Spiers Memorial Lecture: Coordination networks that switch between nonporous and porous structures: an emerging class of soft porous crystals

Coordination networks (CNs) are a class of (usually) crystalline solids typically comprised of metal ions or cluster nodes linked into 2 or 3 dimensions by organic and/or inorganic linker ligands. Whereas CNs tend to exhibit rigid structures and permanent porosity as exempli ﬁ ed by most metal – organic frameworks, MOFs, there exists a small but growing class of CNs that can undergo extreme, reversible structural transformation(s) when exposed to gases, vapours or liquids. These “ soft ” or “ stimuli-responsive ” CNs were introduced two decades ago and are attracting increasing attention thanks to two features: the amenability of CNs to design from ﬁ rst principles, thereby enabling crystal engineering of families of related CNs; and the potential utility of soft CNs for adsorptive storage and separation. A small but growing subset of soft CNs exhibit reversible phase transformations between nonporous (closed) and porous (open) structures. These “ switching CNs ” are distinguished by stepped sorption isotherms coincident with phase transformation and, perhaps counterintuitively, they can exhibit benchmark properties with respect to working capacity (storage) and selectivity (separation). This review addresses fundamental and applied aspects of switching CNs through surveying their sorption properties, analysing the structural transformations that enable switching, discussing structure – function relationships and presenting design principles for crystal engineering of the next generation of switching CNs.

A variety of terms have been coined to describe the exibility of CNs including "accordionlike", 26 "springlike", 27,52 "spongelike", 53,54 "breathing", 55 "swelling", 56,57 "so", 30,31,58 "elastic", 59,60 "dynamic", 24,61,62 "stimuli-responsive", 47,63,64 and "gateopening". 28,65,66 An alternate approach to classify exible CNs is to use their sorption isotherms and we have proposed a classication system based upon sorption isotherm types, namely type F-I to type F-V. 67 IUPAC has classied type I isotherms as being characteristic of rigid microporous adsorbents thanks to the enhanced adsorbent-adsorbate interactions which results in micropore lling at very low relative pressure (Fig. 2a). 68 In contrast, exible microporous CNs usually exhibit distinct "stepped" or "S-shaped" isotherm proles that are yet to be (d) type F-IV isotherms can be sub-divided into single-step type F-IV s and multiplestep ($2) type F-IV m isotherms; (e) type F-V isotherms are indicative of a shape memory effect in second and subsequent consecutive sorption cycles; (f) comparison of working capacity between type I and F-IV isotherms with similar surface areas: black line ¼ adsorption branch of type I isotherm (desorption branch overlays); solid/dash purple lines ¼ adsorption and desorption branches of type F-IV isotherm, respectively; P de : delivery pressure; P st : storage pressure.
formally classied by IUPAC. When exible CNs retain porosity aer activation, a type I-like prole will be followed by the second step at a threshold pressure that coincides with a structural transformation from a less open phase to a more open phase. This transformation could be gradual (type F-I, e.g. the "breathing" effect in MIL-53(Cr) 69 ) or sudden (type F-II, e.g. the "gate-opening" effect in ELM-12 (ref. 70)) ( Fig. 2b). When exible CNs are nonporous aer activation, the transformation from a nonporous (closed) to a porous (open) phase can also occur gradually (type F-III, e.g. the "swelling" effect in [Co 2 (BME-bdc) 2 (dabco)] 71 ) or suddenly (type F-IV, e.g. the "switching" effect in [Zn 2 (BME-bdc) 2 (dabco)] 71 ) (Fig. 2c). In addition, type F-IV isotherms can be further sub-divided depending on the number of sorption steps, i.e., single-step type F-IV s or multiple-step ($2) type F-IV m (Fig. 2d). 72 We note that most of the exible CNs reported to date undergo reversible structural changes in response to the presence/absence of external stimuli and that there are very few examples of exible CNs that do not return to their activated forms aer full desorption. There are, however, CNs that retain the structure of the fully open stage aer the rst sorption cycle and exhibit type I sorption isotherms during the subsequent cycles (type F-V, e.g. the "shape memory" effect in X-pcu-3-Zn-3i 73 ) (Fig. 2e). Amongst the various isotherm types in Fig. 2, exible CNs that exhibit type F-IV isotherms are perhaps the most desirable with respect to gas storage as they can offer higher working capacity than rigid porous materials with type I isotherms (Fig. 2f). 72,74 It should be mentioned that hysteresis is a feature of most type F-IV isotherms and, to be of practical utility, the delivery pressure (P de ) must be lower than the desorption branch rather than the adsorption branch (Fig. 2f).
We adopt the term "switching" herein as it is consistent with the "on/off" nature of CNs that exhibit reversible "closed/open" structural changes of the type that affords type F-IV isotherms. Switching has also been used for other on/off events in materials chemistry such as thermal expansion/shrinkage, spin crossover, redox, photochromism, photoisomerization and valence tautomerism. 42,43,[75][76][77] In the following sections, we analyse and discuss reversible switching in the context of guest sorption by presenting case studies of switching CNs with particular emphasis upon structure-property relationships and the inuence of variables such as metal node, linker ligand, and adsorbate.

