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Porous crystalline frameworks including zeolites, metal-organic frameworks (MOFs), covalent organic frameworks (COFs) and hydrogen-bonded organic frameworks (HOFs) have attracted great research interest in recent years. In addition to their assembly in the solid-state being fundamentally interesting and aesthetically pleasing, their potential applications have now pervaded in different areas of chemistry, biology and materials science. When framework materials are endowed with 'flexibility', they exhibit some properties (e.g., stimuli-induced pore breathing and reversible phase transformations) that are distinct from their rigid counterparts. Benefiting from flexibility and porosity, these framework materials have shown promise in applications that include separation of toxic chemicals, isotopes and hydrocarbons, sensing, and targeted delivery of chemicals. While flexibility in MOFs has been widely appreciated, recent developments of COFs and HOFs have established that flexibility is not just limited to MOFs. In fact, zeolites-that are considered rigid when compared with MOFs-are also known to exhibit dynamic modes. Despite flexibility may be conceived as being detrimental to the formation and stability of periodic structures, the landscape of flexible framework structures continues to expand with discovery of new materials with promising applications. In this review, we make an account of different flexible framework materials based on their framework types with a more focus on recent examples and delve into the origin of flexibility in each case. This systematic analysis of different flexibility types based on their origins enables understanding of structure-property relationships, which should help guide future development of flexible framework materials based on appropriate monomer design and tailoring their properties by bottom-up approach. In essence, this review provides a summary of different flexibility types extant to framework materials and critical analysis of importance of flexibility in emerging applications.

Metal organic framework (MOF) materials have emerged as the adsorbent materials with the highest H2 storage densities on both a volumetric and gravimetric basis. While measurements of hydrogen storage at the material level (primarily at 77 K) have been published for hundreds of MOFs, estimates of the system-level hydrogen storage capacity are not readily available. In this study, hydrogen storage capacities are estimated at the system-level for MOFs with the highest demonstrated volumetric and gravimetric H2 storage densities. System estimates are based on a single tank cryo-adsorbent system that utilizes a type-1 tank, multi-layer vacuum insulation, liquid N2 cooling channels, in-tank heat exchanger, and a packed MOF powder inside the tank. It is found that with this powder-based system configuration, MOFs with ultra-high gravimetric surface areas and hydrogen adsorption amounts do not necessarily provide correspondingly high volumetric or gravimetric storage capacities at the system-level. Meanwhile, attributes such as powder packing efficiency and system cool-down temperature are shown to have a large impact on the system capacity. These results should shed light on the material properties that must to be optimized, as well as highlight the important design challenges for cryo-adsorbent hydrogen storage systems.

While the loading of liquid or solid materials in the pores of metal–organic frameworks (MOFs) can yield composite materials with novel and useful emergent properties, the loading of solids, and ionic solids in particular, can be challenging. We report the loading of the salts NH4NO3 and LiNO3 in the MOF UiO-66. The relatively low-melting NH4NO3 is loaded in UiO-66 in a solvent-free method, and loading is complete in 8 h at 75 °C, far below the melting point of NH4NO3. The higher-melting LiNO3 requires a small amount of solvent (water) for loading, and active removal of water assists in loading of the salt to form a composite that is 38% by mass LiNO3. These and similar salt–MOF composites are of interest for applications such as solid-ion conductors and energetic materials.

