Docking rings in a solid: reversible assembling of pseudorotaxanes inside a zirconium metal–organic framework

An unprecedented zirconium metal–organic framework featuring a T-shaped benzimidazole strut was constructed and employed as a sponge-like material for selective absorption of macrocyclic guests. The neutral benzimidazole domain of the as-synthesized framework can be readily protonated and fully converted to benzimidazolium. Mechanical threading of [24]crown-8 ether wheels onto recognition sites to form pseudorotaxanes was evidenced by solution nuclear magnetic resonance, solid-state fluorescence, and infrared spectroscopy. Selective absorption of [24]crown-8 ether rather than its dibenzo counterpart was also observed. Further study reveals that this binding process is reversible and acid–base switchable. The success of docking macrocyclic guests in crystals via host–guest interactions provides an alternative route to complex functional materials with interpenetrated structures.

A T-shaped ligand was designed as struts for building a zirconium metal–organic framework. Acid–base switchable docking and releasing a 24-membered crown ether inside crystals was successfully accomplished via post-synthetic modification.     

A conductive metal–organic framework photoanode

We report the development of photosensitizing arrays based on conductive metal–organic frameworks (MOFs) that enable light harvesting and efficient charge separation. Zn₂TTFTB (TTFTB = tetrathiafulvalene tetrabenzoate) MOFs are deposited directly onto TiO₂ photoanodes and structurally characterized by pXRD and EXAFS measurements. Photoinduced interfacial charge transfer dynamics are investigated by combining time-resolved THz spectroscopy and quantum dynamics simulations. Sub-600 fs electron injection into TiO₂ is observed for Zn₂TTFTB–TiO₂ and is compared to the corresponding dynamics for TTFTB–TiO₂ analogues that lack the extended MOF architecture. Rapid electron injection from the MOF into TiO₂ is enhanced by facile migration of the hole away from the interfacial region. Holes migrate through strongly coupled HOMO orbitals localized on the tetrathiafulvalene cores of the columnar stacks of the MOF, whereas electrons are less easily transferred through the spiral staircase arrangement of phenyl substituents of the MOF. The reported findings suggest that conductive MOFs could be exploited as novel photosensitizing arrays in applications to slow, and thereby make difficult, photocatalytic reactions such as those required for water-splitting in artificial photosynthesis.

We report the development of photosensitizing arrays based on conductive metal–organic frameworks (MOFs) that enable light harvesting and efficient charge separation.     

Coordination modulated on-off switching of flexibility in a metal–organic framework

Stimuli-responsive metal–organic frameworks (MOFs) exhibit dynamic, and typically reversible, structural changes upon exposure to external stimuli. This process often induces drastic changes in their adsorption properties. Herein, we present a stimuli-responsive MOF, 1·[CuCl], that shows temperature dependent switching from a rigid to flexible phase. This conversion is associated with a dramatic reversible change in the gas adsorption properties, from Type-I to S-shaped isotherms. The structural transition is facilitated by a novel mechanism that involves both a change in coordination number (3 to 2) and geometry (trigonal planar to linear) of the post-synthetically added Cu(i) ion. This process serves to ‘unlock’ the framework rigidity imposed by metal chelation of the bis-pyrazolyl groups and realises the intrinsic flexibility of the organic link.

Stimuli-responsive metal–organic frameworks (MOFs) exhibit dynamic structural changes upon exposure to external stimuli. Here the coordination geometry of a post-synthetically added metal ion drastically changes the adsorption properties.     

Crystals springing into action: metal–organic framework CUK-1 as a pressure-driven molecular spring

Mercury porosimetry and in situ high pressure single crystal X-ray diffraction revealed the wine-rack CUK-1 MOF as a unique crystalline material capable of a fully reversible mechanical pressure-triggered structural contraction. The near-absence of hysteresis upon cycling exhibited by this robust MOF, akin to an ideal molecular spring, is associated with a constant work energy storage capacity of 40 J g⁻¹. Molecular simulations were further deployed to uncover the free-energy landscape behind this unprecedented pressure-responsive phenomenon in the area of compliant hybrid porous materials. This discovery is of utmost importance from the perspective of instant energy storage and delivery.

Mercury porosimetry and in situ high pressure single crystal X-ray diffraction revealed the wine-rack CUK-1 MOF as a unique crystalline material capable of a fully reversible mechanical pressure-triggered structural contraction.

