Binder driven self-assembly of metal-organic cubes towards functional hydrogels

The process of assembling astutely designed, well-defined metal-organic cube (MOC) into hydrogel by using a suitable molecular binder is a promising method for preparing processable functional soft materials. Here, we demonstrate charge-assisted H-bonding driven hydrogel formation from Ga3+-based anionic MOC ((Ga8(ImDC)12)12−) and molecular binders, like, ammonium ion (NH4+), N-(2-aminoethyl)-1,3-propanediamine, guanidine hydrochloride and β-alanine. The morphology of the resulting hydrogel depends upon the size, shape and geometry of the molecular binder. Hydrogel with NH4+ shows nanotubular morphology with negative surface charge and is used for gel-chromatographic separation of cationic species from anionic counterparts. Furthermore, a photo-responsive luminescent hydrogel is prepared using a cationic tetraphenylethene-based molecular binder (DATPE), which is employed as a light harvesting antenna for tuning emission colour including pure white light. This photo-responsive hydrogel is utilized for writing and preparing flexible light-emitting display.

C harge-assisted hydrogen bond (CAHB) is a type of noncovalent interaction (X-H (+) ···Y (−) ) that plays an important role in the structure-property correlation of biomacromolecules and in various biological molecular recognition processes [1][2][3][4] . CAHB is also widely employed in the construction of discrete organic cages, extended crystalline metal-organic architectures 5,6 and soft supramolecular gels 7 . The reason behind such versatility of CAHB is essentially its intrinsic strength (stronger than neutral X-H···Y bond) and directionality, that results in a wide range of materials with an array of exciting and complementary properties 8 . In this regard, CAHB driven selfassembly of predesigned metal-organic polyhedra (MOPs)  that are discrete metal-organic cages with confined cavities and large number of connecting sites, into soft supramolecular hydrogel is yet to accounted.
Among different classes of MOPs, metal organic cubes (MOCs) with a general formula [M 8 L 12 ] x (x = 0, n-), comprising metal ions (M n+ = Ni 2+ , Zn 2+ , In 3+ , Cr 3+ ) as vertices and imidazoledicarboxylate (L) as edges of a cube have been well explored 30 . Aesthetic appeal, structural modularity and robustness pertaining to the MOCs showed great promise for diverse applications 31 . MOCs are neutral (x = 0) or anionic (x = n-, a-MOC) depending upon the charge balance between M n+ and L 32 . MOCs are exploited as molecular building blocks by connecting the peripheral free carboxylate oxygens with metal ions or with H-bond donor molecules and the resulting extended structures showed potential applications in gas storage/ separation and protonconductivity [33][34][35] . Since exteriors of a-MOCs are decorated with free polar carboxylate groups, they could be soluble in polar solvents, like water 36 . We envisioned that interaction of soluble a-MOCs with the positively charged or neutral, H-bond donor molecular binders through CAHB interaction could result in extended, supramolecular network 37,38 . In aqueous solution, such supramolecular assembly between a-MOC and different molecular binders could result in hydrogels. Aida et al. have shown that CAHB interaction between anionic clay nanosheets and dendritic molecular binders containing multiple guanidinium ions facilitated the cross-linking of the clay nanosheets and formed hydrogels 39 . Recently, Johnson et al. and Nitschke et al. reported the self-assembly of soluble polymers having coordinating end groups with metal ions, leading to the formation polymeric gel that consisted of in-situ formed metal-organic cage at the junction of cross-linked polymers [40][41][42] .
Although their approach is inspiring, use of water soluble, preformed a-MOC as a platform to study self-assembly in the presence of different molecular binders is yet to be accounted. We envision that introduction of different molecular binders would tune the nano-morphologies and functionalities of the a-MOChydrogels. For example, the surface charge of the hydrogelnanostructure could be altered by choosing appropriate cationic/ anionic binders, making the hydrogel useful for chromatographic separation of oppositely charged species. Moreover, suitably designed chromophoric molecular binder would result in a processable soft luminescent hybrid hydrogel. The a-MOCs could also act as an excellent template for immobilizing the multichromophoric donors and acceptor binders for light harvesting application. In such system the emission property can be tuned and even processable high quantum efficiency pure white-lightemitting materials can be realized. In addition, stimuli responsive molecular binders can also provide the stimuli-responsive a-MOC-hybrid gels. With the introduction of photoactive molecular binders, photo-responsive hydrogel could also be prepared, which can enable writing on the flexible displays by change in corresponding photochemical reactions.
