Beyond Frameworks: Structuring Reticular Materials across Nano‐, Meso‐, and Bulk Regimes

Abstract Reticular materials are of high interest for diverse applications, ranging from catalysis and separation to gas storage and drug delivery. These open, extended frameworks can be tailored to the intended application through crystal‐structure design. Implementing these materials in application settings, however, requires structuring beyond their lattices, to interface the functionality at the molecular level effectively with the macroscopic world. To overcome this barrier, efforts in expressing structural control across molecular, nano‐, meso‐, and bulk regimes is the essential next step. In this Review, we give an overview of recent advances in using self‐assembly as well as externally controlled tools to manufacture reticular materials over all the length scales. We predict that major research advances in deploying these two approaches will facilitate the use of reticular materials in addressing major needs of society.


Introduction
Thousands of years of history highlight the great human achievements in shaping materials with manual, mechanical, and robotic tools.T his control of material structure and morphology is widely practiced on length scales ranging from millimetres to kilometres,b yu sing external tools with as ize comparable with that of the materials themselves,t hus making structural manipulation feasible.Asignificant effort has been centred on creating materials of smaller sizes to unlock the potential of new functions,s uch as large surface areas and fast charge transport. This objective has been greatly hindered by the difficulty in minimizing the toolmaterial interface,w hich is not completely addressed even with the advent of lithography techniques.C hemists,o nt he other hand, have embarked on accessing the "room at the bottom", [1] by piecing together molecular building units into chemical architectures under the principle of self-assembly. [2] Theorganization of molecules, [3] polymers, [4] and colloids [5] as ac onsequence of specific,l ocal interactions instead of external direction, has led to ap lethora of structures being designed and synthesized particularly on the nanoscale. [6] This synthetic effort, when dedicated to al arger size regime, especially with higher structural complexity and consequently advanced functions,has created alandscape of materials that is often referred to as mesomaterials. [7] Thep ower of selfassembly into ordered structures at the bulk level, however, typically fails to fully manifest itself.Awell-planned strategy to transform the definitiveness of molecular interactions and Reticular materials are of high interest for diverse applications, ranging from catalysis and separation to gas storage and drug delivery. These open, extended frameworks can be tailored to the intended application through crystal-structure design. Implementing these materials in application settings,however,requires structuring beyond their lattices,t oi nterface the functionality at the molecular level effectively with the macroscopic world. To overcome this barrier, efforts in expressing structural control across molecular,nano-, meso-, and bulk regimes is the essential next step.I nt his Review,w eg ive an overview of recent advances in using self-assembly as well as externally controlled tools to manufacture reticular materials over all the length scales.Wepredict that major researchadvances in deploying these two approaches will facilitate the use of reticular materials in addressing major needs of society. bonding across multiple size regimes all the way into defined macroscopic objects is,therefore,r equired. Within the frame of building-up materials,r eticular chemistry-linking molecular building blocks through strong chemical bonds into porous,c rystalline framework structures-provides ah igh level of chemical control. [8] Through reticular synthesis,o rganic molecules of welldefined shape,geometry,and functionality can be assembled with each other into covalent organic frameworks (COFs), [9] or with metal-containing building units to form metal-organic frameworks (MOFs). [8a, 10] Themolecular nodes and struts of framework structures are designed in such away that they remain rigid and directional during their reticulation, thus allowing for structure prediction and design. Thec onnection of these sizable building units opens up the rigid framework backbone to create permanent pores. [11] Thea ccessibility of the interior of these materials to guests, in addition to their chemical designability,h as led to tremendous research interest in reticular materials,especially for their use in gas adsorption, catalysis,s ensing,a nd drug delivery. [10b,12] To meet the requirements of real-world applications,t he structure and morphology of reticular materials on the macroscopic length scale becomes an essential element for achieving optimized performance. [13] Then ecessity of such design beyond the crystal structure is exemplified by packing particles into am onolith to maximize the volumetric efficiency. [14] Additionally,c reating interconnected channels within am onolith significantly reduces the pressure drop (the detrimental effect of resistance to flow), thereby facilitating fast kinetics in gas storage or separation. [15] Aligning crystals on a2 -dimensional (2D) substrate with defined orientation [16] and thickness [17] can lead to the desired anisotropic properties (e.g. conductivity,permeability,optical performance).
Controlling these macroscopic structural features is not straightforward, given that the synthetic repertoire available currently to reticular chemists is largely confined to the molecular regime.Infact, acommon result in the synthesis of frameworks is polycrystalline power, which fails to exhibit well-defined volumetric properties and anisotropic alignment. Efforts have been made to grow large single crystals to fill this design gap, [18] but significant challenges are still present in the engineering of crystal growth and alignment. Theunfavourable mechanical properties of framework single crystals further prohibit their direct use.C onsequently,t ools beyond crystal engineering are required to advance the manufacturing of reticular materials to ah igher level of complexity and functionality.T his budding field of research highlights the importance of physical designability in crafting reticular materials,w hich can be complementary to the chemical control harnessed by reticular synthesis.H ere we adopt the concept of chemical control to describe the common practice of solvothermal synthesis,which is typically governed by thermodynamics.T ohighlight this aspect, we use the term "self-assembly", but do not infer that the connection between framework building units are weak, non-covalent interactions.I nstead, framework structures are distinguished from supramolecules by their strong chemical bonds.O nt he other hand, physical designability refers to the implementation of externally controlled tools,such as microfluids and 3D printing. It is noted that during the process of using external tools,chemical reactions can still take place,but not following the same energy pathway as their bulk-state counterparts. Such external guidance imposes energetic influence on the growth of crystals or directs their spatial arrangement. In this Review,w eg ive an overview of the experimental achievements in the structural and functional control of MOFs and COFs by highlighting three distinctive length scales-nano,m eso,a nd bulk ( Figure 1, Table 1, Figure 5). We hope to bring to the attention of the reticular community the importance of using external control to shape reticular materials,and its cooperative deployment with self-assembly for structuring reticular materials in each size regime.For this purpose,the specific physiochemical properties arising from each length scale and how they relate to the performance in different application fields are discussed in detail. Finally,w ep ropose strategies and tools that can be used for constructing reticular materials into larger size regimes with precise control over the structure, function, and morphology for direct use in real-world applications.T he insights gained across the different length scales can help address current limitations in framework synthesis and promises the future of reticular materials will have ab road impact on science,t echnology, and society.