Reducing the world''s fossil fuel dependence is the focus of many global initiatives,¹ aiming to mitigate the effects of climate change through tapping into sustainable energy resources such as solar and wind power. However, increasing reliance on these renewable energy sources has introduced difficulties due to the offset between power availability and demand peaks. Complementary technologies are necessary to alleviate intermittent supply, such as peaking power plants, demand-side energy management, or large scale energy storage.² The latter is particularly desirable as it can decouple electricity production and consumption, however the lack of a “one size fits all” approach has led the scientific community to envisage unconventional energy storage strategies.One such avenue emerging in recent years is the storage of mechanical energy via the compression of a suitable stimuli-responsive system, either through the intrusion of a non-wetting fluid into hydrophobic porous frameworks,³ or by means of application of an external pressure on flexible materials.⁴The former approach, first pioneered using water intrusion in zeolites and silicas,¹¹ has recently been extended to small pore zeolitic imidazolate frameworks.¹² Unfortunately, besides requiring highly hydrophobic systems, water intrusion achieves a relatively low stored energy density,³ of around 3–25 J g⁻¹. The second strategy takes advantage of the compliant nature of bulk materials. Energy is stored through structural deformations, manifesting as continuous or sudden volume changes under external pressure. The energy stored in flexible materials over a compression/decompression cycle can be an order of magnitude higher compared to the values achieved using fluid intrusion in rigid porous systems.¹³ In theory, three types of pressure-induced structural behaviour can be envisioned for such a responsive system. If the structure contraction is non-reversible, all energy is dissipated and the system is categorized as a nano-shock absorber (Fig. 1b). For structural changes that are reversible upon decompression two families of system can be distinguished, i.e. a nano-damper (Fig. 1c) or an ideal nano-spring (Fig. 1d) when the p–V curves show hysteresis or fully overlap, respectively.¹⁴Open in a separate windowFig. 1(A) Schema of mechanical energy storage in compliant crystalline materials, implying a unit cell volume change between open (op) and contracted (cp) structures, and prototypical pressure-volume curves of stimuli-responsive materials under mechanical pressure for (B) nano-shock absorbers, exemplified by MIL-53(Al),⁵ MIL-53(Ga)-FA⁶ and ZIF-4(Zn),⁷ (C) nano-dampers e.g. MIL-53(Cr),⁸ MIL-47(V)⁹ and MIL-53(Al)-FA¹⁰ and (D) nano-springs, insofar exhibited exclusively by CUK-1 presented herein.Metal–organic frameworks (MOFs), a class of porous, crystalline materials comprised of metal vertices interconnected by organic linkers, are known to exhibit responsiveness to a variety of stimuli,^15,16 including external pressure.¹⁷ Recently, several frameworks of this family of hybrid materials have been shown to act as energy storing nano-dampers or energy dissipative nano-shock absorbers, as is the case for the highly flexible MIL-53(M)^5,8,10 and MIL-47(V)⁹ series and more recently ZIF-4(Zn)⁷ (see Fig. 1b and c for their related structural behaviours). In such flexible crystalline materials compression is associated with a displacive phase transition between distinct structures of differing unit cell volumes, denoted as open (op) and contracted (cp) forms^15,16 and illustrated in Fig. 1a, occurring reversibly or irreversibly for a nano-damper or nano-shock absorber, respectively. The considerable stored energy associated with this transition, in the range of 30–200 J g⁻¹ (up to 4 kJ g⁻¹ for shock absorbers¹⁸) is highly attractive from the perspective of mechanical energy storage. However, the hysteretic compression/decompression curve characterising known nano-damper MOFs leads to a partial loss of work energy, lowering the potential storage efficiency, as well as creating issues through heat dissipation. Insofar, the search for a ideal spring-like crystalline material, capable of reversible pressure-induced structural switching without any hysteresis (Fig. 1d) has been fruitless, precluding their applicability for efficient, high density energy storage applications. Herein, a subtle combination of Hg-porosimetry, high-pressure single crystal X-ray diffraction (SC-XRD) and molecular simulations reveals the 1D-channel CUK-1 (M, M = Co, Mg)¹⁹ MOF as the first compliant hybrid porous material with a spring-back mechanical breathing behaviour.Such unique mechanically-triggered structural response implies a continuous pore contraction/expansion between op and cp forms in a narrow pressure range of 280–290 MPa, accompanied by a unit cell volume change of 20.9%. This optimal scenario paves the way towards fast energy storage/delivery system of about 40 J g⁻¹. The channel-like CUK-1(M) composed of chains of μ₃-OH/O edge and vertex sharing metal octahedra (M = Co,¹⁹ Mg²⁰) coordinated by bidentate 2,4-pyridinedicarboxylic ligands, recently emerged as an attractive porous material owing to its promising sorption performance combined with environmentally-friendly hydrothermal synthesis and high thermal and chemical stability.^20–22 Its wine-rack topology and its relatively rigid behaviour upon guest adsorption are reminiscent to that of MIL-47(V) a MOF which interestingly underwent a hysteretic, reversible structural contraction upon exerting an external pressure of 125 MPa,⁹ associated with a stored/delivered energy of 33 J g⁻¹. Inspired by our previous findings on MIL-47(V), we deliberately explored the pressure-induced structural behaviour of CUK-1 in its isostructural Co and Mg forms. MOF synthesis was performed according to the protocol detailed in ESI.^† Phase purity was confirmed by powder XRD (Fig. S3, S4 and Table S1^†) while their textural features, including BET area and pore volume, were found to match previously reported data.^19,20Mercury intrusion curves were recorded on the powder samples up to a maximum of 413 MPa as shown for CUK-1(Co) in Fig. 2, its Mg variant being reported in Fig. S6, ESI,^† together with full experimental details. A substantial amount of Hg intrudes at low pressure (<10 MPa), due to compaction of the crystals and filling of inter-particle porosity. This is followed by a sudden volume change at 281 MPa where a sharp step is observed (see inset of Fig. 2). By analogy with the conclusions previously drawn for the series of MIL-53(M)/MIL-47(V) frameworks,^5,8–10 this intruded Hg volume increase is associated with a structural contraction of CUK-1(Co), as its channel size (approx. 6.6 Å) is an order of magnitude below the pore dimension where non-wetting mercury can intrude in this pressure range (at 52 Å). The extrusion curve shows a near-perfect overlap, indicating that the framework behaves as an ideal spring, with no hysteresis between the intrusion/extrusion branches.Open in a separate windowFig. 2Sequential mercury intrusion–extrusion curves on CUK-1(Co) powder, in blue line and red, respectively. Line is a guide for eye. Volume below 1 MPa corresponds to powder compaction and intercrystallite void filling. Dotted horizontal lines demarcate contraction lower and upper bounds. Inset highlights the intrusion step in a linear scale with the op/cp contraction marked with an arrow.Moreover, this behaviour is highly repeatable, as confirmed by four consecutive pressure cycles (in Fig. S5^†). Interestingly, the same behaviour also holds true for CUK-1(Mg) (Fig. S6^†), with a similar intrusion pressure of 288 MPa. Since the two metal ions show relatively similar ionic radius (Co²⁺: 1.50(7) Å and Mg²⁺: 1.41(7) Å),²⁴ the averaged metal–oxygen distance is nearly identical in their corresponding coordination: spheres: (Co–O: 2.107(20) Å and Mg–O: 2.073(20) Å). Such analogous metal-linker bond strength is most likely at the origin of the very similar pressure-induced response of the two materials. The high transition pressure of CUK-1(Co) underpins the inability of guest adsorption to induce a breathing effect as observed previously.^20,21 Indeed, the adsorption stresses encountered throughout guest insertion are simply insufficient to overcome the energetic penalty of transition.^14,25 The 0.143 mL g⁻¹ volume change associated with the observed step in the CUK-1(Co) intrusion curve corresponds to a 20.9% change in unit cell volume, lower than in the similar phenyl-based MIL-47(V) of 43%.⁹ However, the stored energy calculated through W = P × ΔV is 40 J g⁻¹, 20% larger than the value reported for MIL-47(V)⁹ of 33 J g⁻¹. Here, the higher pressure of CUK-1(Co) switching, 281 MPa vs. 125 MPa for MIL-47(V) balances out the ΔV term. Moreover, owing to its relatively dense framework, the volumetric energy density of CUK-1(Co) remains attractive when compared to water intrusion systems (Table S4^†).Considering an initial unit cell volume for the CUK-1(Co) op form of 2467 ų from PXRD (see ESI^†), the resulting cp form is estimated to exhibit a unit cell volume of 1950 ų, based on the Hg intruded volume increase at 281 MPa. In order to directly observe the contracted form and identify the mechanism underpinning these intriguing dynamics, high pressure SC-XRD experiments were carried out in a membrane diamond anvil cell (mDAC). Individual CUK-1(Co) crystals were placed in a gasket between the polished diamonds of the mDAC, and immersed in a hydrostatic pressure transmitting medium of silicone oil AP-100, with a gold flake used to monitor inner mDAC pressure (full single crystal synthesis conditions and SC-XRD methodology available in the ESI^†).At ambient pressure, the indexed unit cell volume of the initial op form of CUK-1(Co) is nearly identical (2492 ų) to that of the previously reported²⁰ dehydrated monoclinic phase (2466.72 ų). Upon increasing DAC pressure to around 0.3 GPa, a volume contraction to the cp phase begins, which is in line with Hg porosimetry experiments. Reflections obtained from integrated 2D diffraction images were used to solve the pressure-induced structure through a dual space recycling algorithm in an expanded P1 setting, then further refined on F² using the SHELX suite²⁶ (complete data treatment methodology available in the ESI^†). The structure maintains the same C2/c space group throughout the transition between the two forms, and as such the spring-like dynamics of the framework can be described as a continuous contraction in a narrow pressure range. Above 0.5 GPa, the cp form is attained, with further pressure application leading to a linear decrease of its unit cell volume by 4% up to 1.8 GPa (Fig. 3).Open in a separate windowFig. 3Evolution of the CUK-1(Co) unit cell volume determined through indexation of Bragg reflections as a function of applied pressure as recorded in a DAC. Unit cell parameters corresponding to each pressure point can be found in Table S5, ESI.^†The unit cell dimensions of the solved cp form at 0.5 GPa are provided in Table 1, alongside as-indexed pristine op form parameters with Fig. 4a illustrating the two structures. The anisotropic transition is similar in nature to that of MIL-53(M)/MIL-47(V), characterised by a compaction in the b-direction (from approx. 13 Å to 9 Å) and an elongation along the a-axis (from 18 to nearly 20 Å). The change in the c-parameter is minimal, with only a slight increase, as it lies in the plane of the highly rigid octahedrally coordinated metal chains. A lowering of the angle (from 103 to 99°) is also observed, as the 1D parallel pores are straightened via the linker-induced torsion. A table comparing specific atomic distances, angles and torsions in the two forms is available in Table S7, ESI.^† The unit cell volume of the identified cp phase at 0.5 GPa of 1972 ų is only slightly higher than the value estimated from porosimetry measurements (as 1950 ų). We attribute this offset to the different interactions of the crystal surface with the respective pressure transmitting medium (mercury vs. silicone oil), as observed previously.⁵Crystallographic data of the pristine (op) and high pressure (cp) phases as determined from the CUK-1(Co) SC-XRD

