Small Crystals, Big Insights: How Electron Diffraction is Transforming Materials, Life Science, and Chemistry Research

Metal-Organic Frameworks (MOFs) are a class of crystalline materials characterized by their network-like arrangement of metal ions or clusters connected by organic linkers. This unique structure creates significant voids, or porosity, within the material. The ability to design and control the geometry of both the metal nodes and the organic linkers allows for the creation of diverse MOF structures with tailor-made pores.

MOFs are particularly interesting due to the potential for doing "interesting chemistry" inside their voids. Their applications are diverse and growing, including carbon capture (e.g., CALF-20 in pilot plants for CO₂ capture), gas storage, separation, catalysis, and sensing. A key feature that makes them unique is their tunable porosity and the ability of some MOFs to change their crystal structure in response to external stimuli like temperature or the presence of guest molecules, a phenomenon known as "breathing." This dynamic behavior, where the structure can expand and contract without losing connectivity, opens up further possibilities for smart materials.

Characterizing MOF structures has traditionally been challenging due to several factors. Stable MOFs, especially those made with hard, high-valent cations like zirconium, are often difficult to synthesize as large, high-quality single crystals suitable for conventional single-crystal X-ray diffraction (SC-XRD). This difficulty arises because the coordination bonds holding MOFs together are strong, limiting the "error checking" or reversibility needed for large crystal growth. Early examples, like the first zirconium MOF UiO-66, took two years to determine its structure from powder X-ray diffraction (PXRD) data due to the lack of prior knowledge and the complexity of potential cluster formations.

Many MOFs, particularly flexible ones, also undergo structural changes (e.g., collapse or expansion) when solvents are removed during activation processes, or under specific reaction conditions. These changes often cause SC-XRD quality crystals to fracture, preventing their characterization in these relevant states. Furthermore, PXRD, while useful for highly crystalline bulk materials, often struggles with distinguishing subtle structural differences, identifying mixed phases, or providing atomic-resolution information for entirely new, unpredicted structures.

Electron Diffraction (ED) offers a revolutionary solution by enabling the structural characterization of nanomaterials. It can collect full data sets rapidly (e.g., in 2 minutes) from individual crystals as small as hundreds of nanometers. This means MOFs that form as nanocrystalline materials, or those that fragment during activation or reactions, can now have their single-crystal structures determined. ED is particularly well-suited for activated MOFs, flexible MOFs, and for studying in situ structural changes, providing precise structural information that was previously unattainable with conventional techniques.

Coordination modulation is a synthesis technique that involves adding additional components, known as modulators, to the solvo- or hydrothermal crystallization processes of MOFs. These modulators are typically molecules, often monotopic carboxylic acids, that mimic the donor chemistry of the organic linkers but cannot connect multiple metal clusters.

The primary goal of coordination modulation is to influence the pH and coordination equilibria during MOF self-assembly. By reversibly interfering with the metal-ligand bonding, modulators enhance the "error checking" or reversibility of the coordination chemistry. This improved reversibility allows the MOF structure to effectively self-correct and grow into more crystalline materials.

The effects of coordination modulation can vary significantly:

• Enhancing crystallinity and crystal size: Modulators can promote the growth of larger, higher-quality single crystals by allowing for better structural rearrangement.
• Downsizing crystals: Conversely, some modulators can cap the coordination polymerization process, leading to the formation of smaller nanocrystals.
• Inducing defects: While less desirable, modulators can sometimes be trapped within the structure, leading to defects.
• Diversifying phase landscape: Importantly, by altering the synthetic conditions, modulators can lead to the formation of entirely new MOF structures or polymorphs that might not be accessible otherwise. This expands the range of discoverable MOF phases for a given metal-ligand combination.

The sample environment inside an electron diffractometer is under vacuum, which can lead to the desolvation and potential structural collapse of porous MOFs, especially those containing labile guest molecules. Cryogenic sample loading techniques address this challenge by allowing the characterization of solvated MOF structures and preserving their native state.