Chemical Designability of Reticular Materials
Thec rystal structure of reticular materials are programed and built at the molecular scale, where highly directional and rigid building blocks can be predesigned and synthesized by organic synthesis or metal complex chemistry. [8a, 9b] These building blocks are then joined to each other through chemical bonds in areversible manner to create crystalline solids.T he detailed knowledge of organic synthesis,which has accumulated over the last 200 years, essentially allows the synthesis of any stable molecular building block that can be imagined, only limited by the necessary time and resources.T he rigid and directional nature of the building blocks makes the structural variation highly predictable compared to traditional solid-state chemistry,o ne of the reasons for their wide promise for application. Organic linkers are synthesized with multiple attachment points to lead to extended solids,w ith typically two to four attachment points,b ut many examples exist with more. [19] These attachment points consist of functional groups that coordinate with metal centres in MOFs, [20] or create ac ovalent connection through dynamic covalent chemistry in COFs. [21] Theformation of metal-containing units can be realized in an endless range of possible connectivities,sizes,and geometries,r anging from isolated clusters [22] to infinitely extended one-dimensional (1D) rods [20] and 2D sheets. [23] The geometry of the combined organic/inorganic building blocks defines the structure type of the final framework, thereby allowing the construction of arich chemical space of materials by design. In COFs the linking groups are more limited in their geometries,w ith most employed COF linkages being linear. [9b] Figure 1. Building up reticular materials on all length scales. Atoms are used to construct secondary building units and atoms, which can be used to directly synthesise nano-, meso-, and bulk scale reticular materials. Additionally,meso-and bulk objects can be constructed by the assembly of nanoobjects. Table 1: Important length scales and their associated chemical and physical parameters. At sizes over 1000 nm, these objects display the same properties as their macroscopic counterparts and are, therefore, considered bulk objects.

Molecules
Nanostructures Mesostructures Bulk Scale Thepower of chemical designability in reticular materials not only comes from the rich chemistry and the many possibilities of combining building blocks,b ut also from the ability to selectively vary as ingle structural parameter or functional group without completely changing other parameters of astructure,thus allowing the desired properties of the framework to be tuned for the corresponding applications. [24] This is exemplified by the concept of isoreticular substitution, which describes the synthesis of af ramework with the same overall structure,n ets,a nd often metal nodes but with different linkers. [25] Thea bility to change the linker without altering the net leads to detailed control over the properties of the materials,a nd allows rational design of structures. [24a,b, 26] Ther igidity and directionality of the molecular building blocks create free permanent voids,t hus making these materials porous. [9b] It is possible to functionalize reticular materials on the internal surface,t hat is,o nt he surface of interior pores,a nd also on the external surface,w hich is defined as the limit of the periodic structure terminated by facets of ac rystallite or the interface with the surrounding medium. [27] These fundamentally different approaches, termed internal and external modification ( Figure 2), respec-tively,a llow the properties of reticular materials to be modulated to meet the needs of specific applications.
General approaches to the modification of MOFs and COFs introduce desired functionalities to organic linkers and metal nodes through the formation of new strong interactions with the framework backbone ( Figure 2). [28] These can be by introducing ac ovalent bond to the linker,i onic interactions between the framework backbone and guest species,acoordination bond formed by binding new ligands to coordinatively unsaturated sites (CUS) of metal nodes,o ra dditional metals to linkers. [27a,c] Methods specific to internal and external modification will be covered in detail below.
Thechemical designability has an immediate effect on the properties of the materials.T ypical design parameters are the size and geometry of the framework pores,w hich are of importance to porosity and the adsorption of guest molecules. In MOFs,t he metals carry much of the functionality of the framework, and can lead to the material having catalytic, [29] optical, [30] and magnetic [31] properties,a nd also serve as selective adsorption sites. [32] In COFs and MOFs,t he linker is also used to influence the properties.T he linkers of af ramework can be used to modulate the optical properties, [33] adsorption properties, [34] work as catalysts, [35] or serve as anchoring points for additional functional groups. [26c, 28d] Often, tuning the linker ensemble can be used to fine-tune the guest adsorption [36] or the catalytic performance of apore-mounted catalyst. [37] Linker interactions are of special importance in controlling the conductivity [38] and the optoelectronic properties. [24b,39] The chemical designability also affects important parameters for application, such as chemical stability. [28e, 40]

Internal Modification
Thetypical approach to modification of af ramework is the chemical alteration of reactive moieties present within the crystal. [27a, 28c] During internal modification, the reactive moieties are accessible to incoming reagents that diffuse across interconnected pores,thus enabling the chemical reaction to take place.T hese functional groups are often deliberately included in the framework design to provide anchoring sites for post-synthetic modification ( Figure 2). Therobust framework backbone ideally allows for structural modifications without com- promising the integrity of the crystalline architecture.O ther than addressing reactive moieties of the building units, functionalization strategies such as linker exchange [41] and transmetalation [42] are employed to substitute the whole building units with new types of building blocks or those with functionalities of interest. As pecial type of internal modification is post-metalation, where metal species are introduced post-synthetically to defined binding sites in the framework. In COFs,t he post-metalation of linkers is very important for the introduction of functionality; [28e, 43] this approach has also been used with MOFs. [44] Most strategies to modify the internal surfaces of reticular materials aim to uniformly modify the properties of the parent framework materials.H owever,i ti sa lso possible to create structural changes beyond the periodic structure,such as creating af unctionality gradient across ac rystal. The heterogeneous modification can be facilitated by using lightinduced reactions that allow localized post-synthetic modification. Thethiol-ene click reaction has been used to cross-link the organic linkers of aM OF. [45] This approach allows functionally graded materials to be produced or composites with abrupt changes in properties from the initially uniform materials,b ut it can also be used to selectively dissolve the non-cross-linked MOF to obtain structured gels,based on the previous patterning.The inherent diffusion limitation of postsynthetic reactions,a st hey start at the faces of ac rystallite that are in contact with the solvent and then react further into the inside of the crystallite,c an be used to structure heterogeneous reticular crystals.T his effect has been demonstrated in post-synthetic topotactic linker exchange and has been visualized by fluorescence and Raman microscopy, [46] and can be used to generate well-defined core-shell structures. [47] Internal post-synthetic modification is effective in tuning bulk characteristics such as porosity [48] as well as magnetic, [49] optical, [50] and electronical properties. [49,51] It can also be used to modulate the flexibility [52] of as tructure,t he mechanical properties, [53] and change its polarity. [54] Post-synthetic modification is mostly used to tether functional groups to the inner pore surface, [55] such as biologically active groups, [55g] ionic functional groups, [56] catalysts, [55c] and gas-releasing agents. [57] This allows the introduction of fundamentally new properties to the reticular material or the stabilization of tethered guests that would otherwise deactivate in solution. [58] In addition to the designed and desired changes in properties,areduction in pore size and porosity is often observed. [27a] In COFs,p ostsynthetic modification of the linkage that connects the building blocks is used to alter the chemical and physical properties of the framework. Themost drastic effect was seen in the stability of the frameworks,w here this strategy shows promise for generating ultra-stable frameworks that are much more stable than most MOFs. [40][41]59]