Infrared crystallography for framework and linker orientation in metal–organic framework films

Pore alignment and linker orientation influence diffusion and guest molecule interactions in metal–organic frameworks (MOFs) and play a pivotal role for successful utilization of MOFs. The crystallographic orientation and the degree of orientation of MOF films are generally determined using X-ray diffraction. However, diffraction methods reach their limit when it comes to very thin films, identification of chemical connectivity or the orientation of organic functional groups in MOFs. Cu-based 2D MOF and 3D MOF films prepared via layer-by-layer method and from aligned Cu(OH)₂ substrates were studied with polarization-dependent Fourier-transform infrared (FTIR) spectroscopy in transmission and attenuated total reflection configuration. Thereby, the degrees for in-plane and out-of-plane orientation, the aromatic linker orientation and the initial alignment during layer-by-layer MOF growth, which is impossible to investigate by laboratory XRD equipment, was determined. Experimental IR spectra correlate with theoretical explanations, paving the way to expand the principle of IR crystallography to oriented, organic–inorganic hybrid films beyond MOFs.

Polarization-dependent infrared spectroscopy of oriented metal organic framework films fills the information gap left by diffraction methods and gives access to the orientation of the aromatic linker and initial orientation of ultra-thin films.     

Multiple rotational rates in a guest-loaded,amphidynamic zirconia metal–organic framework

Amphidynamic motion in metal–organic frameworks (MOFs) is an intriguing emergent property, characterized by high rotational motion of the phenylene rings that are embedded within an open, rigid framework. Here, we show how the phenylene rings in the organic linkers of the water stable MOF PEPEP-PIZOF-2 exhibit multiple rotational rates as a result of the electronic structure of the linker, with and without the presence of highly interacting molecular guests. By selective ²H enrichment, we prepared isotopologues PIZOF-2d4 and PIZOF-2d8 and utilized solid-state ¹³C and ²H NMR to differentiate the dynamic behavior of specific phenylenes in the linker at room temperature. A difference of at least one order of magnitude was observed between the rates of rotation of the central and outer rings at room temperature, with the central phenylene ring, surrounded by ethynyl groups, undergoing ultrafast 180° jumps with frequencies higher than 10 MHz. Moreover, loading tetracyanoquinodimethane (TCNQ) within the pores produced significant changes in the MOF''s electronic structure, but very small changes were observed in the rotational rates, providing an unprecedented insight into the effects that internal dynamics have on guest diffusion. These findings would help elucidate the in-pore guest dynamics that affect transport phenomena in these highly used MOFs.

Zirconia based metal–organic framework PEPEP-PIZOF-2 exhibits the emergent property of amphidynamic motion with organic links that show multiple rotational rates related to their molecular structure and independently from presence of molecular guests.

