Taming Multiscale Structural Complexity in Porous Skeletons: From Open Framework Materials to Micro/Nanoscaffold Architectures

Recent developments in the design and synthesis of more and more sophisticated organic building blocks with controlled structures and physical properties, combined with the emergence of novel assembly modes and nanofabrication methods, make it possible to tailor unprecedented structurally complex porous systems with precise multiscale control over their architectures and functions. By tuning their porosity from the nanoscale to microscale, a wide range of functional materials can be assembled, including open frameworks and micro/nanoscaffold architectures. During the last two decades, significant progress is made on the generation and optimization of advanced porous systems, resulting in high‐performance multifunctional scaffold materials and novel device configurations. In this perspective, a critical analysis is provided of the most effective methods for imparting controlled physical and chemical properties to multifunctional porous skeletons. The future research directions that underscore the role of skeleton structures with varying physical dimensions, from molecular‐level open frameworks (<10 nm) to supramolecular scaffolds (10–100 nm) and micro/nano scaffolds (>100 nm), are discussed. The limitations, challenges, and opportunities for potential applications of these multifunctional and multidimensional material systems are also evaluated in particular by addressing the greatest challenges that the society has to face.


Introduction
Nature is an endless source of inspiration for the development of novel materials because it offers a wealth of complex structures that have evolved to perform specific functions with unparalleled efficiency. [1]From the honeycombs assembled by bees to the delicate patterns on butterfly wings, the structural blueprint defines the very position of each component to enable the tailoring of highly functional and versatile materials.Porosity is a key aspect in many of these natural architectures as it endows materials with some important functions such as filtration, storage, and ionic transport.The ability to precisely control the porosity is therefore essential for the emergence of the next-generation materials and nanostructures featuring low density and multifunctionality for applications which are highly sought after, for example, in the context of energy storage, water purification, and environmental monitoring. [2]By mimicking nature and applying advanced micro-and nanofabrication techniques, it is possible to develop a range of complex and functional materials with desired architectures, porosities, and morphologies.
Molecular materials and nanostructures display architectures similar to those of macroscopic scaffolds that can be found in nature.The synergetic integration of wide-ranging structural properties and physical dimensions in artificial porous material systems offers limitless avenues for future advancement of this field.Porous skeletons can be categorized into three classes as depicted in Figure 1: open framework materials (OFMs), supramolecular scaffolds, and micro/nano scaffolds.During the last 2 decades, significant progress has been made in the design, synthesis, and assembly of molecular building blocks into ordered architectures resulting in the development of 3D functional materials and nanostructures. [3]By tuning their specific chemical and physical properties, these porous scaffold structures have already been used for applications in diverse fields including gas storage, filtration, and catalysis, sensing, and ionic conduction. [4]In view of such a fast moving research landscape, there is a need for a critical assessment covering the methods for generating such porous architectures with tailored skeletons at different length scales and a profound analysis of their physical and chemical properties.
This perspective article offers an in-depth and comprehensive discussion of the enlightening 3D scaffold materials/nanoarchitectures at different length scales, with a special focus on their composition, preparation, and processing methods; structural characteristics; as well as their tailored optical and electrical properties.Most of the recent review articles have focused on the synthesis of new porous skeletons and their applications in gas adsorption and catalysis.The unique optical and electrical properties of these multiscale porous skeleton systems seem to have been overlooked by scientists.We have highlighted the future directions, challenges, and opportunities of the 3D scaffold materials/architectures in the area optoelectronics by offering guidelines to leverage their progress toward innovative technologies.
Open framework materials with crystalline nanoporous structures have gained widespread popularity during the last 18 years, with the two most famous representatives being metal-organic frameworks (MOFs) and covalent-organic frameworks (COFs). [5]hese two nanostructured materials exhibit remarkable degree of tunability and structural diversity, as well as intriguing chemical and physical properties. [6]MOFs and COFs can be precisely tailored by modifying their secondary building units (SBUs) and linkers, resulting in tunable porosity, periodicity, and stability.It has been experimentally shown that MOFs assembled in 2D or 3D structures are predominantly made of micropores (i.e., with pore size being below 2 nm), whereas most of the COFs are meso-porous (i.e., with pore size spanning between 2 and 50 nm). [7]4h,8] As an emerging class of molecular-scale porous structures, MOF-COF hybrid materials are thought to combine the advantages of MOF and COF and have recently attracted tremendous attention.The MOF-COF hybrid system has shown good performance in diverse fields including pollutant adsorption, gas separation, catalysis, energy storage, chemical sensing, and biomedicine. [9]n this perspective, we will primarily focus on 3D MOFs and COFs as functional scaffolds, addressing their unique characteristics that make them particularly suitable for (opto)electronics such as field-effect transistors, bioelectronics, and heterojunction devices.
In addition to MOFs and COFs, the formation of multifunctional supramolecular scaffolds with well-defined microporous or mesoporous structures via the spontaneous molecular selfassembly of building blocks connected by weak and reversible intermolecular interactions such as hydrogen bonding, van der Waals forces, hydrophobic effects, and - stacking interactions, has been extensively demonstrated. [10]4f,11] Significantly, the size and arrangement of the composing building blocks play a critical role in the construction of structurally stable, porous supramolecular nanostructures, enabling the rapid development of non-covalent scaffolds holding huge potential for various technological applications such as drug delivery, gas absorption and separation, and chemical sensing.
The third type of porous 3D structures is micro/nanoscaffolds.They exhibit pore dimensions in the nanoscale to microscale (i.e., pore sizes > 50 nm) and can be realized by using unconventional nanofabrication technologies.These architectures, such as the self-suspended nanomesh scaffolds, have gained significant attention due to their unique motifs defining the appearance of exceptional optoelectronic properties. [12]For the integration into functional devices, it is important that such scaffolds are highly versatile in their compatibility with a variety of interfaces and environments, including rigid planar supports and flexible polymer substrates.Further, high mechanical stability and structural integrity are also crucial requirements.
Figure 2 illustrates the most common preparation strategies for the construction of the three classes of porous scaffold materials and structures, together with their selected applications, whereas their descriptions are detailed in the following sections.

Open Framework Materials
The field of reticular chemistry has offered powerful tools for the coupling of well-defined molecular building blocks through strong bonds to create extensive crystalline open frameworks. [13]urrently, over 100 000 MOFs and 500 COFs have been successfully synthesized and reported in the Cambridge Structural Database (CSD) or the clean, uniform, refined with automatic tracking from experimental database (CURATED)-COFs database. [14]The number of MOFs and COFs continues to grow, and their potential for applications in emerging technologies is unceasingly being explored.

Composition, Structure, and Morphology
The use of multidentate components enables the generation of structurally well-defined and spatially extended periodic frameworks such as MOFs and COFs.While these two architectures are assembled using different types of interactions, that is, non-covalent and covalent bonds, respectively, their porous structures and functional design can be similarly tuned and tailored.By controlling the dynamic self-assembly of molecular building units, either 0D finite cages or 2D/3D extended polymeric frameworks can be formed.Specifically, metalorganic cages (MOCs) result from the coordination of organic ligands to metals, yielding the discrete and soluble systems with a limited amount of pores, whereas MOFs can be generated as infinite 3D porous networks, as shown in Figure 3a,b. [15]However, the formation of an extended network leads to a significant loss of solubility, which hinders the processability of the materials.COFs are another kind of porous crystalline frameworks made up of light elements (B, C, N, O, and Si), first reported by Yaghi  et al. in 2005, in which small organic monomers are connected through dynamic covalent chemistry bonds. [16]By using building blocks that do not form periodic extended structures, discrete architectures called covalent-organic cages (COCs) can also be produced (Figure 3c,d). [17]In this context, porous frameworks have a clear advantage that they can be designed with controlled geometries yielding high performances when exploited as active materials to fulfill some complex application demands.
The understanding and modulation of the complex structures of 3D MOFs and COFs are essential to achieve greater functional tunability of the frameworks.To achieve this goal, three key aspects need to be fulfilled:

Design and Selection of the Modular Building Blocks
15b,17c,18] For MOFs, the connectivity and geometry of the inorganic components, including the single metal-ion and metal-cluster nodes, need to be carefully considered.The choice of "hard" or "soft" metals and the electronic configurations for central metal ions also have an impact on the desired properties such as electronic, magnetic, optical, mechanical, and structural characteristics of the frameworks. [19]

Synthetic Strategies and Morphology Control
State-of-the-art chemical synthesis enables the generation of the open framework materials in different form, with morphologies and properties at will depending on the targeted application. [20]OFs and COFs are usually synthesized under solvothermal conditions as microcrystalline powders or polycrystalline powders due to the disparities in bond strength and reversibility.Integrating and processing crystalline MOFs and COFs nanostructures is highly challenging due to their inherent attributes.As an alternative, researchers are pursuing new synthetic methods that precisely control material structures and morphologies to unlock their full potential for a wide range of technological applications.