Switching CNs
Whereas coordination complexes and organic molecules are long-known for the ability to exhibit guest inclusion or clathration that accompanies switching between closed and open phases, [78][79][80][81][82][83] the rst examples of 2D and 3D switching CNs were not reported until the beginning of the 21 st century (Fig. 3). 28,51 Werner complexes are prototypal coordination compounds and their guest-clathration ability was systematically studied by Schaeffer in 1957. 78 The associated sorption isotherms were reported in 1969 by Barrer's group, who studied the sorption behaviour of several Werner complexes upon exposure to a range of gases and vapours. 79 Type F-IV isotherms were observed when the Werner complex [Co(etpy) 4 (NCS) 2 ] was exposed to benzene, toluene and xylenes and this phenomenon was attributed to closed/open phase transformations. We recently reported a related example of such a Werner complex and termed it as a Switching Adsorbent Molecular Material (SAMM). 83 The rst 2D switching CN, [Cu(bpy) 2 (BF 4 ) 2 ], ELM-11, 28 was reported in 2001 and can be regarded as being comprised of linked Werner complexes which serve as molecular building blocks (MBBs 84 ). The rst 3D switching CN, [Cu 2 (bdc) 2 (bpy)], 51 was reported in 2002 and features a 2D square lattice layer comprised of linked "paddle-wheel" MBBs that is pillared by bpy linkers to form a 2-fold interpenetrated 3D network. Altering the length of the vertical pillar and horizontal linker afforded a non-interpenetrated switching CN DUT-8(Ni) which set benchmark uptakes for N 2 (77 K) and CO 2 (195 K) in switching CNs. 113,114 Hybrid CNs 6 with primitive cubic (pcu) topology can be generated from 2D square lattice (sql) CNs when an inorganic counter anion such as hexauorosilicate can serve as a pillar ligand, as exemplied by the rst switching hybrid CN, SIFSIX-23-Cu. 151 Whereas the coordination spheres of Werner complexes, 2D sql CNs and 3D pcu CNs are chemically related, their dimensional rigidities can be different. 40 In 2008, type F-IV m isotherms were observed in Co(bdp), 101 which later set a benchmark for volumetric CH 4 working capacity in the context of switching CNs. 74 Although MIL-53 CNs were reported as early as 2002, a switching variant, MIL-53(Fe), was not introduced until 2009. 106 Xdia-1-Ni, reported in 2018, was the rst example of a diamondoid (dia) topology switching CN and it was found to exhibit the second highest CH 4 working capacity in switching CNs. 67 Overall, around 60 switching CNs have now been reported (Table 1), with sql and pcu topology CNs being the most common 2D and 3D switching CNs, respectively. Whereas 2D switching CNs were reported rst, the proportion of 3D switching CNs is currently much higher than 2D switching CNs  . This is likely a reection of the larger number of 3D CNs (especially MOFs) that have been studied for their sorption properties in the past two decades but should not imply any prejudice in favour of modularity or properties for 3D CNs versus 2D CNs. Switching between a closed and a single open phase generally results in singlestep type F-IV s isotherms 51,65,66,71,74 whereas switching between closed and multiple open phases usually affords multiple-step type F-IV m isotherms ( Fig. 2d  and 4). 72,[85][86][87] The number of structural transitions and/or sorption steps in a switching CN can be inuenced by factors such as temperature, pressure and adsorbate as exemplied by ELM-11. 72,85-87 Several of the prototypal switching CNs are modular and amenable to crystal engineering through systematic variation of metal moieties, linker ligands, sorbates or a combination thereof. Importantly, in many published reports, both the closed and open phases are structurally characterized and/or computationally modelled, thereby providing insight into both the mechanism of switching and the nature of the sorbent-sorbate interactions  that drive the switching event. In the following sections, we present case studies of representative switching CNs and succinctly analyse their switching behaviour. . The MIL-53 family of CNs represents perhaps the most widely studied family of exible CNs thanks to the contributions of Férey and others. 155 The metals (M) of MIL-53 are trivalent, e.g. Cr, Al, Fe, Sc, Ga or In. [156][157][158][159][160][161] The structure of MIL-53(M) CNs is sustained by innite trans corner sharing [MO 4 (OH) 2 ] octahedra that form 1D chains by serving as rod building blocks, RBBs, [162][163][164] which are cross-linked through bdc ligands (bdc ¼ 1,4-benzenedicarboxylate) to afford sra topology networks (Fig. 5). 162 The resulting 1D rhombic tunnels are occupied by solvent or guest molecules. The rst members of the MIL-53(M) family, MIL-53(Cr) and MIL-53(Al), were reported in 2002 and 2003, respectively. 156,157 Initial studies focused on structural transformations during hydration-dehydration processes. 165,166 Large and reversible phase transformation between the activated "open" phase (MIL-53ht, ht ¼ high temperature) and hydrated "contracted" phase (MIL-53lt, lt ¼ low temperature) was revealed and this "breathing" phenomenon remains a rarity even aer two decades. A superhydrated "open" phase was obtained by immersing MIL-53(Cr) in water and was reported in 2011. 167 Gas sorption studies involving N 2 , H 2 , CH 4 and CO 2 on MIL-53(Cr, Al) revealed guest-dependent sorption proles. 69,156,157 For N 2 , H 2 and CH 4 , the sorption isotherms are type I as expected for rigid microporous CNs and zeolites (Fig. 2a). In contrast, for CO 2 MIL-53(Cr, Al) were observed to exhibit type F-I sorption isotherms (Fig. 6a). 69 This was attributed to CO 2 molecules interacting strongly with the hydroxide moieties that line the rhombic tunnels, resulting in pore contraction at low pressure and pore opening at high pressure. 168 A two-step type F-I isotherm was observed with polar vapours (e.g. MeOH, EtOH) and C 2 -C 8 hydrocarbon sorbates. [169][170][171] Substitution of the metal nodes by Fe(III) or Sc(III) resulted in nonporous "closed" phases in MIL-53(Fe or Sc) aer activation. 122,123,172 Compared to MIL-53(Sc), MIL-53(Fe) has been more widely subjected to study. CO 2 sorption on MIL-53(Fe) revealed a multi-step type F-IV m isotherm (Fig. 6b). 107 Other gases such as light hydrocarbons were observed to exhibit the same trend but with  (Table 2).
Ligand functionalization impacts the exibility of MIL-53(Fe) with various adsorbates (Table 3) as exemplied by MIL-53(Fe)-X variants prepared from a library of functionalised bdc ligands (Fig. 8a). [179][180][181][182][183] Except for MIL-53(Fe)-2OH, the activated "dry" phases of MIL-53(Fe)-X exhibited larger volumes than MIL-53(Fe), consistent with the ip1 or ip2 phase of MIL-53(Fe) (Fig. 8b, black line). 181 77 K N 2 sorption studies revealed no porosity (closed phases) except for MIL-53(Fe)-2CF 3 . The larger pore volumes of activated MIL-53(Fe)-X phases were attributed to steric hindrance between functional groups which prevent pore closure. Hydration was found to result in only negligible changes to the unit-cell volumes (Fig. 8b, red line). MIL-53(Fe)-2OH is an exception as it was found to exhibit high water uptake thanks to hydrogen bonding between hydroquinolic OH groups and water molecules. Regarding other solvents, MIL-53(Fe)-2COOH was observed to be an outlier. It remained in its closed phase regardless of the liquid used and exhibited no signicant solvent uptake. This outcome was  attributed to strong intraframework hydrogen bonding interactions that drive the pores to remain closed. In general, pore opening in functionalised MIL-53(Fe)-X variants is governed by a balance between the intrinsic stability of the closed and open phases and guest-framework interactions. The inuence of the linker on the switching of MIL-53(Fe) upon CO 2 and C 1 -C 9 hydrocarbon adsorption was also systematically studied (Fig. 8c). 182,183 With the exception of methane, closed to open transformations occurred through an intermediate phase (X¼ Cl, Br, CH 3 ), thus differing from the parent, MIL-53(Fe), for which two intermediate phases were observed (Fig. 8b). MIL-53(Fe)-2COOH and MIL-53(Fe)-NH 2 remained closed when exposed to CO 2 or hydrocarbons. 182,183 A combination of steric effects and intraframework interactions in MIL-53(Fe)-X was deemed responsible for these observations.