External Modification
External modification encompasses the chemistry at the 2D interface between areticular structure and its surrounding medium, mostly performed on nanoparticles and mesostruc-tured materials because of their large ratio of outer surface area to volume. [27b, 60] Tw os trategies have been developed to specifically functionalize the outer surface,b ut not the internal surface of reticular materials:b yt argeting reactive sites present exclusively on the outer surface [27b,61] or confining reactions through steric hindrance. [31,62] Thef ormer strategy takes advantage of the accumulation of coordinatively unsaturated metal sites [63] or dangling terminal linkers on the external surface. [64] Thes uccess of the latter strategy relies on am olecular-sieving effect, which precludes the penetration of reagents into the crystal interior. [65] Thesurface confinement of the modification reaction has also been enabled by hydrophilic/hydrophobicp hase separation, [66] electrostatic adsorption, [67] and covalent binding. [68] External modification enables the modulation of the external surface polarity of nanoparticles by introducing hydrophobic coatings,which leads to improved chemical stability in water and colloidal stability in non-polar solvents. [69] Fluorescent molecules are also frequently introduced to label framework nanoparticles for bio-imaging purposes. [27b, 70]

Physical Designability beyond Crystal Structure
An important limitation of self-assembly is the difficulty in controlling the physical properties of the final material, for example,m echanical performances and morphology.T herefore,there has been significant interest in applying externally controlled tools that offer accurate spatial control to shape materials over multiple length scales. [71] Many of these techniques using external control for structuring solids originate from the field of micro-and nanofabrication technology developed for microelectronics, which has branched and spawned across many new areas such as microfluidics, [72] micro-optics, [73] and micro-electromechanical systems for reticular materials. [74] Although the number of applications and their diversity have multiplied over the years,t he pool of materials that are compatible with routine micro-and nanofabrication processes has long been dominated by inorganic dielectrics and semiconductors.W e envision that this limitation will be overcome by ar ange of new and highly versatile fabrication technologies that may vastly expand the choice of materials and have ac ontinuing impact on the manufacture of reticular materials.
Thefuture development of physical control over reticular materials also relies on the advancement of additive manufacturing techniques,s uch as 3D printing, [75] two-photon polymerization, [76] and focused ion-beam deposition. [77] These techniques are becoming mature and widely available. Many subtractive methods,s uch as ultra-short pulsed laser ablation have been or are becoming available for the processing of MOFs and COFs,s uch as nanotemplating techniques,m echanical shaping methods,a nd 3D printing ( Figure 3). [78] Ac entral aspect in the physical designability is the external control, which is used to position and shape reticular materials.E xternal control over MOFs and COFs can be exerted by many different means,r anging from casting into template shapes [18a, 79] (Figure 4i), direct ink writing into 2D [80] and 3D [81] architectures (Figure 4f), whereby ar eticular material is selectively added at ad esired position, to dip pen lithography,w here particles can be grown in confined volumes. [82] Thegrowth of particles in microfluidic channels is similar, where either the extent of the channel limits the growth [18b] or particles are grown in droplets flowing in the microfluidic channel. [83] Theg rowth of thin films exploit nucleation [16] and epitaxial growth on sacrificial substrates. [84] Additionally,t he growth of MOF films on self-assembled monolayer surfaces allows precise control over their thickness. [85] These techniques balance the self-assembly and the externally guided processes to achieve astructured material.
Thechallenges that arise with externally controlled tools are the minimum feature size,w hich limit the accuracya nd level of detail, and the possibility for scale-up,w hich determines how much material can be produced by these methods.O ften these two features are inversely related to each other, where accuracy, complexity,and small feature size are in opposition to the amount of product that can be patterned. Self-assembly can help in producing structuring at al ength scale that cannot be achieved with an externally guided method. Methods based on self-assembly can be scaled up more easily.
Themethods used to alter the properties at different length scales differ significantly,w ith smaller structural features being dominated by methods controlled by self-assembly and larger structural features by externally guided methods ( Figure 3). When multiple size regimes are affected at the same time,they are often achieved by ac ombination of selfassembly at small length scales and an externally guided method at larger length scales. [86] Different size regimes can therefore be addressed independently.  . Examples of reticular objects synthesized over awide range of length scales through either self-assembly (left) or external guidance (right). a) CuBr(4,4',4'',4''-tetrakis(4-pyridylethen-2-yl)tetraphenylmethane)microstars (solvothermal), [98] b) millimetre-sized Cu 3 (BTC) 2 crystals (solvothermal), [99] c) assembly of truncated rhombic dodecahedral ZIF-8 nanoparticles (evaporation), [100] [101] f) TPE-COF-precursor/F127-ink pyramid (3D printing), [81a] g) MOF glass corresponding to Ti 16 [102] h) HKUST-1, ZIF-8, and NH 2 -UiO-66b ased foams (freeze-drying), [103] i) asingle crystal of ZIF-8 with ordered macropores (templating), [79] and j) ZIF-67 microcubes( etching). [104] 6. Reticular Nano-Objects Decreasing the size of particles leads to adrastic increase in their exterior surface area relative to their volume,which is the defining characteristic of nanosized frameworks.A st he key parameter of reticular materials in the nanoregime,t he determination of particle size is particularly important and generally not trivial:d ifferent specific physical characterization techniques yield different results as they describe either the crystallite size,p article size,o rh ydrodynamic radius. [87] In addition to reducing the diameter of ap article, another key criterion for realizing the promise of extensive surface areas requires the particles to be isolated and handled as well-dispersed solutions.
Nanoparticles (Figure 4d,e) of reticular materials naturally inherit the bulk properties such as ultra-high porosity and internal surface areas,but display new properties that are dominant on the nanoscale,i ncluding short diffusion distances across the whole particle,abundant surface defects,facile chemical functionalization, cooperative structural dynamics, and higher chemical reactivity. [6,88] Decreasing the size of reticular materials to the nanoregime can increase the adsorption kinetics,w hereas interparticle mesopores and non-closing hysteresis loops in sorption isotherms can be caused by nanoparticle aggregation. [89] When MOF nanoparticles are used as catalysts,t he conversion rates can increase significantly compared to the bulk counterpart, because of enhanced diffusion into the particles making all the particlesinside available to function as acatalyst. Alternately,alarger outer surface area can lead to improved catalytic activity when active sites reside on the external surface. [29] Small crystal sizes can also bring about an ew dynamic behaviour:T he adsorption of guest molecules can result in reversible changes in the crystal structure of ar eticular material:o pen-framework structures can be stabilized by guest molecules,w hile the removal of guest molecules can lead to closed non-porous forms. [90] Thed ownscaling of the crystals to the nanoscale additionally gives access to transient, otherwise inaccessible,m etastable structures,s uch as open frameworks,even after guest removal. [91] Thet ailorable nature of the individual building blocks of reticular materials provides them with ah igh functionalization efficiency, which can be increased even further for nanospecies.G rafting different functional groups on the external surface of framework nanoparticles can introduce multivariate functionalities carried by as ingle particle to the surface. [92] These functionalities-such as being fluorescent, magnetic, highly charged, and having molecule recognition sites-which can operate both individually and synergistically, thus allow one sophisticated nanoparticle to perform multiple tasks required for demanding applications such as drug delivery ( Figure 5).