Amphidynamic crystals are an emerging class of materials made of molecular components that exhibit fast internal motion within a rigid lattice.^1,2 Metal–organic frameworks (MOFs) can be considered as intrinsically amphidynamic materials, because they are formed by the assembly of organic molecules that carry high degrees of freedom linked to inorganic clusters that form an extended solid matrix.³ This assembly allows for the organic components to behave like rotators, while the solid matrix/framework acts as a stator, with gyroscope-type motion enabled by the open architecture of the MOF with motion modulated by the molecular structure of the linker.^4–6 In order to create materials with targeted dynamic properties for real-life applications, like molecular machines, it is important to determine whether the chemical environment of the linkers can produce dynamics at multiple rates and how the presence of molecular guests affect such dynamics. To do so, it is important to use MOFs that are chemically stable to water and humidity, because this robustness increases the reproducibility of the results and the applicability of the MOF. The interplay between guest diffusion, linker dynamics and the overall framework flexibility has been actively investigated in recent years.⁷Here, we prepared a water-stable MOF, PEPEP-PIZOF-2 (Fig. 1a), strategically labelled with deuterium atoms to probe the multiple segmental motion in the pristine and guest-loaded materials. Utilizing solid-state NMR techniques, we elucidated that this MOF exhibits bimodal rotational rates, with the central ring of the linker having free rotation above the 10 MHz limit of quantitation, and with the outer rings having slower rotation. This double-rate internal dynamics is preserved even in the presence of a very “sticky,” electron-deficient guest such as tetracyanoquinodimethane, TCNQ. Studying the molecular dynamics of this class of MOFs helps in accelerating their use as applied materials and for the fundamental studies of transfer phenomena that occur in MOFs such as mass, heat, and momentum transfer.Open in a separate windowFig. 1(a) Crystal structure of the double interweaved MOF PIZOF-2, showing each framework in separate colors. (b) Molecular structure of the PEPEP link. (c) Deuterium enriched linkers used in this study emphasizing the deuterium location in the link and the different chemical environments.Zirconia MOFs have been shown to exhibit unprecedented chemical stability, of which the family of Porous Interpenetrated Zirconia Organic Frameworks (PIZOFs) features superior stability combined with a unique molecular composition of their linkers.^8,9 The linkers in PIZOF MOFs are linear and made by a combination of phenylene rings and ethynylene groups, where multiple chemical environments can be created around the phenylenes, thus altering their rotational behavior. Of the series, PEPEP-PIZOF-2 (hereafter PIZOF-2) is a high symmetry interweaved MOF (interweaved = interpenetrated with minimally displaced frameworks^10,11) made with linkers that contain three phenylenes (P) and two ethynylenes (E) in an alternating form (hence PEPEP, Fig. 1b), creating two different types of chemical and crystallographic environments around the rotor moieties: the central phenylene ring is surrounded by two alkyne groups that provide a negligible electronic barrier for rotation and two outer phenylene rings surrounded by an alkyne and a carboxylate. So, we expect to observe significant differences in dynamics for each component of the linker.¹² To properly observe the gyroscope-like rotation, protons were replaced with deuterons either in the inner ring (PIZOF-2d4, Fig. 1c) or in the outer rings (PIZOF-2d8, Fig. 1c). These two modes of isotopic labeling allowed the isolation of each ring to study of their dynamics by ²H NMR.Samples of the PIZOF-2 MOF containing natural and isotopically enriched PEPEP links were prepared from adapted published procedures (ESI^†).¹³ The MOFs were prepared via solvothermal crystallization of the respective linkers in DMF in the presence of ZrCl₄ and proline-HCl at 120 °C for 24 h, resulting in crystalline powder samples of formula Zr₆O₄(OH)₄[PEPEP]₆, Zr₆O₄(OH)₄[PEPEP-d4]₆, and Zr₆O₄(OH)₄[PEPEP-d8]₆. Powder X-ray diffraction (PXRD) patterns of all three isotopologues exhibited sharp diffraction lines starting at 3.84° 2θ (CuKα radiation) characteristic of the cubic PIZOF-2 MOF phase (Fd$\bar{3}$m space group symmetry) (Fig. 2a).¹⁴ Phase purity was assessed using Rietveld refinement of the experimental patterns using the single crystal unit cell data resulting in phase pure samples with low residuals (Fig. S1–S4^†).Open in a separate windowFig. 2(a) Powder X-ray diffraction of the natural and isotopically enriched PIZOF-2 MOFs demonstrating their isoreticular nature. Miller indices of the most intense peaks are indicated. (b) ¹³C CPMAS NMR spectra of natural and isotopically enriched PIZOF-2 MOFs.The internal structure of the MOFs was analyzed using ¹³C Cross-Polarization with Magic Angle Spinning (CP MAS) NMR spectroscopy, where the intensities of ¹³C signals varied according to the level of deuteration of the linker in each MOF (Fig. 2b). PIZOF-2 exhibits a ¹³C spectrum with signals at around 92 ppm, corresponding to the internal ethynyl, signals between 120 and 140 ppm corresponding to the phenylene carbons, and signals at 173 ppm that correspond to carboxylates, consistent with the expected structure. In PIZOF-2d4 the signals that correspond to the central phenylene ring are attenuated (Fig. 2b, signal 8) compared to the natural material, whereas in PIZOF-2d8, the only visible signals are those of the central ring, due to the absence of vicinal protons required for CP. Peaks associated with solvents and other reagents were not observed indicating a successfully evacuated framework, which in addition to high crystallinity and the magnetic field produces changes in the spectral line shape that can be associated with different types of motion.¹⁵ In the case of the PIZOF MOFs, the differences in the molecular substructure and porosity ensured having optimal samples for dynamic studies. Despite being double interweaved, the distances between centroids of the aromatic rings of the interpenetrating frameworks have values in the range of 6.23 Å to 8.04 Å (Fig. S7^†). Considering that the volume of revolution of the phenylene is ca. 6 Å, significant changes in the internal rotational dynamics caused by interpenetration were ruled out. Besides, it is expected that the phenylene rings have sufficient space to undergo fast rotational displacement, as it has been observed in other MOFs.