Structural Analysis
The detailed structure of the framework materials can be elucidated by means of single crystal X-ray or electron diffraction measurements and high-resolution transmission electron microscopy (HRTEM) supported by theoretical simulations to enable highest precision in the interpretation of the diffraction data. [21]The crystal structures contain crucial information about the pore characteristics (i.e., pore size, pore volume, and specific surface area), structural motifs, molecular packing, topological representation, and non-covalent interactions between neighboring moieties.An in-depth understanding of these structures can be instrumental to unveil the relationship between structures and properties.

Tailored Properties and Preparation Strategies
6c,22] To unlock the full potential of these materials, their structural design and synthesis should target at endowing specific properties to the system.14c,23] Second, structural, thermal, and chemical stability are issues of great concern that must be controlled and maximized for all the practical application of these frameworks. [24]For example, it is important that both MOFs and COFs can maintain their 3D structures, even when guest molecules are removed from the cavities.Third, rigidity of the structure is highly relevant for structure adaptivity.A sufficient mechanical flexibility in the framework architecture is essential for the development of dynamic structures which are capable to respond to external stimuli such as mechanical pressure and thermal gradients. [25]Typically, MOFs offer greater flexibility compared to COFs as dynamic coordination bonds guarantee higher elasticity of the entire architecture when compared to covalent bonds.In order to increase the mechanical flexibility of the framework, the well-defined precursors and post-synthetic modification approach can be employed.Overall, these design considerations are critical for the optimal properties tuning and further exploitation of MOFs and COFs.
Despite the rapid progress achieved in the development of MOFs and COFs, a significant imbalance between the realization of novel structures and their practical applications is still present.It is indeed fair to state that many structurally sophisticated MOFs and COFs have been generated without a clear understanding of how their unique properties could be leveraged for specific purposes.To bridge this gap and to exploit the full potential of these materials, it is important that the design of new structures is accomplished with an explicit focus, with the ultimate goal of endowing the final materials with targeted properties that are relevant for practical applications.Besides more goaloriented synthesis approaches that take the desired end-use of the material into consideration, such target can also be achieved by optimizing the preparation and processing methods for these materials.Currently, various synthetic techniques have been exploited for the fabrication of thin films typically for 2D MOFs and COFs, for example, the Langmuir-Blodgett (LB) method, in-terfacial synthesis, liquid-phase epitaxy (LPE) method, and electrochemical deposition approach. [26]However, due to the limited structural anisotropy and unbalanced interlayer interactions, the controlled growth of 3D MOFs and COFs based thin films or layers is cumbersome.Despite these challenges, some exploratory studies have been reported on the successful fabrication of 3D MOF and COF films using general or dedicated techniques; for example, LPE, established by Wöll et al. in 2007, which was employed to fabricate 3D MOF thin films of HKUST-1 (Cu 3 (BTC) 2 , BTC = 1,3,5-benzenetricarboxylate) through the layer-by-layer dipping assembly process. [27]The preparation of 3D COFs thin films remains relatively underdeveloped.Börjesson et al. recently demonstrated the successful synthesis of a free-standing and uniform 3D COF film through liquid-liquid interfacial synthesis, achieved by optimizing the reaction conditions. [28]They also introduced another interesting approach for fabricating all-carbon linked COFs with desired thickness and significant crystallinity by using a stable continuous flow system. [29]Advantages of this method include the possibility to manually control the reaction rate and the use of the quartz crystal microbalance (QCM) to monitor the growth of thin films in real-time during the entire process; thus, ensuring the film quality and crystallinity.

Applications
Organic field-effect transistors (OFETs) are key building blocks in modern organic electronic devices as they are gifted by current amplification and signal processing. [31]A typical OFET device comprises a thin layer of organic semiconductor, three electrodes (source, drain, and gate), and an insulator layer (gate dielectric).During the last decade, a lot of efforts have been devoted to exploring the potential of MOFs as effective building blocks for OFETs due to their diverse structures, long-range crystallinity, low density, and tunable electronic properties. [32]27b] The design concept relied on the generation of a modified layer of gate dielectric with a low dielectric constant (k) by using MOF thin films to facilitate the formation of charge transport channels with low trap density owing to the reduced dipolar disorder.The OFET device was fabricated by the spin-coating of a p-type semiconducting polymer of PTB7-Th (poly[4,8-bis(5-(2-ethylhexyl) ) onto the SURMOF HKUST-1 modified SiO 2 /Si substrates and thermal evaporation of top gold electrodes, as schematically shown in Figure 4a-i,ii.XRD patterns in Figure 4b-i indicate that HKUST-1 thin films epitaxially grown on the pretreated SiO 2 /Si substrates with different number of LPE cycles displayed an identically dominant [111] orientation.SEM and AFM images clearly revealed the homogeneous HKUST-1 film with a thickness of ≈9 nm under three growing cycles (Figure 4b-ii-iv).The device performance of these HKUST-1/SiO 2 -based OFETs was determined by recording the output and transfer curves depicted in Figure 4c-i,iii.27b] Copyright 2017, American Chemical Society.d) Schematic of the preparation procedures of the molecular textiles using the multi-heteroepitaxial sandwich-layer SURMOF system.e) Illustration of the formation of molecular textiles in the active MOF layer constructed by BAB-TPDC organic linker.f-i,iii) SEM and f-ii,iv) AFM images of ten and five cycles of molecular weavings, respectively.g) The thickness of the molecular textiles as a function of the number of cycles to form SURMOF active layers.Reproduced with permission. [30]Copyright 2017, Springer Nature.
below 10 V, compared to those of OFETs based on a bare SiO 2 substrate.Such result is a clear demonstration that OFET properties can be enhanced by using porous, crystalline, and low-k MOF thin films as a modification layer of the gate dielectric.
Another interesting example of the SURMOF thin films being used as porous templates for 2D molecular weaving was reported by Mayor et al. in the same year. [33]To form the woven polymer networks, a heteroepitaxial technique was first employed to construct an active MOF layer that was structurally sandwiched between two sacrificial layers (Figure 4d).Then, acetylene side groups of the quadratic organic linkers of BAB-TPDC (bis(acetylene-biphenyl)terphenyl dicarboxylic acid) were coupled via the Glaser-Hay reaction, and finally, the metal ions in the MOFs were removed with diluted solutions of hydrochloric acid (Figure 4e).The as-produced molecular textiles were transferred to TEM grids and Si substrates using a lift-off technique, making it convenient for morphology characterization.SEM and AFM images revealed clear and continuous planar textiles consisting of different cycles of active layers, as shown in Figure 4fi-iv.The experimental investigation showed that the thickness of molecular textiles decreased from ≈20 to 10 nm when the cycles of active layers were reduced from 10 to 5 (Figure 4g).These results demonstrate that SURMOF thin films are truly versatile and efficient materials for treating surfaces and imparting them with novel properties.They can form clear and continuous planar textiles through a series of chemical reactions, and the thickness can be easily tuned by adjusting the number of active layers.Overall, SURMOFs can offer new tools for the generation of innovative materials and peculiar properties.
As an alternative class of porous molecular systems, 3D COFs were introduced by Yaghi et al. in 2007. [34]3e,g,35] Moreover, the  [36] Copyright 2021, Wiley.d) Scheme of the preparation of SBFdiyne-COF films via the substrate-catalysed synthesis in continuous flow.e-i) High magnification SEM image of SBFdiyne-COF film.e-ii) Raman spectra of the SBFyne monomer and SBFdiyne-COF film tested at four randomly selected positions.f-i) Device structure of the semiconductor DPP4T/SBFdiyne-COF heterojunction.f-ii) Current output curves of the DPP4T/SBFdiyne-COF heterojunction device.f-iii) Current output curve of a single-layer DPP4T device.29c] Copyright 2022, Elsevier.
judicious choice of the starting monomers can lead to the formation of (semi)conducting 3D COFs.However, low stability and solubility, lack of conjugation, and difficulty in chemical synthesis are the main challenges that hamper their practical applications, especially in electronic devices.Recently, Cao et al. synthesized a 3D COF called BUCT-COF-1 featuring full -conjugation via the Schiff base condensation reaction between a saddle-shaped linker of aldehyde-substituted cyclooctatetrathiophene (COThP-CHO) and 1,4-diaminobenzene (DAB) (Figure 5a). [36]Structural simulation and PXRD analysis sug-gested that BUCT-COF-1 had a 3D lattice with a tetragonal unit cell (a = b = 45.287Å, c = 3.865 Å) in a space-fill mode and topologically indicated a 13-fold-interpenetrated diamond (dia-c13) net with I4 1 /a space group (Figure 5b).N 2 sorption isotherms at 77 K and solid-state fluorescence spectrum of BUCT-COF-1 confirmed a BET surface area of ≈976.6 m 2 g −1 and a turn-on fluorescence emission at  max = 583 nm; thus, revealing an intrinsically high porosity and enhanced fluorescence performance compared with the COThP-CHO linker (Figure 5c-i,ii).Hall effect measurements at room temperature revealed that 3D BUCT-COF-1 behaved as a typical n-type semiconductor with electron mobility up to ≈3.0 cm 2 V −1 s −1 in pressed pellets with an average thickness of 0.5 mm (Figure 5c-iii).Notably, temperature dependence of Hall mobility at T > 100 K, portrayed in Figure 5civ revealed that BUCT-COF-1 exhibits a behavior of band-like charge transport.By taking advantage of the full sp 2 -electron conjugation, charge delocalization takes place throughout the porous skeletons of 3D BUCT-COF-1, providing a new avenue to advance the field of organic optoelectronic materials.
The development of 3D COF thin films as porous materials with homogeneous morphology for use in film-based devices is still in its infancy compared to that of their 2D analogs.To date, only a few works about the controlled growth of 3D COF thin films on the supported substrates have been reported, usually with a film thickness being on the micrometer scale. [37]29c] Figure 5d portrays the reaction flow cell; a space-confined self-coupling Glaser reaction was taken between the acetylenic monomers SBFyne (3,3′,6,6′-tetraethynyl-2,2′,7,7′-tetramethoxy-9,9′-spirobifluorene) and Cu substrates.As a result, the asprepared COF films with a thickness of ≈100 nm exhibited high morphological uniformity and good flatness, along with a new peak in the Raman spectra at 2196 cm −1 which can be ascribed to the conjugated diacetylene linkages (Figure 5e-i,ii).Such a high-quality film can be detached and transferred onto any desired substrates making it possible to implement exploratory attempts to create, for example, electroactive structures.As a proof-of-concept, the 3D COF film was applied to construct heterojunction with a semiconducting polymer of DPP4T (poly[2,5-bis(2-octyldodecyl)pyrrolo [3,4-c]pyrrole-1,4(2H,5H)dione-3,6-diyl)-alt-(2,2′;5′,2′′;5′′,2′′′-quaterthiophen-5,5′′′-diyl)]) and the formed semiconductor/COF heterojunction was sandwiched between two vertically stacked gold electrodes to fabricate the device, as shown in Figure 5f-i.The conductivity of the heterojunction was studied by applying a fixed voltage sweep from +8 to −8 V and it was found to have a significant asymmetry in the output current and an outstanding rectification ratio exceeding 10 4 , largely outperforming single layer of the semiconducting polymer shown in Figure 5f-ii,iii.Such results indicate the exciting opportunities and potential applications of 3D COF films in the field of organic electronics.
Although challenging, the development of simple and efficient methods to create porous framework materials and integrate them into electronic devices is a key step toward realizing high-performance (opto)electronics.Here, we delineate two efficient strategies for developing functional 3D porous frameworks films, namely the liquid epitaxial technique and continuous flow method.Applying these processing strategies, the porous materials can be successfully incorporated in electronic devices such as OFETs and heterojunction devices.

