Molecular cannibalism: Sacrificial materials as precursors for hollow and multidomain single crystals

Crystal properties are strongly influenced by shape and structure in both natural and synthetic systems. Biological single-crystalline scaffolds with hierarchical architectures (e.g., sea urchin spines) display complex morphologies with curved features, while in synthetic systems nanoparticle shape affects optical, mechanical, and biological interactions. For MOFs, morphology control remains underdeveloped compared to structural design, limiting applications that exploit 3D object shape. The authors previously showed metallo-organic crystals with unusual morphologies, including a yoyo-shaped, chiral, multidomain single crystal that challenged the conventional link between single crystallinity and single-domain morphology. This study addresses whether paradoxical multidomain-yet-single-crystalline morphologies can be rationally formed in synthetic metallo-organic systems, and what mechanisms and growth conditions enable such structures while preserving single-crystallinity and chiral packing.

- Biological systems guide crystal shaping for function; examples include calcite spines and hierarchical architectures in marine organisms. - In synthetic nanocrystals, shape modulates optical properties, mechanical performance, and cell uptake relevant to drug delivery. - Facet-specific growth can be tuned via solvents/additives; interfacial synthesis, microemulsions, and templates are widely used to control morphology across organic and inorganic materials. - MOF morphology control is nascent; size/shape can influence porosity, catalysis, and cellular uptake, but most efforts focus on framework design and porosity-driven applications. - Prior work from the authors yielded diverse metallo-organic crystal morphologies, including a yoyo-shaped, chiral, multidomain single crystal, suggesting that single-crystallinity can coexist with complex multidomain morphology, echoing biominerals. - Sonochemistry is known to generate radicals and influence crystallization kinetics; it has been used in MOF synthesis typically via in situ ultrasonication of reactants leading to local heating/pressure from bubble cavitation.

Synthesis and growth conditions: - Components: tetrahedral ligand TPVA (1,3,5,7-tetrakis(4-[(E)-2-pyridin-4-yl-vinyl]phenyl)adamantane) and NiBr2. - Solvent system: DMF/CHCl3 mixtures. Sonochemical-solvothermal route (to hollow multidomain single crystals): - Pre-sonicate solvent mixture of CHCl3 (1.0 ml) and DMF (2.0 ml) in a glass vial using an ultrasonic cleaner (33–40 kHz) for 1.5 h in an ice bath. - Immediately dissolve TPVA (3.0 mg, 3.5 μmol; 1.2 mM) in the sonicated solvent. - Dissolve NiBr2 (5.0 mg) in 3.3 ml DMF (7.0 mM). - Mix 3.0 ml of TPVA solution (3.5 μmol) with 1.0 ml of metal salt solution (7.0 μmol) in a glass pressure tube (final [TPVA]=0.9 mM, [Ni salt]=1.8 mM; TPVA:NiBr2 = 1:2). - Seal, exclude light, and heat at 105 °C for 48 h without stirring. Cool 5 min to room temperature, open, isolate green precipitate by centrifugation, wash with ethanol. Solvothermal control (to monodomain prisms): - Identical to above but without the 1.5 h solvent sonication step. Time-resolved morphology tracking: - Stop reactions at set times (e.g., 30 min, 1.5 h, 3 h, 24 h, 48 h) for ex situ analyses. Characterization and analyses: - SEM: HRSEM ZEISS instruments at 1.5 kV; Everhart-Thornley detector. Samples drop-cast on Si and dried. - FIB cross-sections: Helios 600 dual-beam; Pt deposition to immobilize; used to expose interiors. - Micro-CT: For sonicated samples, Zeiss Micro-XCT400 (40 kV, 200 μA, 1200 projections over 180°, pixel 0.33 μm). For non-sonicated samples, Zeiss Xradia 520 Versa (100 kV, 90 μA, 2401 projections over 360°, pixel 0.39 μm). 3D reconstructions and analysis with Avizo 9.5. - TEM nanobeam electron diffraction: FEI Tecnai F20 Twin in STEM microprobe mode; ~10 nm defocused beam, 10 pA, 0.1 s per raster point; simulations via SingleCrystal. - PXRD: Rigaku Ultima III, Cu Kα, 40 kV/40 mA, scan 5–30° 2θ, step 0.025°, 12 s/step; phase analysis and Rietveld/whole-pattern refinement (Jade 2010). - SXRD: Synchrotron ID-29 ESRF (λ ≈ 0.700 Å) and Rigaku XtaLabPro (Cu Kα, λ=1.5418 Å) with 100 K data collection. Structure solution by SHELXT and refinement by SHELXL; Platon SQUEEZE applied; graphics via Mercury and PLATON. - Raman: LabRAM HR Evolution, 632.8 nm HeNe, 600 gr/mm, 100× objective; pixel spacing 1.3 cm−1. - FT-IR: Monitored pyridine bands and C=C bands during growth. - EPR: Bruker ELEXSYS 500 X-band; 20 mW, 0.1 mT modulation, 100 kHz, sweep 10 mT; spin trapping with DMPO; MATLAB EasySpin-based simulation; assessed radical formation post-sonication and lifetime (~45 min) in presence of NiBr2. - Elemental analysis: C, H, N, Cl, Ni, Br (Kolbe Laboratorium); inferred anion exchange and charge balance (Cl− from CHCl3) with Cl/Ni ≈ 2.2; Br− only traces (0.32–0.65%). - TGA: TA SDT Q600 under N2, 30–800 °C; compared sonicated vs non-sonicated products (t=48 h). - CD/UV: Chirascan-plus Auto CD over 200–600 nm, 1 mm pathlength; assessed bulk enantiomeric excess (zero signal indicates racemate). Mechanistic probes: - Ex situ SEM/micro-CT at staged times to map evolution from early parallelogram particles to cylinders to hollow multidomain structures. - Solvent sonication as key variable; also tested reduced NiBr2 concentration under solvothermal conditions and cross-swapping early parallelogram crystals into sonicated vs non-sonicated solvents to test conversion pathways.