Herein, we report synthesis of a Ga 3+ based metal-organic cube, ((Me 2 NH 2 ) 12 (Ga 8 (ImDC) 12 )·DMF·29H 2 O) (1), extended into three dimension through CAHB interaction between anionic [Ga 8 (ImDC) 12 ] 12− (MOC) and Me 2 NH 2 + (DMA) cations. Compound 1 is highly soluble in water, where discrete MOCs remain intact in solution. This particular phenomenon provides an opportunity to crosslink the MOCs with a wide range of molecular binders that lead to the formation of charge-assisted hydrogels (Fig. 1a). Different molecular binders are assembled with MOC and the resulting hydrogels show different morphologies and properties (Fig. 1b). When ammonium cation (NH 4 + ) is used as molecular binder, the resulting hydrogel with negatively charged, tubular nanostructures is exploited for gel chromatographic separation of positively charged species. We also extend the concept to form stimuli responsive luminescent hydrogel by rationally designing an aggregation induced emission (AIE)active molecular binder containing tetraphenylethene (TPE) core. The photoresponsive behaviour of this hydrogel is further exploited for writing on flexible displays based on photocyclization of TPE core. Such photoresponsive behaviour of the hydrogel is also explored for tuning the excitation energy transfer from TPE segment to encapsulated acceptor dye. Finally, a pure white-light-emitting hydrogel with Commission Internationale de L'Eclairage (CIE) co-ordinates of (0.33, 0.32) is achieved.

Results
Synthesis and structural characterization of 1. Solvothermal reaction of Ga(NO 3 ) 3 .6H 2 O and 4,5-imidazoledicarboxylic acid (H 3 ImDC) in N,N′-dimethylformamide (DMF) in presence of triethylamine (Et 3 N) at 120°C affords a pale yellow powder. Aqueous solution of the powder on slow evaporation yields block-shaped single crystals of 1. The asymmetric unit contains two Ga 3+ (Ga1, Ga2) centres, two ImDC 3− , two dimethyl ammonium cations (Me 2 NH 2 + , DMA) and eight guest water and one DMF molecules (Supplementary Figure 4 and Supplementary  Table 1). The DMA cations are formed in-situ from DMF under solvothermal condition 43,44 . Each ImDC 3− (ImDC_a or ImDC_b) linker chelates two Ga 3+ (Ga1···Ga2 or Ga1···Ga1) centres in a bis(bidentate) fashion through two imidazole nitrogen atoms (N1, N2 or N3, N4) and two carboxylate oxygen atoms (O1, O2 or O5, O8), while other two oxygen atoms (O3, O4 or O6, O7) remain free ( Fig. 2a and Supplementary Figure 4). Twelve ImDC 3− alternatively connect eight Ga 3+ centres to form an anionic metal-organic cube, (Ga 8 (ImDC) 12 Figure 5). The negative charge from 12 carboxylate group of each cube is neutralized by twelve surrounding DMA cations. Two DMA cations (N5 and N7) play an important role in extending of MOCs as they connect two adjacent cubes through N-H···O Hbonding interaction with pendent carboxylate oxygen atoms (O3, O4 and O7, O6) ( Fig. 2b and Supplementary Figure 5). The N-H···O H-bond distances and ∠N-H···O H-bond angles are in the range of 1.992-2.127 Å and 127-166°, respectively, which indicate the presence of strong charge-assisted intermolecular H-bonding interaction. The minimum distance between two adjacent cube (O3···O7) is 3.429 Å. Each cube is concomitantly connected to six neighbouring ones through H-bonding with DMA cations and forming a 3D extended structure (Fig. 2c). The periodic arrangement of cubes generates an open framework which exhibits two types of alternative 3D channels with an approximate window size of (5.9 × 4.4 Å 2 ) and (5.4 × 0.18 Å 2 ), respectively (Fig. 2d). These 3D channels are occupied by disordered water and DMF guest molecules (Supplementary Figure 6). Thermogravimetric analysis (TGA) of as-synthesized 1 shows an initial weight-loss of 15% at 200°C that corresponds to a loss of guest water and DMF molecules (Supplementary Figure 7). The similar powder X-ray diffraction (PXRD) pattern of the assynthesized powder and the simulated one indicates purity of the compound (Supplementary Figure 8). N 2 adsorption isotherm of desolvated 1 (1′) at 77 K shows a type-II profile indicating surface adsorption (Supplementary Figure 9). However, CO 2 adsorption isotherm of 1′ at 195 K shows type I behaviour with the total uptake of~75 mL g −1 suggesting microporous nature of the extended framework of 1 (Supplementary Figure 9).  