The process involves:

1. Flash freezing: MOF samples are flash-frozen in liquid nitrogen.
2. Cryo-transfer: A cryo-transfer holder is used to transfer the frozen sample into the diffractometer under cryogenic conditions.
3. Low-temperature measurement: The sample is maintained at low temperatures (e.g., 100-175 Kelvin) during data collection.

This method offers several key advantages:

• Preservation of solvation: It prevents the evaporation of guest molecules (solvents, water) from the MOF pores, allowing for the characterization of their solvated structures. This is crucial for understanding host-guest interactions, such as hydrogen bonding between the MOF framework and the guest molecules.
• Access to native states: By preserving the solvent, ED can characterize MOFs in their "as-synthesized" states, which might be different from their activated (solvent-free) states.
• Study of hydrates: It is particularly useful for studying MOF hydrates, preventing water loss during analysis and enabling the distinction of subtle differences in water ordering or occupancy within the pores.
• Mitigation of vacuum effects: While the sample is still exposed to vacuum, the cryogenic temperatures minimize structural changes that would occur at room temperature, thus allowing for more accurate representation of the MOF's structure in its solvated form.

Electron Diffraction (ED) is exceptionally well-suited for studying the "breathing" and other dynamic behaviors of flexible MOFs, which involve significant structural changes (expansion and contraction) in response to external stimuli. This is challenging with conventional methods because:

• X-ray quality crystals often fragment: Flexible MOF crystals, especially larger ones, typically do not survive the mechanical stress of large amplitude volume changes during activation (solvent removal) or gas adsorption processes, making SC-XRD impossible for these states.
• PXRD limitations: Powder X-ray diffraction can show phase changes but struggles to provide detailed atomic resolution of intermediate or transient structures, particularly if multiple phases coexist or if the transition is continuous.

ED overcomes these limitations by:

• Analyzing nanocrystals: It can obtain single-crystal structures from the smaller, still-crystalline particles that result from the fracture of larger crystals during activation or flexing.
• In-situ measurements: The vacuum environment of the diffractometer, combined with the ability to control temperature, allows for in situ observation of structural changes. For example, by gradually increasing temperature, researchers can induce desolvation and observe the resulting structural collapse or expansion in real-time.
• Trapping transient intermediates: Unlike stepwise breathing observed under ambient conditions (where only initial and final states are seen), the vacuum environment of ED can reveal "continuous breathing," enabling the trapping and characterization of transient intermediate phases that are otherwise unobservable. This provides a more complete picture of the structural pathway during flexing.
• Particle-to-particle heterogeneity: ED's ability to analyze multiple individual crystallites on a single grid allows researchers to probe heterogeneity within a bulk sample, revealing subtle differences in crystal structures or degrees of opening/closing between particles that might lead to broadened PXRD peaks.

Electron Diffraction provides unprecedented insights into the relationship between MOF structure and their gas adsorption properties, particularly by allowing the characterization of structures in states relevant to absorption measurements.

Conventional techniques often struggle to obtain high-resolution structures of MOFs in their activated (solvent-free) state or under the specific conditions of gas adsorption. However, ED can:

• Characterize activated structures: ED excels at determining the precise crystal structures of activated MOFs, which are typically fragile and prone to fracturing when solvents are removed. This provides the exact "empty" framework structure that gases will encounter.
• Mimic adsorption conditions: The sample environment within the electron diffractometer (high vacuum, variable temperature) can closely mimic the conditions under which gas adsorption isotherms are performed (e.g., 77 Kelvin and microbar vacuum for nitrogen absorption). This means ED can characterize the exact crystal structures of MOFs in the state they are in prior to or during absorption measurements.
• Explain physical phenomena: By comparing the ED-derived structures with gas adsorption data, researchers can directly correlate structural features (e.g., pore opening, internal flexibility) with observed adsorption behavior. For example, if a MOF remains partially open at 77 Kelvin as observed by ED, it might show porosity to nitrogen, whereas a MOF that is completely closed under the same conditions might be non-porous.
• Future directions with environmental cells: The development of environmental cells for TEM (Transmission Electron Microscopy) suggests that similar advancements for ED could allow for in operando characterization of MOFs directly during gas adsorption, observing real-time structural changes as gases are introduced.