¹⁶ To determine this, the deuterated samples were studied using solid-state quadrupolar echo ²H NMR spectroscopy. The reorientation of the C–²H bond vectors with respect to the external between outer and inner rings is expected to afford different rotational rates.The ²H NMR line shape at room temperature of PIZOF-2d8 displays signals characteristic of motions in the intermediate exchange regime. A successful fitting of the spectrum using NMRweblab¹⁷ was obtained using a model that assumed two-fold flip jumps, indicating a rotational rate at room temperature of the outer rings of k_rot = 2.10 MHz (Fig. 3 top). The rate of rotation of the deuterated outer rings is similar to that reported in UiO-66(Zr)¹⁸ (k_rot = 2.3 MHz at rt) and much larger than that of other simple MOFs like MOF-5(Zn),¹²MIL-47(V),¹⁹ and MIL-53(Cr)¹⁹ (k_rot < 0.001 MHz at rt). The rotation of the outer rings could be then regulated by the electronic conjugation of the phenylene with the carboxylates and/or affected by the interactions with the metal oxide clusters.Open in a separate windowFig. 3Experimental (blue) and calculated (orange) deuterium line shapes of PIZOF-2 at 295 K: (top) PIZOF-2d8 and (bottom) PIZOF-2d4.Conversely, in the case of PIZOF-2d4 (Fig. 3 bottom) the narrow ²H NMR spectrum is characteristic of ultrafast reorientations about the –C C– axis. A fitting of the spectrum was carried out assuming fast 180° jumps and large amplitude vibrations, indicating a rate of rotation of k_rot > 10 MHz, the upper limit of the ²H NMR sensitivity, so at 295 K the inner rings are rotating freely. This rate correlates with the minimal electronic barrier given by the flanking alkynes as has also been observed in a Zn-pyrazolate MOF that contains the same diethynyl-phenylene-diethynyl moiety.¹⁶ To date, this is the first time a MOF exhibits multiple rotational rates of their phenylene rings, which has implications for understanding and improving guest-diffusion related phenomena such as guest storage, catalysis, and separations.As the transport of guests throughout the MOF would be affected by the interactions between the guest and the static and dynamic components of the framework, we impregnated deuterated PIZOF-2 samples with tetracyanoquinodimethane (TCNQ). Given the electron-rich nature of the linker, electron deficient TCNQ was selected because it fits into the pores and has a high propensity to form strong π–π stacking bonds, often in the form of charge transfer complexes.²⁰ In other words, TCNQ is a very sticky molecule known to affect the electronic structure of MOFs and has been used as an additive to enhance their charge conduction properties for device applications.^21,22The incorporation of TCNQ into the MOF was performed by immersing MOF powder samples in CH₂Cl₂ solutions for a minimum of 6 h at 295 K followed by rinsing, resulting in a loading capacity of 28.6 ± 0.2 TCNQ molecules per unit cell. At this saturated state, the white crystals changed to a green color and showed a strong EPR signal with g = 2.0025 (Fig. S8c^†), compared to the pristine MOF. This could be attributed to a charge transfer event that produces organic radicals which overshadows the intrinsic paramagnetism of the zirconia oxoclusters.²³ We also observed a quench of the emission, with a significant change in the quantum yield from Φ_F = 8.5% to Φ_F < 0.1% (Fig. S8d^†). Fluorescence quenching was expected due to the interaction of electron deficient molecules with the conjugated oligo-phenylene-ethynylene linkers that make the MOF emissive.²⁴The ¹³C CPMAS spectrum of TCNQ loaded PIZOF-2d4 not only confirmed the guest within the pores (Fig. S12^†), but it also revealed the changes in the chemical environment around the linkers: the appearance of a second carboxylate signal around δ = 174 ppm and a second quaternary carbon signal around δ = 128 ppm, with higher intensities with an increased loading time (Fig. 4a), attributable to the interaction of TCNQ with the outer rings of the PEPEP links, closer to the Zr cluster. Surprisingly, despite the evidence of the diffusion of TCNQ into the MOF, the solid-state ²H NMR spectrum of PIZOF-2d4 loaded with TCNQ for 6 h remained unaltered (Fig. 4b). Increasing the impregnation time to 72 h or increasing the temperature to 60 °C resulted in similar line shapes. These results suggest that the guest may have adsorbed near the outer phenylene rings of the linker. To demonstrate this, PIZOF-2d8 loaded with TCNQ for 6 h (Fig. 4c) was studied by ²H NMR. Interestingly, the fitting of the ²H line shape indicated slightly faster rotational rates compared to pristine PIZOF-2d8, changing from k_rot = 2.1 MHz to k_rot = 3.3 MHz. Only rising the impregnation temperature to 60 °C for 24 h allowed faster adsorption equilibration, decreasing the rotational rate to k_rot = 1.2 MHz. This indicates that the diffusion of TCNQ is slow and may require longer equilibration times at higher temperature to reach an equilibrium. Furthermore, considering the changes in the chemical shift of the carboxylate peak observed by ¹³C NMR CP MAS upon the diffusion of the guest (Fig. 4a, pink mark), as well as the minor changes in the rotational dynamics of the aromatic rings, we postulate that the TCNQ is located closer to the metal cluster, which agrees well with previously observed guest-loaded Zr-based MOFs.^25,26Open in a separate windowFig. 4(a) ¹³C CPMAS of PIZOF-2d4 at different TCNQ loading times. (b) Experimental and simulated ²H NMR spectra of PIZOF-2d4 revealing that the signal from the central phenylene remains unaffected. (c) Experimental and simulated ²H NMR of PIZOF-2d8 under different TCNQ loading conditions.This work highlights that our approach can tackle one of the challenges in guest-loaded MOFs, which is the understanding of the interactions between the guest and the framework, a problem often exacerbated by the difficulty of acquiring high-quality single crystals. Furthermore, even after obtaining suitable crystals, X-ray diffraction studies provide only averaged space and time information. Conversely, solid-state NMR, as it is time-resolved, is ideal to analyze guest loaded MOFs in bulk samples, providing kinetic information such as transient π-interaction sites,²⁷ gas-absorption diffusional rates,²⁸ internal rotational dynamics,⁶ and other kinetic details.^29,30     