Supramolecular Scaffolds
Supramolecular organic frameworks (SOFs), a newly discovered type of porous materials, characterized by well-defined topologies and intrinsic porosities, can perform specific functions of great interest for various applications.Besides well-known MOFs and COFs, SOFs offer advantages in which their structure and chemical properties can be deliberately regulated.During the last 5 decades, supramolecular chemistry has progressed by enhancing compositional, structural, and functional complexity through the use of more and more sophisticated molecular and macromolecular starting building blocks. [38]3b,39] Selfassembly has established itself thanks to its programmability, simplicity, scalability, versatility, and low-cost, which has made it an ideal tool for a range of applications, including nanotechnology, imaging sciences, sensing techniques, and biomedical sciences. [40]

Morphology and Structural Characteristics
The leitmotif of supramolecular chemistry is the ability of achieving a full control over the correlation between structure and function as a prerequisite for the development of high-performance inorganic and organic materials.In this framework, elucidating morphology and structural characteristics of supramolecular assemblies is key to tailoring 0D, 1D, 2D, and 3D architectures.
Generally, 0D supramolecular assemblies are nanospheres with the strict limitation on their structures in all three dimensions.A well-known example of 0D supramolecular assembly consists in nanospheres of TAPP (5,10,15,20tetrakis(4-aminophenyl)−21H,23H-porphyrin) produced through the solvent-induced precipitation (SIP) in a DMSO/H 2 O (Figure 6a). [41]In contrast, 1D molecular assemblies have a structural anisotropy that results from the preferential expansion in one direction, yielding a variety of nanostructures such as nanoribbons, nanowires, nanorods, nanofibers, and nanotubes.Figure 6b shows a discrete molecularly engineered 1D graphitic-like nanotube with an aspect ratio exceeding 10 3 , produced by Aida et al. via the controlled self-assembly of an amphiphilic hexa-peri-hexabenzocoronene (HBC) derivative in THF solution. [42]The formation of this tubular architecture is believed to result from a helical coil composed of loose rolling-up of the bilayer graphitic tape as the coiled and tubular structures were both successfully observed by TEM during the self-assembly process.By expanding order and periodicity to an additional dimension, 2D supramolecular assemblies can be tailored, forming single or few-layer thick architectures such as thin films and nanosheets.These materials display a high surface-to-volume ratio and hold great potential for specific applications due to their ultrathin and spatially extended geometries.However, there is a limited number of building blocks that can exhibit these properties.As an example, Fukushima et al. in 2016 reported an impressive bridgehead-substituted tripodal triptycene as a building block, driving the arrangement and orientation of C 60 molecular units to enable multi-layered 2D supramolecular structures (Figure 6c). [43]On the other hand, 3D supramolecular assemblies are formed through the non-covalent bridging of molecules in all three dimensions, resulting in the nanobrick-like microscopic morphologies.A typical example is the bottle-brush polymer Poly-1, which was Figure 6.Examples of the controlled supramolecular assembly of organic functional molecules forming structures with different dimensionalities, from 0D to 3D structures.a) 0D nanospheres of a -conjugated porphyrin molecule TAPP using a mixed-solvent-induced reprecipitation method; Reproduced with permission. [41]Copyright 2021, Wiley.b) 1D supramolecular graphitic nanotube obtained via self-assembly of amphiphilic hexa-perihexabenzocoronene.Reproduced with permission. [42]Copyright 2004, American Association for the Advancement of Science.c) 2D assembly of C 60appended tripodal triptycene 2 TEG into a thin film.Reproduced with permission. [43]Copyright 2016, American Chemical Society.d) Large-area 3D molecular assembly of polymer brush poly-1 via one-step hot-pressing with uniaxially stretched Teflon sheets.Reproduced with permission. [44]Copyright 2010, American Association for the Advancement of Science.
reported to self-assemble into a 5-μm-thick freestanding film after a one-step hot-pressing treatment with stretched Teflon sheets, as shown in Figure 6d. [44]The aggregated polymer film displayed a highly ordered 3D structure over a large scale, as revealed by synchrotron radiation small-angle X-ray scattering (SAXS).
10d,45] Crystals of SOFs are easy to be produced and are very useful for characterization and analysis, enabling a comprehensive understanding of their structures.These appealing features of SOFs have garnered interest from both academic and industrial communities.

Tailored Properties and Preparation Strategies
Novel design principles focused on the fine tuning of noncovalent interactions have made it possible to advance the field of 3D SOFs, despite only a limited number of 3D SOFs being currently available.In particular, the use of geometry control ensured by the choice of the molecular components and their assembly modes represents powerful driving forces to achieve supramolecular 3D structures with programmed architectures upon operating under thermodynamic control. [46]In addition, high directionality can be achieved by exploiting coordination bonds, hydrogen bonds, and - forces rather than van der Waals and ionic interactions. [47]When choosing organic building blocks, it is crucial to ensure that the formed 3D supramolecular framework is enthalpically favored, in particular, when using non-planar molecules exposing multiple non-covalent anchoring sites, such as moieties capable to undergo the hydrogen-bonding, as in polycarboxylic acid systems.
Synthetically, 3D SOFs can be prepared by making use of hydrothermal and solvothermal synthesis.These processes, involving the crystallization of materials from high-temperature aqueous or organic solutions at high vapor pressures, have proven to be a highly effective way to produce large-scale, high-quality crystals, especially for materials with high vapor pressure near the melting point.For example, Vaidhyanathan et al. synthesized a tricarboxylic acid SOF with a non-planar 3D framework consisting of tricarboxytriphenylamine in acetic acid at 150 °C for 3 days. [48]In addition, other relatively mild preparation methods, such as SIP and solvent evaporation/diffusion, have also been shown to produce promising results. [49]Liu et al. reported an interesting example of modulating reaction conditions to synthesise two 3D SOF isomers (denoted as JLU-SOF2 and JLU-SOF3) with high thermal stability and permanent porosity. [50]he hexagonal prism-shaped crystals of JLU-SOF2 were collected after solvothermally heating a mixture of organic monomers in dioctyl adipate (DOA) at 85 °C for 2 days.Conversely, the rodlike crystals of JLU-SOF3 were obtained by slowly diffusing acetonitrile into the DOA solution of monomers for 1 day at ambient temperature.In another example, the 3D SOF crystals were not prepared under mild conditions but instead under a certain degree of external disturbance.45a] The feasibility of synthesis and solution processability of 3D SOFs has greatly promoted the use of these structures in some specific applications.

