Multifunctional Supramolecular Hybrid Materials Constructed from Hierarchical Self‐Ordering of In Situ Generated Metal‐Organic Framework (MOF) NanoparticlesA synergistic approach is described to engineer supramolecular hybrid materials based on metal-organic frameworks, encompassing HKUST-1 nanoparticles formed in situ, coexisting with an electrically conducting gel fiber network. The following findings are made: i) multistimuli-responsive structural transformation via reversible sol–gel switching and ii) radical conversion of a soft hybrid gel into a mechanically malleable, viscoelastic matter. Major advances in supramolecular science1 have triggered a new wave of discoveries of next-generation functional materials, especially stimuli-responsive soft matter,2 which could afford multiple functions applicable to emergent technologies. It is envisioned that the unique combination of self-organized supramolecular systems,3 self-healing4 and thermo-reversibility,5 and on-demand sol–gel transformation6 makes stimuli-responsive materials not only extremely versatile, but also straightforward to process into novel multifunctional devices.7 Yet, the precise control over its microstructural alignment and detailed understanding of complex organization, constituting a self-assembly,1 remain some of the biggest challenges facing supramolecular materials science today. One of the best examples featuring self-organization of lamellar hierarchical growth lies in the naturally occurring protein assembly, known as collagen,8 whose highly aligned fibrous microstructure not only dictates its biomechanical stability, but also underpins numerous biostimuli functions.9 To this end, exploration of synthetic routes mimicking the self-directed molecular assemblies evidenced in nature has attracted considerable interests, further broadening the scope of applications linked to novel functional materials and soft matter. For example, oriented growth of supramolecular assemblies allows switchable or tunable material properties, which have utility in emerging areas ranging from microfluidics and molecular filtration, to nanowires and microelectronics devices.7 Recent studies have also focused on the cooperative effects arising from nanoparticle–polymer pairs (or block copolymers) to create energetically favorable multicomponent systems, incorporating complex patterned growth of bespoke nanomaterials.10 In this study, we report a rare type of supramolecular hybrid material built from self-organization of nano-metal-organic framework (NMOF) particles, which are accommodated within highly aligned fiber network scaffolding. Metal-organic framework (MOF)11 is a rapidly expanding class of crystalline nanoporous materials, whose 3D framework consists of ordered units of metal ions or clusters bridged by organic linkages. MOFs offer rich chemical functionalities combined with vast structural versatility.12 The high uniformity and 3D microporous architecture of MOF crystals could provide the unique platform for symbiotic effects, yielding orthogonal interactions13 central to achieve supramolecular self-assembly. It is worth noting that, although metal-organic gels (MOGs)14 are a very similar type of self-assembled hybrid compound, they comprise randomly cross-linked metal ions by organic linkers (without forming an ordered framework), subsequently trapping solvent molecules to form a more conventional gel network material. The microporous material designated as HKUST-115 represents one of the most intensely studied MOFs today due to its wide-ranging potential applications. Typically, it can be obtained via solvothermal reactions between Cu(II) and BTC3− (1,3,5-benzene tricarboxylic acid), yielding crystalline HKUST-1 [Cu3(BTC)2] as the most thermodynamically favorable product.16 In fact, HKUST-1 is renowned for its ease of synthesis using different solvents, temperatures, or bases,17 in addition to membranes, hollow capsules, and superstructures derived from it.18 Herein, we demonstrate that, by employing a high concentration of standard reactants of HKUST-1 in a relatively small quantity of solvents, yields previously unreported gel-like hybrid materials that exhibit counterintuitive chemico-physical properties. In the present work, we discovered that room-temperature reaction between Cu(NO3)2 solution and deprotonated BTC, using triethylamine base (NEt3), yields facile formation of an unconventional gel-like supramolecular self-assembly. Interestingly, the aforementioned reactions can be accomplished in both polar-protic and polar-aprotic solvents, resulting in an