Pillared-layer CNs.
Pillared-layer coordination networks (PLCNs) are a diverse class of CNs that have been extensively studied in the past two decades. 6,184-188 Most PLCNs feature pcu or "jungle-gym" geometry and the most commonly studied subset has the general formula [M 2 L 2 P] n (M ¼ divalent metal cation; class I: L ¼ dicarboxylate linker and P ¼ N-donor pillar; class II: L ¼ Ndonor linker and P ¼ inorganic anionic pillar). 188 Class I PLCNs comprise twodimensional sql topology networks linked by dicarboxylate linker ligands (L) which are pillared in the third dimension by neutral N-donor linker ligands (P) to form pcu topology frameworks (Fig. 9). The metal nodes of the prototypal PLCNs are dinuclear paddle-wheel [M 2 (COO) 4 ] that serve as 6-connected (6c) MBBs.
Kaskel's group subsequently reported that other metals can sustain DUT-8(M) analogues (M ¼ Co, Zn, Cu) and exhibit distinct sorption proles. 114 Depending on the metal ion used, DUT-8 variants were found to exhibit reversible (DUT-8(Ni), DUT-8(Co)), irreversible (DUT-8(Zn)) or no (DUT-8(Cu)) transformation upon activation and/or guest sorption. It was noted that the particle size of DUT-8(Ni) can inuence sorption proles, which were either type F-IV s (particle size > 1 mm) or type I (particle size < 500 nm). 116,193,194 A similar "downsizing" effect was also reported for the DMOF-1 analogue [Cu 2 (bdc) 2 (bpy)]. 93 Recently, our group reported the structures and sorption properties of a series of DMOF-1 variants, namely [Zn 2 (DMTDC) 2 (P)], X-pcu-n-Zn (n ¼ 5, P ¼ 1,2-di (4-pyridyl)-ethylene (dpe); n ¼ 6, P ¼ 1,2-bis(4-pyridyl)ethane (bpe); n ¼ 7, 146,147 The four pillar ligands used are longer than dabco and enable 2-fold interpenetration in the X-pcu-n-Zn family (Fig. 11a). The as-synthesised "open" phases, X-pcu-n-Zna, were obtained by solvothermal synthesis with solvent molecules occupying voids and calculated guest-accessible volumes of ca. 45% (Fig. 11b). Single-crystal X-ray diffraction (SCXRD) studies revealed that activation of X-pcu-n-Zn-a resulted in "closed" nonporous phases, X-pcu-n-Zn-b, with unit cell volumes reduced by ca. 35% (Fig. 11c). CO 2 , C 2 H 2 and C 2 H 4 sorption on X-pcu-n-Zn-b revealed switching behaviour with comparable uptakes but different switching pressures ( Fig. 11d-f). Up to 250 cm 3 g À1 CO 2 uptake and good recyclability (>35 sorption cycles) make Xpcu-n-Zn only the second family of exible CNs to exhibit both high uptake and reversibility. The switching pressures for the three gases studied followed a consistent trend: X-pcu-6-Zn-b < X-pcu-5-Zn-b < X-pcu-7-Zn-b < X-pcu-8-Znb which was attributed to the relative degree of conformational exibility of the pillar ligands. 147 By exploring longer linkers and extended pillar ligands, other interpenetrated derivatives of DMOF-1 have been obtained. For example, with 4,4 0 -biphenyldicarboxylate as the linker and 1,4-bis(4-pyridyl)benzene or 4,4 0 -bis(4-pyridyl) biphenyl as the pillar, 3-fold interpenetrated CNs X-pcu-3-Zn and X-pcu-1-Zn were synthesised, respectively. 73,195 Although interpenetration inevitably reduces guest-accessible voids, X-pcu-3-Zn and X-pcu-1-Zn nevertheless exhibited ca. 45% guest-accessible volume. Both PLCNs were found to exhibit complicated structural transformations through solvent exchange, heating, vacuum and gas sorption processes. Interestingly, a rare example of "shape memory" was observed for X-pcu-3-Zn and X-pcu-1-Zn, which exhibited irreversible phase transitions. The only other reported example of this phenomenon was for [Cu 2 (bdc) 2 (bpy)]. 93 A second diverse class of PLCNs is exemplied by the SIFSIX-n-M family of general formula [M(L) 2 (SiF 6 )] (M ¼ divalent transition metal ions, L ¼ N-donor ditopic ligands). 6 This family is diverse in composition since the SIFSIX pillar can be replaced by other uorinated divalent anions such as TiF 6 2À , GeF 6 2À and SnF 6 2À and there are numerous N-donor linkers available. A difference between the SIFSIX-n-M and DMOF-1 families is that N-donor ligands act as linkers in the former whereas they serve as pillars in the latter. Cationic sql layers in the SIFSIXn-M family are charge balanced by anionic pillars. The SIFSIX-n-M family is of particular interest because several members have been reported to exhibit benchmark separation performances towards a variety of gas mixtures. 196,197 Whereas SIFSIX-n-M sorbents generally remain rigid during sorption cycles, rotation of linkers can cause inections in gas adsorption isotherms. 198,199 Recently, our group introduced the rst switching SIFSIX-n-M PLCN, [Cu(L) 2 (SiF 6 )] (SIFSIX-23-Cu, L ¼ 1,4-bis(1-imidazolyl)benzene), which was found to exhibit dramatic structural distortions. 151 Unlike previous SIFSIX-n-M sorbents, the SIFSIX pillars in SIFSIX-23-Cu adopt a cis-bridging mode ( Fig. 12a and b) and the sql layers undulate thanks to the "V"-shaped L(syn) linker ligands (Fig. 12c). Desolvation induces SIFSIX-23-Cu to undergo structural transformations involving multiple intermediate phases (SIFSIX-23-Cu-g1, -g2, -g3) prior to forming a solvent-free closed phase, SIFSIX-23-Cu-b1 (Fig. 12d). N 2 and CO 2 sorption on SIFSIX-23-Cu-b1 reveal type F-IV s and type F-IV m isotherms, respectively (Fig. 12e). SIFSIX-23-Cu is one of the few switching CNs that exhibits >200 cm 3 g À1 gas uptake and good recyclability (>40 sorption cycles). SIFSIX-23-Cu was also found to be stable in water for at least one year. 2.1.3 Diamondoid CNs. Diamondoid CNs feature dia topology and are amongst the earliest and most comprehensively studied CNs. They are also highly amenable to crystal engineering. 