Thes ynthesis of reticular materials in the nanoregime is mostly performed by self-assembly (Figure 4). In these techniques,aprecise optimization of the precursor concentration, feed rate,r eaction temperature,a nd reaction time is necessary to control both the nucleation and growth of framework crystals to achieve narrow size distributions. [93] Often modulators and surfactants are employed to gain control over the size of the particles. [94] These modulators can be,f or example,m onofunctional derivatives of the linkers, surfactants,orpolymers. [95] Modulators enable the formation of nanoparticles by inhibiting growth, slowing down the reaction, and by inhibiting aggregation in solution, thus creating stable dispersions and allowing control over the size of the particles.Modulators further enable the stabilization of the nanoparticles in solution and thus inhibit aggregation, which would otherwise lead to undesired particle sizes and instability of colloidal solutions. [96] Modulators can also be used to control the facets present on the nanoparticles,a nd tuning the modulator type and concentration can yield objects with increasing size and morphologies ranging from nanoparticles to 1D or 2D meso-objects. [97] Similar to molecular systems,t he purification and recycling of colloidal dispersions of reticular materials is possible through techniques such as centrifugation, dialysis,a nd filtration, which effectively remove excess reactants,g uests, and reaction products after catalytic reactions.T hese separation methods can also be applied to purify framework nanoparticles according to their sizes and functionalities.

Reticular Meso-Objects
Reticular structures at alength scale of 100 nm to 1000 nm is no longer governed by principles of the nanoscale,and their properties are typically also not accessible in the bulk. Reticular materials on this meso length scale are defined by the size of their characteristic structural features (minimal fragment of the structure that defines the property of the material). Fore xample,t he characteristic feature of am embrane is its permeation pathway;t herefore,amembrane on the mesoscale is defined by its thickness even though it may possess extensive lateral dimensions.Atthis length scale,the properties exhibited by the material are often affected by the interplay of multiple components of the structure,such as the alignment of constituent crystallites in their assemblies or the combined sieving effects between different layers in coreshell structures ( Figure 5). Meso objects bridge the world of nano-and bulk materials and are able to combine the features common to both. On one hand, meso-objects such as thin films display high external surface areas commonly found in nanomaterials. [105] On the other hand, meso-objects are optimized for performances that are characteristic of bulk materials,s uch as conductivity over al arge length scale. [106] Ty pical meso-objects are structured such that the morphology and spatial arrangement of subunits can lead to the enhancement of properties or the emergence of new ones that cannot be derived from the crystal structure alone.M eso structures can be synthesized in the form of single or polycrystals through self-assembly,e xternally directed approaches,o racombination of both (Figure 4a). [107] The synthesized mesostructures are classified here depending on their overall dimensionality:z ero-dimensional (0D,F igure 4ai,j), 1D,2 D, and three-dimensional (3D,F igure 4c) systems.
0D meso-objects often combine the structure features of nanoparticles with ah igher level of complexity,s uch as compartmentalization. Tw oofthe most common examples of this class are hollow particles and core-shell structures. Manufacturing these MOF and COF mesostructures has been realized by al arge number of synthesis methods. [108] In self-assembly-based approaches,t he morphology of these reticular materials can be controlled through surface-energydriven mechanisms (Figure 4a), [98,108f] modulating surfactants, [109] or self-templating. [107e,110] Methods based on external control include etching [104,111] and templating, which is based on the heterogeneous nucleation and assembly on the surface of the template and the replication of the template morphology. [107a-d, 112] Ty pically,t hese templates are then removed in as ubsequent step to yield core-shell structures. [113] When using reticular structures themselves as templates,i ti s possible to generate mesostructures with multiple layers, with each layer comprising adistinct framework. [112c, 114] 1D mesostructures can exhibit helical, fibrous, [115] and needle-shaped morphologies.Although there are examples of generating reticular 1D materials with specialized methods such as electrospinning [116] or using external electric fields to produce 1D particle strings, [117] these materials are typically synthesised by templated crystallization. [118] This process can be carried out in the hollow channels of ah ard template followed by template dissolution. [119] Alternatively,this can be achieved by heterogeneous nucleation on 1D objects,f acilitated by surface groups that induce crystallisation [120] Other approaches use sacrificial templates to grow 1D MOF structures on surfaces. [121] Similar to 0D mesostructures,t he templated synthesis of 1D reticular materials allows the transfer of template morphology to the resulting meso-object, which can either be used to increase the external surface area of the materials [121] or to introduce specific functionality,such as chirality. [120a] 2D mesostructures include free-standing membranes and surface-supported films.Astraightforward method of producing thin films and thin-film heterostructures is the deposition of nanoparticle suspensions on as ubstrate by spin or dip coating. [122] This method circumvents the necessity for nucleation and growth on the surface,b ut it is limited by the necessity to synthesise nanoparticles.Another approach is the direct growth of reticular materials on the surface of asubstrate from amother liquor/synthesis solution using the nucleation at the surface of the substrate for the initiation of growth. [122] Va riations of this direct growth have been used for the synthesis of MOF and COF thin films.Anotable extension of this method is the vapour-assisted growth of reticular thin films,w here the reactive precursors are deposited on the substrate as asmall droplet and the reaction mixture is heated in solvent vapour. [105b,c,123] Theg rowth of MOFs and COFs can lead to oriented growth even in the absence of special functional groups at the surface.
[38b] However,s urface-supported reticular materials are often grown on surfaces that are functionalized to facilitate the growth of the crystalline materials and to improve the adhesion between the reticular material and the substrate.