This capability significantly enhances the ability to design MOFs with targeted gas adsorption selectivities and capacities.

Electron Diffraction is highly effective in resolving ambiguities and identifying heterogeneity in MOF structures that often challenge Powder X-ray Diffraction (PXRD). This is because ED provides single-crystal structural information from individual particles, rather than an averaged bulk signal.

Key ways ED helps:

• Distinguishing polymorphs/close structures: For materials like MIL-53 hydrates, where PXRD patterns of different polymorphs (e.g., primitive and centered monoclinic forms) are so similar they are almost indistinguishable, ED can isolate and characterize single crystals of both forms from the same sample grid. This directly proves their coexistence, resolving debates that even advanced spectroscopic techniques struggled with.
• Identifying phase impurities: ED can quickly analyze multiple crystallites within a bulk sample. This allows researchers to detect and characterize phase impurities or subtle structural variations (e.g., different levels of solvation, slight changes in crystal parameters) that might be averaged out or simply undetectable in a bulk PXRD pattern.
• Explaining broadened PXRD peaks: Broad PXRD peaks are sometimes interpreted as low crystallinity. However, ED can reveal that such broadening might instead be due to particle-to-particle heterogeneity, where the bulk sample consists of numerous crystalline particles with very slightly different crystalographic parameters (e.g., varying degrees of opening/closing in a flexible MOF). ED can quantify these individual variations.
• Characterizing unpredictable structures: When synthesis methods lead to completely new or unexpected phases whose PXRD patterns cannot be readily indexed or modeled (as seen with the formic acid-modulated zirconium MOF), ED can directly solve the novel single-crystal structure, providing the necessary foundational knowledge.

By offering a single-particle view, ED complements PXRD, allowing chemists to accurately understand the precise structural composition and variations within their bulk MOF samples.

A surprising and crucial finding from the application of Electron Diffraction to MOFs is their remarkable beam stability. Contrary to the common assumption that MOFs, being porous and often flexible, would be fragile and degrade under an electron beam, they exhibit significant stability. Researchers have found that they can run numerous data sets on individual crystallites repeatedly without observing any decrease in diffraction intensity. This is attributed to working with a low dose of electrons, suggesting that MOFs are "brilliantly suited" for structural characterization by ED. This high beam stability is a significant advantage, as it allows for extensive and reliable data collection.

This unexpected stability, combined with the sample environment of an electron diffractometer, opens up vast potential for new types of in situ experiments:

• Temperature-dependent studies: While the vacuum already induces desolvation, the ability to control temperature within the diffractometer allows for in situ observation of thermal transitions. This can reveal continuous breathing processes and enable the characterization of transient intermediate phases that are otherwise unobservable under ambient conditions, providing a more detailed understanding of MOF flexibility.
• Monitoring post-synthetic modifications: ED can be used to track structural changes resulting from post-synthetic modification reactions, where MOFs are chemically altered after their initial synthesis. These reactions often destroy SC-XRD quality crystals, but ED can analyze the resulting crystalline (albeit often small) products.
• Gas adsorption studies (future): While not yet routine, the development of environmental cells (like gas cells) for TEM suggests a future where MOFs can be characterized by ED in operando during gas adsorption. This would allow for direct observation of structural changes as gases penetrate the pores, providing unprecedented insight into their adsorption mechanisms.
• Probing solvent interactions: ED, especially with cryogenic loading, can reveal how different solvents interact with the MOF framework, including specific host-guest hydrogen bonding, which is vital for understanding MOF performance in various applications.

In essence, the combination of MOF beam stability and the controlled environment of the ED system pushes the boundaries of MOF crystallography beyond conventional structure determination, enabling the study of dynamic processes and heterogeneous materials at a single-particle level.