13,14,200,201 Interpenetration in diamondoid CNs tends to reduce surface area but can enhance rigidity. 202 Nevertheless, structural exibility was observed in the 2-fold interpenetrated diamondoid CN [In(ABDC) 2 ] (SHF-61, ABDC ¼ 2-aminobenzene-1,4-dicarboxylate) with continuous breathing/ swelling behaviour during desolvation/solvation. 203 This is a rare phenomenon that was previously observed in MIL-88 CNs. 56,57 The 8-fold interpenetrated diamondoid CN [Zn(oba)(pip)] (JUK-8), which is based on 4,4 0 -oxybis(benzenedicarboxylate) (oba) and 4-pyridyl functionalised benzene-1,3dicarbohydrazide (pip) linkers, exhibited switching upon exposure to H 2 O and CO 2 . 154 Recently, our group reported a exible diamondoid CN [NiL 2 ], X-dia-1-Ni, which is based upon a mixed N/O-donor ligand, 4-(4-pyridyl)-biphenyl-4carboxylic acid (HL). 67 Despite 6-fold interpenetration, the accessible void volume in X-dia-1-Ni is 49% thanks to the mode of interpenetration and the rectangular channels sustained by ligand L (Fig. 13a-d). X-dia-1-Ni was observed to undergo single-crystal-to-single-crystal (SCSC) transformations through solvent exchange with CH 2 Cl 2 and transform to its closed phase X-dia-1-Ni-c1 by heating under vacuum (Fig. 13e). 77 K N 2 sorption indicated that X-dia-1-Ni-c1 is nonporous whereas 195 K CO 2 sorption revealed multiple steps in both the adsorption and desorption branches. These proles indicate reversible structural changes between closed and open phases. Interestingly, the CH 4 adsorption isotherm measured at 298 K (Fig. 13f) revealed a type F-IV s isotherm that combined an appropriate switching pressure (5-35 bar) and high saturation uptake (>200 cm 3 g À1 ). These metrics made X-dia-1-Ni only the second exible CN aer Co(bdp) to exhibit such a high working capacity for CH 4 (Fig. 14a,  bdp ¼ 1,4-benzenedipyrazolate), 74,101-103 exhibits an unusual ve-step N 2 sorption isotherm at 77 K (Fig. 14b). A combination of in situ PXRD and molecular simulation studies revealed that the activated "closed" phase, Co(bdp)-dry, transformed to its fully "open" phase via three distinct intermediate phases (Fig. 14c). In addition, H 2 sorption on Co(bdp)-dry revealed a phase transformation which was not seen in other switching CNs. 101 Most importantly, Co(bdp)-dry exhibits a stepped type F-IV s isotherm when exposed to CH 4 (Fig. 14d). 74 The gate adsorption and desorption pressures are ca. 15 and 5 bar, respectively, suitable for CH 4 storage and delivery in the context of vehicular transport (5-35 bar). High uptake and the right type of F-IV isotherm prole made Co(bdp) the benchmark for CH 4 working capacity (ca. 200 cm 3 cm À3 ). The Fe  analogue, Fe(bdp), was also found to exhibit a type F-IV s isotherm but with a much higher switching pressure, highlighting the profound impact that metal centers can exert on switching pressures. In contrast, the Zn and Ni analogues, Zn(bdp) and Ni(bdp), were only found to exhibit type I gas sorption isotherms. 204 Furthermore, ligand functionalization in Co(bdp) can be used to tune the switching pressures. 133 2.2. Examples of 2D switching CNs 2.2.1 Square lattice (sql) CNs. 2D CNs may exhibit switching through claylike intercalation. One would intuitively anticipate that such a switching mechanism is facile because the attractive forces between layers are relatively weak and little strain upon the CN is likely to be required. Square lattice (sql) CNs are quite prevalent and account for nearly half of reported 2D CNs. 205 A well-studied family of sql CNs is comprised of octahedral metal ions (M), axial counter anions (A) and linear linker ligands (L). 206 When M : L is 1 : 2, sql CNs of general formula [M(L) 2 (A) 2 ]$xguest can be formed (Fig. 15). [207][208][209] From a crystal engineering perspective, this family of CNs exemplify the "node and linker" strategy rst developed by Robson and Hoskins over thirty years ago. 13,14 An emphasis upon design and structural characterization of sql CNs preceded sorption studies until the ELM (elastic layer-structured MOF) family was investigated. 28,59,72,[85][86][87][88][89][90] The rst reported sorption study on sql CNs was conducted upon [Cu(bpy) 2 (BF 4 ) 2 ], ELM-11. 59 (Fig. 16a) 28 and its activated form, ELM-11, was observed to exhibit phase switching when exposed to N 2 , Ar and CO 2 . To our knowledge, this is the rst example of type F-IV sorption isotherms in switching CNs. This behaviour was initially attributed to hydrogen bond regulation but there was no structural information concerning the activated form. 28 It was not until 2006 that the structure of the activated form was determined by synchrotron PXRD to be the sql topology CN [Cu(bpy) 2 (BF 4 ) 2 ] (Fig. 16b). 88 It was later found that ELM-11 underwent a multi-step rather than single-step phase transition induced by CO 2 at 1 bar when the temperature was decreased from 273 to 195 K (Fig. 16c), 86,87 revealing a type F-IV m isotherm. The mechanism of the switching behaviour was reinterpreted as expansion/shrinkage between adjacent layers triggered by inclusion of CO 2 molecules (Fig. 16d). With respect to N 2 sorption, ELM-12 exhibited a two-step type F-II isotherm distinct from that observed in ELM-11. On the other hand, both ELM-13 and ELM-31 were observed to exhibit type F-IV isotherms like ELM-11 but with different uptakes and switching pressures.
