Designing Glass and Crystalline Phases of Metal-Bis(acetamide) Networks to Promote High Optical Contrast

Carboxylate-Based Metal-Organic Framework and Coordination Polymer Glasses: Progress and Perspectives

ConspectusCoordination polymers (CPs) and metal-organic frameworks (MOFs) represent versatile materials with diverse structural and functional properties, making them appealing for various applications. However, their conventional forms, which are typically synthesized as powders or crystals, pose challenges due to limited processability and mechanical fragility. Recently, CP/MOF glasses have emerged as promising alternatives, offering enhanced processability while retaining some of the unique characteristics shown in the mother crystalline materials. Despite the prevalence of carboxylate ligands in CP/MOF synthesis, the development of carboxylate-based CP/MOF glasses has been limited compared to that of zeolitic-imidazole framework (ZIF)-based glasses. This is attributed to the strong metal-ligand bonds and low thermal stability of carboxylic acids, which hinder their melting in CP/MOF structures. Nonetheless, recent advancements have led to a surge in methods for synthesizing carboxylate-based CP/MOF glasses. So far, desolvation and melt-quenching have been introduced for achieving glass structures from CP/MOF precursors.The first melt-quenched MOF glass was reported in 2015 with ZIFs. However, we informally observed the melting of the MOF during thermal decomposition research of aliphatic carboxylate-based MOFs as a sacrificial template dating back to 2013. In that study, aliphatic ligands, instead of aromatic carboxylate, were employed due to their high lability, lower thermal stability, and high degree of freedom, which facilitated pyrolysis. The results were published with a focus on synthesizing hierarchically porous MgO via the pyrolysis of an aliphatic ligand-based Mg-MOF in an inert environment. A decade later, it was revisited and studied as the first melt-quenched carboxylate-based MOF glass, converted from a crystalline MOF through the liquid phase before decomposition during the heating process.This Account aims to introduce six studies, including the aforementioned example, on the synthesis of CP/MOF glasses from carboxylate-based CPs/MOFs that have been published so far. To overcome the challenges with aromatic carboxylates in CP/MOF glass formation, the metal coordination sphere should be altered and the degree of freedom in the ligands should be increased. Based on these approaches, the strategies for vitrification of carboxylate-based CPs/MOFs can be divided into two categories: desolvation and melt-quenching. Desolvation can be preceded by vapor perturbation such as hydration. Carboxylate-based CP/MOF glasses possess the potential to expand into a broader range of applications beyond those of existing CP/MOF glasses. Alongside the diversity offered by carboxylic acid ligands, these materials mirror the extensive range of applications previously explored in the existing carboxylate-based CP/MOF crystals. Moreover, their high processability, inherent to glass materials, enables their applications in various industrial fields. This versatility may extend to previously unexplored areas of utilization such as a novel class of bioactive glass.

Functional metal-organic liquids

For decades, the study of coordination polymers (CPs) and metal-organic frameworks (MOFs) has been limited primarily to their behavior as crystalline solids. In recent years, there has been increasing evidence that they can undergo reversible crystal-to-liquid transitions. However, their "liquid" states have primarily been considered intermediate states, and their diverse properties and applications of the liquid itself have been overlooked. As we learn from organic polymers, ceramics, and metals, understanding the structures and properties of liquid states is essential for exploring new properties and functions that are not achievable in their crystalline state. This review presents state-of-the-art research on the liquid states of CPs and MOFs while discussing the fundamental concepts involved in controlling them. We consider the different types of crystal-to-liquid transitions found in CPs and MOFs while extending the interpretation toward other functional metal-organic liquids, such as metal-containing ionic liquids and porous liquids, and try to suggest the unique features of CP/MOF liquids. We highlight their potential applications and present an outlook for future opportunities.

Controlling glass forming kinetics in 2D perovskites using organic cation isomers

The recent discovery of glass-forming metal halide perovskites (MHPs) provides opportunities to broaden the application domain beyond traditionally celebrated optoelectronic research fueled by associated crystalline counterparts. In this regard, it is crucial to diversify the compositional space of glass-forming MHPs and introduce varied crystallization kinetics via synthetic structural engineering. Here, we compare two MHPs with slightly varying structural attributes, utilizing isomer organic cations with the same elemental composition, and demonstrate how this change in functional group position impacts the kinetics of glass formation and subsequent crystallization by multiple orders of magnitude. (S)-(-)-1-(1-Naphthyl)ethylammonium lead bromide (S(1-1)NPB) exhibits a lower melting point (Tm) of 175 °C and the melt readily vitrifies under a critical cooling rate (CCR) of 0.3 °C s-1. In contrast, (S)-(-)-1-(2-naphthyl)ethylammonium lead bromide (S(1-2)NPB) displays a Tm ∼193 °C and requires a CCR of 2500 °C s-1, necessitating the use of ultrafast calorimetry for glass formation and study of the underlying kinetics. The distinct Tm and glass-formation kinetics of the isomer MHPs are further understood through a combination of calorimetric and single-crystal X-ray diffraction studies on their crystalline counterparts, highlighting the influence of altered organic-inorganic hydrogen bonding interactions and entropic changes around melting, providing insights into the factors driving their divergent behaviors.