Applications
By taking advantage of their structures, composition, and dynamic nature, 3D SOFs have showcased promise for functional applications such as gas adsorption, separation, and drug delivery.For gas separation, the performance of the adsorbent is dependent on its adsorption capacity and selectivity; and therefore, engineering the host-guest interactions, pore size, shape, and surface function of 3D SOFs is crucial to discriminate the adsorption of specific components in the presence of a mixture of interfering agents and effectively targets the separation of important gases, such as H 2 , O 2 , CO 2 , and light hydrocarbons.For drug delivery, the design of 3D SOFs that can maintain stable and appropriate interactions with drugs is a primary concern.The drug or guest molecule can be chemically coupled or physically encapsulated within the carrier through covalent or non-covalent interactions.In order to improve the drug molecules' loading efficiency and bioavailability, 3D SOFs should not only possess high porosity and large surface areas but also need to be watersoluble, biocompatible, and biodegradable.The versatility of the synthesis process makes 3D SOFs easily customized to fit specific applications and a promising area of research for scientists and engineers.
In 2014, Li et al. first reported the spontaneous co-assembly of a tetrahedral molecular block 1 and cucurbit[8]uril (CB[8]) into a highly ordered porous 3D SOF in aqueous solution, referred to as Com-Tetra (Figure 7a). [51]The periodicity of the solutionprocessed 3D supramolecular framework was confirmed by the presence of sharp peaks in SAXS and synchrotron XRD profiles.SEM and TEM images further revealed the microcrystallization process of the 3D SOF upon solvent evaporation, as evidenced by a gradual increase in structural ordering observed in the selected area electron diffraction (SAED) patterns.HRTEM analysis of the microcrystals confirmed a pronounced porosity of this 3D SOF with 1.7 nm spacing (Figure 7b-i).Its high crystallinity has been demonstrated by the foursquare order with reciprocal lattice observed for the {100} facet through Cryo-TEM and corresponding SAED patterns (Figure 7ii).Thanks to the porous and polycationic features, this SOF exhibited a marked ability to adsorb organic anionic guests and enhance drug delivery in different solutions.As shown in Figure 7c, the drugs loaded in the SOF microcrystals can be selectively released into water under different pH values and operating conditions.
4f,11b] In 2019, Liu et al. reported a "direction-oriented" strategy for the synthesis of hydrogen-bonded 3D SOFs, also known as HOFs. [50]Using a non-planar 2,4,6-trimethyl benzene-1,3,5-triyl-isophthalic acid (TMBTI) as a building block, two isomers of JLU-SOF2 and JLU-SOF3 were synthesized and characterized.Single crystal structural analyses revealed that JLU-SOF2 and JLU-SOF3 exhibit different hydrogen-bonding interactions and packing motifs, despite having similar pore sizes.Of them, JLU-SOF3 showed a threefold symmetry framework directed by C─H••• interactions, forming honeycomb-like open channels with an internal diameter of ≈19.3 Å along the crystallographic z-axis direction, as schematically illustrated in Figure 7d.It was clear that the prominent CO 2 uptake, as evidenced by CO 2 adsorption isotherms at 298 K, demonstrated the potential of these two 3D SOFs as gas adsorbents for light hydrocarbons under ambient conditions.Adsorption isotherms of three common hydrocarbons are shown in Figure 7e-i-iii.It has been found that JLU-SOF3 outperforms JLU-SOF2 with higher uptake capacities toward CH 4 (1.05 mmol g −1 for JLU-SOF3 and 0.96 mmol g −1 for JLU-SOF2), C 2 H 6 (4.41 mmol g −1 for JLU-SOF3 and 4.04 mmol g −1 for JLU-SOF2), and C 3 H 8 (4.70 mmol g −1 for JLU-SOF3 and 4.11 mmol g −1 for JLU-SOF2) at 298 K and 1 bar.In addition to that, the C 2 H 6 /CH 4 and C 3 H 8 /CH 4 selectivity for JLU-SOF3 was calculated to be 17.8 and 89.2, and that of JLU-SOF2 was 16.3 and 48.1, respectively.The difference in adsorption ability and selectivity of JLU-SOF2 could be interpreted as the stronger dispersion interaction for larger and more readily polarizable molecules, which was proved by the isosteric adsorption enthalpy (Q st ) in Figure 7e-iv.
Inspired by the recent progress in conductive MOFs and COFs, (semi)conducting SOFs are attracting increasing attention.Several crystalline 3D or quasi-3D HOF materials displaying high charge carrier mobilities and conductivity have been developed.The first example of porous HOFs that showed electrical conductivity was recently reported by Farha and co-workers. [52]They synthesized a HOF (named HOF-110) based on tetrathiafulvalene tetranaphthoic acid (H 4 TTF-TN) using a precipitation process from a mixed solvent of DMF and DCM.In HOF-110, the adjacent naphthyl rings and in-plane neighboring carboxylic acid groups demonstrated distinct - and hydrogen-bonding interactions (Figure 8a,b).TTF moieties in HOF-110 were found to ) Drug release profiles from microcrystal at 37 °C under c-i) pH = 6.5-6.8 and c-ii) pH = 4.5.Note that the release experiments were performed with the samples being in the static state (blue triangles: Drug-3, black squares: Drug-1, and red circles: Drug-2) or in the shaking state (green triangles: Drug-1 and purple triangles: Drug-2).Reproduced with permission. [51]Copyright 2014, Springer Nature.Different intermolecular ─COOH•••HOOChydrogen-motifs of d-i) JLU-SOF2 and d-ii) JLU-SOF3.View of the porous structures of d-iii) JLU-SOF2 and d-iv) JLUSOF3 along the x-axis and z-axis, respectively.Gas adsorption isotherms for e-i) CH 4 , e-ii) C 2 H 6 , and e-iii) C 3 H 8 at 273 and 298 K together with e-iv) their corresponding Q st for JLU-SOF2 and JLUSOF3.Reproduced with permission. [50]Copyright 2019, Royal Society of Chemistry.isotherms for HOF-110 and HOF-110@I 2 −1 and −2 at 77 K. Reproduced with permission. [52]Copyright 2021, American Chemical Society.
exhibit dimeric stacks, and the closest intermolecular S•••S distance and the shortest S•••S contact between adjacent dimers were 3.6 and 4.4 Å, respectively (Figure 8b).In terms of its electrical property, the pressed pellet of HOF-110 displayed a conductivity of 2.2 × 10 −8 S cm −1 at room temperature (Figure 8c).After exposure to I 2 vapors for 24 and 48 h, the bulk conductivity of the as-synthesized HOF-110@I 2 -1 and −2 revealed a 12-and 27-fold increase when compared with the pristine HOF-110, yielding values up to 2.7 × 10 −7 and 6.0 × 10 −7 S cm −1 , respectively.The doping process was accompanied by a decrease in both the gravimetric N 2 uptake and BET surface areas, as shown in Figure 8d.Interestingly, based on a similar ligand of tetrathiopentanedicarboxylic acid (H 4 TTFTB), Espallargas et al. subsequently synthesized two new porous semiconducting HOFs (MUV-20a and MUV-20b). [53]he addition of diethyl ether to the THF solution of NaH 3 TTFTB at 80 °C could generate red needle-like crystals of MUV-20a after cooling down to room temperature.Upon solvent washing of MUV-20a crystals with diethyl ether, MUV-20b could be obtained by exchanging THF molecules with diethyl ether.Single crystal structural analyses suggested that two HOFs were held together by a 2D hydrogen-bonded supramolecular network that involved three carboxylic acids from each molecule, with O•••O hydrogen-bond distances in the range of 2.60-2.73Å.The structural difference of them consisted in the presence of structural interpenetration between adjacent layers in MUV-20a.Two-point probe measurement based on pellets at 300 K showed record conductivities of 6.07 × 10 −7 and 1.35 × 10 −6 S cm −1 for MUV-20a and MUV-20b, respectively.The enhancement of the electronic transport in MUV-20a and MUV-20b can be attributed to their zwitterionic nature composed of a positively charged TTF •+ and a negatively charged COO − .This conclusion was reached based on the analysis of their electronic band structures and density of states (DOS).
Since their introduction as multifunctional scaffold structures, 3D supramolecular frameworks have been rapidly developing.The integration of adaptive molecular building blocks into the non-covalent 3D structures enables exceptional performance in various aspects.To achieve more intriguing applications, further research is required to fully exploit the potential of these supramolecular materials.

Morphology and Structural Characteristics
Unlike the 3D networks described above, micro/nanoscaffold structures are a unique class of stereoscopic architectures with dimensions in micron to nanometer range.Micro/nanoscaffold typically comprises mono-or multi-composite porous frameworks in which the mesopores and/or macropores could serve as active channels for material filling, charge transport, and confined/epitaxial crystallization.Hence, porous micro/nanoscaffold structures have two important advantages: 1) The entire micro/nanoscaffold structure can be precisely assembled from multiple components, allowing for precise control and adjustment of the overall performance from individual parts.2) The tunable porous structure makes it compatible with a wide range of processing methods and different shapes of nanomaterials.Notably, micro/nanoscaffold architectures are prototypical systems for nanoscience and nanotechnology with unique optical and electrical properties, whose first few examples of applications when integrated in optoelectronic devices have appeared in the literature.