[105a] Fore xample,s elf-assembled monolayers with functional groups such as carboxylic or amine moieties can undergo crystal growth on many different surfaces and can even direct the crystallographic orientation of MOF and COF films. [85c, 124] Films of 2D COFs are often grown on highly oriented pyrolytic graphene (HOPG) or graphene,a st his substrate facilitates p-p interactions between the aromatic backbone of the COF and the graphene,t hereby leading to oriented growth. [24a, 125] Under well-defined conditions,p atterned single-layer graphene on SiO 2 can be used to achieve selective nucleation on graphene and thereby lead to patterned COF surfaces. [126] Thegrowth of reticular materials can be highly controlled by using aSAM in combination with alayer-by-layer growth or liquid-phase epitaxy (LPE) to obtain highly oriented surface-anchored MOFs (SURMOFs). [78a] These thin films are obtained by the controlled sequence of adding the precursors,whereby asingle layer of linker or metal precursor are deposited. Thealternation of both steps with intermittent washing steps allows fine-tuning of the height of the films,and allows the sequential deposition of MOF-on-MOF heteroepitaxial films. [127] In am odified approach, 2D MOFs are synthesized by aL angmuir-Blodget approach and then transferred layer-by-layer to as ubstrate. [105d, 106] Thed eposition of MOFs by chemical vapour deposition has recently emerged as am ethod that is compatible with standard microelectronic structuring techniques,w here ad ense metal oxide,d eposited in af irst step,i st ransformed into ap orous ZIF by reaction with methylimidazole vapour. [105e] One interesting outcome of growing thin films on substrate surfaces is interfacial strain. [128] In the case of MOF nanofilms,t his can be used to access otherwise only metastable structures with lattice parameters deviating from the default values of their bulk counterpart, modulated by the magnitude of the strain. Thestructural distortion can lead to an increase in the symmetry of the crystal structure of the MOF film [129] and stabilization of non-interpenetrated structures. [130] Synthesis on a2 Ds ubstrate allows for controlled growth of framework structures perpendicular to the substrate plane into the nanoscale.S uch spatial confinement results in thin films that feature short diffusion distances. Additionally,c ompared to the bulk equivalent, MOF thin films present ah igh concentration of coordinatively unsaturated metal sites at the extensive interface with the adjacent medium. [131] Reticular thin films further display increased structural rigidity,w hich can inhibit the expansion of the crystal structure upon guest uptake,abehaviour called "breathing", thus leading to unique sorption properties. [132] Free-standing reticular 2D films can be manufactured by externally controlled exfoliation of pre-synthesized crystals of stacked 2D layers into well-dispersed solutions. [105b,133] This process can be assisted by mechanical techniques, [134] sonication, [135] and chemical modifications.F acile exfoliation was enabled by introducing bulky functional groups to aC OF backbone [136] that decrease the interlayer interactions. Another example is to replace bridging linkers with nonbridging ligands between layers,t hereby leading to exfoliation of the MOF. [137] In recent years,f ree-standing 2D reticular films have also been synthesised through selfassembly at the interface [138] or in bulk solution. [139] 3D mesostructures,which are often called superstructures, exhibit ahierarchical arrangement of morphologically distinguishable components,w hich differentiates them from bulk materials.
[108b] They have been realized in the form of both single objects or extended assemblies of reticular components. 3 Dm esostructures have been synthesised by selfassembling objects with lower dimensionality,s uch as nanoparticles, [100,140] 1D helices, [120a] and those capable of selftemplating. [141] External means to produce 3D mesostructures include templating, [79,142] and manually stacking thin-film layers into 3D architectures. [122, 138d] Theordered arrangement of reticular constituents into superstructures can provide the resulting material with superordinate pores and other large cavities, [143] thereby leading to better accessibility for guest molecules.T he 3D mesostructuring of reticular materials can additionally increase the mechanical stability ( Figure 5) and provide improved flexibility. [144] Bicontinuous structures in monoliths can be achieved by spinodal decomposition during the synthesis,t hereby leading to am icroporous monolith, [145] or by coordination replication, which can create ar ange of mesostructured monoliths and materials through the transformation of astructured monolith consisting of,for example, ap recursor metal oxide into aM OF while maintaining its mesostructural characteristics. [71,84,146]

Reticular Bulk Objects
Reticular materials are manufactured into bulk sizes to exploit their extensive properties arising from the sum of its parts,f or example,a bsolute gas storage capacity,r ather than the intensive properties.R eal-world applications require ac onsiderable output of specific extensive properties,t hus demanding the large-scale use of reticular materials.T ypical conditions for MOF and COF synthesis produce loose powders which exhibit undefined volumes and are challenging to process.Single crystal, [99,147] monolith, [148] pellet, [149] and glass [150] (Figure 4g)a re the physical forms favoured for application scenarios,w ith each form exhibiting ad efined volume,adesired geometry,and atuneable size.T he specific physical form of the final reticular material is chosen based on the properties required for different applications.F or example,t ransparencyi sf ound in as ingle crystal and glass; unidirectional mass transport can be enabled by films and pellets;complex architectures can be presented by amonolith. Fors tructuring reticular materials on this macroscopic scale, various externally controlled tools have been developed, including casting,pressing,and printing. These techniques,in parallel with the chemical control exerted by reticular design, produce bulk reticular objects that display optimized proton and thermal conductivity,e lasticity,a nd gas capture and storage capabilities for aw ide range of applications ( Figure 5).
Monoliths are key players in the industrial use of reticular materials (Figure 4h). MOFs and COFs are capable of gas uptake with high gravimetric and volumetric capacities, exceeding the performance of other porous counterparts that have been made.A lthough the values of these parameters are typically obtained by measuring reticular materials in the form of single crystals or polycrystalline powders,realworld applications require the same measurement undertaken with their monoliths. [151] Whether it is methane storage in ac ar fuel tank or CO 2 capture from industry-scale pipelines,p acking as much material in ag iven volume is one of the priorities.T herefore,i nterstitial space between particles and crystallites,w hich do not contribute to gas uptake,should be minimised. By structuring frameworks into as haped monolith, typically with binders, [152] empty space is significantly reduced and the density of the final material is increased. Theshape of the monolith can be tuned to meet the specification of the container used.
An advanced level of structuring reticular materials is to shape the monolith into complex architectures.C atalytic converters epitomise the concept of using interconnected channels to decrease flow resistance and thus facilitate mass transport. Implementing architecturally optimized monoliths into ar elevant application context promotes gas uptake and release kinetics. [81b] Furthermore,r eticular materials can be combined with other types of materials into one monolith. Theh ybrid construct will benefit the interplay between multiple material performances in as ingle process,f or example,i ntegrated water harvesting [153] (Figure 5) and photothermal conversion. [154] Finally,t he architecture of the monolith determines its overall mechanical stability,and this aspect has to be taken into consideration to satisfy the need of the application environment.
