A review of the effect of multiple conformers on crystallization from solution and strategies for crystallizing slow inter-converting conformers

Growth "self-inhibition" of irbesartan desmotrope: surface intra-annular tautomer inter-conversion is the culprit

The newly discovered growth self-inhibition phenomenon of tautomeric crystals is now generalized to the demostrope (form B) of irbesartan that displays intra-annular tautomerism in neutral aqueous solutions. The dynamic intra-annular tautomer inter-conversion on the surface is the key factor. Our findings provide implications for producing and engineering tautomeric materials.

Highly Ordered Small Molecule Organic Semiconductor Thin-Films Enabling Complex, High-Performance Multi-Junction Devices

Organic semiconductors have opened up many new electronic applications, enabled by properties like flexibility, low-cost manufacturing, and biocompatibility, as well as improved ecological sustainability due to low energy use during manufacturing. Most current devices are made of highly disordered thin-films, leading to poor transport properties and, ultimately, reduced device performance as well. Here, we discuss techniques to prepare highly ordered thin-films of organic semiconductors to realize fast and highly efficient devices as well as novel device types. We discuss the various methods that can be implemented to achieve such highly ordered layers compatible with standard semiconductor manufacturing processes and suitable for complex devices. A special focus is put on approaches utilizing thermal treatment of amorphous layers of small molecules to create crystalline thin-films. This technique has first been demonstrated for rubrene─an organic semiconductor with excellent transport properties─and extended to some other molecular structures. We discuss recent experiments that show that these highly ordered layers show excellent lateral and vertical mobilities and can be electrically doped to achieve high n- and p-type conductivities. With these achievements, it is possible to integrate these highly ordered layers into specialized devices, such as high-frequency diodes or completely new device principles for organics, e.g., bipolar transistors.

Molecular Assembly and Nucleation Kinetics during Nucleation of 3,5-Dinitrobenzoic Acid

As one of the most important processes in the process of crystallization, nucleation determines the physicochemical properties of the crystal products. The mechanism of nucleation has not been sufficiently understood due to the complexity of the molecular assembly process. In this work, a rigid molecule of 3,5-dinitrobenzoic acid (DNBA) was selected as the model compound to investigate the connection between nucleation kinetics and solution chemistry and to investigate the mechanism of nucleation. The nucleation induction period was determined by the nonrandom method, and the parameters including interfacial energy γ and collision frequency f₀C₀ were calculated. FTIR, NMR, and MS were used to analyze the existing form of DNBA molecules in solutions. It was found that the solute exists in the form of monomer, multimers, and solvates in different solvents. Besides, molecular simulation and calculation were also used to investigate the intermolecular interactions of DNBA in different solvents, and the relationship between the molecular existing form and the nucleation kinetics was revealed. Finally, a possible nucleation mechanism of DNBA molecules in solution was proposed.

Tautomerism unveils a self-inhibition mechanism of crystallization

Modifiers are commonly used in natural, biological, and synthetic crystallization to tailor the growth of diverse materials. Here, we identify tautomers as a new class of modifiers where the dynamic interconversion between solute and its corresponding tautomer(s) produces native crystal growth inhibitors. The macroscopic and microscopic effects imposed by inhibitor-crystal interactions reveal dual mechanisms of inhibition where tautomer occlusion within crystals that leads to natural bending, tunes elastic modulus, and selectively alters the rate of crystal dissolution. Our study focuses on ammonium urate crystallization and shows that the keto-enol form of urate, which exists as a minor tautomer, is a potent inhibitor that nearly suppresses crystal growth at select solution alkalinity and supersaturation. The generalizability of this phenomenon is demonstrated for two additional tautomers with relevance to biological systems and pharmaceuticals. These findings offer potential routes in crystal engineering to strategically control the mechanical or physicochemical properties of tautomeric materials.

Insight into the Nucleation Mechanism of p-Methoxybenzoic Acid in Ethanol-Water System from Metastable Zone Width

The metastable zone width (MSZW) of p-methoxybenzoic acid (PMBA) in an ethanol-water system was measured using the polythermal method. The nucleation order m obtained by the Nývlt’s model indicates the nucleation of PMBA following a progressive nucleation mechanism at low saturation temperature (m = 3.18–7.50) and an instantaneous nucleation mechanism at high saturation temperature (m = 1.46–2.55). Then, combined with the metastable zone experiment and the Sangwal model, we found that the MSZW and the interfacial energy reached the maximum when the mass fraction of ethanol was 0.8, which resulted in the smallest crystal product size. Meanwhile, the maximum rcrit and ΔGcrit obtained based on the modified Sangwal model indicating the PMBA needs to overcome a higher nucleation barrier in the ethanol mass fraction of 0.8. Finally, we proposed a preferential strategy for adjusting MSZW by correlating the interfacial energy with the change in ethanol mass fraction, saturation temperature, and cooling rate, respectively.

On the Supra-LUMO Interaction: Case Study of a Sudden Change of Electronic Structure as a Functional Emergence

