Metal-organic frameworks (MOFs) are porous materials with diverse applications, but their synthesis into high-quality bulk single crystals remains a challenge. While crystals of some MOFs have reached lengths of 6–7 mm using methods like vapor diffusion and reseeding, bulk crystal growth for fundamental studies and practical applications is largely unmet. This research focuses on overcoming this limitation by investigating the growth of a novel MOF using a less conventional solvent, DMSO, offering a wider concentration range compared to commonly used solvents like DMF or DEF. The authors hypothesize that using DMSO and a specific combination of metal and organic components will lead to a MOF with properties conducive to bulk crystal growth via a homoepitaxial process.

The literature highlights the limited size of MOF single crystals typically synthesized, with examples such as MOF-5 (3 × 3 × 2 mm), HKUST-1 (2 × 3 × 4 mm), and Rb/Cs cyclodextrin MOFs (6–7 mm). Previous attempts to achieve larger crystals have involved techniques such as vapor diffusion, reseeding, and exploiting the Marangoni effect, resulting in centimeter-long, yet thin, needle-shaped crystals. However, the growth of bulk crystals of MOFs remains elusive. The existing literature lacks a comprehensive understanding of the crystal growth mechanisms and required conditions for achieving truly scalable MOF crystal growth. This study addresses this gap by exploring the impact of solvent choice and host-guest interactions on MOF crystal size and quality.

The researchers synthesized the MOF by mixing zinc acetate and terephthalic acid (TPA) in DMSO, a solvent rarely used for MOF synthesis. The crystal structure was analyzed using single-crystal X-ray diffraction (SXRD) and density functional theory (DFT) calculations. The synthesis involved two steps: (1) initial crystal growth by mixing zinc acetate and TPA in DMSO at 40°C, resulting in crystals up to 3.7 mm in length; and (2) homoepitaxy, where the grown crystal served as a seed for further growth in a fresh solution with adjusted molar ratios of zinc to terephthalate (4:1 to 5:1). The solution was replaced periodically, and any small crystals on the surface were carefully removed to ensure continued homoepitaxial growth. DFT calculations were used to optimize the molecular structure of Zn(DMSO)2²+ within the framework and to analyze the electrostatic potential. The detailed methods for reagent preparation, the initial crystal growth procedure, homoepitaxy, DFT calculations and data availability are clearly documented in the paper.

SXRD analysis revealed a three-dimensional MOF structure built from 2D Zn-terephthalate square lattices interconnected by acetate pillars through diatomic zinc nodes. Every second 1D channel along the b-axis is filled with chains of Zn(DMSO)2²+ cations, leading to charge neutralization. DFT calculations confirmed the host-guest electrostatic interactions between the anionic framework and the cationic 1D fillers. The homoepitaxial growth yielded large square cuboid crystals reaching dimensions of 19.8 × 4.2 × 4.2 mm, with growth continuing at the top and bottom ends. The continuous growth with no apparent size limit is primarily due to strong host-guest electrostatic attraction between the anionic framework and the cationic fillers, leading to a stable and continuously growing crystal structure. The crystal structure reveals that growth along the c-axis occurs by alternating deposition of the 2D Zn-terephthalate layer, acetate anion pillars, and 1D Zn(DMSO)2²+ chains. The charge compensation between the monoanionic pillars and the dicationic filler ensures the open porosity of every second 1D channel.

The findings directly address the research question of achieving bulk MOF crystal growth. The centimeter-scale crystals obtained represent a significant advancement in MOF synthesis. The key to the success lies in the host-guest charge-transfer mechanism, where the electrostatic attraction between the anionic framework and the cationic 1D fillers drives the continuous homoepitaxial growth. This mechanism contrasts with previous approaches that relied solely on thermodynamic factors. The use of DMSO as a solvent and careful control of solution chemistry during the homoepitaxy step were crucial for avoiding the nucleation of smaller crystals and ensuring a uniform growth of the larger crystals. This approach opens new avenues for synthesizing large single crystals of other MOFs and potentially explores heteroepitaxial growth with tailored linkers and metal species.

This research demonstrates successful homoepitaxy of an anionic MOF incorporating 1D arrays of Zn(DMSO)2²+ cations, resulting in centimeter-scale (2 cm) crystals. The host-guest charge-transfer mechanism is identified as the driving force behind this scalable epitaxial growth. The method holds significant potential for extending this approach to other MOF systems, offering the possibility of fabricating large, high-quality MOF crystals for diverse applications. Future research could explore the synthesis of other charge-transfer MOFs using different combinations of metal acetates and linear dicarboxylic acids, as well as examining heteroepitaxy with patterned linkers and metal species.

While the method successfully produces large crystals, the transparency of the crystal decreases in the older section, possibly due to aging. The growth process also leads to the formation of small crystals on the surface of the main crystal, necessitating a cleaning step to maintain the epitaxial growth. The long growth time (five months) for the largest crystal also represents a practical limitation. Further optimization of the growth parameters might help to reduce the growth time and improve crystal quality.