Recently, our group studied the sorption properties of the previously known sql CN [Co(bpy) 2 (NCS) 2 ], 208 sql-1-Co-NCS, 144,145 which is isostructural to the ELM family. sql-1-Co-NCS is sustained by Co(II) ions coordinated at equatorial coordination sites to bpy linker ligands with terminal NCS À anions occupying the axial positions. Whereas the N 2 uptake of sql-1-Co-NCS at 77 K was found to be negligible (Fig. 17a), the 195 K CO 2 isotherm was observed to exhibit a type F-IV s isotherm. 144 Such an isotherm is consistent with the switching behaviour seen in ELM-11. High-pressure CO 2 sorption isotherms of sql-1-Co-NCS were recorded at different temperatures and the relationship between temperature and switching pressure was found to obey the Clausius-Clapeyron equation. In addition, recyclability tests revealed that sql-1-Co-NCS exhibits good recyclability and fast kinetics. C 8 aromatic vapour sorption isotherms were also collected on sql-1-Co-NCS at 298 K (Fig. 17b). 145 It was observed that sql-1-Co-NCS switches in the presence of each of the C 8 aromatic isomers but with marked differences in terms of adsorption capacity and switching pressure. Single-crystal and powder XRD studies revealed that the interlayer distance in the closed phase (4.5Å) increased to 5.4Å (CO 2 loaded phase) and to 9.2Å (xylene loaded phase) (Fig. 17c-e), the largest interlayer separation yet reported for bpy-based sql CNs.

Other examples of switching 2D CNs.
Kitagawa and co-workers documented several examples of 2D interdigitated layered CNs and studied their switching properties. 65,66 [Cu(dhbc) 2 (bpy)] was prepared from copper nitrate, 2,5dihydroxybenzoic acid (Hdhbc) and 4,4 0 -bipyridine (bpy). Cu(II) ions are linked by bpy linkers to generate chains that are further linked by two carboxyl groups of dhbc to afford a 2D layer motif. 65 These layers are mutually interdigitated by the phenolic group of dhbc to generate 1D channels (Fig. 18a). Despite no change to the Cu(II) coordination environment aer activation, the dhbc ligands tilt signicantly and transform from cis-mode to trans-mode arrangement which results in layer shrinkage (Fig. 18b). 94 High pressure N 2 , O 2 , CH 4 and CO 2 sorption studies demonstrated that the activated phase of [Cu(dhbc) 2 (bpy)] exhibits a switching effect with type F-IV s isotherms (Fig. 18c). This study was the rst report of a stepped sorption in switching CNs at ambient temperature and high pressure. Adsorbate-dependent switching pressures were observed at 50, 35 and 0.4 bar for N 2 , O 2 and CO 2 , respectively.

Factors that impact switching between phases
Various factors can inuence the exibility of switching CNs. With respect to extrinsic factors, temperature, pressure and the nature of the adsorbate all impact switching events. Since switching between the closed and open phases is generally a thermodynamic event, low temperature and high pressure tend to promote more facile switching. To this end, in order to screen for switching it is recommended that sorption isotherms are collected at the boiling point of the adsorbate (e.g. 77 K for N 2 or 195 K for CO 2 ). High-pressure gas sorption might be necessary at ambient temperatures. For example, at 298 K and 1 bar, sql-1-Co-NCS does not exhibit switching upon exposure to CO 2 . Rather, switching requires either decreasing the temperature to 195 K or increasing pressure to around 30 bar. 144 The adsorbate is also a key factor since adsorbate-adsorbent interactions play a role in triggering the phase transformations. 106 In general, nonpolar gases such as H 2 , Ar, O 2 and N 2 tend to exhibit relatively weak interactions with most CNs. Hydrocarbons with unsaturated bonds may favour host-guest interactions with decreasing affinity as follows: aromatics > alkynes > alkenes > alkanes. The number of carbon atoms of hydrocarbon adsorbates also inuences switching since more carbon atoms generally results in stronger host-guest interactions. However, this empirical rule-of-thumb is limited by the need for suitable pore size and shape in the open phase of switching CNs. With respect to intrinsic factors, the composition of a CN is key to enabling and controlling phase transformations including switching. Ligand functionalization has been established as an effective route to ne-tune switching pressure as demonstrated with Co(bdp)-X 133 and MIL-53(Fe)-X. 182,183 Pillar ligand substitution of X-pcu-n-Zn PLCNs was also effective. 146,147 However, linker ligand substitution in PLCNs (e.g. DUT-8(Ni) 113,114 vs. DMOF-1 (ref. [189][190][191]) can stop switching. This can also occur with metal-node substitution. For three wellstudied families of switching CNs, MIL-53(M), DUT-8(M) and M(dpd), switching was profoundly affected by metal node substitution. Only specic metal nodes, i.e., MIL-53(Fe/Sc), DUT-8(Ni) and Co/Fe(dpd), were observed to exhibit switching behaviour. Particle size can also impact switching behaviour but remains largely understudied. 140,194 3. Switching mechanisms  Table 4). [38][39][40][41] Intra-network distortions include ligand motions ( Fig. 20a: bending, twisting and rotation), metal node coordination sphere changes (Fig. 20b: deformation and reconstitution/isomerism) and overall intranetwork motions (Fig. 20c: edge bending/straightening and net shearing). Internetwork motions include subnetwork displacements such as interpenetrated net sliding (Fig. 20d) and layered net expansion (Fig. 20e).
