Probing the Crystal Structure Landscape by Doping: 4-Bromo, 4-Chloro, and 4-Methylcinnamic Acids

Polymorphic Solid Solutions in Molecular Crystals: Tips, Tricks, and Switches

Crystal polymorphism has been a topic of much interest for the past 20 years or so, especially since its scientific (and legal) importance to the pharmaceutical industry was realized. By contrast, the formation of solid solutions in molecular crystals has been overlooked despite its long-standing prevalence in the analogous field of inorganic crystals. Wilfully forgotten, crystalline molecular solid solutions may be very common in our world since molecular compounds are rarely produced with 100% purity, and impurities able to form solid solutions are difficult to reject via recrystallization. Given the importance of both polymorphism and solid solutions in molecular crystals, we share here some tips, tricks, and observations to aid in their understanding. First, we propose a nomenclature system fit for the description of molecular crystalline solid solutions capable of polymorphism (tips). Second, we highlight the challenges associated with their experimental and computational characterization (tricks). Third, we show that our recently reported observation that polymorph stabilities can change by virtue of solid solution formation is a general phenomenon, reporting it on a second system (switches). Our work focuses on the historically important compound benzamide forming solid solutions with nicotinamide and 3-fluorobenzamide.

Seeking Rules Governing Mixed Molecular Crystallization

Mixed crystals result when components of the structure are randomly replaced by analogues in ratios that can be varied continuously over certain ranges. Mixed crystals are useful because their properties can be adjusted by increments, simply by altering the ratio of components. Unfortunately, no clear rules exist to predict when two compounds are similar enough to form mixed crystals containing substantial amounts of both. To gain further understanding, we have used single-crystal X-ray diffraction, computational methods, and other tools to study mixed crystallizations within a selected set of structurally related compounds. This work has allowed us to begin to clarify the rules governing the phenomenon by showing that mixed crystals can have compositions and properties that vary continuously over wide ranges, even when the individual components do not normally crystallize in the same way. Moreover, close agreement of the results of our experiments and computational modeling demonstrates that reliable predictions about mixed crystallization can be made, despite the complexity of the phenomenon.

Tuning of bandgaps and emission properties of light-emitting diode materials through homogeneous alloying in molecular crystals

Alloy formation is ubiquitous in inorganic materials science, and it strongly depends on the similarity between the alloyed atoms. Since molecules have widely different shapes, sizes and bonding properties, it is highly challenging to make alloyed molecular crystals. Here we report the generation of homogenous molecular alloys of organic light emitting diode materials that leads to tuning in their bandgaps and fluorescence emission. Tris(8-hydroxyquinolinato)aluminium (Alq₃) and its Ga, In and Cr analogues (Gaq₃, Inq₃, and Crq₃) form homogeneous mixed crystal phases thereby resulting in binary, ternary and even quaternary molecular alloys. The M_xM′_(1−x)q₃ alloy crystals are investigated using X-ray diffraction, energy dispersive X-ray spectroscopy and Raman spectroscopy on single crystal samples, and photoluminescence properties are measured on the exact same single crystal specimens. The different series of alloys exhibit distinct trends in their optical bandgaps compared with their parent crystals. In the Al_xGa_(1−x)q₃ alloys the emission wavelengths lie in between those of the parent crystals, while the Al_xIn_(1−x)q₃ and Ga_xIn_(1−x)q₃ alloys have red shifts. Intriguingly, efficient fluorescence quenching is observed for the M_xCr_(1−x)q₃ alloys (M = Al, Ga) revealing the effect of paramagnetic molecular doping, and corroborating the molecular scale phase homogeneity.

Multicomponent molecular alloy crystals exhibit intriguing effects of tuning and quenching in their photoluminescence, suggesting ‘alloy-crystal engineering’ as a useful design strategy for molecular functional materials.

Synthetic Approaches to Halogen Bonded Ternary Cocrystals

Higher cocrystal synthesis depends acutely on a knowledge of supramolecular synthons. We report three synthetic approaches towards ternary halogen bonded cocrystals that illustrate specificity and generality. Electrophilicity/nucleophilicity differences are needed among alternative sites of halogen bond formation. The two halogen bonds A⋅⋅⋅B and B⋅⋅⋅C in a halogen bonded ternary cocrystal ABC need to be of different strength. Interaction mimicry of hydrogen bonds by halogen bonds is a viable approach towards ternaries as illustrated with the pyrene structure. Finally, the crystal engineer should well be able to anticipate halogen bonds that are stronger than hydrogen bonds.