Guest-mediated phase transitions in a flexible pillared-layered metal–organic framework under high-pressure

The guest-dependent flexibility of the pillared-layered metal–organic framework (MOF), Zn₂bdc₂dabco·X(guest), where guest = EtOH, DMF or benzene, has been examined by high-pressure single crystal X-ray diffraction. A pressure-induced structural phase transition is found for the EtOH- and DMF-included frameworks during compression in a hydrostatic medium of the guest species, which is dependent upon the nature and quantity of the guest in the channels. The EtOH-included material undergoes a phase transition from P4/mmm to C2/m at 0.69 GPa, which is accompanied by a change in the pore shape from square to rhombus via super-filling of the pores. The DMF-included material undergoes a guest-mediated phase transition from I4/mcm to P4/mmm at 0.33 GPa via disordering of the DMF guest. In contrast, the benzene-included framework features a structure with rhombus-shaped channels at ambient pressure and shows direct compression under hydrostatic pressure. These results demonstrate the large influence of guest molecules on the high-pressure phase behavior of flexible MOFs. Guest-mediated framework flexibility is useful for engineering MOFs with bespoke pore shapes and compressibility.

The guest-dependent flexibility of the pillared-layered metal–organic framework (MOF), Zn₂bdc₂dabco·X(guest), where guest = EtOH, DMF or benzene, has been examined by high-pressure single crystal X-ray diffraction.     

Interplay of structural dynamics and electronic effects in an engineered assembly of pentacene in a metal–organic framework

Charge carrier mobility is an important figure of merit to evaluate organic semiconductor (OSC) materials. In aggregated OSCs, this quantity is determined by inter-chromophoric electronic and vibrational coupling. These key parameters sensitively depend on structural properties, including the density of defects. We have employed a new type of crystalline assembly strategy to engineer the arrangement of the OSC pentacene in a structure not realized as crystals to date. Our approach is based on metal–organic frameworks (MOFs), in which suitably substituted pentacenes act as ditopic linkers and assemble into highly ordered π-stacks with long-range order. Layer-by-layer fabrication of the MOF yields arrays of electronically coupled pentacene chains, running parallel to the substrate surface. Detailed photophysical studies reveal strong, anisotropic inter-pentacene electronic coupling, leading to efficient charge delocalization. Despite a high degree of structural order and pronounced dispersion of the 1D-bands for the static arrangement, our experimental results demonstrate hopping-like charge transport with an activation energy of 64 meV dominating the band transport over a wide range of temperatures. A thorough combined quantum mechanical and molecular dynamics investigation identifies frustrated localized rotations of the pentacene cores as the reason for the breakdown of band transport and paves the way for a crystal engineering strategy of molecular OSCs that independently varies the arrangement of the molecular cores and their vibrational degrees of freedom.

Pentacene assembled into 1D arrays using a metal–organic framework (MOF) approach. This cofacial packing motif, which is not present in pentacene bulk, shows an interesting interplay of band-like and hopping-type transport.     

Identification of a metastable uranium metal–organic framework isomer through non-equilibrium synthesis

Since the structure of supramolecular isomers determines their performance, rational synthesis of a specific isomer hinges on understanding the energetic relationships between isomeric possibilities. To this end, we have systematically interrogated a pair of uranium-based metal–organic framework topological isomers both synthetically and through density functional theory (DFT) energetic calculations. Although synthetic and energetic data initially appeared to mismatch, we assigned this phenomenon to the appearance of a metastable isomer, driven by levers defined by Le Châtelier''s principle. Identifying the relationship between structure and energetics in this study reveals how non-equilibrium synthetic conditions can be used as a strategy to target metastable MOFs. Additionally, this study demonstrates how defined MOF design rules may enable access to products within the energetic phase space which are more complex than conventional binary (e.g., kinetic vs. thermodynamic) products.

Identifying the relationship between structure and energetics in a uranium MOF isomer system reveals how non-equilibrium synthetic conditions can be used as a strategy to target metastable MOFs.     

Single-step synthesis and interface tuning of core–shell metal–organic framework nanoparticles

Control over the spatial distribution of components in metal–organic frameworks has potential to unlock improved performance and new behaviour in separations, sensing and catalysis. We report an unprecedented single-step synthesis of multi-component metal–organic framework (MOF) nanoparticles based on the canonical ZIF-8 (Zn) system and its Cd analogue, which form with a core–shell structure whose internal interface can be systematically tuned. We use scanning transmission electron microscopy, X-ray energy dispersive spectroscopy and a new composition gradient model to fit high-resolution X-ray diffraction data to show how core–shell composition and interface characteristics are intricately controlled by synthesis temperature and reaction composition. Particle formation is investigated by in situ X-ray diffraction, which reveals that the spatial distribution of components evolves with time and is determined by the interplay of phase stability, crystallisation kinetics and diffusion. This work opens up new possibilities for the control and characterisation of functionality, component distribution and interfaces in MOF-based materials.

Core–shell metal–organic framework nanoparticles have been synthesised in which the internal interface and distribution of components is found to be highly tunable using simple variations in reaction conditions.     

Exploiting in situ NMR to monitor the formation of a metal–organic framework

The formation processes of metal–organic frameworks are becoming more widely researched using in situ techniques, although there remains a scarcity of NMR studies in this field. In this work, the synthesis of framework MFM-500(Ni) has been investigated using an in situ NMR strategy that provides information on the time-evolution of the reaction and crystallization process. In our in situ NMR study of MFM-500(Ni) formation, liquid-phase ¹H NMR data recorded as a function of time at fixed temperatures (between 60 and 100 °C) afford qualitative information on the solution-phase processes and quantitative information on the kinetics of crystallization, allowing the activation energies for nucleation (61.4 ± 9.7 kJ mol⁻¹) and growth (72.9 ± 8.6 kJ mol⁻¹) to be determined. Ex situ small-angle X-ray scattering studies (at 80 °C) provide complementary nanoscale information on the rapid self-assembly prior to MOF crystallization and in situ powder X-ray diffraction confirms that the only crystalline phase present during the reaction (at 90 °C) is phase-pure MFM-500(Ni). This work demonstrates that in situ NMR experiments can shed new light on MOF synthesis, opening up the technique to provide better understanding of how MOFs are formed.

A new in situ NMR methodology for studying the formation processes of MOFs is reported, supported by SAXS and PXRD experiments. Synthesis of a phosphonate-based MOF is described, from molecular aggregation through to nucleation and crystallisation.     

Aperiodic metal–organic frameworks

Metal–organic frameworks (MOFs) represent one of the most diverse structural classes among solid state materials, yet few of them exhibit aperiodicity, or the existence of long-range order in the absence of translational symmetry. From this apparent conflict, a paradox has emerged: even though aperiodicity frequently arises in materials that contain the same bonding motifs as MOFs, aperiodic structures and MOFs appear to be nearly disjoint classes. In this perspective, we highlight a subset of the known aperiodic coordination polymers, including both incommensurate and quasicrystalline structures. We further comment upon possible reasons for the absence of such structures and propose routes to potentially access aperiodic MOFs.

This perspective discusses progress and future directions in metal–organic frameworks with aperiodic structures. Reported quasicrystalline and incommensurate materials are presented, and pathways towards designing new such materials are provided.     