The synergistic functioning of redox-active components that emerges from prototypical 2,2'-di(N-methylpyrid-4-ylium)-1,1'-biphenyl is described. Interestingly, even if a trans conformation of the native assembly is expected, due to electrostatic repulsion between cationic pyridinium units, we demonstrate that cis conformation is equally energy-stabilized on account of a peculiar LUMO (SupLUMO) that develops through space, encompassing the two pyridiniums in a single, made-in-one-piece, electronic entity (superelectrophoric behavior). This SupLUMO emergence, with the cis species as superelectrophore embodiment, originates in a sudden change of electronic structure. This finding is substantiated by insights from solid state (single-crystal X-ray diffraction) and solution (NOE NMR and UV-vis-NIR spectroelectrochemistry) studies, combined with electronic structure computations. Electrochemistry shows that electron transfers are so strongly correlated that two-electron reduction manifests itself as a single-step process with a large potential inversion consistent with inner creation of a carbon-carbon bond (digital simulation). Besides, absence of reductive formation of dimers is a further indication of a preferential intramolecular reactivity determined by the SupLUMO interaction (cis isomer pre-organization). The redox-gated covalent bond, serving as electron reservoir, was studied via atropisomerism of the reduction product (VT NMR study). The overall picture derived from this in-depth study of 2,2'-di(N-methylpyrid-4-ylium)-1,1'-biphenyl proves that trans and cis species are worth considered as intrinsically sharply different, that is, as doubly-electrophoric and singly-superelectrophoric switchable assemblies, beyond conformational isomerism. Most importantly, the through-space-mediated SupLUMO may come in complement of other weak interactions encountered in Supramolecular Chemistry as a tool for the design of electroactive architectures.

Unveiling the self-association and desolvation in crystal nucleation

As the first step in the crystallization process, nucleation has been studied by many researchers. In this work, phenacetin (PHEN) was selected as a model compound to investigate the relationship between the solvent and nucleation kinetics. Induction times at different supersaturation in six solvents were measured. FTIR and NMR spectroscopy were employed to explore the solvent-solute interactions and the self-association properties in solution. Density functional theory (DFT) was adopted to evaluate the strength of solute-solvent interactions and the molecular conformations in different solvents. Based on these spectroscopy data, molecular simulation and nucleation kinetic results, a comprehensive understanding of the relationship between molecular structure, crystal structure, solution chemistry and nucleation dynamics is discussed. Both the solute-solvent interaction strength and the supramolecular structure formed by the self-association of solute molecules affect the nucleation rate. The findings reported here shed new light on the molecular mechanism of nucleation in solution.

Molecular conformational evolution mechanism during nucleation of crystals in solution

Nucleation of crystals from solution is fundamental to many natural and industrial processes. In this work, the molecular mechanism of conformational polymorphism nucleation and the links between the molecular conformation in solutions and in crystals were investigated in detail by using 5-nitro-furazone as the model compound. Different polymorphs were prepared, and the conformations in solutions obtained by dissolving different polymorphs were analysed and compared. The solutions of 5-nitro-furazone were proven to contain multiple conformers through quantum chemical computation, Raman spectra analysis, 2D nuclear Overhauser effect spectroscopy spectra analysis and molecular dynamics simulation. The conformational evolution and desolvation path was illustrated according to the 1H NMR spectra of solutions with different concentrations. Finally, based on all the above analysis, the molecular conformational evolution path during nucleation of 5-nitro-furazone was illustrated. The results presented in this work shed a new light on the molecular mechanism of conformational polymorphism nucleation in solution.

Probing the structural pathway of conformational polymorph nucleation by comparing a series of α,ω-alkanedicarboxylic acids

Herein the nucleation pathway of conformational polymorphs was revealed by studying the relationships and distinctions among a series of α,ω-alkanedicarboxylic acids [HOOC-(CH2) n-2-COOH, named DAn, where n = 5, 7, 9, 11, 13, 15] in the solid state and in solution. Their polymorphic outcomes, with the exception of DA5, show solvent dependence: form I with conformation I crystallizes from solvents with hydrogen-bond donating (HBD) ability, whereas form II with conformation II crystallizes preferentially from solvents with no HBD ability. In contrast, form II of DA5 does not crystallize in any of the solvents used. Quantum mechanical computation showed that there is no direct conformational link between the solvents and the resultant polymorphic outcomes. Surprisingly, solute aggregates were found in no-HBD solvents by Fourier transform infrared spectroscopy, and only monomers could be detected in HBD solvents, suggesting stronger solvation. Furthermore, it was found that all six compounds including DA5 followed the same pattern in solution. Moreover, crystal-packing efficiency calculations and stability tests stated that dimorphs of DA5 bear a greater stability difference than others. These suggest that the rearrangement from conformation II to I could not be limited by hard desolvation in HBD solvents, where form I was also obtained. In other systems, metastable II was produced in the same solvents, probably as a result of the rearrangement being limited by hard desolvation. In this work, a comparative study uncovers the proposed nucleation pathway: difficulty in desolvation has a remarkable effect on the result of rearrangement and nucleation outcome.

Review and Modeling of Crystal Growth of Atropisomers from Solutions

In this paper, theories on anisotropic crystal growth and crystallization of atropisomers are reviewed and a model for anisotropic crystal growth from solution containing slow inter-converting conformers is presented. The model applies to systems with growth-dominated crystallization from solutions and assumes that only one conformation participates in the solute integration step and is present in the crystal lattice. Other conformers, defined as the wrong conformers, must convert to the right conformer before they can assemble to the crystal lattice. The model presents a simple implicit method for evaluating the growth inhibition effect by the wrong conformers. The crystal growth model applies to anisotropic growth in two main directions, namely a slow-growing face and a fast-growing face and requires the knowledge of solute crystal face integration coefficients in both directions. A parameter estimation algorithm was derived to extract those coefficients from data about temporal concentration and crystal size during crystallization and was designed to have a short run time, while providing a high-resolution estimation. The model predicts a size-dependent growth rate and simulations indicated that for a given seed size and solvent system and for an isothermal anti-solvent addition crystallization, the seed loading and the supersaturation at seeding are the main factors impacting the final aspect ratio. The model predicts a decrease of the growth inhibition effect by the wrong conformer with increasing temperature, likely due to faster equilibration between conformers and/or a decrease of the population of the wrong conformer, if of low energy, at elevated temperatures. Finally, the model predicts that solute surface integration becomes the rate-limiting mechanism for high solute integration activation energies, resulting in no impact of the WC on the overall crystal growth process.