Tailored Properties and Preparation Strategies
Porous anodized aluminum oxide (AAO) is a well-established micro/nanoscaffold that offers a unique platform for developing high-density arrays of functional nanostructures and nanocomposites. [54]The honeycomb-like structure of AAO, which is formed through the electrochemical oxidation of metallic aluminum in acid electrolytes such as sulfuric acid, phosphoric acid, chromic acid, and oxalic acid, is characterized by densely arranged uniform and parallel pores.By adjusting parameters such as the anodizing potential, the temperature of the electrolyte, the electrolyte concentration, the processing time, the pore diameter of AAO can be tuned from 5 nanometers to several hundred nanometers whereas the length can be controlled from tens of nanometers to several hundred micrometers.The tunability in the AAO's pore size and internal surface nature, as well as the thickness of the AAO layer, has offered good opportunities for researchers operating in different fields.54c,56] Apart from the AAO, other porous metal oxide scaffolds such as TiO 2 and ZnO are usually exploited as the electron transport layers in perovskite solar cells (PSCs) and dye-sensitized solar cells (DSSCs). [57]For example, the mesoporous TiO 2 scaffold has a large specific surface area to facilitate the adsorption of active perovskite materials and control the morphology of perovskite films.Besides, it can be in full contact with the light absorption layer to ensure the maximum photogenerated charge separation and injection.
In the field of optoelectronics, the development of micro/nanoscaffold structures with precise and controlled functionalities is beneficial to the integration of complex functions in devices.To achieve this goal, several key properties must be taken into consideration.First, the micro/nanofabrication and processing of functional scaffold structures with high-quality structural properties is a challenge that requires the use of advanced and precision fabrication techniques.3D porous nanoscaffolds can be fabricated using photolithography or sacrificial template methods.These techniques have been successfully applied in the fabrication of metal nanoscaffolds, nanoscaffold-structured polymer membranes, and electrodes with functional porous layers. [58]In addition, it is possible to fabricate high-quality functional scaffolds directly by electrospinning techniques such as poly(vinyl alcohol) (PVA) nanofibrous scaffolds. [59]12a,61] Second, the integration of these scaffolds into various devices requires proper physical interfacing and contact with other layers or components to ensure efficient charge transport and device performance.Third, it is essential for the correct function and operation of devices, and as such, scaffolds must meet rigorous requirements of high structural and mechanical stability.Last, in view of the increasing request for lightweight and flexible devices, the integration of multifunctional scaffold structures onto flexible and lightweight substrates, or the formation of composite scaffold structures for desired device fabrication and application, is becoming a cutting-edge research direction and a future hotspot in this field.Our group has made progress in the integration of micro/nanoscaffolds in optoelectronic and photonic devices, including OFETs, organic solar cells (OSCs), organic photodetectors (OPDs), and organic light-emitting diodes (OLEDs).
In the following section, we will present a comprehensive overview of the latest advancements in various micro/nanoscaffold architectures and delve into the underlying principles and design strategies that guide the development of these scaffolds.Our focus will be on highlighting the key features that enable the customization of these scaffolds to meet the demands of specific applications, with a view to enhancing device performance.