A semi-rigid organic linker, namely, 3,3′,5,5′-tetrakis(4-(α-carboxy)methoxyphenyl)-2,2′,6,6′-tetramethoxy-1,1′-biphenyl (H4L), was designed and synthesized to access metal–organic frameworks (MOFs). While the ortho-methoxy substituents in the biphenyl core of H4L were surmised to twist the aromatic planes and impart porosity to the resultant MOFs, the (α-carboxy)methoxyphenyl moieties at the periphery were envisaged to enable requisite flexibility for metal–ligand coordination polymerization. The reactions of H4L with Cd, Mn, and Zn salts indeed yielded MOFs, i.e., Cd-L, Mn-L, Zn-Lsqc, and Zn-Ldia, with interesting structural features and unusual inorganic SBUs. In particular, the reaction of H4L with ZnI2 in DMF at 90 °C over 2 days led to concomitant formation of a pair of compositionally distinct Zn-MOFs, i.e., Zn-Ldia and Zn-Lsqc, each of which could be accessed exclusively by controlling the reaction conditions. The diversity observed in the structures of MOFs formed with the linker H4L with a limit...

White organic light-emitting diodes (WOLEDs) are at the center stage of OLED research today because of their advantages in replacing the high energy-consuming lighting technologies in vogue for a long time. New materials that emit white light in simple devices are much sought after. We have developed two novel electroluminescent materials, referred to as BABZF and BATOMe, based on a twisted bianthryl core, which are brilliantly fluorescent, thermally highly stable with high Td and Tg, and exhibit reversible redox property. Although inherently blue emissive, BABZF leads to white-light emission (CIE ≈ 0.28, 0.33) with a moderate power efficiency of 2.24 lm/W and a very high luminance of 15 600 cd/m2 in the fabricated multilayer nondoped OLED device. This device exhibited excellent color stability over a range of applied potential. Remarkably, similar white-light emission was captured even from a double-layer device, attesting to the innate hole-transporting ability of BABZF despite it being non-nitrogenous, that is, lacking any traditional hole-transporting di-/triarylamino group(s). Similar studies with BATOMe led to inferior device performance results, thereby underscoring the importance of dibenzofuryl groups in BABZF. Experimental as well as theoretical studies suggest the possibility of emission from multiple species involving BABZF and its exciplex and electroplex in the devices. The serendipitously observed white-light emission from a double-layer device fabricated with an unconventional hole-transporting material (HTM) opens up new avenues to create new non-nitrogenous HTMs that may lead to more efficient white-light emission in simple double-layer devices.

Two energetic metal-organic frameworks (MOFs), Co-ANTA and Zn-ANTA, are synthesized from 5-amino-3-nitro-1H-1,2,4-triazole (ANTA) and exhibit superior oxygen balance, density, and thermal stability compared to ANTA. The superior oxygen balance is achieved through a combination of hydroxide ligands and deprotonated linkers. Although the materials are isostructural and have similar density, oxygen balance, and sensitivity to heat, their impact sensitivities are significantly different. Similar to ANTA, Zn-ANTA is fairly insensitive to impact. By contrast, the impact sensitivity of ANTA is increased significantly after coordination polymerization with cobalt. The disparate impact sensitivities of the compounds might be attributed to the different electronic configurations of the metal ions constituting the frameworks.

It is not unusual for advances in science to run ahead of nomenclature for the discovered phenomena. However, in the case of metal–organic frameworks (MOFs), research activity has been booming for more than a decade, and yet there is no agreed upon definition for the term. Different structural aspects of these materials are critically analyzed with examples and counterexamples to arrive at a definition of MOF that is both definite and consistent with common usage.

High-energy coordination polymers (CPs) based on nitrogen-rich ligands are an emerging class of explosives. However, modulation of the energetic properties of high-energy CPs and the establishment of their structure–function relationship remain in their infancy. In the present study, the utility of coordination polymerization as a technique to modulate the application of critical energetic properties, such as density and thermal stability, of a secondary explosive, 5,5′-dinitro-2H,2H′-3,3′-bi-1,2,4-triazole (DNBT), is presented. Ni-DNBT is a discrete octahedral complex with density lower than that of DNBT. Cu-DNBT also contains octahedral metal coordination, similar to that in Ni-DNBT, as the building unit; however, the partial reduction of CuII to CuI ions during the reaction and their unique geometrical preferences lead to linking of the octahedral CuII complexes by tetrahedral CuI ions and render the resultant material a one-dimensional polymer with high density. In fact, Cu-DNBT has the highest densit...

