The shape and structure of crystals significantly influence their properties and functionalities, a relationship observed in both biological and synthetic systems. Biomineralization often produces single crystals with complex, hierarchical architectures and curved features, unlike synthetic methods. While synthetic approaches to shape control exist, including interfacial synthesis, microemulsion, and template-assisted growth, achieving the multidomain single crystallinity found in biominerals remains a challenge, especially in metal-organic frameworks (MOFs). Current MOF research largely focuses on structure design and the exploitation of porosity, neglecting the potential of tailored morphologies as three-dimensional objects. This study aims to address this gap by introducing a novel method for creating hollow, multidomain single crystals with a unique morphology, and investigates the underlying mechanism.

Previous research has highlighted the importance of crystal shape in various applications, including impacting optical and mechanical properties, as well as cell membrane permeability for drug delivery. Controlling crystal shape in synthetic systems requires careful manipulation of parameters such as solvents and additives. While the synthesis of morphologically tailored MOFs is still in its early stages, studies have shown that controlling crystal size and shape can influence porosity, catalytic activity, and cellular uptake. However, most efforts have concentrated on designing crystal structures, limiting applications to those exploiting porosity. The authors’ group has previously demonstrated the formation of unique metallo-organic crystals with varying morphologies, including a yoyo-shaped single crystal with a multidomain and chiral morphology. This broke the traditional assumption that single-unit morphology is linked to monocrystallinity.

The study synthesized hollow, multidomain crystals using a tetrahedral organic ligand (TPVA) and nickel bromide (NiBr2) in a sonochemical-solvothermal process. A 1:2 molar ratio of TPVA:NiBr2 was used in a DMF/chloroform solvent mixture. Crucially, the solvents were sonicated for 1.5 hours before adding TPVA. The mixture was then heated at 105 °C for 48 hours. Control experiments were performed without sonication. The resulting crystals were characterized using various techniques: Scanning electron microscopy (SEM) and focused ion beam (FIB)-assisted cutting were employed to visualize the morphology and internal structure of the crystals. Transmission electron microscopy (TEM) and nanobeam electron diffraction were used to confirm single crystallinity. X-ray diffraction (SXRD), both home-source and synchrotron, provided detailed structural information. Powder X-ray diffraction (PXRD) was used to compare the structures of crystals obtained at different stages of the reaction. Raman and IR spectroscopy investigated structural changes during the synthesis. Electron paramagnetic resonance (EPR) measured the radical species generated by sonication. Elemental analyses determined the composition of the crystals. Thermogravimetric analysis (TGA) assessed thermal stability. Circular dichroism (CD) measured the optical activity. Micro-computed tomography (micro-CT) was performed to provide 3D reconstruction of the crystals' internal structures. The researchers conducted a series of ex situ experiments at various time points (30 min, 1.5 h, 3 h, 24 h, and 48 h) to monitor the morphological transformation from initial parallelogram-shaped structures to the final hollow, multidomain crystals.

The study successfully synthesized uniform, hollow, multidomain single crystals with a unique morphology consisting of six interconnected half-rods forming a hexagonal-like tube. The crystals exhibited high uniformity (length = 35.7 ± 5.1 µm, outer diameter = 13.6 ± 2.3 µm). Micro-CT analysis confirmed their hollow nature and double-cone-shaped channel structure. Despite their complex morphology, detailed crystallographic studies showed that the crystals were single crystalline, evidenced by well-defined lattice spacings in X-ray and electron diffraction patterns. The crystals are isostructural across different morphologies, belonging to the rare space group P622, indicating chiral packing. This chirality arises from the arrangement of achiral components. The synthesis process involves an inside-out Ostwald ripening mechanism, where initial, defective monodomain crystals dissolve from the inside while material is added to the outside, shaping the final multidomain structures. Sonication of the solvent is essential; it generates radicals that reduce the concentration of the active metal salt, influencing growth kinetics and leading to the formation of the hollow, multidomain crystals, instead of the solid, monodomain prisms observed in the absence of sonication. The morphological transformation was observed by ex situ SEM measurements and micro-CT measurements at different reaction times (30 minutes, 1.5 h, 3 h, 24 h, and 48 h). The parallelogram-shaped structures initially formed in both conditions transformed into prisms under solvothermal conditions and hollow multidomain crystals under sonochemical-solvothermal conditions. Differences in Raman and FT-IR spectra confirm the structural rearrangement during the morphological transformation. The Ni-N bond distances and lengths are within the range commonly found for coordinately saturated nickel complexes, but the axial position is longer than usual.

The findings reveal a unique crystal growth mechanism involving a transition from monodomain to multidomain single crystals via an inside-out Ostwald ripening process. The chirality and the rare P622 space group are conserved through the morphological transformation. The crucial role of sonication in reducing the metal salt concentration and accelerating the transformation emphasizes the importance of reaction kinetics in achieving these complex morphologies. The results expand the understanding of crystal growth and provide a novel strategy for producing chiral, porous, single-crystalline materials with complex morphologies, which may have implications for various applications.

This research successfully synthesized hollow, multidomain single crystals exhibiting chirality and belonging to a rare space group. The inside-out Ostwald ripening mechanism, coupled with sonication-induced radical generation and reduced metal salt concentration, was identified as crucial for obtaining this unique morphology. The findings represent a significant advancement in crystal engineering and open avenues for the design and synthesis of advanced materials with tailored properties. Future studies should explore the influence of other parameters (ligand modifications, different metal salts) to further optimize the formation of multidomain crystals and to expand the range of attainable morphologies.

The study primarily focuses on a specific ligand-metal combination. Further research is needed to determine the generality of the observed mechanism and its applicability to other systems. While the chiral nature of the crystals is established, the precise control over enantiomeric excess requires further investigation. The exact role of the radical species generated by sonication needs more detailed mechanistic study.