Biomimetic mimicry of formaldehyde-induced DNA–protein crosslinks in the confined space of a metal–organic framework

DNA–protein crosslinks (DPCs) are highly toxic DNA lesions induced by crosslinking agents such as formaldehyde (HCHO). Building artificial models to simulate the crosslinking process would advance our understanding of the underlying mechanisms and therefore develop coping strategies accordingly. Herein we report the design and synthesis of a Zn-based metal–organic framework with mixed ligands of 2,6-diaminopurine and amine-functionalized dicarboxylate, representing DNA and protein residues, respectively. Combined characterization techniques allow us to demonstrate the unusual efficiency of HCHO-crosslinking within the confined space of the titled MOF. Particularly, in situ single-crystal X-ray diffraction studies reveal a sequential methylene-knitting process upon HCHO addition, along with strong fluorescence that was not interfered with by other metabolites, glycine, and Tris. This work has successfully constructed a purine-based metal–organic framework with unoccupied Watson–Crick sites, serving as a crystalline model for HCHO-induced DPCs by mimicking the confinement effect of protein/DNA interactions.

An amine-functionalized biological metal–organic framework serving as a crystalline model for mimicking formaldehyde-induced DNA–protein crosslinks in a confined space.     

Switchable electrical conductivity in a three-dimensional metal–organic framework via reversible ligand n-doping

Redox-active metal–organic frameworks (MOFs) are promising materials for a number of next-generation technologies, and recent work has shown that redox manipulation can dramatically enhance electrical conductivity in MOFs. However, ligand-based strategies for controlling conductivity remain under-developed, particularly those that make use of reversible redox processes. Here we report the first use of ligand n-doping to engender electrical conductivity in a porous 3D MOF, leading to tunable conductivity values that span over six orders of magnitude. Moreover, this work represents the first example of redox switching leading to reversible conductivity changes in a 3D MOF.

Redox-active ligands are used to reversibly tune electrical conductivity in a porous 3D metal–organic framework (MOF).     

Efficient solid-state photoswitching of methoxyazobenzene in a metal–organic framework for thermal energy storage

Efficient photoswitching in the solid-state remains rare, yet is highly desirable for the design of functional solid materials. In particular, for molecular solar thermal energy storage materials high conversion to the metastable isomer is crucial to achieve high energy density. Herein, we report that 4-methoxyazobenzene (MOAB) can be occluded into the pores of a metal–organic framework Zn₂(BDC)₂(DABCO), where BDC = 1,4-benzenedicarboxylate and DABCO = 1,4-diazabicyclo[2.2.2]octane. The occluded MOAB guest molecules show near-quantitative E → Z photoisomerization under irradiation with 365 nm light. The energy stored within the metastable Z-MOAB molecules can be retrieved as heat during thermally-driven relaxation to the ground-state E-isomer. The energy density of the composite is 101 J g⁻¹ and the half-life of the Z-isomer is 6 days when stored in the dark at ambient temperature.

4-Methoxyazobenzene can be occluded into the pores of a MOF and show near-quantitative E → Z photoisomerization under irradiation with 365 nm light. The energy density of the composite is 101 J g⁻¹ and the half-life of the Z-isomer is 6 days.     

A cofacial metal–organic framework based photocathode for carbon dioxide reduction

Innovative and robust photosensitisation materials play a cardinal role in advancing the combined effort towards efficient solar energy harvesting. Here, we demonstrate the photocathode functionality of a Metal–Organic Framework (MOF) featuring cofacial pairs of photo- and electro-active 1,4,5,8-naphthalenediimide (NDI) ligands, which was successfully applied to markedly reduce the overpotential required for CO₂ reduction to CO by a well-known rhenium molecular electrocatalyst. Reduction of [Cd(DPNDI)(TDC)]_n (DPNDI = N,N′-di(4-pyridyl)-1,4,5,8-naphthalenediimide, H₂TDC = thiophene-2,5-dicarboxylic acid) to its mixed-valence state induces through-space Intervalence Charge Transfer (IVCT) within cofacial DPNDI units. Irradiation of the mixed-valence MOF in the visible region generates a DPNDI photoexcited radical monoanion state, which is stabilised as a persistent species by the inherent IVCT interactions and has been rationalised using Density Functional Theory (DFT). This photoexcited radical monoanion state was able to undergo charge transfer (CT) reduction of the rhenium molecular electrocatalyst to effect CO generation at a lower overpotential than that required by the discrete electrocatalyst itself. The exploitation of cofacial MOFs opens new directions for the design philosophy behind light harvesting materials.

The photocathode functionality of a Metal–Organic Framework (MOF) featuring cofacial photo- and electro-active ligands provides a new approach to CO₂ reduction via charge transfer with a rhenium electrocatalyst.