Ligand motion is oen observed, not just in exible CNs, but also in rigid CNs. [210][211][212] When ligand motion occurs in switching CNs it tends to occur over a relatively large amplitude. Compositional analysis of switching CNs reveals that the most commonly used organic ligands are the linear linkers bpy and bdc, which occur in ca. 25% of switching CNs. The N-donor linker bpy can rotate and twist even without ligand bending, as exemplied by sql CNs such as sql-1-Co-NCS (Fig. 21a). 144,145 The O-donor ligand bdc rotates, as exemplied by MIL-53(Fe) (Fig. 21b and 7b), 106 and bending behaviour was reported for DMOF-1. 189 Ligand rotation is a feature of functionalised MIL-53(Fe) analogues, MIL-53(Fe)-X, whereby the functional groups X in the closed phases project towards the channels and prevent full contraction. 181 Ligand rotation and twisting was also reported for Co(bdp) (Fig. 21c), 74 whereas more extreme ligand bending that could be described as contortion was seen for longer ligands such as pbpc in X-dia-1-Ni (Fig. 13e). 67 Metal node motion does not always occur in switching CNs. For example, the coordination environments of metal nodes in sql-1-Co-NCS (octahedral geometry), MIL-53(Fe) (octahedral geometry) and Co(bdp) (tetrahedral geometry) were unchanged in their closed and open phases. Conversely, metal node deformation was observed in the [Zn 2 (COO) 4 ] and [Ni 2 (COO) 4 ] paddle-wheel units of MOF-508 (ref. 97 and 99) and DUT-8(Ni), 115 respectively (Fig. 10a). Metal node isomerism involving coordination bond breakage remains relatively rare but plays an important role in some switching CNs, e.g. [Cu(pyrdc)(bpp)], [Zn 2 (bdc) 2 (dpb)] and [Co(VTTF)]. 96,120,138 Intra-network motion can oen be attributed to cooperative ligand and metal node motions, e.g. net shearing and edge bending/strengthening (hinge motion) that arise thanks to nonlinear coordination modes between metal ions and organic ligands. Overall, our analysis indicates that intra-network motions are quite common in switching CNs that feature square or rhombic cavities (Fig. 21). This is presumably attributable to the rhombus being a basic exible building block of a "truss network", as observed even at the macro scale. 213 For example, at the macro scale, a common use of "rhombus deformation" in a chemistry lab would be the lab jack. Integration of square or rhombic units into a CN is likely to be a viable crystal engineering strategy to design new switching CNs from rst principles. Subnetwork displacement is a phenomenon restricted to CNs that comprise individual subnets that interact with each other by weak forces, e.g. hydrogen bonding or van der Waals interactions. This includes interpenetrated net sliding as well as layered net expansion in 3D and 2D switching CNs, respectively. Interpenetrated net sliding usually occurs between centered interpenetrated nets (closed phase) and offset interpenetrated nets (open phase), thereby creating voids to accommodate guest molecules as exemplied by 2-fold interpenetrated PLCNs such as the X-pcu-n-Zn family. 146,147 Layered net expansion occurs in 2D switching CNs thanks to their ability to offer clay-like intercalation of guest molecules as observed in switching sql CNs. 144,145 A question that remains to be answered is why do isostructural CNs exhibit very different structural exibility and sorption behaviour, e.g. MIL-47 does not exhibit breathing behaviour whereas MIL-53 does? 214,215 Addressing this matter requires a quantitative understanding of the energy landscape including the energy barriers between multiple phases and the energy derived from host-guest and guest-guest interactions. [216][217][218] In addition, kinetic factors such as linker rotation dynamics can affect the framework transition, especially across transient phases. 219 In many cases, the precise mechanisms of phase transformations are beyond the scope of SCXRD, which only provides the static perspective of crystal structures. Further mechanistic insight will likely come through in situ experiments coupled with molecular modeling studies.

Gas storage
Gas storage is one of the most important and well-studied properties of porous CNs. [220][221][222][223] Indeed, we are in the "age of gas" 224 and physisorption is recognised as an energy-efficient approach to mitigate the energy footprint associated with traditional compression and liquefaction-based technologies. A variety of gases such as non-polar gases (H 2 , O 2 , N 2 , Ar, Xe), polar gases (CO, NO, CO 2 ) and C 2 -C 4 light hydrocarbon gases and vapours have been investigated in rigid porous CNs. 220-223 77 K N 2 sorption serves as the most common characterisation tool to experimentally determine the porosity of an adsorbent. The greenhouse gas CO 2 is another extensively studied gas at low (e.g. 195 K and 1 bar) and high (e.g. 298 K and 50 bar) temperatures and pressures, driven by the importance of carbon capture from ue gases (bulk) and air (trace). On the other hand, as a relatively inexpensive and clean fuel gas, CH 4 is of particular interest and importance given the energy footprints and limitations of liquied and compressed nature gas storage.
Recent studies have demonstrated that switching nonporous CNs that exhibit type F-IV isotherms can enable higher working capacities than physisorbents with type I isotherms as explained above (Fig. 2f). There are several other potential advantages of switching CNs for gas storage. First, since type F-IV isotherms usually exhibit hysteresis between their adsorption and desorption branches, it allows for a gas to be adsorbed at high pressure and stored at relatively low pressure. 72 Second, switching pressure and temperature follow the Clausius-Clapeyron equation, 72,86,144 which means that switching pressure can be calculated for a given temperature. This enables the selection of a bespoke sorbent that offers optimised parameters for temperature or pressure swing adsorption processes or combinations thereof. Last, but not least, endothermal/exothermal structural expansion/contraction in switching CNs naturally offsets the exothermal/endothermal nature of adsorption/desorption processes, thereby facilitating improved thermal management. 74,87 As detailed earlier herein, various gases have been studied to determine if they can induce switching in CNs (Table 1). 77 K N 2 and 195 K CO 2 sorption isotherms are well documented as summarised in Table 5. Thus far, around 20 switching CNs exhibit >100 cm 3 g À1 CO 2 or N 2 uptake at cryogenic temperatures and their switching pressures vary from ca. 0.1 to 80 kPa ( Fig. 22a and b). DUT-8(Ni) exhibits the highest N 2 (670 cm 3 g À1 ) and CO 2 (590 cm 3 g À1 ) uptakes yet reported at 77 and 195 K, respectively. 