Rational Design of 2D Metal Halide Perovskites with Low Congruent Melting Temperature and Large Melt-Processable Window

Metal halide perovskites (MHPs) have garnered significant attention due to their distinctive optical and electronic properties, coupled with excellent processability. However, the thermal characteristics of these materials are often overlooked, which can be harnessed to cater to diverse application scenarios. We showcase the efficacy of lowering the congruent melting temperature (Tm) of layered 2D MHPs by employing a strategy that involves the modification of flexible alkylammonium through N-methylation and I-substitution. Structural-property analysis reveals that the N-methylation and I-substitution play pivotal roles in reducing hydrogen bond interactions between the organic components and inorganic parts, lowering the rotational symmetry number of the cation and restricting the residual motion of the cations. Additional I···I interactions enhance intermolecular interactions and lead to improved molten stability, as evidenced by a higher viscosity. The 2D MHPs discussed in this study exhibit low Tm and wide melt-processable windows, e.g., (DMIPA)2PbI4 showcasing a low Tm of 98 °C and large melt-processable window of 145 °C. The efficacy of the strategy was further validated when applied to bromine-substituted 2D MHPs. Lowering the Tm and enhancing the molten stability of the MHPs hold great promise for various applications, including glass formation, preparation of high-quality films for photodetection, and fabrication of flexible devices.

Organic-Inorganic Hybrid Glasses of Atomically Precise Nanoclusters

Organic-inorganic atomically precise nanoclusters provide indispensable building blocks for establishing structure-property links in hybrid condensed matter. However, robust glasses of ligand-protected nanocluster solids have yet to be demonstrated. Herein, we show [Cu4I4(PR3)4] cubane nanoclusters coordinated by phosphine ligands (PR3) form robust melt-quenched glasses in air with reversible crystal-liquid-glass transitions. Protective phosphine ligands critically influence the glass formation mechanism, modulating the glasses' physical properties. A hybrid glass utilizing ethyldiphenylphosphine-based nanoclusters, [Cu4I4(PPh2Et)4], exhibits superb optical properties, including >90% transmission in both visible and near-infrared wavelengths, negligible self-absorption, near-unity quantum yield, and high light yield. Experimental and theoretical analyses demonstrate the structural integrity of the [Cu4I4(PPh2Et)4] nanocluster, i.e., iodine-bridged tetranuclear cubane, has been fully preserved in the glass state. The strong internanocluster CH-π interactions found in the [Cu4I4(PPh2Et)4] glass and subsequently reduced structural vibration account for its enhanced luminescence properties. Moreover, this highly transparent glass enables performant X-ray imaging and low-loss waveguiding in fibers drawn above the glass transition. The discovery of "nanocluster glass" opens avenues for unraveling glass formation mechanisms and designing novel luminescent glasses of well-defined building blocks for advanced photonics.

Melt-quenched carboxylate metal-organic framework glasses

Although carboxylate-based frameworks are commonly used architectures in metal-organic frameworks (MOFs), liquid/glass MOFs have thus far mainly been obtained from azole- or weakly coordinating ligand-based frameworks. This is because strong coordination bonds of carboxylate ligands to metals block the thermal vitrification pathways of carboxylate-based MOFs. In this study, we present the example of carboxylate-based melt-quenched MOF glasses comprising Mg2+ or Mn2+ with an aliphatic carboxylate ligand, adipate. These MOFs have a low melting temperature (Tm) of 284 °C and 238 °C, respectively, compared to zeolitic-imidazolate framework (ZIF) glasses, and superior mechanical properties in terms of hardness and elastic modulus. The low Tm may be attributed to the flexibility and low symmetry of the aliphatic carboxylate ligand, which raises the entropy of fusion (ΔSfus), and the lack of crystal field stabilization energy on metal ions, reducing enthalpy of fusion (ΔHfus). This research will serve as a cornerstone for the integration of numerous carboxylate-based MOFs into MOF glasses.

Creating glassy states of dicarboxylate-bridged coordination polymers

We report the direct formation of dicarboxylate-based coordination polymer glasses through thermal dehydration. The rearrangement of the coordination networks caused by dehydration was monitored by in situ powder X-ray diffraction, infrared spectroscopy, and synchrotron X-ray characterizations. The microporosity and mechanical properties of these glasses were investigated.

Water as a Modifier in a Hybrid Coordination Network Glass

Chemical diversification of hybrid organic-inorganic glasses remains limited, especially compared to traditional oxide glasses, for which property tuning is possible through addition of weakly bonded modifier cations. In this work, it is shown that water can depolymerize polyhedra with labile metal-ligand bonds in a cobalt-based coordination network, yielding a series of nonstoichiometric glasses. Calorimetric, spectroscopic, and simulation studies demonstrate that the added water molecules promote the breakage of network bonds and coordination number changes, leading to lower melting and glass transition temperatures. These structural changes modify the physical and chemical properties of the melt-quenched glass, with strong parallels to the "modifier" concept in oxides. It is shown that this approach also applies to other transition metal-based coordination networks, and it will thus enable diversification of hybrid glass chemistry, including nonstoichiometric glass compositions, tuning of properties, and a significant rise in the number of glass-forming hybrid systems by allowing them to melt before thermal decomposition.

Generalizable Synthesis of Highly Fluorinated Ionic Liquids

The unique chemistry of fluorocarbons (in particular, their weak intermolecular interactions and high degree of intrinsic free volume) makes them promising building blocks for ionic liquids with high gas capacities. Here, we report a generalizable method for the synthesis of fluorinated ionic liquids, which relies on the evolution of gaseous byproducts to drive product formation. This synthetic strategy overcomes solubility challenges that can hinder the synthesis of highly fluorinated ionic liquids via conventional methods and enables a systematic investigation of the effect of fluorination on ionic liquid viscosity and gas solubility.