The development of photocathode materials for CO₂ reduction and hydrogen evolution catalyses has traditionally focussed on photosensitising transition metal complexes or nanostructured solid state semiconductors.^1,2 At the nascent frontier between robust solid state semiconductors and synthetically protean metal complexes are photo-/electro-active Metal–Organic Frameworks (MOFs) that consolidate the flexibility of homogeneous systems into the robust heterogeneous phase.³ Contrasting with reported MOF examples, natural photosynthesis remains one of the most efficient light harvesting systems.⁴ One common reaction centre adopted in photosynthesis features a redox-active cofacial dimer of chlorophyll pigment molecules.⁵ This cofacial moiety stabilises the photoexcited charge separated state through intra-dimer Intervalence Charge Transfer (IVCT) interactions, enabling the trapping and conversion of light to chemical energy. Recently, we characterised IVCT interactions upon reduction to the mixed-valence state in the MOF [Zn₂(TDC)₂(DPPTzTz)₂]_n (DPPTzTz = 2,5-bis(4-(4-pyridyl)phenyl)thiazolo[5,4-d]thiazole and H₂TDC = thiophene-2,5-dicarboxylic acid) featuring cofacial dimers of the thiazolothiazole redox-active core, and probed its structure–activity dependence computationally and experimentally.^6–9 Subsequently, we sought design a new MOF featuring cofacial pairs of the photo- and redox-active N,N′-di(4-pyridyl)-1,4,5,8-naphthalenediimide (DPNDI) ligand, as a conceptually neoteric photosensitiser for incorporation into systems relevant towards artificial photosynthesis.The naphthalene diimide (NDI) core was selected for its photoactive radical monoanion state.¹⁰ For a number of discrete systems, Wasielewski and coworkers have computationally and experimentally demonstrated the ability to photoexcite the easily accessible NDI radical monoanion using visible light, facilitating its transient photoelectrochemical reduction of Re based catalytic CO₂ reduction sites.^2,11–14 Recently, Goswami et al. synthesised a Zr NDI-based MOF, applying this as a radical state heterogeneous photosensitiser to decompose dichloromethane.¹⁵Here, we describe the synthesis of a new photo- and redox-active MOF [Cd(DPNDI)(TDC)]_n, denoted csiMOF-6 (cofacial stacked IVCT), featuring cofacial dimers of the DPNDI ligand. Cofacial DPNDI MOFs have been reported previously by Takashima et al.¹⁶ and Sikdar et al.,¹⁷ where guest dependent charge transfer (CT) and neutral state photoexcitation behaviours were examined. Dinolfo et al. also incorporated DPNDI into a rhenium based cofacial complex, where its mixed-valence IVCT behaviour was probed using electrochemical and spectroelectrochemical (SEC) techniques.¹⁸ We envisaged that the cofacial NDI units in csiMOF-6 would stabilise its photoexcited radical monoanion state by IVCT interactions, akin to cofacial moieties in natural photosynthsesis processes. This strengthens the persistence of the NDI photoexcited radical monoanion state, thereby improving its efficacy at photoelectrochemical reduction of catalytically active sites. Effectiveness of the cofacial design principle behind csiMOF-6 photocathodes was verified using a combined experimental and computational approach. The successful photocathode performance of csiMOF-6 under broad band visible light irradiation encompassed its photoelectrochemical reduction of the [Re(bipy-tBu)(CO)₃Cl] (bipy-tBu = 4,4′-di-tert-butyl-2,2′-bipyridine, developed by Smieja et al.¹⁹) CO₂ reduction electrocatalyst, resulting in CO generation at reduced overpotential requirements.     

Isomer recognition by dynamic guest-adaptive ligand rotation in a metal–organic framework with local flexibility

Local flexibility in a metal–organic framework is intriguing for reconstructing a microenvironment to distinguish different guest molecules by emphasizing their differences. Herein, guest-adaptive flexibility is observed in a metal–organic framework for efficiently discriminating aromatic isomers. Microcrystal electron diffraction directly reveals that the anthracene rings can rotate around the single bond with the adsorption of guest molecules. Disorder transformation of the ligand enables the preferential adsorption of ethylbenzene over other xylene isomers. Especially, a coated capillary column combining single/multi-component adsorption confirms a unique separation order of ethylbenzene > p-xylene > m-xylene > o-xylene with excellent selectivities, which has not been reported in other materials. Density functional theory calculations and the calculated Hirshfeld surface of guest molecules in the framework demonstrate that a guest-induced splint-like confinement structure makes the main contribution to such separation performance. This finding will provide a rational strategy for molecular recognition utilizing the local flexibility of metal–organic frameworks.

Local flexibility in a metal–organic framework is intriguing for reconstructing a microenvironment to distinguish different guest molecules by emphasizing their differences.     

Modulated self-assembly of metal–organic frameworks

Exercising fine control over the synthesis of metal–organic frameworks (MOFs) is key to ensuring reproducibility of physical properties such as crystallinity, particle size, morphology, porosity, defectivity, and surface chemistry. The principle of modulated self-assembly – incorporation of modulator molecules into synthetic mixtures – has emerged as the primary means to this end. This perspective article will detail the development of modulated synthesis, focusing primarily on coordination modulation, from a technique initially intended to cap the growth of MOF crystals to one that is now used regularly to enhance crystallinity, control particle size, induce defectivity and select specific phases. The various mechanistic driving forces will be discussed, as well as the influence of modulation on physical properties and how this can facilitate potential applications. Modulation is also increasingly being used to exert kinetic control over self-assembly; examples of phase selection and the development of new protocols to induce this will be provided. Finally, the application of modulated self-assembly to alternative materials will be discussed, and future perspectives on the area given.

This Perspective gives an overview of the modulated self-assembly of MOFs – incorporating additives and alternative precursors into syntheses – focusing on its varying influences on crystallization mechanisms, physical properties, and applications.     

Transient self-assembly of metal–organic complexes

Implementing transient processes in networks of dynamic molecules holds great promise for developing new functional behaviours. Here we report that trichloroacetic acid can be used to temporarily rearrange networks of dynamic imine-based metal complexes towards new equilibrium states, forcing them to express complexes otherwise unfavourable in their initial equilibrium states. Basic design principles were determined for the creation of such networks. Where a complex distribution of products was obtained in the initial equilibrium state of the system, the transient rearrangement temporarily yielded a simplified output, forcing a more structured distribution of products. Where a single complex was obtained in the initial equilibrium state of the system, the transient rearrangement temporarily modified the properties of this complex. By doing so, the mechanical properties of an helical macrocyclic complex could be temporarily altered by rearranging it into a [2]catenane.

Trichloroacetic acid was used to temporarily rearrange networks of imine-based metal complexes, enabling the expression of complexes unfavourable in the initial state of the networks. Basic design principles were determined for these networks.     

Applications of metal–organic framework-based bioelectrodes

Metal–organic frameworks (MOFs) are an emerging class of porous nanomaterials that have opened new research possibilities. The inherent characteristics of MOFs such as their large surface area, high porosity, tunable pore size, stability, facile synthetic strategies and catalytic nature have made them promising materials for enormous number of applications, including fuel storage, energy conversion, separation, and gas purification. Recently, their high potential as ideal platforms for biomolecule immobilization has been discovered. MOF-enzyme-based materials have attracted the attention of researchers from all fields with the expansion of MOFs development, paving way for the fabrication of bioelectrochemical devices with unique characteristics. MOFs-based bioelectrodes have steadily gained interest, wherein MOFs can be utilized for improved biomolecule immobilization, electrolyte membranes, fuel storage, biocatalysis and biosensing. Likewise, applications of MOFs in point-of-care diagnostics, including self-powered biosensors, are exponentially increasing. This paper reviews the current trends in the fabrication of MOFs-based bioelectrodes with emphasis on their applications in biosensors and biofuel cells.

Metal–organic frameworks based bioelectrodes.