entirely new system of MOF-based supramolecular hybrid materials (see Figure S1 in the Supporting Information), which we termed: G⊃ACN, G⊃DMF, G⊃DMSO, G⊃ETH, and G⊃MEH (where G denotes gel obtained using solvent: ACN: acetonitrile; DMF: N,N-dimethyl formamide; DMSO: dimethyl sulfoxide; ETH: ethanol; MEH: methanol; see Figure S2 and Table S1 in the Supporting Information). The formation mechanism was investigated utilizing different solvents, which allowed us to study the rich morphological and structural diversity of these novel hybrid materials. We found that the use of different solvents yields hybrid materials that exhibit distinct structural, mechanical, chemical, and electrical properties. Noteworthy, sol–gel transitions occur only in the case of G⊃DMSO, while viscoelastic phase conversion (from soft to rigid network) is evident only in G⊃ACN. Moreover, detailed characterization of physico-mechanical properties has enabled us to pinpoint structural dependencies of uncommon functions. The abovementioned are key highlights of the current study which are explored in detail in this work. Application of DMSO as a solvent for making solutions of Cu(NO3)2 and BTC produces a hybrid gel compound featuring unique properties, as depicted in Figure 1. Although the reaction of BTC and Cu(II) in DMSO solvent has previously been reported,19 in our approach, the addition of triethylamine (NEt3) triggers an unexpected auxiliary effect leading to the formation of a new supramolecular material. Remarkably, this hybrid gel compound, G⊃DMSO, exhibits multiple responses against thermal, mechanical, and chemical stimuli (Figure 1a). First, it was found that subsequent alternate additions of 1 equivalent of BTC and/or 1.5 equivalent of Cu(II) solution cause the hybrid assembly to undergo a rapid phase transformation, i.e., switching from gel to sol, and vice versa. This observation confirms that the hybrid gel can be broken down by dissolution, but subsequently reassembled upon the addition of cationic Cu(II) or anionic BTC3−, such that chemical triggering accounts for the reversible nature of sol–gel transformation observed. Nevertheless, as this process is repeated with successive cycles of additions of BTC and/or Cu(II) solutions, an increasingly longer time was required to recover the gel form (Table S2, Supporting Information). Second, mechanical stresses invoked by shaking could disrupt the structural integrity of the G⊃DMSO hybrid assembly, causing the gel to collapse into a viscous sol (Figure 1a). This gel phase, however, can be regained either by subjecting the sol to sonication for ≈1–2 min, or simply by leaving it uninterrupted for ≈10–15 min. The gel was found to be extremely sensitive to application of external forces such that phase recovery (sol to gel) takes longer to occur when gel is mechanically agitated for a longer period of time. Third, further to chemical and mechanical responses, this particular hybrid gel also demonstrates thermo-responsive behavior; for instance, the sol of G⊃DMSO can be transformed back into a gel when heated at 80 °C for just ≈1–2 min. We identified another striking phase change phenomenon conferred by the acetonitrile solvent (ACN), wherein the corresponding hybrid gel synthesized is designated as G⊃ACN. Initial observation of G⊃ACN gels transforming into a semi-solid-like material has prompted us to study this effect in detail. We established that by allowing an G⊃ACN sample (sealed in a vial) unperturbed for more than 48 h would eventually transform the gel phase into a viscoelastic (VE) material (Figure 1b,c), which we termed VE⊃ACN (detailed viscoelastic mechanical behavior presented in later sections). Its two major characteristics are: (1) if gentle mechanical stress is imposed onto G⊃ACN in the gel state (for instance, by shaking sample vial), this causes the fibrous gel scaffolding to break down to yield a sol; and (2) more vigorous mechanical agitation of the reactant mixture ultimately produces only precipitation, prohibiting any further gelation from occurring. Above findings support the notion that gel to viscoelastic phase change can be accomplished, only if the underlying gel scaffolding (i.e. building skeleton) is kept stress-free and maintained stable with encapsulated solvent molecules. In light of this, the foundation for constructing a rigid fiber network stems from the gel itself, acting as the 3D scaffold; but without which, phase change into the viscoelastic material cannot occur. An attractive physical property of the VE⊃ACN material lies in its shape-persistent capacity; it is