- Novel morphology: Uniform hollow metallo-organic single crystals composed of six-connected half-rods arranged in a circular hexagonal-like tube. - Dimensions and uniformity (sonochemical-solvothermal, t=48 h): length l = 35.7 ± 5.1 μm; outer diameter ≈ 13.6 ± 2.3 μm. Early cylindrical monodomain intermediates at t=1.5 h: l = 29.9 ± 4.8 μm; ø = 8.2 ± 2.0 μm. - Internal architecture: Micro-CT and FIB-SEM show a continuous, hollow, double-cone-shaped channel; termini openings up to ~15 μm at 48 h; edge diameter ~10× core diameter. Interfaces between half-rods show relatively low electron density. - Surface features: Rounded termini with large micro-scale cavities with fine-textured inner surfaces resembling leaf veins; external surfaces evolve from smooth to layered, protruding six half-rods with three subdomains per half-rod. - Single-crystallinity: Entire crystals (100 μm beam home source; 30 μm synchrotron beam) display single-crystal diffraction with no evidence of multiplicity or twinning; TEM nanobeam diffraction aligns with SXRD-calculated patterns in corresponding zone axes. - Crystallography: Hexagonal space group P622 (a Sohncke group indicating chiral packing). Unit cells: t=1.5 h a=b=25.719 Å, c=17.870 Å; t=48 h a=b=25.961 Å, c=17.818 Å. Atomic resolution 1.19 Å (t=1.5 h) and 1.10 Å (t=48 h). Final R factors: 0.0762 and 0.0941 [I > 2σ(I)]. Flack parameters: 0.11(9) and 0.13(8), suggesting minor enantiomeric twinning. CD indicates racemic bulk samples (zero CD signal). - Framework details: Achiral TPVA and Ni2+ assemble into continuous chiral networks with hexagonal (~9.1 Å) and trigonal (~11.6 Å) channels along c-axis. Ni(II) octahedral coordination: four equatorial pyridines (Ni–N ≈ 2.021–2.037 Å at 1.5 h; 2.08–2.10 Å at 48 h) and two axial water ligands elongated (Ni–O ≈ 2.427–2.451 Å). - Growth mechanism: Ostwald ripening from early small parallelogram particles to larger cylinders, followed by inside-out Ostwald ripening where defective monodomain cylinders dissolve internally while material redeposits anisotropically on the shell, producing hollow multidomain single crystals. Evidence from Raman/IR spectral evolution and morphological time series. - Role of sonication/radicals: Solvent sonication generates DMF radicals (EPR lifetime ~45 min at RT in presence of NiBr2). Sonication is essential to obtain hollow multidomain single crystals. Lowering NiBr2 concentration by ≥50% under non-sonicated solvothermal conditions also yields hollow multidomain crystals, indicating that radical-induced reduction of active NiBr2 concentration drives the pathway. - Controls without sonication: Monodomain hexagonal prisms form (l = 19.9 ± 0.5 μm, Ø = 4.3 ± 0.4 μm at 48 h), solid and defect-free by micro-CT, isostructural (P622) and with similar PXRD, Raman, IR to sonicated products but lacking morphological transformation to multidomain hollow forms. - Anion exchange: Products contain primarily Cl− (Cl/Ni ≈ 2.2) with only trace Br−, implying post-coordination bromide-to-chloride exchange (CHCl3 source). Composition similar between sonicated and non-sonicated syntheses. - Thermal stability: TGA shows similar stability for both morphologies (t=48 h), indicating comparable 3D frameworks despite morphological differences.