Theapproaches used for the production of monoliths are largely based on externally controlled tools.T he monoliths can be prepared by casting precursor mixtures into adesired shape in combination with wetting the solid precursors, [155] evaporation, [156] extrusion, [157] as ol-gel method [158] or an organic terra cotta approach in COFs. [159] Alternatively,p resynthesised powders of reticular structures are pressed into amonolith shape. [160] Thepressure used needs to be controlled within arange that does not lead to structural collapse of the framework structures. [14] Pre-existing monolith scaffolds [161] and 3D printing [78d, 81a,b] techniques have been developed for constructing reticular monoliths of complex architectures. Additive manufacturing or 3D printing (Figures 3a nd 4f)o f MOFs and COFs is challenging due to the necessity for rapid solidification of the product to be able to print it into monoliths.S everal approaches to print 3D monoliths from reticular materials have been developed. As traightforward approach is the production of ag el from only the MOF precursors and solvent which can be directly printed. [78d] By embedding pre-formed materials in as hear thinning liquid that can be directly printed and do not flow freely anymore as soon as it has been deposited, the shear thinning liquid can be removed after the printing.T his approach largely allows the retention of the porosity and enables high loading and low amounts of binder in the final monoliths.Often atype of gel composite is used here,s uch as one based on hydroxyethylcellulose. [81a,152g,162] Embedding the reticular material in apolymer matrix allows printing by the typical melt extrusion used in commercial 3D printers,b ut this approach does not allow high loading and ar eduction in the porosity can be observed. [163] In reactive printing, precursors are reacted in situ and solidification is used as the printing principle. [164] In addition, soft substrates such as cloths,f oams,a nd sponges,which exhibit both porosity and flexibility,have been employed for constructing reticular monoliths.S trategies to synthesise these materials include the introduction of MOF particles into af oam-synthesis mixture, [103] dip-coating preexisting foams with pre-synthesized nanoparticles, [165] synthesizing MOF particles in the pores of foams, [166] by nucleation of MOF or COF crystals on the fabric surface, [12i, 167] and by synthesising monoliths by coordination replication. [168] Pelleting particles into ac ylinder shape allows large lateral areas with small thicknesses to be achieved. This form has as hape that is similar to that of am embrane,w hich features an even smaller thickness.T he reason for having as mall dimension is to take advantage of the short mass transport pathway along this direction to make electron-or ion-conductive materials.F or example,c rystals of an anionic framework have been mechanically pressed into ap ellet to serve as as olid electrolyte for al ithium-ion battery. [169] As presented in this work and many other cases,w hen small crystals (powder) are available,pellets are the preferred form of final material, since the thickness can be tuned.
Glasses are amorphous structures that exhibit adisordered arrangement of their chemical building blocks.S imilarly, reticular glasses show al ack of long-range periodic order, while still preserving the short-range order in the connection of their building units.The production of reticular glass occurs through the combined synthetic control from both selfassembly and external tools (Figure 4). They can be synthesised directly by using modulating and high-viscosity solvents; [102] however, the majority of MOF glasses have been manufactured by the amorphization of their parent framework structures by applying high pressure [170] or melting and quenching. [171] Compared to their parent compounds,M OF glasses display transparency, improved mechanical stability, and an increased density, [172] but can still remain microporous ( Figure 5). [173] 9. Applications of Reticular Materials Benefiting from Structural Control Theapplication of reticular materials is highly dependent on their structuring across all length scales ( Figure 5). With structural features deliberately controlled by self-assembly and external tools,f rameworks have been developed into materials that exhibit unique properties at every level.
Thef ollowing section, therefore,d escribes the most important application fields for reticular nano-, meso-, and bulk objects.

Specific Applications of Nano-Objects
Combining the versatile chemistry of MOFs and COFs with the properties of the nanoworld opens adoor for alarge variety of applications for nanoparticles. [88,174] These properties include short diffusion paths,f ast kinetics,a nd sorption properties,a sw ell as size-dependent optical, electrical, and magnetic properties.
As ar esult of their enhanced conversion rates and sorption kinetics,M OF nanoparticles find applications in fields such as catalysis [175] and adsorption, [89] as well as in electrical applications,for example,ascomposite materials in supercapacitors. [176] Reducing the size of MOF particles down to the nanoscale to form Zr-based MOF nanoparticles has led to ad ramatic enhancement in the catalytic degradation of an erve agent simulant. [175] Significant rate enhancements were observed for the nanocrystals compared to the microcrystals of the same materials.These effects are due to the larger external surface area relative to the internal surface area and/or faster diffusion. [175] Many reticular materials show flexibility or breathing during gas adsorption, where structural changes induced by the gas adsorption lead to dramatic changes in the ability to take up gases.T hese phenomena are typically cooperative phenomena that are significantly affected by particle size. Therefore,the particle size can be used to tune gas adsorption and cooperative adsorption features in particles of reticular materials.I tw as observed that the gate-opening pressure shifts with the crystallite size, [177] thus indicating that particle size can be used to tune the gate-opening for specific pressure ranges and applications.T he crystallite size also affects negative gas adsorption/pressure amplification. [132b] In these Angewandte Chemie Reviews 22360 www.angewandte.org cases,acomplete suppression of the cooperative adsorption phenomena is also observed for sufficiently small crystallites. [177] Reducing the diffusion length by increasing the external surface area through the use of nanosized particles is also beneficial for electrode materials for batteries and supercapacitors.I nt hese cases,t he faster diffusion rates lead to improved rate capabilities and higher deliverable capabilities, as smaller particles increase the contact with the electrolyte and the device benefits from shortened electron and ion transport pathways. [176,178] Theshortened diffusion lengths can also increase the utilization of electroactive components,such as sulfur in lithium sulfur batteries,w here af aster and more efficient conversion is possible with smaller sizes. [179] Although ad ecreased particle size might lead to af urther increase in diffusion and utilization, acompromise with other device characteristics is necessary,s uch as the stability or cycling performance of the device. [176] Them ain focus in the application of MOF and COF nanoparticles lies in the field of biomedicine ( Figure 5). [180] Nanosized particles are favourable in biomedical applications as they exhibit improved endocytosis and are well-dispersed in body fluids.Reticular nanoparticles are investigated in this field because of their high internal surface area and functionalizability.Both these features can be tuned through control of the inner pore wall by reticular chemistry.T his allows the precise tuning of host-guest interactions and ultra-high porosity,b oth of which are central for the high loading and release of active molecules. [28a, 36, 181] Reticular chemistry allows not only to use reticular nanoparticles as transport vehicles,b ut enables their scaffolds to be turned into active components for cancer treatment, [182] by providing linkers as active components [180c] or with metal species that affect cell