Applications
In 2019, Lee et al. demonstrated the use of nanoporous AAO scaffolds to steer the molecular orientation of solutionprocessed small molecule organic semiconductors. [55]As shown in Figure 9a-i, the simple dip-coating of a solution of bis(triisopropylsilylethynyl)pyranthrene (TIPS-PY) onto a bare SiO 2 substrate makes it possible to produce needle-like crystals lying flat on the surface of the substrate.However, when the same substrate was coated with nanoporous AAO before the dip-coating process, the resulting TIPS-PY crystals displayed a portion of vertical molecular orientation, with their long axes perpendicular to the substrate surface, as shown in Figure 9aii.This growth behavior can be attributed to the nucleation of TIPS-PY within the cylindrical AAO nanopores and the preferred crystal growth along the unconfined direction of the scaffolds (Figure 9b).In addition, the versatility of this deposition strategy enabling the growth of out-of-plane oriented organic semiconductors has been demonstrated by a variety of acene derivatives, including bis(triisopropylsilylethynyl)anthanthrene (TIPS-AT), bis(triisopropylsilylethynyl)bistetracene (TIPS-BT), bis(triisopropylsilylethynyl)dibenzopyrene (TIPS-DBP), and TIPS-PY (Figure 9c).
The use of nanoporous AAO has successfully been applied to the field of perovskites, where researchers have attempted to overcome the hurdles of environmental unfriendliness and the thermal instability nature of these materials.In an early study, Moon et al. described a nanoporous AAO-induced spatial confinement effect based on facile strain engineering to prepare the phase stability of -CsPbI 3 . [63]More recently, Míguez et al. have successfully fabricated optoelectronic devices based on the nonmetallic oxide scaffold-stabilized CsPbI 3 nanocrystals. [64]They prepared a  and c-iv) TIPS-PY films that were dip-coated onto nanoporous AAO scaffolds.Reproduced with permission. [55]Copyright 2019, American Chemical Society.d) Schematic illustration of a 3D conducting polymer transistor in a tube.e) Photograph of a tubistor with an amplified image of the scaffold inside the tube.f-i) Transfer curve and the corresponding transconductance of a tubistor at V DS = −0.6V. f-ii) Switching performance of the tubistor to periodic square gate pulses.g) SEM images of the neat PEDOT:PSS, PEDOT:PSS/DBSA, PEDOT:PSS/DBSA/collagen, and PEDOT:PSS/DBSA/SWCNT scaffolds.Scale bar: 100 μm.h) Output transistor curves for various conducting scaffolds.Reproduced with permission. [62]Copyright 2018, American Association for the Advancement of Science.
nanoporous SiO 2 scaffold by dip-coating a diluted colloidal suspension of 30 nm SiO 2 nanoparticles on a glass substrate; and then, thermal annealing at 450 °C for 30 min.Results of the performance analysis of the scaffold-supported solar cells indicated dot-to-dot charge transport and an optimum power conversion efficiency (PCE) of 5.1%.Further, when the fabricated LED device was applied with a voltage of ≈2.3 V, electroluminescence of the scaffold-stabilized ensembles of CsPbI 3 nanocrystals was observed, with a peak luminance of 2.5 cd m −2 at 6 V and a full width at half maximum (FWHM) of 33 nm.The improved optical prop-erties could be caused by the positive interaction between quantum dot nanocrystals and the embedding SiO 2 matrix through quantum size effects.
The advancement of porous scaffold-based OFETs in the field of biological applications is truly remarkable.In 2018, Owens et al. devised a novel organic bioelectronic device in a tube called "tubistor".This device, based on 3D conducting polymer scaffolds, was designed to monitor the growth of 3D cell cultures in real-time. [62]As illustrated in Figure 9d,e, the tubistor is a unique electrochemical transistor configuration, with a device configuration consisting of two uniaxial inlet and outlet ports, source-drain electrodes integrated into the central opening, a gate electrode in the extension of the tube near the inlet port, and the most important feature being a 3D conducting channel of the device consisting of a porous PEDOT:PSS scaffold.This scaffold was produced in situ via a freeze-drying process and was instrumental in enabling the dynamic monitoring of cell culture growth.When a positive gate bias was applied, cations from the electrolyte were injected into the complex porous scaffold and anions were compensated, resulting in the extraction of holes and a decrease in the drain current as demonstrated in Figure 9f-i, namely, the dedoping of PEDOT:PSS.Further, upon application of periodic square gate pulses, the current recovery time (V GS = 0 V) was longer than the dedoping process (V GS = 0.2 V) as the ion trapping slowed down the cations' drift from the conducting scaffold to the electrolyte (Figure 9f-ii).Before using the tubistor for cell growth, the authors conducted SEM and electrical characterization of four different conducting scaffolds to understand the impact of their morphological and structural properties on the electrical performance.As shown in Figure 9g,h, the electrical performance of the tubistor was found to be influenced by the morphological and structural properties of the scaffold.Later on, it was also observed that both epithelial and fibroblast cells can easily grow on PEDOT:PSS scaffolds, and the corresponding tissue formation can be shortened to 2 days of culture, as evidenced by fluorescence images and realtime changes in the steady-state electrical characteristics of the transistor.This result is encouraging as the favorable biocompatibility and mechanical properties of the 3D conducting scaffolds should enable their application in complex biological systems.
40c,67] However, efficient photogenerated carrier transport in such devices has been a significant challenge due to the difficulty in controlling the interface between the nanostructures and electrodes.61a] Figure 10b-i,ii shows AFM images of the monolayered polystyrene (PS) nanospheres and the resulting nanomesh scaffold.Importantly, the regular nanoholes in such a scaffold could accommodate flexible n-type PTCDI-C8 (N,N′-dioctyl-3,4,9,10-perylenedicarboximide) nanowires (Figure 10b-iii,iv).To achieve more efficient exciton separation, transport, and suppression of non-radiative recombination, the selective modification of the Si/nanowire interface represents an efficient approach.Accordingly, we modified the Si surface with three different types of hole-transport layers including P3HT (poly(3-hexylthiophene-2,5-diyl)), IIDDT-C3 (poly(3decyltetradecyl isoindigo-alt-dithiophene)), and F8T2 (poly(9,9dioctylfluorene-alt-bithiophene)), thereby forming p-n junctions with PTCDI-C8 nanowires and leading to an outstanding photovoltaic performance (Figure 10c).Our optimized device demonstrated a significant improvement in both short-circuit current (I SC ) and open-circuit voltage (V OC ).Remarkably, it exhibited a signal-to-noise ratio of up to 10 7 in the photovoltaic mode, an ultrafast photo response time of 10 ns, and external quantum efficiency (EQE) exceeding 55%.This sort of vertical-channel device configuration based on porous nanomesh scaffolds represented a new architecture of general interest for fundamental studies in organic nanostructured optoelectronic devices, in the first instance, by expanding the type of chosen active materials.Based on this idea, we further reported a self-suspended nanomesh scaffold compatible with plastic substrates and enabled the integration of crystalline p-n heterojunctions into high-performance flexible photovoltaic detectors. [65]The multistep fabrication of the self-suspended nanomesh scaffold is schematically illustrated in Figure 10d.The as-produced empty scaffold showed remarkable mechanical stability, making it possible to sustain a large number of PTCDI-C8 crystals on top of the gold nanomesh electrode, and the TIPS-PEN (bis(triisopropylsilylethynyl)pentacene) crystals were allowed to grow at the hollow space at the bottom; thus, forming the asymmetric p-n heterojunctions (Figure 10e).Thanks to the unique device architecture and highly ordered supramolecular assemblies, the photovoltaic detector supported on flexible polyethylene terephthalate (PET) substrates demonstrated a very fast photo response time of fewer than 10 ns along with a signal-to-noise ratio reaching 10 5 under photovoltaic mode.Besides, the photoresponsivity could remain at ≈80% of its initial performance after 1000 bending fatigue tests, suggesting the hollow nanomesh scaffold was robust enough to retain ultralow leakage current (Figure 10f).
As a proof-of-concept, our group recently devised a novel foldable inverted polymer LED (iPLED) based on a vertical-yet-open nanoscaffold as an alternative structure to replace the conventional sandwich-like OLEDs. [66]Figure 10g shows the unprecedented device configuration based on a flexible but robust asymmetric nanoscaffold.It is composed of a cathode (aluminium), an electron transport layer (ZnO), an electron injection layer (EA, ethanolamine), a dielectric nanopyramid spacer (PI, polyimide), a light-emitting layer (SY (super yellow)/F8BT (poly(9,9dioctylfluorene-alt-benzothiadiazole)), a nanomesh anode (gold), and an encapsulation layer (PMMA, poly(methyl methacrylate)).The electroluminescence performance of the iPLED, luminance, and current density as a function of the applied voltage are presented in Figure 10h.The nanoscaffold iPLED using SY as light-emitting materials exhibited a turn-on voltage at 1.8 V and reached a maximum brightness of 2300 cd m −2 at 18 V, which was comparable with the reported sandwich-like iPLEDs.Further, owing to the outstanding structural and mechanical robustness of this nanoscaffold, the iPLEDs could be recycled by removing; for example, the given SY emitting layer, and reusing it after refilling with a fresh layer of F8BT.One distinguishing feature of the vertical nanoscaffold is its ability to be compatible with a variety of solution-processing techniques such as drop-casting, spin-coating, brush-coating, and inkjet-printing.In addition, the nanoscaffold iPLEDs displayed good mechanical flexibility and stability as demonstrated by device fabrication on flexible PI substrates and the corresponding bending tests (Figure 10i).Our robust vertical self-suspended nanoscaffolds emerged as powerful device architectures, revealing a major step forward in the field of solution-processed OLEDs, and particularly, in flexible optoelectronics.(red) and light illumination at 470 nm and 212.9 mW cm −2 (blue).61a] Copyright 2016, Springer Nature.d) Fabrication procedures for self-suspended nanomesh scaffolds through NSL technique and oxygen plasma etching, with multiple p-n heterojunctions sustained by the hollow nanomesh scaffold in the last cartoon.e) SEM images showing e-i) the bare suspending gold nanomesh and e-ii) PTCDI-C8 crystalline nanowires (CNWs) on top of hollow gold nanomesh.SEM images illustrating e-iii) large TIPS-PEN crystalline domains grown in between top and bottom electrodes using the drop-casting method and e-iv) organic crystalline p-n heterojunctions sustained by the gold nanomesh scaffold.e-v) Optical photograph of the optoelectronic device made up of PTCDI-C8 nanowires and TIPS-pentacene crystalline domains.f) Bending fatigue test of the photodetector.Reproduced with permission. [65]opyright 2018, Wiley.g) Cartoon portraying the device configuration of iPLEDs realized by the vertical nanoscaffold.h) Current density and luminance versus driving voltage curves of a vertical nanoscaffold iPLED.The inset is the photograph of an iPLED biased at 16 V.i) Photograph of i-i) the empty vertical nanoscaffolds on the free-standing flexible PI substrate and i-ii) the flexible nanoscaffold iPLEDs being folded.Reproduced with permission. [66]opyright 2022, American Association for the Advancement of Science.
As discussed at the beginning of this article, nature provides an impressive arsenal of micro-and nanostructures with diverse functions, and these bio-based architectures have offered a wealth of inspiration for researchers in their quest to create new and innovative materials and systems.Among these structures, the honeycomb pattern is one of the most prominent examples.
The uniformity and stability of the hexagonal pattern seen in these structures have captured the attention of researchers and inspired the creation of novel materials.A recent study by Lei  et al. designed ultrathin and stiff nanoelectrodes based on honeycomb alumina nanoscaffolds (HAN) for high-energy microsupercapacitors (MSCs). [56]Figure 11a shows the schematic Cross-sectional SEM images of a HAN.c) Top-view SEM images of HAN@SnO 2 electrodes c-i) before and c-ii) after 30 000 CV cycles.c-iii) Top-view and c-iv) cross-sectional SEM image of HAN@SnO 2 electrodes after CV measurement at a continuous mechanical extrusion pressure of 10 MPa.Reproduced with permission. [56]Copyright 2020, Springer Nature.d) Schematic representation of the fabrication steps for bioinspired patterned thin photovoltaic absorbers.e-i) Photographs of the patterned 130 nm thin a-Si:H layers on glass substrates with completely etched disordered nanoholes and unpatterned samples.e-ii) Top-view SEM image of the nanostructured a-Si:H thin layer.The inset is the ring-shaped pattern in the 2D Fourier power spectrum with respect to the SEM image.e-iii) Statistical histogram of the nanohole diameters of this sample.f-i) 3D AFM image of the bioinspired a-Si:H thin film, together with a 2D surface profile.f-ii) Correlation of the disordered nanoholes on the absorption wavelength recorded at normal AOI using unpolarized light.f-iii) Integrated absorption as a function of AOI for the patterned and unpatterned samples.Reproduced with permission. [68]Copyright 2017, American Association for the Advancement of Science.g) Schematic diagram of the fabrication process of 3D nanoarchitected materials.Lowmagnification and higher-magnification SEM micrographs of g-i) an IP-Dip photoresist sample and g-ii) the final pyrolytic carbon material.h) Schematic illustration of the LIPIT showing Au ablation process accelerating microscopic spherical projectiles toward targeted materials and the impact process in real time being captured by ultrahigh-speed imaging with a micrometer and nanosecond resolution.i) Mass-normalized inelastic energies as a function of impact velocity in ballistic impact experiments.Color bar corresponds to a logarithmic density scale.Reproduced with permission. [69]Copyright 2021, Springer Nature.
diagram of the HAN fabrication process.First, anodization of a surface-nanopatterned aluminum foil in a 0.4 m H 3 PO 4 aqueous solution at 160 V produced a double-layer nanoporous Al 2 O 3 with a hexagonal cell arrangement and 400 nm inter-cell spacing (Figure 11b-i).Then, a finely tuned two-step chemical etching was employed to remove the acid anion contaminated Al 2 O 3 layers.The resulting HAN exhibited a clear layer of Al 2 O 3 with an ultrathin cell wall of 16 ± 2 nm and a cell depth of 25 μm (Figure 11b-ii-iv).To fabricate nanostructured current collectors, nickel layers with a thickness of ≈10 μm as the supporting substrates were first electrochemically deposited onto the top of the HAN, and 12-nm-thick tin oxide (SnO 2 ) layers were subsequently coated using the atomic layer deposition (ALD) technique (Figure 11c-i).Results indicated that nearly 98% of the initial current density was retained with negligible structural changes for the HAN@SnO 2 current collector after long-termed 30 000 CV cycles, suggesting superior electrochemical performance and structural stability (Figure 11c-ii).Alongside, its excellent mechanical property was proven by the absence of structural damage after electrochemical measurements under the continuous mechanical extrusion pressure of 10 MPa (Figure 11c-iii,iv).For the performance of MSCs based on the HAN-based nanoelectrodes, the biggest capacitance value of this device was 128 mF cm −2 at the current density of 0.5 mA cm −2 , and the peak energy and power densities were 160 μWh cm −2 and 40 mW cm −2 , surpassing all the reported MSCs up to date.
In another work, inspired by the scales of the black butterfly (Pachliopta aristolochiae), Hölscher et al. reported a porous photonic nanoscaffold based on phase-separated binary polymer blends for the construction of bioinspired thin photovoltaic absorbers. [68]The disorderly arranged hierarchical micro-and nanostructures in the black butterfly scales could facilitate light absorption and collection with strong angular and polarization robustness. [70]Based on the results of the microspectroscopy and 3D optical simulation of the photonic biostructures of the butterfly by finite element method (FEM), the authors developed a 3D structural model of butterfly scales with disordered hole arrangement and non-uniform hole diameters for the thin-film absorbers.The main fabrication procedures are schematically illustrated in Figure 11d, involving first spin-coating of a mixed solution of PMMA and PS in methyl ethyl ketone (MEK) on the hydrogenated amorphous silicon (a-Si:H) supported by a glass substrate; then, a selective wet etching of the PS, and finally, a pattern transfer into the a-Si:H using dry etching.Photographs of a patterned absorber with a 130-nm-thin a-Si:H layer and an unpatterned sample are shown in Figure 11e-i.It was found that the average diameter of the disorderly positioned nanoholes was 238 nm, which was very close to the optimized value of 240 nm from optical simulations (Figure 10e-ii,iii).Regarding the optical performance of the thin-film PV absorber, its relative integrated absorption (IA) increase can reach 93% at a normal angle of incidence (AOI) using unpolarized light in comparison to a flat slab, and a higher value of > 200% can be obtained when a large AOI of 50°is applied (Figure 11f).The photonic structures in nature can provide new valuable insights into the optimal fabrication and functionality of photovoltaic devices by integrating efficient porous light-harvesting micro-and nanostructures.
The potential of 3D micro-and nano-architectural scaffolds as lightweight mechanical materials is attracting growing attention in the scientific community. [71]One notable example is the work of Greer et al., who recently fabricated a nanoarchitecture carbon lattice material through a unique pyrolysis process. [69]The 3D nanoarchitected material was created by exposing a 3D patterned cross-linked polymeric precursor made of IP-Dip photoresists to a high temperature in a vacuum (900 °C), using a sophisticated two-photon lithography method (Figure 11g).SEM images of both the polymer precursor and the resulting pyrolytic nanoarchitecture are shown in Figure 11g-i,ii.To assess the dynamic response of this novel material under the supersonic impact, the researchers employed a laser-induced particle impact test (LIPIT) which was coupled with an ultra-high frame rate camera to capture the impact process in real-time.They also used laser-confocal and electron microscopy techniques to characterize the ballistic response of the material both qualitatively and quantitatively (Figure 11h).The results were surprising, demonstrating an incredible impact energy dissipation of ≈70% in this 3D nanoarchitected material.Its specific inelastic energies were determined to be between 0.19 and 1.1 MJ kg −1 when thickness of 3D nanoarchitected material increased from 400 μm to a few millimeters (Figure 11i), making it significantly superior to traditional impact-resistant materials such as steel, aluminum, and PMMA.These findings demonstrate the tremendous potential of architected micro-and nanomaterials as components for the design and development of ultra-lightweight, impact-resistant protective materials.