114 However, at 298 K, DUT-8(Ni) adsorbs negligible N 2 and only 58% of its saturation CO 2 uptake (345 cm 3 g À1 ) at 50 bar. For CH 4 , DUT-8(Ni) was found to exhibit a characteristic type I isotherm and adsorbed only 70 cm 3 g À1 at 50 bar, 298 K. Other switching CNs such as X-pcu-n-Zn, ELM-11 and M(bdp) exhibit >200 cm 3 g À1 uptakes at cryogenic temperatures. With respect to the Xpcu-n-Zn family (n ¼ 5, 6, 7, 8), X-pcu-6-Zn exhibits the lowest switching pressures for N 2 and CO 2 . 147 At 298 K, X-pcu-n-Zn CNs exhibit negligible CO 2 uptake except for the "soest" variant, X-pcu-6-Zn, which adsorbs 50 cm 3 g À1 of CO 2 at 33 bar. Unsurprisingly, like DUT-8(Ni), high pressure CH 4 adsorption isotherms to 50 bar revealed negligible uptake for all four X-pcu-n-Zn CNs. ELM-11 exhibits a type F-IV s isotherm for N 2 at 77 K and a type F-IV m isotherm for CO 2 at 195 K with almost identical saturation uptakes (ca. 250 cm 3 g À1 ). 59,86,87 High pressure CO 2 sorption conducted upon ELM-11 at room temperature produces the type F-IV m isotherm but with slightly lower uptakes. 86 High pressure CH 4 sorption on ELM-11 at 303 K revealed a type F-IV s isotherm with 84 cm 3 g À1 uptake, 59 close to the CO 2 uptake registered at the rst step. This adsorbed volume is comparable to that of active carbon ber (ACF), but the recovery percentage (i.e., working capacity) of ELM-11 is much higher, 59 thanks to its isotherm prole. Very recently,  and another switching PLCN, MOF-508, 225 were studied for a different fuel gas storage application, the storage of the explosive gas acetylene, C 2 H 2 , at ambient temperature. ELM-11 exhibits a four-step type F-IV m isotherm and it can deliver 163 cm 3 g À1 or 174 cm 3 cm À3 of C 2 H 2 between the second and third steps. However, the switching pressure at the third step is higher than 2 bar, beyond the safety pressure limit for pure C 2 H 2 . It is reasonable to assume that the inclusion and intercalation of C 2 H 2 into ELM-11 will desensitize C 2 H 2 but this matter must be veried or a porous monolith might be exploited. In the case of the MOF-508 series, a C 2 H 2 working capacity of 106 cm 3 cm À3 can be delivered under a relatively narrow pressure range . Compared to the benchmark rigid CNs which are more suited for C 2 H 2 sequestration rather than C 2 H 2 storage/delivery, 72 these two reports underline the high upside potential of switching CNs for practical C 2 H 2 storage. The switching pressure, one of the two most critical parameters for utility in the context of gas storage, is strongly affected by temperature. For example, the CO 2 switching pressure for sql-1-Co-NCS at 195 K is 0.1 bar but it increases to 26.7 bar at 298 K. 144 The multi-step switching nature of ELM-11 was overlooked for some time until low-temperature or high-pressure gas sorption isotherms were collected. 72,86 Although the switching pressure for a given adsorbate can be accurately calculated using the Clausius-Clapeyron equation, once the phase transformation enthalpy is determined, it is difficult to predict the switching pressure across a range of adsorbates. For instance, it is a challenge to quantitatively predict the CH 4 switching pressure even if we know the N 2 and/or CO 2 switching pressures of a given CN. Thus far, most switching CNs that exhibit high N 2 and/or CO 2 uptake at low (e.g. cryogenic) temperatures exhibit low or negligible CH 4 uptake at higher temperatures and pressures. This is likely because CH 4 tends to form relatively weak interactions with most CNs, especially compared to CO 2 . The switching pressure of CH 4 is expected to be much higher than that of N 2 or CO 2 at the same conditions and usually falls beyond the measurement limits of most studies reported in the literature.
With respect to methane storage, there are currently <10 switching CNs with CH 4 uptake measured at 298 K and only three (Co(bdp), Fe(bdp) and X-dia-1-Ni) exhibit >200 cm 3 g À1 CH 4 uptake (Fig. 22c). 67,74 Considering its relevance to adsorbed natural gas (ANG) storage, 226-233 the working capacity is typically determined as the quantity of NG that is deliverable between 5-35 or 5-65 bar, pressure ranges that are relevant for commercial combustion engines. In this context, Co(bdp) and Fe(bdp) would be expected to deliver around 100% of the stored NG thanks to their type F-IV isotherms. 74 To put this in context, rigid CNs that exhibit benchmark gravimetric uptake (e.g. >500 cm 3 g À1 ) tend to have much lower volumetric working capacities because of their relatively low densities (e.g. <0.3 g cm À3 ). 233 Overall, Co(bdp) and Fe(bdp) exhibit comparable or superior volumetric working capacities ($200 cm 3 cm À3 ) compared to the benchmark rigid CNs (Fig. 22d), and meet the early DOE (U.S. Department of Energy) target for volumetric CH 4 uptake (180 cm 3 cm À3 ). Unfortunately, these working capacities remain well below the current DOE target of 263 cm 3 cm À3 (350 cm 3 cm À3 considering ca. 25% packing loss). 234 Indeed, regardless of structural rigidity or exibility, there is no adsorbent that yet meets the ambitious target for methane storage. Accordingly, development of new switching CNs with improved uptakes, the right switching pressures and proper densities is needed to properly exploit their intrinsically advantageous isotherms.

Separations
Separations are of industrial relevance since most commodities are obtained as mixtures and require further purication steps prior to downstream processing and utilisation. [235][236][237][238][239][240][241] Unfortunately, the energy footprint of separation/ purication is generally high and currently represents $15% of the global energy consumption because of our reliance upon energy-intensive separation methods such as distillation, evaporation, chemisorption, and crystallization. 242 Given that a three-fold increase in demand for commodities is projected to occur by 2050, 224 more energy-efficient purication technologies are urgently required. In this context, adsorptive separation using porous solids is increasingly recognised as a promising alternative to traditional methods. Purication of a wide range of mixtures, from gaseous (e.g. CH 4 /CO 2 , CH 4 /N 2 , C 2 H 2 /CO 2 , C 2 H 2 /C 2 H 4 / C 2 H 6 , C 3 H 4 /C 3 H 6 /C 3 H 8 ) to vapour and liquid forms (e.g. C 5 , C 6 alkanes and C 7 , C 8 aromatics) have been extensively studied using rigid porous CNs. [235][236][237][238][239][240][241] Fig . 22 Comparison of switching CNs with respect to (a) N 2 (77 K), (b) CO 2 (195 K) and (c) CH 4 (298/303 K) sorption. Black and red symbols denote CNs with type F-IV s and F-IV m isotherms respectively. For type F-IV m isotherms, only the switching pressures of the last step are shown. (d) Comparison of gravimetric and volumetric working capacities for CH 4 at 298 K between current benchmarks for switching and rigid CNs.