growth. [183] TheM OF nanoparticles can also exhibit optical and catalytic properties at the targeted site in vivo to help treat cancer, by catalyzing the formation of reactive oxygen species for photodynamic treatment. [184] Straightforward functionalization of the external surface of reticular nanoparticles facilitates cell uptake and can be used to introduces targeted functional groups for cell uptake, or even be used to deliver drug molecules. [31] External functionalization of reticular nanoparticles also affects the stability in vivo and enables the breakdown and application of their function only upon uptake by the cell. [183] In addition to therapeutics,reticular nanoparticles can be used for different biomedical imaging techniques (e.g.p ositron emission tomography,m agnetic resonance imaging) [27b,185] or in combination with drug delivery. [186] Endowing ar eticular material with biocompatible building blocks to minimize toxicity,m akes MOF nanoparticles ideal candidates to utilize in vivo. [97b] Thes ize of MOF nanoparticles enable easy uptake by the cell, which can be enhanced further by coating strategies,s uch as polymer or lipid coatings. [27b,62b,187] Luminescent functionalization allows the MOFs to be used as chemical sensors, [188] with easy monitoring of their pathways and metabolism within living cells. [189] Furthermore, the tailorable nature of MOFs allows the use of biocompatible building blocks and their combination with the hybrid composition of the MOF scaffold result in high biodegradability and biocompatibility,a st he toxicity of MOF nanoparticles can be influenced by the choice of the metal and organic component, independently. [190] Importantly,t he toxicity of nanoparticles also highly depends on size,s hape, surface area, surface charge,a nd dose and cannot be limited to composition only ( Figure 5). [92,191] In comparison to MOFs,C OFs benefit from their metalfree nature and avoid incompatibility with cells because of the presence of heavy or toxic metals. [192] Thec ytotoxicity and water instability of MOFs limit their cellular and biological applications,w hen afast breakdown of the framework is not desired. [193] MOFs can, therefore,b eu sed to facilitate the release of active species by degradation of the MOF itself. These features can be overcome by using COFs for drug delivery and applications where biocompatibility is required. [36, 181g, 192]

Specific Applications of Meso-Objects
Applications of mesostructures rely on the strong hierarchy in their respective systems.0 DM OF and COF mesostructures offer both compartmentalization and al arge external surface area for functionalization, which is why they are mainly examined for catalysis, [112a,b] drug delivery, [194] and in ion conduction [118] (Figure 5). Similarly,3 Dr eticular mesostructures can be used as precursor materials for the generation of sophisticated catalysts.Additionally,assemblies of 3D mesostructures have been used for filtering applications [195] and in sensors based on ac hange in the optical properties of the materials upon guest uptake. [196] The majority of the applications of COF and MOF mesostructures, however, are film-based. Here,1Dspatial confinement results in reduced transport distances for electron and ion conduction. Additionally,mesomaterials provide an increased interface between asurface and its surrounding medium, which is beneficial for catalytic applications.T he growth of MOF and COF films on supports is as traightforward method to augment ac arrier material with advantages such as porosity, crystallinity,i on conductivity,a nd new optical properties. These properties make the thin-film framework materials interesting for sensing applications. [197] Thep orous nature allows the inclusion of guest molecules within the frameworks,w hich then cause,f or example,c hanged absorption spectra and altered refractive indices that can be read out, thereby leading to potential uses in the detection of small molecules. [198] Porosity,ahigh interface with the surrounding medium, and ahigh concentration of reactive sites are reasons for the use of framework films in catalysis. [199] Similarly,aframework surface coating can enhance the reactivity of electrodes by facilitating charge-carrier separation and providing enhanced light absorption for application in water splitting. [200] The surface can also template ac rystalline co-orientation of the resulting thin film and result in enhanced catalytic performance for the electrocatalytic reduction of CO 2 ,where facile charge transport is enhanced by alignment. [24a, 124a] Framework thin films have also been examined for ion conduction such as with lithium and protons:T he porous structure of MOFs can either be functionalized with protonconducting dangling groups on the surface,s uch as sulfonate moieties,o rb el oaded with proton-conducting guest molecules such as imidazole and histamine. [201] Furthermore,t hin films composed of COF nanosheets have been used for lithium-ion conduction and as separators in batteries. [38c,202] Thecombination of aporous structure and large interface area of 2D reticular mesostructures is strongly related to their applications in water purification [203] and gas separation. [204] Films of MOF nanosheets can selectively filter mixtures of gas molecules containing molecules such as H 2 or CO 2 .Selection criteria include the size of the gas molecules or their chemical nature,asthey have to interact with the pores of the material. This advantage can be further emphasized through postmodification of the pore size or the introduction of functional groups such as amino groups into pores to further improve the selectivity.Gas separation based on mixed-matrix membranes (MMMs) composed of either COF particles or nanosheets has been used in COFs,w here ab inder is used to improve the processability of the COF. [152d-f,205] In the past, MOFs have been understood as not being suitable as electron conductors because of their metal centres generally preventing electron resonance delocalization and the lack of redox-active ligands.T his is slowly changing with the development of new electron-rich linker molecules,t he inclusion of guest molecules,a nd the development of MOFs with "through-space" mechanisms,s uch as p-p stacking, which have opened up the use of MOF films for such applications. [206] Frameworks with improved charge mobility have also been realized by the synthesis of mixed-valence frameworks [207] and the introduction of charge-transport pathways through heteroatoms such as sulfur. [208] In contrast, COFs have often been used as semiconducting materials,a s most demonstrated COFs are two dimensional, with p-p stacking in the third dimension that facilitate charge transport. In many 3D COFs,e xtensive p-p contacts are even present due to interpenetration. [209] The p-p interaction facilitated by the stacking allows for facile charge conduction. [210] Thein-plane conductivity is often hampered in COFs by the intrinsic polarization of the COF linkages,s uch as in boronate COFs and imine-based COFs.H owever,r ecent developments in the development of new COF linkages has improved the in-plane conductivity through the use of linkages such cyanovinyl or phenazine linkages. [211] Theporous nature of reticular materials makes them ideal materials for sensing applications,a st heir porosity enables access of analytes to the inside of the material and thus,i n contrast to dense materials,not only is the outside surface of the materials active but also the inside surface.S ensing applications often requires as pecific mesostructure that is highly dependent on the fundamental sensing principle.T hin films are needed for sensing changes in electrical and ionic conductivity, [212] mass changes in aq uartz microbalance, [105a] or changes in the optical properties of Fabry-Perótd evices. [213] Theporous nature of MOFs and COFs in combination with conductivity present an ideal case for sensing applications,s ince the conductivity allows for facile readout of the signal ( Figure 5). Electrical conductivity is used to detect ac hange in the resistivity of as ample in response to as timulus,s uch as gas molecules that can be detected with MOFs or COFs. [211b, 212, 214] Electrical conductivity and