Figure 12). Addition of 3.5% aq. NH 3 into the aqueous solution of 1 results in stable, transparent hydrogel (MOC-G1) after 8 h at room temperature (critical gelation concentration = 15 mg ml −1 ) (Fig. 2e). Notably, no gelation is observed when aq. NH 3 is added into the aqueous solution of ligand (4,5-imidazoledicarboxylic acid), confirming importance of anionic MOC in hydrogel formation ( Supplementary Figure 13). MOC-G1 does not show any visible weakening over a month. The sol-gel transition is completely reversible after multiple shaking-resting cycles, indicating its thixotropic behaviour. Upon heating, the MOC-G1 loses entrapped water molecules and does not show thermo-reversibility. However, MOC-G1 exhibits excellent pH responsive behaviour. When 0.1 N HCl (pH = 4-5) is added to MOC-G1 (intrinsic pH = 11) a precipitate forms which reforms gel after addition of aq. NH 3 (pH = 12) (Supplementary Figure 14). This indeed suggests the interaction of NH 4 + with MOC is crucial for hydrogelation and it is feasibly driven through charge-assisted H-bonding between NH 4 + and peripheral carboxylate oxygens of MOC (Fig. 2f) Such anisotropic growth to 1D tape is probably governed by the competitive binding of DMA and NH 4 + cations [45][46][47] . As the reaction proceeds, these 1D tapes are further assembled to form partially grown nanotubes in which three sides of nanotubes are  Figure 22). It is interesting to note that the solution converts to viscous liquid at this stage. After 8 h, stable transparent gel is formed and FESEM images show complete formation of nanotubes (Fig. 3d).
Gel-column chromatographic separation of charged species. Zeta potential MOC-G1 xerogel is found to be −22 mV, indicating negatively charged surfaces of the nanotube (Supplementary Figure 34). The negatively charged surface of one dimensionally aligned nanotubes prompted us to use MOC-G1 for gel chromatographic separation of oppositely charged species from their mixture (Fig. 3f). For this study, we have used anionic (sulforhodamine G, SHG) and cationic (nile blue, NB and acridne orange, AO) dye molecules, since they would give better visual demonstration (Supplementary Figure 35). Initial studies indicate that the hydrogel can fully adsorb (~100%) the cationic dyes (10 −5 M solution) but not the anionic dyes (10 −5 M solution) as observed from UV-Vis spectra (Supplementary Figures 36-38). For demonstrating separation of dyes from their mixture, a chromatographic column (2.5 cm long) is packed with hydrogel (stationary phase) and eluted with mixture of NB (1 × 10 −6 M) and SHG (1 × 10 −6 M) in methanol (Fig. 3f, g). On passing the feed solution the column first becomes orange because SHG elutes rapidly through the column due to electrostatic repulsion with nanotubes (Fig. 3g). Gradually, all SHG molecules pass through the column and at that point the column turns completely blue indicating the adsorption of NB in hydrogel (Fig. 3g). Such selective adsorption of NB in MOC-G1 is resulting due to the electrostatic interaction of the cationic dye onto the surface of  Figure 45). Since the hydrogel is basic (pH = 11) the −COOH group of RhB gets deprotonated easily while passing through the gel and hence is repelled by the surface of anionic nanotubes. On the other hand, the cationic part of RhB ((Et) 2 N + ) force the molecule to be attached with the gel matrix. Because of such two opposite forces, only 34% dye is absorbed by MOC-G1.
Preparation of luminescent MOC-G5 hydrogel. A cationic, AIEactive molecular binder, 1,2-bis(4-(2-diethylammonioethoxy) phenyl)-1,2-diphenylethenedihydrochloride (DATPE) having flexible ethoxy chain functionalized with terminal quaternary ammonium (REt 2 NH + ) groups is designed to achieve CAHB assisted assembly of MOCs (Fig. 5a). It is envisioned that assembly of DATPE and MOCs will result in highly emissive hydrogel due to restricted phenyl rotation of TPE segments.  Figure 50). The self-assembly of MOC and DATPE is explained by a proposed mechanism, shown in Fig. 5a. Both titration study and molecular dimension of DATPE (size: 19.0 × 7.0 Å 2 ) suggest that two cubes can be connected to maximum two DATPE molecules in a face-to-face manner via CAHB interaction between free oxygen atoms of MOC and amine groups of DATPE ( Supplementary Figures 51-52). Therefore, each MOC would be attached to six neighbouring MOCs by 12 DATPE molecules and forms smaller aggregate which extend three dimensionally to generate a network like structure entrapping DMA cations and water molecules to form hydrogel (Fig. 5a). MOC-G5 also shows strong cyan emission at 474 nm (λ ex = 350 nm) similar to molecular aggregates formed during titration study, with a CIE coordinates of (0.17, 0.24) (Fig. 5b, c). The restricted phenyl rotation in gel initially occurs through immobilization of DATPE over MOC followed by aggregation effect, which is due to π-π interactions between two neighbouring DATPE moieties. Therefore, the emission in MOC-G5 is a combination of immobilization and AIE phenomena as previously observed in TPE-based MOFs and coordination polymer gels 48,49 .