Paper
Faraday Discussions  Switching nonporous CNs remain underexplored in the context of separation but they are attracting increasing attention as stepwise adsorption can endow switching CNs with high adsorption selectivities, at least in principle (e.g. based on Henry's law and/or IAST calculations). This is especially the case when a single component of a mixture exclusively induces structural transformation(s). However, in practice, separation performances tend to be inferior to the theoretical values derived from pure-component sorption isotherms. This is because of the complexities of dynamic conditions, especially sorption/diffusion kinetics and coadsorption. Switching CNs might also be unsuited for trace gas removal (e.g. <1000 ppm, or 0.1%) because the switching pressure is normally well above 0.1 kPa at room temperature. Nevertheless, switching CNs hold promise for adsorptive separation as reected in some of the benchmark separation performances registered in Table 6. For example, ELM-11 exhibits good selectivity for CO 2 over CH 4 (66.7) at 273 K and 1 bar as veried by breakthrough experiments. 243 Importantly, ELM-11 also retains this CO 2 /CH 4 separation performance at 298 K and 20 bar, making it favourable for separation of a CH 4 /CO 2 stream obtained directly from a natural gas source. Similarly, high-pressure purication of CH 4 is also observed in Co(bdp), which exhibits "near-perfect" selectivity of CO 2 over CH 4 as conrmed by binary equilibrium adsorption experiments. 103 Another sql CN, sql-1-Co-NCS, was found to exhibit excellent separation performance towards C 8 aromatic isomers, 145 one of the seven separation challenges that could "change the world". 242 This switching adsorbent layered material (SALMA) is the rst sorbent that combines high xylene uptake (>50 wt%) with high selectivity (>5). 145 MIL-53(Fe) also exhibited potential to separate xylene isomers, 177 but its selectivity was determined to be lower than those of sql-1-Co-NCS and MIL-53(Al). 145,244,245 Another recent example of a high performing switching CN is NTU-65 (also termed SIFSIX-23-Cu). 152 Compared to zeolites and rigid CNs, NTU-65 exhibited comparable or superior separation performance for C 2 H 4 purication from C 2 H 2 and CO 2 impurities. In summary, there is much scope to further explore the separation properties and performance of switching CNs which already show, perhaps counterintuitively, great promise for several separations that have been a great challenge for rigid physisorbents.
Apart from working capacity and selectivity as discussed above, recyclability and kinetics are two other key metrics that must be evaluated for practical application, yet both remain largely understudied. Despite the strong recyclability demonstrated by Co(bdp) 74 and X-pcu-n-Zn, 146,147 several 3D switching CNs, such as X-dia-1-Ni, 67 DUT-8(Ni) 246 and [Zn 2 (DPT) 2 (bpy)], 136 suffered from degradation in terms of switching pressure and uptake aer multiple gas sorption cycles. In contrast, 2D layered switching CNs such as ELM-11 (ref. 246) and sql-1-Co-NCS 144 demonstrated excellent recyclability over at least 100 cycles. The strong recyclability performance of several 2D CNs might be attributed to the relatively low structural strain and/or weak inter-network interactions that accompany expansion/contraction during recycling. With respect to kinetics, the CH 4 adsorption kinetics of ELM-11 reached saturation uptake in 10 min (ref. 59) and we demonstrated that the CO 2 adsorption of sql-1-Co-NCS requires less than 30 min. 144 In general, the adsorption kinetics of switching CNs is even less wellstudied than recyclability.

Conclusions
We detail herein the scope of CNs that switch between closed and open phases, an emerging class of physisorbents, and discuss the impact of metal ions, linker ligands and adsorbates on switching parameters upon properties of relevance to gas storage and separations. We also address the mechanisms associated with switching between closed and open phases. Switching CNs offer potential advantages over rigid CNs which results from their extreme exibility, the specic stimulus-response and unusual sorption properties. However, both scientic and practical hurdles remain to be overcome before there can be utility.
From a scientic perspective, the synthetic design principles for switching or, more generally, even so CNs largely remain to be established. This is despite the relative maturity of crystal engineering with respect to design of CNs from rst principles. The introduction of switching elements into CNs and realisation of structural dynamism remains somewhat unpredictable due to uncertainties over their self-assembly processes and the complex interplay of local and global exibility. Indeed, as revealed by Table 5, other than the effect of temperature and pressure upon switching pressure, there is no obvious correlation between sorbates and switching pressure other than the general observation that polar sorbates tend to promote switching. Improvements in understanding and predictability of switching will come from in-depth investigation of the origin and mechanism of switching, which remains limited by a reliance on X-ray diffraction. 247 It is therefore important to further exploit in situ characterization using techniques such as nuclear magnetic resonance, Raman spectroscopy, transmission electron microscopy and neutron scattering to broaden the scopes offered by diffraction experiments. 45 In this context, given the dynamics of switching transformations, time-resolved in situ techniques are of particular importance. 176,248 It is also worth mentioning that the performance of CNs is determined not only by their atomic level structure, but also by their macroscopic features, i.e., morphology, particle size, etc. Therefore, further insights into the roles of particle size, morphology, defects and disorder are needed. [249][250][251] Finally, development of machine learning and task-specic theoretical modelling approaches merit increased attention since they might not only address the underlying mechanisms, but also offer predictable modelling of the performance of switching CNs with pre-dened functions. [252][253][254] From a practical viewpoint, the hygroscopicity of CNs must be taken into account in addition to the aforementioned performance metrics (i.e., recyclability and kinetics). This is because hygroscopicity can strongly impact both the lifetime and the performance of switching CNs. 255 In addition, that switching between closed and open phases usually results in large volume changes in switching CNs may pose challenges to downstream processes such as formulation/shaping. It should be noted that the activation can also inuence the switching pressure and uptake. 28,136 Cost of ingredients and processing is also a consideration that must be addressed when assessing the viability of CN-based products and technologies. Ironically, even though this is not considered by most academic researchers, this is perhaps the one factor that a chemist can control. With respect to processing, the use of mechanochemistry such as ball milling and twin-screw extrusion 256 offers promise for scale-up and several well-known rigid CNs have been prepared with high efficiency using mechanochemistry. 257 The suitability of mechanochemistry for synthesis of switching or so CNs is even more understudied though.
In conclusion, we are perhaps approaching the "end of the beginning", but we are not yet there in terms of either design or properties. With respect to design, there are still just a few families of switching CNs and we remain mainly at the stage of empirical observation. With respect to properties, apart from adsorptive gas storage and mixture separation as discussed above, switching CNs and derived composites have been noted as having the potential to serve other emerging applications thanks to combination(s) of different applied stimuli such as light, redox and mechanical stress. [258][259][260][261] Whereas these applications emphasise the relevance of switching CNs more than ever, there remains a large gap between a property serving as an indicator of performance and real-world applications. At this stage, ANG storage is perhaps the most promising short to medium term opportunity for end-use of switching CNs.

Conflicts of interest
There are no conicts to declare.