mechanically resilient enough to be dissected, deformed, or molded into a range of 3D shapes, a few examples of which are illustrated in Figure 1b,c. Figure 2 shows microstructural characteristics of the supramolecular hybrid gel materials obtained from scanning electron microscopy (SEM), revealing exquisite fine-scale morphologies that correlate to their chemical structures and physical properties (Figure 2 and Figures S3–S8, Supporting Information). Comparing Figure 2a to b, we recognize that the viscoelastic material (VE⊃ACN) consists of fine fibrous filaments hierarchically packed within fiber bundles, which are reminiscent of fibrils present in structural biological materials.8, 9 Such a microstructure, however, is absent in the initial gel material (G⊃ACN) prior to its viscoelastic conversion. It can be seen that images of the G⊃DMSO system that displays multistimuli response reveal relatively thicker rods of gel-like blocks; two examples of which are shown in Figure 2c,d with dimensions of ≈200 × 50 μm. In contrast, the remaining gel samples (Figure 2e–j) feature appreciably smaller bundles of microscopic fibers, growing unidirectionally in the form of strips, whose lengths may extend from several micrometers up to hundreds of micrometers. It is worth noting that recent literature on MOF-based aerogels,20 xerogels,21 and metal-organic gels (MOGs)[14],22 exhibits substantially different fine-scale microstructures, compared with the distinctive fiber microarchitectures we reported here. Given the identical chemical precursors employed in all syntheses, apart from the choice of solvent, it is intriguing that sol–gel transformation was identified only in the case of G⊃DMSO, whereas viscoelastic conversion evidenced only in the case of G⊃ACN. This result underscores the critical roles played by the specific solvent molecules (Figure S2, Supporting Information), which in turn affect the microstructures of the resultant fiber network assembly (Figure 2) and henceforth its physico-chemical characteristics. Electron donor atoms, viz., O, N, S, which are present in certain solvent molecules enhance interaction with Cu(II) centres, and may extend the structural connectivity by means of hydrogen bonding. Especially in the DMSO solvent, for example, we propose that soft S and hard O donor atoms (at opposing ends of molecule) facilitate switchable connections for weak bond making-breaking process; this mechanism could accommodate supramolecular association–dissociation processes, thus triggering sol–gel conversion of G⊃DMSO (Figure 1). Moreover, because S and O readily interact with soft and hard electrophiles, DMSO forms strong hydrogen bonding (Kamlet–Taft parameters: α = 0.00, β = 0.76)23 with BTC (Figures S9 and S10, Supporting Information).24 Of the systems we studied, G⊃DMSO requires the longest gelation time (Table S1, Supporting Information), suggesting that the reaction responsible for fiber formation is occurring at a relatively slower pace, due to effects of solvent interaction that may cage BTC3− linkers approaching the Cu(II) cations. Although O donor is also present in other solvent molecules, we found that monodentate ligation property is inadequate to trigger microstructural reintegration fundamental to reversible sol–gel conversions. Turning to G⊃ACN, the linear molecular structure of acetonitrile (ACN) means that, not only its associated geometrical constraints should be weaker, but also it offers opportunity for coordination to the Cu(II) centers to further stabilize the overall supramolecular network assembly. Together, it is projected that such auxiliary effects, over time, perpetuate transformation of gel into the viscoelastic material, which constitutes fine-scale fibrous architecture specific to VE⊃ACN (Figure 2b and Figure S3, Supporting Information). By virtue of the rapid formation of G⊃MEH gels from methanol (≈2 min), we probed this material to gain insights into the basic structural development mechanisms. During the reaction, we observed intense color emerging from nanoparticles embedded in the fibers as time advances; these nanoparticles can be easily harvested from embedding fibrous matter by washing the gel thrice using excessive methanol (Figure S11, Supporting Information). Bulk amount of nanoparticles collected this way has been examined by powder X-ray diffraction (PXRD, Figure S12, Supporting Information), scanning electron microscopy Figure S11, Supporting Information), and microscopy Figure Supporting Information), that they are in MOF (NMOF) of phase We that the mechanism the of supramolecular MOF