Conclusion and Outlook
In this perspective, we have introduced recent enlightening methods for the development of a range of multifunctional porous skeletons from nano to micron scale.By sub-dividing these skeletons on the basis of their pore size, we have performed a thorough and critical analysis of molecularly engineered 3D MOFs (<10 nm), 3D COFs (<10 nm), supramolecular scaffolds (10-100 nm), and micro/nanoscaffolds (>100 nm).Such an investigation offers a deep understanding of the relationship between chemical structure and physical properties of the open framework materials and their relevant applications.3D MOFs and COFs are particularly fascinating due to their unique stereoscopic structures, which can lead to additional versatility and adaptability, making them suitable for optoelectronic devices, catalysis, and other special manufacturing.On the other hand, the development of supramolecular architectures can be attained through fine-tuning of the intermolecular interactions.3D SOFs with permanent porosity can be assembled by exploiting coordination bonds, hydrogen bonds, and - interactions.The self-assembly of 3D supramolecular architectures has been demonstrated to be effective for gas storage, purification, and biomedical science, owing to their highly defined structures, solution processability, and facile regeneration via recrystallization.Micro/nanoscaffold structures represent promising multifunctional architectures that hold fundamental relevance in materials science and nanotechnology.The use of artificial micro/nanoscaffold structures, such as self-standing nanomesh scaffolds, enables the development of novel device architectures composed of an open multicomponent system targeting highperformance optoelectronics devices and lightweight mechanical materials.The diversity in the size, composition, and properties of these families of porous structures offers the greatest variety of architectures for specific technological applications.
In the near future, we foresee three main hot topics which will attract the attention of the scientific community: i) In the view of practical applications, it is important to further develop film-forming methods for open framework materials to study their physical properties and real applications.For instance, MOFs and COFs have demonstrated excellent insulating and (semi)conducting properties when coupled with metal ions, monomers, or ligands.The fabrication of high-quality MOFs and COFs films could pave the way for applications in OFETs, lightemitting devices, thermoelectric devices, and low-trap density insulation or encapsulation layers.ii) The combination of deep learning and artificial intelligence (AI) could be instrumental for accelerating the screening process of open framework materials with specific functionalities.By unravelling the structureproperty relationship in these materials, the design of multifunctional materials with outstanding electrical or thermal conductivity, in analogy to AlphaFold in protein structure prediction, can be optimized.iii) The integration of a multifunctional nature in nanoscaffold structures can be accomplished by incorporating thermally, optically, or magnetically sensitive functional components yielding a variety of stimuli-responsive devices that are capable to execute complex functions such as neuromorphic operations.On the longer term, the advances in porous skeletons tailoring will provide decisive contributions toward emerging technologies for addressing various societal challenges, for example, in the field of energy storage, water purification, environmental, and health monitoring, as well as drug release.

Figure 1 .
Figure 1.The overview of porous skeletons from open frameworks to micro/nano architectures based on the physical dimension of materials.

Figure 2 .
Figure 2. Schematic diagram of selected preparation protocols used for three categories of porous frameworks together with their applications.

Figure 3 .
Figure 3. Molecular assembly via the coordination of organic linkers with inorganic nodes to form a) discrete hybrid cage structures or b) periodic 3D MOFs.Covalent assembly of organic building blocks via dynamic covalent chemistry bonds to yield c) discrete organic cages or d) periodic 3D COFs.

Figure 4 .
Figure 4. a-i) Schematic diagram of polymer-based OFETs with the interfacial modification of the gate dielectric by SURMOF thin film.a-ii) Scheme for the fabrication of SURMOF HKUST-1 through LPE approach.b-i) XRD patterns of SURMOF HKUST-1 with different LPE growing cycles.b-ii-iv) SEM and AFM images of SURMOF HKUST-1 with three growing cycles.c-i) Output and c-ii) transfer curves of OFETs device with HKUST-1(3 cycles)/SiO 2 /Si.Reproduced with permission.[27b]Copyright 2017, American Chemical Society.d) Schematic of the preparation procedures of the molecular textiles using the multi-heteroepitaxial sandwich-layer SURMOF system.e) Illustration of the formation of molecular textiles in the active MOF layer constructed by BAB-TPDC organic linker.f-i,iii) SEM and f-ii,iv) AFM images of ten and five cycles of molecular weavings, respectively.g) The thickness of the molecular textiles as a function of the number of cycles to form SURMOF active layers.Reproduced with permission.[30]Copyright 2017, Springer Nature.

Figure 5 .
Figure 5. a) The design and synthesis of fully -conjugated 3D COF with the incorporation of sp 2 -carbon based saddle-shaped COThP-CHO linker into frameworks.b) Model of BUCT-COF-1 displaying a 13-fold interpenetrated dia topology with I4 1 /a space group.c-i) N 2 adsorption and desorption isotherms of BUCT-COF-1 at 77 K. c-ii) Solid-state fluorescence spectra of COThP-CHO and BUCT-COF-1 ( ex = 469 nm).The inset is the photograph of BUCT-COF-1 powders under daylight condition.c-iii) Hall effect measurement of BUCT-COF-1 at 298 K. c-iv) Temperature dependence of the Hall mobility of BUCT-COF-1 on a logarithmic scale.Reproduced with permission.[36]Copyright 2021, Wiley.d) Scheme of the preparation of SBFdiyne-COF films via the substrate-catalysed synthesis in continuous flow.e-i) High magnification SEM image of SBFdiyne-COF film.e-ii) Raman spectra of the SBFyne monomer and SBFdiyne-COF film tested at four randomly selected positions.f-i) Device structure of the semiconductor DPP4T/SBFdiyne-COF heterojunction.f-ii) Current output curves of the DPP4T/SBFdiyne-COF heterojunction device.f-iii) Current output curve of a single-layer DPP4T device.Reproduced with permission.[29c]Copyright 2022, Elsevier.