A fluorescent organic linker, namely, 10,10′-bis(4-carboxyphenyl)-9,9′-bianthryl (H2L), was rationally designed and synthesized to access luminescent metal–organic materials (MOMs). A series of structurally diverse MOMs was synthesized with the diacid linker H2L by reacting it with main group, transition, and lanthanide metal ions under different conditions. Among them, Zn-L MOM is a 1D polymeric chain, while Cd-L is a 2D structure in which 4,4′-bipyridyls mediate the formation of 2D networks by linking up the 1D metal–carboxylate chains. The Pb-L MOM is found to be a 2D polymeric net, while Sr-L, obtained under similar reaction conditions, is a noninterpenetrated 3D polymeric structure; the extension from 2D to 3D framework occurs by mediation of Cl– ions. Notably, the reaction of H2L with the lanthanide ions yielded isostructural 3D MOFs, i.e., Tb-L, Eu-L, Sm-L, Nd-L, La-L, Pr-L, Gd-L, and Yb-L, which are noninterpenetrated and porous. The Ln-MOFs are highly robust and stable to solvent exclusion. The r...

Coordination polymers (CPs) have recently emerged as a promising class of high energy materials useful for the synthesis of tailored energetic materials. CPs have shown potential to improve energetic performance relative to conventional organic energetic materials with regard to density, oxygen balance, sensitivity, and heat of detonation. Thus far, a variety of energetic linkers have been applied, and success has been achieved across a number of structure classes including nitrogen-rich heterocycles and azides. Here, the current progress in the field of energetic CPs, both from the standpoint of structure and properties, is reviewed, and a perspective on current challenges and promising future directions for the field are delineated. Inasmuch as structure/function relationships have been elucidated for CPs with particular applications in mind, these are discussed as well as shortcomings in experimental work required to firmly establish predictive principles within the realm of energetic materials.

A semirigid tetraacid linker H4L functionalized with 1,2,3-triazole was rationally designed and synthesized to access nitrogen-rich MOFs for selective adsorption of CO2. The cadmium MOF, that is, Cd-L, obtained by the reaction of H4L with Cd(NO3)2, is found to be a 3D porous framework structure that is robust to desolvation. Crystal structure analysis reveals channels that are decorated by the triazole moieties of L. Gas adsorption studies show that Cd-L MOF permits remarkable CO2 uptake to the extent of 99 and 1000 cc/g at 1 and 30 bar, respectively, at 0 °C. While literature survey reveals that MIL-112, constructed from a 1,2,3-triazole functionalized linker, exhibits no porosity to gas adsorption due to structural flexibility, the results with Cd-L MOF described herein emphasize how rigidification of the organic linker improves gas uptake properties of the resultant MOF.

The domino oxidation of diols to lactones is an important transformation, and catalytic protocols that allow this conversion smoothly are scarce. Capitalizing on the established reactivity of tetramethyl-IBX ( TetMe-IBX ) and its in situ generation in the presence of a co-oxidant, such as oxone, we have shown that a variety of diols can be converted to the corresponding lactones in respectable yields by employing the precursor of TetMe-IBX , namely, tetramethyl- o -iodobenzoic acid ( TetMe-IA ), as a catalyst in 5 mol % in the presence of 2 equiv of oxone.

The twisted 3,3′-diiodo-2,2′,6,6′-tetramethoxybiphenyl-4,4′-dicarboxylic acid (DIDA) was designed and synthesized for the in situ generation of Bis-IBX and catalytic oxidations. The seemingly better solubility of the in situ-generated Bis-IBX and the attenuated reactivity arising from its unique structural features and methoxy substituents allowed the catalytic oxidation of activated alcohols selectively using DIDA/oxone. Chemoselective oxidations were demonstrated for substrates containing two different hydroxy groups. Furthermore, the unique reactivity of DIDA was demonstrated for sequential oxidation reactions of 1,4- and 1,6-diols to give lactones catalytically in respectable yields.