structuring as thin films is also necessary for applications in electrocatalysis, [24a, 215] for supercapacitors, [210c, 216] photovoltaic devices, [217] and for photoconductivity. [33a, 218] More complex architectures are constructed for sensing based on responsive photonic crystals ( Figure 5). In these cases,o ne-, two-, or three-dimensional periodic structures with ah igh degree of regularity need to be generated to achieve good optical properties.Since the sensing principle of photonic crystals is based on general properties,s uch as the refractive index of the analyte and the uptake in the porous structure,photonic crystals are generally applicable to sensing applications and, therefore,h ighly sought after.O ne-dimensional photonic crystals can be constructed from MOFs by constructing optical lattices of repeated layers of thicknesscontrolled thin films of the MOF and either as econd inert layer or as econd MOF layer with different adsorption properties,w hich can be constructed by spin-coating nanoparticle suspensions,o rb yg rowing MOF thin films in alternation with titanium dioxide layers. [122] 3D photonic crystals are also called opals and possess periodicity in three dimensions;these can be constructed from MOFs by coordination replication of am etal precursor lattice that has been templated with polymer beads to have 3D periodicity,where the metal precursor lattice gets transformed into aM OF by reaction with al inker solution [84] or by filling the voids in ap re-assembled opal of polymer particles. [198] Ad ifferent approach is the synthesis of highly size-and habit-controlled MOF nano-or microparticles,which are then assembled into 3D crystallite superlattices,w hich possess sufficient domain size and low polydispersity as well as sufficient optical properties to allow the direct optical sensing of guest molecules in the pores (Figure 4c). [100] 12. Specific Applications of Bulk Objects Reticular materials that are used in bulk-scale applications rely mainly on properties that originate from their framework structure.T he extension of the dimensions of the frameworks to materials in the bulk regime amplifies the physical behaviour of aunit cell in the collective performance of the whole material. These applications require as pecific macroscopic shape,s uch as columns and reaction beds,i nto which framework powder can be packed.
MOF and COF bulk materials offer diverse applications in heterogeneous catalysis.F or one thing,their porous structure allows the loading of catalytic active species such as metal nanoparticles [24b,219] or single-site metal complexes [220] into their pores,t hus making MOFs and COFs excellent carrier materials.T he heterogeneous build-up of the scaffold structure can provide catalytic active centres through both the metal centres [221] as well as the linker molecules [55c, 222] or the COF linkages. [223] Thep recisely defined pores of aM OF and COF structure deliver their catalytic potential by providing the material with astructural selectivity towards both starting Angewandte Chemie Reviews 22362 www.angewandte.org materials and products,f iltering molecules by size [223,224] as well as chirality. [225] New gas storage technologies are readily sought after as they are especially relevant in fuel technology.Materials that are used in this field need to exhibit ac ombination of high storage capacities,h igh cycling stability,l ow adsorption/ desorption enthalpies,ahigh thermal conductivity,a nd high safety ( Figure 5). Fuel tanks based on COFs and MOFs can meet many of these criteria:the adsorption of gas molecules such as hydrogen [226] and methane [226b,227] into the porous structures allows high volumetric storage capacities at reduced pressures,w hich results in easier refuelling and increased safety because of the lower pressurization of the fuel container.
Their porous nature has also opened up the field of heat transfer to MOFs.H eat in MOFs is transported primarily along the chemical bonds of their porous structure.T he low atomic density of their crystal structure results in al ow thermal conductivity (around 0.1 Wm À1 K). [228] Here,s tructures act as heat pumps that distribute energy through the adsorption and desorption enthalpies of small molecules. [229] Research in this field is most advanced in the case of water harvesting,w here MOFs are already surpassing benchmark materials. [230] Similarly,t hese sorption properties have resulted in MOFs being used in air dehumidification. [231] Species with as ufficiently small desorption enthalpy can even be used to extract water from air through solar powering. [232] 13. Outlook Thes ynthetic effort made by reticular chemists has been largely focused on periodic, crystalline structures,w hich has led to the discovery of frameworks of new structure types and the use of such structures for specific purposes.W eencourage as hift in the attention from crystal structures towards their morphological manifestation at the final stage.T his requires raising the awareness of how much the physical forms that the reticular materials are structured into can affect the properties relevant to application scenarios. [233] This aspect has been taken into consideration when volumetric gas adsorption capacity is calculated using the geometrical volume of the pellet, instead of using the one calculated from the crystallographic density. [158] We hope that such ac onsideration can permeate into many other application fields,s ot hat the parameters of reticular materials are measured and compared in aw ay that is directly expressed in ap ractical setting.I n many other applications,a nisotropic properties displayed by framework materials are valued, for example,e lectron and ion conductivity,w hich are much more dependent on the orientation, arrangement, and distribution of reticular crystals and the chemistry of their interfaces.Itiswidely observed that as ingle crystal of high conductivity does not simply guarantee the conductivity of the membrane in which it is incorporated. As such, there remains much to be explored for structuring reticular materials beyond their unit cells to excel for specific tasks.
Manufacturing reticular materials in the nano-, meso-, and bulk regimes can be facilitated by self-assembly or external control;a te ach scale,t he importance of these two approaches are weighted differently and as pecialised set of methods have become available.W eb elieve the research on using external tools to structure reticular materials will continue to expand, especially those tools that work in parallel with self-assembly for achieving advanced structure control. However,s uch ac ooperation is applicable only in arelatively narrow range of size,typically spanning not more than one to two orders of magnitude (Table 1) and mostly converging at the mesoscopic regime.I nt he history of discovery,e very time that chemical and physical control have been combined together through the advent of at echnique (e.g. layer-by-layer growth), [85b] anew field of study and apreviously unexplored landscape of chemical structures has emerged. [234] We expect that the interplay between chemical and physical structuring will yield new results for reticular materials,c onsidering the high level of chemical control expressed by the linking of molecular building units.
We envision that the development of external tools will enable that even larger length scales could be addressed when shaping reticular materials.Framework structures that can be made into monoliths in sizes of centimetres or even decimetres are appealing,asthey could be directly used as afuel tank in ac ar, as mall-scale catalytic converter, aw aterharvesting chamber, structural materials for heat insulation and acoustic absorption, paint for modulating humidity,o r window glasses for formaldehyde photo-degradation. To achieve manufacturing capability at this level, external tools other than mechanical shaping and templating (the two physical methods mostly used so far) are needed for structuring reticular materials.Potential candidate techniques are two-photon polymerization (for photo-induced linkage chemistry), ion-beam deposition (with ionic building units), advanced 3D printing, and etching processes compatible with linkage chemistry.