect would drive the pre-nucleation clusters to exhibit very different structures. How the clusters vary with concentration and temperature, and how they differ before and after nucleation have been systematically studied. In addition, the molecular dynamics of the evolution of clusters, the effect of initial mixing process on clusters and the nucleation dynamics were also investigated. The results suggested that the pre-nucleation clusters played a key role in the process of crystallization of organic small molecules, indicating that the dynamics of nucleation could be regulated by changing the structure and size of the pre-nulceation clusters.

Evolution of CdTe Magic-Size Clusters with Single Absorption Doublet Assisted by Adding Small Molecules during Prenucleation

An approach is reported for the exclusive production of CdTe magic-size clusters (MSCs) that exhibit an optical absorption doublet peaking at 385/427 nm, with an explanation of the synthesis procedure. The MSCs, defined as dMSC-427, were produced from the reaction of cadmium oleate (Cd(OA)₂) and tri-n-octylphosphine telluride in octadecene at 100 °C, with the addition of acetic acid (HOAc) or acetate (M(OAc)₂) during the prenucleation stage (40 °C). Without such an addition or when it was performed in the postnucleation stage (100 °C), quantum dots (QDs) developed. The production of dMSC-427 or QDs is hypothesized to be related to the solubility of the Cd precursor, such as Cd(OA)₁(OAc)₁ or Cd(OA)₂, respectively. Also, the reactions that lead to Cd(OA)₁(OAc)₁ are proposed. The present study provides an in-depth understanding of the two-pathway model proposed for the prenucleation stage of binary colloidal QDs, as well as of the formation of MSCs and/or QDs.

Visualising early-stage liquid phase organic crystal growth via liquid cell electron microscopy

Here, we show that the development of nuclei and subsequent growth of a molecular organic crystal system can be induced by electron beam irradiation by exploiting the radiation chemistry of the carrier solvent. The technique of Liquid Cell Electron Microscopy was used to probe the crystal growth of flufenamic acid; a current commercialised active pharmaceutical ingredient. This work demonstrates liquid phase electron microscopy analysis as an essential tool for assessing pharmaceutical crystal growth in their native environment while giving insight into polymorph identification of nano-crystals at their very inception. Possible mechanisms of crystal nucleation due to the electron beam with a focus on radiolysis are discussed along with the innovations this technique offers to the study of pharmaceutical crystals and other low contrast materials.

Harmonic light scattering study reveals structured clusters upon the supramolecular aggregation of regioregular poly(3-alkylthiophene)

Solubilized poly(3-alkylthiophene)s are known to self-assemble into well-ordered supramolecular aggregates upon lowering the solvent quality. This supramolecular organization largely determines the optical and electronic properties of these polymers. However, despite numerous studies the exact mechanism and kinetics of the aggregation process and the role of external stimuli are still poorly understood. Classical characterization techniques such as electronic spectroscopy, dynamic light scattering, and diffraction-based techniques have not been able to provide a full understanding. Here we use second-harmonic scattering (SHS) and third-harmonic scattering (THS) techniques to investigate this supramolecular aggregation mechanism. Our results indicate that the actual supramolecular aggregation is preceded by the formation of structured polymer-solvent clusters consistent with a nonclassical crystallization pathway.