Reply to the 'Comment on "Trimorphs of 4-bromophenyl 4-bromobenzoate. Elastic, brittle, plastic"' by J. J. Whittaker, A. J. Brock, A. Grosjean, M. C. Pfrunder, J. C. McMurtrie and J. K. Clegg, Chem. Commun., 2021, , DOI: 10.1039/D0CC07668F

Crystals that differ in their molecular constitution may yet share the same mechanical property, such as plastic deformation, because they are equivalent in a supramolecular sense.

Bandgap Tuning in Molecular Alloy Crystals Formed by Weak Chalcogen Interactions

We demonstrate systematic tuning in the optical bandgaps of molecular crystals achieved by the generation of molecular alloys/solid solutions of a series of diphenyl dichalcogenides-characterized by weak chalcogen bonding interactions involving S, Se, and Te atoms. Despite the variety in chalcogen bonding interactions found in this series of dichalcogenide crystals, they show isostructural interaction topologies, enabling the formation of solid solutions. The alloy crystals exhibit Vegard's law-like trends of variation in their unit cell dimensions and a nonlinear trend for the variation in optical bandgaps with respect to their compositions. Energy-dispersive X-ray and spatially resolved Raman spectroscopic studies indicate significant homogeneity in the domain structure of the solid solutions. Quantum periodic calculations of the projected density of states provide insights into the bandgap tuning in terms of the mixing of states in the alloy crystal phases.

Color- and Dimension-Tunable Light-Harvesting Organic Charge-Transfer Alloys for Controllable Photon-Transport Photonics

An electron donor/acceptor pair comprising perylene (Pe) and 9,10-dicyanoanthracene (DCA) was specifically designed to construct organic charge-transfer (CT) alloys via weak CT interaction through a solution co-assembly route. By adjusting the molar ratio between Pe and DCA, we achieve color- and dimension-tunable CT alloy assemblies involving one-dimensional (1D) (DCA)_1-x (Pe)_x (0 ≤ x ≤10 %) microribbons and two-dimensional (2D) (Pe)_1-y (DCA)_y (0 ≤ y ≤5 %) nanosheets as a consequence of energy transfer from DCA or α-Pe to Pe-DCA CT complex. Importantly, dimension-related optical waveguiding performances are also revealed: continuously adjustable optical loss in 1D (DCA)_1-x (Pe)_x microribbons and successive conversion from isotropic waveguide to anisotropic waveguide in 2D (Pe)_1-y (DCA)_y nanosheets. The present work provides a desired platform for in-depth investigation of light-harvesting organic CT alloy assemblies, which show promising applications in miniaturized optoelectronic devices.

Consistency and variability of cocrystals containing positional isomers: the self-assembly evolution mechanism of supramolecular synthons of cresol-piperazine

The disposition of functional groups can induce variations in the nature and type of interactions and hence affect the molecular recognition and self-assembly mechanism in cocrystals. To better understand the formation of cocrystals on a molecular level, the effects of disposition of functional groups on the formation of cocrystals were systematically and comprehensively investigated using cresol isomers (o-, m-, p-cresol) as model compounds. Consistency and variability in these cocrystals containing positional isomers were found and analyzed. The structures, molecular recognition and self-assembly mechanism of supramolecular synthons in solution and in their corresponding cocrystals were verified by a combined experimental and theoretical calculation approach. It was found that the heterosynthons (heterotrimer or heterodimer) combined with O-H⋯N hydrogen bonding played a significant role. Hirshfeld surface analysis and computed interaction energy values were used to determine the hierarchical ordering of the weak interactions. The quantitative analyses of charge transfers and molecular electrostatic potential were also applied to reveal and verify the reasons for consistency and variability. Finally, the molecular recognition, self-assembly and evolution process of the supramolecular synthons in solution were investigated. The results confirm that the supramolecular synthon structures formed initially in solution would be carried over to the final cocrystals, and the supramolecular synthon structures are the precursors of cocrystals and the information memory of the cocrystallization process, which is evidence for classical nucleation theory.