hybrid in with by the of the organic and basic building via weak in a network of gel material. This development is subsequently by coordination forming between Cu(II) and molecular it collapse to the assembly, yielding HKUST-1 nanoparticles in the fiber network. Recent on framework hybrid materials and conversion of MOFs into gel-like are worth for In the case of donor atoms interact with metal ions to form hybrid material. MOF on the other is to form by of organic linkers present in the In contrast, the supramolecular assembly of the current of hybrid materials an entirely different MOF nanoparticles by the present in the assembly, which ultimately yields hierarchical architecture the major crystalline phase of the overall soft matter. of HKUST-1 and metal in HKUST-1 offers to other molecules to either or the hollow MOF effects, We that the of BTC molecules, with coordination to Cu(II) centres, may growth of fibrous network in a highly aligned Moreover, the use of for BTC cationic we from the the of to with Cu(II) in the of organic Such a termed the molecular between multiple molecular formation due to the resultant synergistic effects To further the growth in another we have the effects of of materials on the A of BTC3− solution in DMSO × was on the by (Figure Supporting Information), on the of which was subsequently several of Cu(II) solution This approach a concentration to the of organic and DMSO is a solvent, the sample of G⊃DMSO obtained was to at images from the sample microstructural the synergistic see This not only that, in fibers could as small that to the formation of HKUST-1 but also because of molecular crystals featuring have on the hybrid gel fiber (see also Figures and in the Supporting Information). it is worth that synthesis of hybrid materials in with of not result in the formation of such to study the structural integrity of the supramolecular hybrid when to and corresponding stress The are in Figure the and are of the response and viscous It that the = whose is the mechanical i.e., its structural against by a Figure it can be seen that the and of the gel samples are distinct from one which tunable mechanical properties of this of hybrid materials based on choice of solvents (Figures Supporting Information). one of the of have been for G⊃ACN, G⊃DMF, and on the other found for G⊃DMSO and G⊃MEH that an is also (Figure Supporting Information). The = in Figure shows in G⊃ACN, and G⊃ETH, corresponding to phase associated with between and was observed in G⊃ACN, that its hybrid network is relatively against any forces imposed at G⊃DMSO demonstrates a phase change mechanical of the G⊃DMSO sample has been investigated by which the reversible recovery of the gel phase min, over from to at see Figure in the Supporting Information). Herein, we the of the supramolecular network assembly using the in Figure in with the observed fibrous architecture HKUST-1 as in Figure the of the of have been established in G⊃DMSO, for which could be linked to the high concentration of the nature of supramolecular hybrid and its subsequent (i.e. recovery upon By the structural transitions of G⊃ACN via we have established the conversion into subsequent viscoelastic phase (VE⊃ACN) the overall network Figure the and corresponding to and structural evidenced from in with time in a For at the of was at but to and at we found a of than that of a the in Figure shows that, of is phase change 48 This further the that, over time, weak supramolecular network in G⊃ACN gel has more rigid see Figure in the Supporting and thus mechanically against further as it into constituting a microstructure (Figure on VE⊃ACN has established a of which was from the between the and (Figure Supporting Information). Moreover, we the recovery of samples at h in which the case corresponding to VE⊃ACN shows and recovery response (Figure Supporting Information). current using a potential of The presented in Figure tunable electrical properties, for which the of can be for samples synthesized from different the sample of G⊃MEH exhibits the electrical at S whereas the was for G⊃DMSO at S i.e., approaching two of that of the The was in G⊃ACN S which is by S a relatively has been for G⊃ACN S the viscoelastic material (VE⊃ACN) derived from the gel sample yields a of S (Figure Supporting Information). the unique for the electrical properties of MOF materials is of that VE⊃ACN is malleable, (see Figure in the Supporting and it can be easily or molded into stable structures (Figure its electrical property may utility in bonding and the fiber network architecture could an in of multiple (Figure Supporting Information), which are responsible for facile in the current of supramolecular MOF In formation of in the supramolecular assembly also functions as molecular to fibrous in which MOF with facilitate Nevertheless, electrical in the case for example, S and G⊃DMSO S could be by the of whose is by solvent molecules the stable gel phase, thus its overall In fact, that electrical mechanism in HKUST-1 can be by the molecules interact with Cu(II) the framework forming an uninterrupted linear of clusters this of we propose that electrical in the current of supramolecular fiber may be to the effects of encompassing BTC3−, and HKUST-1 nanoparticles metal all of which are in the hybrid supramolecular assemblies (Figure Supporting Information). In addition to the unique physical properties of supramolecular MOF another of such sol–gel lies in the making of HKUST-1 By hybrid gels as we have established the of different to HKUST-1 and uniformity in to the of and without to to the use of relatively self-assembled For the in Figure addition of solvent to G⊃MEH down rapidly the gel fibrous network to yield HKUST-1 nanoparticles whose sol can be onto a Such for instance, sol obtained from G⊃DMSO is especially for use with the to with (Figures Supporting Information), which have been via microscopy (Figure Supporting Information). and by into methanol for this thus leaving HKUST-1 (Figures S12, Supporting Information). color upon to while the shows color change of an HKUST-1 material, i.e., switching from to (Figure upon of from the centers within the by sol–gel approach uniformity with of and a of as by (Figure the high of is for fine-scale mechanical properties of In a we the combined approach using by and by to the behavior of HKUST-1 Figure shows the of HKUST-1 the was found to between this is with for a range of MOF materials, Herein, we demonstrate that the approach in an is for properties of MOF which be to other mechanical characterization development of multifunctional materials is ultimately by precise control over its fine-scale microstructural organization, the mechanism of molecular self-assembly. it is central to the effects complex structural to gain insights into the multiple and In this work, we that an unconventional system MOF-based supramolecular hybrid gels can be via hierarchically and building The highly tunable microstructural illustrated in this study are different from the hybrid gels and soft matter. By the synergistic formation of highly aligned fiber coexisting with MOF this offers an new to multistimuli-responsive properties, yielding chemical, structural, and mechanical all of which are key to and other microelectronics and technologies. The novel viscoelastic material (VE⊃ACN) derived from a hybrid gel is also worth its combination of mechanical and electrical by virtue of the chemical and structural of it is that the facile synthetic we illustrated the for rapid to wide-ranging hybrid systems, up of the emerging of MOFs with the of supramolecular gels and soft matter. was in solvents and triethylamine was in to achieve in the This solution was for prior to reaction with the which was in a corresponding solvent by of and further with sonication for min. of was to with vigorous shaking for a few the mixture is it shows gel-like behavior as by the hybrid gel materials obtained from the aforementioned synthesis as precursors for MOF material was using of methanol and to the at the of was collected and later to of MOF onto via the with between the of the and of the a few micrometers up to of micrometers The has also been for and for 50 for 50 and for on the with a was for all studies by a 1 between the and the A stress of was for and stress recovery with a of using the the scanning electron images and of MOF using 3D X-ray powder diffraction characterization of nanoparticles and gel samples using the with diffraction collected at from to using a and microscopy and using the in with of and a of on the was for the was using a standard sample of with an established = of gel samples was using the and a with at 1 The of the viscoelastic was from a of VE⊃ACN material between a of This was by the and of and the The are to and for to the and their electrical The the at for to the materials characterization The also the and and and for the of materials characterization The to and at for to the viscoelastic a to our and this by the Such materials are and may be for but are not or support arising from than should be to the The is not responsible for the or of any by the than should be to the corresponding for the