Simple synthetic manipulation of S-proline allows access to prolinamides 5-7 as organocatalysts capable of double hydrogen bonding for enantioselective Michael addition reactions of carbonyl compounds to β-nitrostyrenes. It is shown that prolinamide catalyst 7 leads to addition products with a high diastereo- as well as enantioselectivity. The transition state structure involving the binding of electrophilic nitrostyrene via two H-bonds is believed to be further stabilized by π,π stacking interactions mediated by the tosyl ring.

Despite the fact that benzophenone has traditionally served as a prototype molecular system for establishing triplet state chemistry, materials based on molecular systems containing the benzophenone moiety as an integral part have not been exploited as generic host materials in phosphorescent organic light-emitting diodes (PhOLEDs). We have designed and synthesized three novel host materials, i.e., BP2-BP4, which contain benzophenone as the active triplet sensitizing molecular component. It is shown that their high band gap (3.91-3.93 eV) as well as triplet energies (2.95-2.97 eV) permit their applicability as universal host materials for blue, green, yellow, and red phosphors. While they serve reasonably well for all types of dopants, excellent performance characteristics observed for yellow and green devices are indeed the hallmark of benzophenone-based host materials. For example, maximum external quantum efficiencies of the order of 19.2% and 17.0% were obtained from the devices fabricated with yellow and green phosphors using BP2 as the host material. White light emission, albeit with rather poor efficiencies, has been demonstrated as a proof-of-concept by fabrication of co-doped and stacked devices with blue and yellow phosphors using BP2 as the host material.

The domino oxidation of diols to lactones is an important transformation, and catalytic protocols that allow this conversion smoothly are scarce. Capitalizing on the established reactivity of tetramethyl-IBX ( TetMe-IBX ) and its in situ generation in the presence of a co-oxidant, such as oxone, we have shown that a variety of diols can be converted to the corresponding lactones in respectable yields by employing the precursor of TetMe-IBX , namely, tetramethyl- o -iodobenzoic acid ( TetMe-IA ), as a catalyst in 5 mol % in the presence of 2 equiv of oxone.

Porous coordination polymers (CPs) with partially uncoordinated pyridyl rings based on rationally designed polypyridyl linkers are appealing from the point of view of their application as nucleophilic catalysts. A D2d -symmetric tetradentate organic linker L, that is, 2,2',6,6'-tetramethoxy-3,3',5,5'-tetrakis(4-pyridyl)biphenyl, was designed and synthesized for metal-assisted self-assembly aimed at porous CPs. Depending on the nature of the metal ion and the counter anion, the ligand L is found to function as a 3- or 4-connecting building block leading to porous CPs of diverse topologies. The reaction of L with Zn(NO3 )2 and Cd(NO3 )2 yields porous 2 D CPs of "fes" topology, in which the tetrapyridyl linker L serves as a 3-connecting unit with its free pyridyl rings well exposed into the pores. The functional utility of these porous CPs containing uncoordinated pyridyl rings is demonstrated by employing them as efficient heterogeneous nucleophilic catalysts for acetylation of a number of phenols with varying electronic properties and reactivities.

Different substituted primary alcohols can be converted to acids or aldehydes by simply changing the solvent and the temperature.

Simple synthetic manipulation of S-proline allows access to prolinamides 5-7 as organocatalysts capable of double hydrogen bonding for enantioselective Michael addition reactions of carbonyl compounds to β-nitrostyrenes. It is shown that prolinamide catalyst 7 leads to addition products with a high diastereo- as well as enantioselectivity. The transition state structure involving the binding of electrophilic nitrostyrene via two H-bonds is believed to be further stabilized by π,π stacking interactions mediated by the tosyl ring.