Through comprehensive analysis of carboxylatebased metal−organic frameworks (MOFs), we present general evidence that challenges the common perception of MOF metallinker bonds being static. Structural dynamics in MOFs, however, typically refers to the "breathing" behavior of cavities, where pores open and close in response to guest molecules, and to the transient binding of guest molecules, but dynamic bonding would explain important MOF phenomena in catalysis, postsynthetic exchange, negative thermal expansion, and crystal growth. Here, we demonstrate, through use of variable-temperature diffuse reflectance infrared Fourier transform spectroscopy (VT-DRIFTS) aided by ab initio plane wave density functional theory, that similar evidence for melting behavior in zeolitic imidazolate frameworks (ZIFs), i.e., reversible metal-linker bonding, driven by specific vibrational modes, can be observed for carboxylate MOFs by monitoring the redshifts of carboxylate stretches coupled to anharmonic metal-carboxylate oscillators. To demonstrate the generality of these findings, we investigate a wide class of carboxylate MOFs that includes iconic examples with diverse structures and metal-linker chemistry. As the very vibrations invoked in ZIF melting but heretofore unobserved for carboxylate MOFs, these metal-linker dynamics resemble the ubiquitous soft modes that trigger important phase transitions in diverse classes of materials while offering a fundamentally new perspective for the design of next-generation metal−organic materials.

The diverse optical, magnetic, and electronic behaviors of most colloidal semiconductor nanocrystals emerge from materials with limited structural and elemental compositions. Conductive metal−organic frameworks (MOFs) possess rich compositions with complex architectures but remain unexplored as nanocrystals, hindering their incorporation into scalable devices. Here, we report the controllable synthesis of conductive MOF nanoparticles based on Fe(1,2,3-triazolate) 2 . Sizes can be tuned to as small as 5.5 nm, ensuring indefinite colloidal stability. These solutionprocessable MOFs can be analyzed by solution-state spectroscopy and electrochemistry and cast into conductive thin films with excellent uniformity. This unprecedented analysis of MOF materials reveals a strong size dependence in optical and electronic behaviors sensitive to the intrinsic porosity and guest−host interactions of MOFs. These results provide a radical departure from typical MOF characterization, enabling insights into physical properties otherwise impossible with bulk analogues while offering a roadmap for the future of MOF nanoparticle synthesis and device fabrication.

Assigning optical band gaps to MOFs is paramount for understanding their optical, electronic, and reactivity properties, but literature reports have produced a wide range of values for even the same MOF. Despite the molecular nature of MOF electronic structures, experimental assignments employ Tauc analysis, a method applied to semiconductors. Here, we report optical band gaps of common MOFs and demonstrate that Gaussian fitting is more appropriate for assigning accurate gap energies. We further support this claim with DFT simulation, providing a reliable method for estimating optical band gaps from ground-state hybrid-GGA. MOFs that require Tauc analysis exhibit overlapping optical transitions uncommon for typical carboxylate-based MOFs and more akin to narrow-gap semiconductors. Taken together, these results provide a simple roadmap for assigning MOF optical band gaps.

Cooperative interactions are responsible for the useful properties of spin crossover (SCO) materialslarge hysteresis windows, critical temperatures near room temperature, and abrupt transitionswith hybrid framework materials exhibiting the greatest cooperativity and hysteresis of all SCO systems. However, little is known about the chemical origin of cooperativity in frameworks. Here, we present a combined experimental−computational approach for identifying the origin of cooperativity in the metal−organic framework (MOF) Fe(1,2,3-triazolate) 2 (Fe(TA) 2 ), which exhibits the largest known hysteresis window of all SCO materials and unusually high transition temperatures, as a roadmap for understanding the manipulation of SCO behavior in general. Variable-temperature vibrational spectroscopy provides evidence that "soft modes" associated with dynamic metal−linker bonding trigger the cooperative SCO transition. Thermodynamic analysis also confirms a cooperativity magnitude much larger than those of other SCO systems, while electron density calculations of Fe(TA) 2 support previous theoretical predictions that large cooperativity arises in materials where SCO produces considerable differences in metal−ligand bond polarities between different spin states. Taken together, this combined experimental−computational study provides a microscopic basis for understanding cooperative magnetism and highlights the important role of dynamic bonding in the functional behavior of framework materials.

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