Figure 7 .
Figure 7. a-i) Chemical structures of compound 1 and CB[8] and a-ii) CPK model of their 3D SOF Com-Tetra.b-i) From left to right: SEM images of Com-Tetra recorded right after evaporating the solvent and standing for 3 and 14 days.The respective SAED patterns, in the insets, indicate the change in structural order.The last one is the HRTEM image of the periodic and porous microcrystal.b-ii) From left to right: schematic representation, cryo-TEM image, SAED pattern, and its model illustration of the microcrystal showing a reciprocal lattice observed for the {100} facet.c) Drug release profiles from microcrystal at 37 °C under c-i) pH = 6.5-6.8 and c-ii) pH = 4.5.Note that the release experiments were performed with the samples being in the static state (blue triangles: Drug-3, black squares: Drug-1, and red circles: Drug-2) or in the shaking state (green triangles: Drug-1 and purple triangles: Drug-2).Reproduced with permission.[51]Copyright 2014, Springer Nature.Different intermolecular ─COOH•••HOOChydrogen-motifs of d-i) JLU-SOF2 and d-ii) JLU-SOF3.View of the porous structures of d-iii) JLU-SOF2 and d-iv) JLUSOF3 along the x-axis and z-axis, respectively.Gas adsorption isotherms for e-i) CH 4 , e-ii) C 2 H 6 , and e-iii) C 3 H 8 at 273 and 298 K together with e-iv) their corresponding Q st for JLU-SOF2 and JLUSOF3.Reproduced with permission.[50]Copyright 2019, Royal Society of Chemistry.

Figure 9 .
Figure 9. a) Side-view SEM micrographs illustrating deposited TIPS-PY films onto a-i) a SiO 2 substrate and a-ii) an AAO scaffold via dip-coating.The insets show side-view SEM micrographs of a flat SiO 2 /Si substrate and a nanoporous AAO scaffold on the top of SiO 2 with a substrate angle of 35°, respectively.b-i) Schematic diagram of a single crystal of TIPS-PY, with the b-axis or -stack direction to the long axis of the crystal.b-ii) Schematic of the TIPS-PY vertical crystal growth process on AAO scaffolds during dip-coating.c) Top-view SEM images of c-i) TIPS-AT, c-ii) TIPS-BT, c-iii) TIPS-DBP,and c-iv) TIPS-PY films that were dip-coated onto nanoporous AAO scaffolds.Reproduced with permission.[55]Copyright 2019, American Chemical Society.d) Schematic illustration of a 3D conducting polymer transistor in a tube.e) Photograph of a tubistor with an amplified image of the scaffold inside the tube.f-i) Transfer curve and the corresponding transconductance of a tubistor at V DS = −0.6V. f-ii) Switching performance of the tubistor to periodic square gate pulses.g) SEM images of the neat PEDOT:PSS, PEDOT:PSS/DBSA, PEDOT:PSS/DBSA/collagen, and PEDOT:PSS/DBSA/SWCNT scaffolds.Scale bar: 100 μm.h) Output transistor curves for various conducting scaffolds.Reproduced with permission.[62]Copyright 2018, American Association for the Advancement of Science.

Figure 10 .
Figure10.a) Cartoon illustrating the high-density nanowire photovoltaic devices realized by the vertical-channel nanomesh scaffold.b) Topographical AFM images of b-i) the monolayer and b-ii) prepared nanomesh scaffold.SEM micrographs of b-iii) the bare nanomesh electrodes and b-iv) nanomesh electrodes bearing PTCDI-C8 nanowires.c) I-V traces at dark (red) and light illumination at 470 nm and 212.9 mW cm −2 (blue).The inset is the photo switching cycles under 0 and 1.5 V. Reproduced with permission.[61a]Copyright 2016, Springer Nature.d) Fabrication procedures for self-suspended nanomesh scaffolds through NSL technique and oxygen plasma etching, with multiple p-n heterojunctions sustained by the hollow nanomesh scaffold in the last cartoon.e) SEM images showing e-i) the bare suspending gold nanomesh and e-ii) PTCDI-C8 crystalline nanowires (CNWs) on top of hollow gold nanomesh.SEM images illustrating e-iii) large TIPS-PEN crystalline domains grown in between top and bottom electrodes using the drop-casting method and e-iv) organic crystalline p-n heterojunctions sustained by the gold nanomesh scaffold.e-v) Optical photograph of the optoelectronic device made up of PTCDI-C8 nanowires and TIPS-pentacene crystalline domains.f) Bending fatigue test of the photodetector.Reproduced with permission.[65]Copyright 2018, Wiley.g) Cartoon portraying the device configuration of iPLEDs realized by the vertical nanoscaffold.h) Current density and luminance versus driving voltage curves of a vertical nanoscaffold iPLED.The inset is the photograph of an iPLED biased at 16 V.i) Photograph of i-i) the empty vertical nanoscaffolds on the free-standing flexible PI substrate and i-ii) the flexible nanoscaffold iPLEDs being folded.Reproduced with permission.[66]Copyright 2022, American Association for the Advancement of Science.

Figure 11 .
Figure 11.a) Schematic diagram of the HAN fabrication process.b) SEM images of b-i) nanoporous alumina after anodization treatment and bii) honeycomb nanoscaffolds with hexagonal cell arrangement.b-iii) Illustration of the evolution of the HAN by the second etching treatment.b-iv)Cross-sectional SEM images of a HAN.c) Top-view SEM images of HAN@SnO 2 electrodes c-i) before and c-ii) after 30 000 CV cycles.c-iii) Top-view and c-iv) cross-sectional SEM image of HAN@SnO 2 electrodes after CV measurement at a continuous mechanical extrusion pressure of 10 MPa.Reproduced with permission.[56]Copyright 2020, Springer Nature.d) Schematic representation of the fabrication steps for bioinspired patterned thin photovoltaic absorbers.e-i) Photographs of the patterned 130 nm thin a-Si:H layers on glass substrates with completely etched disordered nanoholes and unpatterned samples.e-ii) Top-view SEM image of the nanostructured a-Si:H thin layer.The inset is the ring-shaped pattern in the 2D Fourier power spectrum with respect to the SEM image.e-iii) Statistical histogram of the nanohole diameters of this sample.f-i) 3D AFM image of the bioinspired a-Si:H thin film, together with a 2D surface profile.f-ii) Correlation of the disordered nanoholes on the absorption wavelength recorded at normal AOI using unpolarized light.f-iii) Integrated absorption as a function of AOI for the patterned and unpatterned samples.Reproduced with permission.[68]Copyright 2017, American Association for the Advancement of Science.g) Schematic diagram of the fabrication process of 3D nanoarchitected materials.Lowmagnification and higher-magnification SEM micrographs of g-i) an IP-Dip photoresist sample and g-ii) the final pyrolytic carbon material.h) Schematic illustration of the LIPIT showing Au ablation process accelerating microscopic spherical projectiles toward targeted materials and the impact process in real time being captured by ultrahigh-speed imaging with a micrometer and nanosecond resolution.i) Mass-normalized inelastic energies as a function of impact velocity in ballistic impact experiments.Color bar corresponds to a logarithmic density scale.Reproduced with permission.[69]Copyright 2021, Springer Nature.
www.small-methods.comYifan Yao is a professor at the College of Chemistry and Chemical Engineering at the Hunan University.He received his Ph.D. degree under the supervision of Prof. Wenping Hu and Prof. Huanli Dong from the Institute of Chemistry, Chinese Academy of Sciences in 2016.He was a postdoc at the University of Strasbourg, France, from 2016 to 2021 (supervisor: Prof. Paolo Samorì).At present, his main research interest is on high-performance organic (opto)electronic devices by supramolecular self-assembly, nanofabrication, and organic semiconductor physics.Paolo Samorì is distinguished professor at the University of Strasbourg and director of the Institut de Science et d'Ingénierie Supramoléculaires.He is Member of the Académie des Technologies, Fellow of the Royal Society of Chemistry (FRSC), Fellow of the European Academy of Sciences (EURASC), Member of the Academia Europaea, Foreign Member of the Royal Flemish Academy of Belgium for Science and the Arts (KVAB), and Senior Member of the Institut Universitaire de France (IUF).His research interests comprise nanochemistry, supramolecular sciences, and materials chemistry focusing on 2D materials and organic/polymeric components for opto-electronics, sensing, and energy storage.