The study demonstrates that complex multidomain morphologies can emerge while preserving single-crystallinity and chiral packing in a synthetic metallo-organic system. Initial monodomain cylinders formed rapidly under conditions that reduce the effective metal salt concentration (via solvent sonication and radical generation) are structurally defective internally. These unstable crystals act as sacrificial templates: dissolution from the interior combined with anisotropic redeposition on specific external regions yields a hollow, six-half-rod morphology with curved features, consistent with inside-out Ostwald ripening. Throughout this morphological evolution, the crystallographic framework remains essentially unchanged and single-crystalline, as verified by PXRD, SXRD, and TEM nanobeam diffraction, indicating that the transformation occurs via dissolution–reprecipitation that preserves lattice continuity. The sonication step is pivotal, not by cavitation during reaction (sonication is pre-treatment), but by generating radicals that persist for ~45 min and likely reduce the concentration of reactive NiBr2. This shift in metal-to-ligand balance alters growth kinetics, promoting fast formation of defective monodomain intermediates that subsequently undergo inside-out ripening. The observation that simply decreasing NiBr2 concentration in non-sonicated reactions reproduces the hollow multidomain morphology supports this mechanistic picture. In contrast, in the presence of excess active metal salt (no sonication), crystals ripen via conventional Ostwald ripening to solid, monodomain prisms. These findings bridge bioinspired paradoxical morphologies with synthetic crystal engineering, suggesting that octahedral coordination of metals by four equatorial pyridines predisposes to chiral P622 packing, while morphology is governed by growth kinetics, defect formation, and local supersaturation gradients. The work expands the design space of single-crystalline, chiral MOFs with complex 3D shapes.

This work introduces a generalizable route to hollow, multidomain, yet single-crystalline metallo-organic crystals by leveraging solvent sonication to modulate reactive metal salt concentration and growth kinetics. The resulting crystals exhibit a unique six half-rod, hexagonal-like tubular morphology with continuous double-cone channels, while retaining an isomorphous chiral framework (P622) assembled from achiral components. The study elucidates a mechanism of inside-out Ostwald ripening where early unstable monodomain cylinders serve as sacrificial precursors, dissolving internally with anisotropic redeposition on the outer shell. Main contributions: - Discovery of uniform hollow multidomain single crystals with chiral P622 packing. - Identification of solvent sonication (radical-mediated reduction of active NiBr2) and metal-to-ligand ratios as key levers controlling morphology versus framework identity. - Direct evidence for single-crystallinity coexisting with complex multidomain morphology, verified across multiple diffraction modalities. Potential future directions: - Explore generality across different metals, ligands, and solvent systems to access a library of paradoxical single-crystalline morphologies. - Quantify radical chemistry and speciation to precisely control active metal salt concentration during growth. - Real-time in situ studies (e.g., liquid-cell TEM, in situ SAXS/WAXS) to capture dissolution–redeposition dynamics. - Investigate functional implications of hollow, chiral single crystals (e.g., transport, catalysis, optics) and post-synthetic modifications.

- Mechanistic role of radicals is inferred: EPR confirms radical formation and lowering NiBr2 concentration mimics sonication effects, but the precise chemical pathways reducing active NiBr2 and their kinetics are not fully resolved. - Generality is shown for one ligand–metal system (TPVA–Ni2+ in DMF/CHCl3); applicability to broader chemistries remains to be established. - Minor enantiomeric twinning is indicated by Flack parameters, and bulk samples are racemic (zero CD), which may limit chiroptical applications without enantioselection. - Ex situ time-resolved analysis provides snapshots; continuous in situ monitoring could better capture transient intermediates and rate processes.