Photomodulated emission in MOC-G5. TPE shows strong AIE emission in solid state and it is well-known to undergo photocyclization to diphenylphenanthrene (DPP), which is nonemissive in solid state and blue emissive in solution ( Fig. 6a and Supplementary Figure 53 Figure 55). With these convincing results, we further study the photocyclization of DATPE in MOC-G5 hydrogel. The emission spectrum of MOC-G5 gel film prepared on quartz substrate exhibits strong AIE emission peak at 474 nm due to DATPE ( Supplementary Figure 56). However, after photo-irradiating (with 365 nm light) the film for 20 min the emission gets completely quenched, which is also clear from images of the film under UV lamp (Supplementary Figure 56). This quenching of emission is attributed to the aggregation caused quenching (ACQ) effect of DPPQA formed after photo-cyclization of DATPE in the film. However, on dissolving this photo-irradiated film in water a strong blue emission at 410 nm is observed indicating the presence of molecularly dissolved DPPQA units (Supplementary Figure 57). It is indeed possible to photopattern/write the desired shapes/letters with this hydrogel. To elucidate this, a MOC-G5 film is prepared on glass substrate and a light opaque template with unmasked pattern of 'J N C' is made. The opaque template is placed over the MOC-G5 glass substrate. Upon irradiation (λ = 365 nm) for about 20 min the unmasked area turns to non-luminescent due to formation of DPPQA (ACQ effect) while the masked area remains luminescent (Fig. 6e).
Processable white-light-emitting hydrogel. White light emission can be achieved by balancing the ratio of cyan and orange or RGB colours [52][53][54] . Here, a dichromatic approach is utilized to achieve white-light-emitting MOC hybrid gel. As mentioned earlier, MOC-G5 showed strong AIE emission at 474 nm (λ ex = 350 nm) with a CIE coordinates of (0.17, 0.24) (Fig. 5c, e). We envision that such luminescent hybrid gel can act as donor scaffold for partial energy transfer to immobilized acceptor dye and thus could exhibit tunable emission property. Rhodamine 6 G (Rh6G) is chosen as an acceptor because its absorption spectrum overlaps partially with emission spectrum of MOC-G5 hybrid and also its cationic nature would help immobilization over the hydrogel matrix (Fig. 5b). In-situ mixing of Rh6G (0.02 mol%) during gelation under sonication triggers instant formation of a tricomponent hydrogel, Rh6G 0.02% @MOC-G5. Interestingly, the emission of Rh6G 0.02% @MOC-G5 at 474 nm (λ ex = 350 nm) decreases to an extent while a new band appears at 560 nm from encapsulated Rh6G (Fig. 5d). With gradual increase of Rh6G concentration from 0.02 to 0.08%, the emission intensity at 474 nm exhibits a gradual decrease with an simultaneous increase in intensity of Rh6G band (Fig. 5d). Interestingly, the emission colour of hydrogels changes from cyan to strong pink when observed under UV light (Fig. 5c). CIE coordinates of Rh6G 0.08% @MOC-G5 are calculated to be (0.30, 0.28), which is near white light as seen in CIE diagram (Fig. 5e). Energy transfer in the mixed hybrid is evident from direct excitation of Rh6G 0.08% @MOC-G5 at 530 nm which shows less emission intensity compared to the indirect excitation at 350 nm (Supplementary Figure 58). The excitation spectrum of with anioinc MOCs electrostatically and their appropriate dipoledipole orientation with DATPE molecules in gel system results in energy transfer through FRET mechanism. Further increasing the Rh6G loading to 0.1% yields a viscous liquid and no gel formation is observed even after 24 h. The emission feature of Rh6G 0.08% @MOC-G5 gel with CIE coordinates (0.30 0.28) is further tuned towards white-lightemission by increasing the contribution of Rh6G emission. To do this, photocyclization property of TPE segment is utilized (Fig. 6a). We envisioned that the controlled photo-irradiation on Rh6G 0.08% @MOC-G5 could lead to the formation of DPPQA in hydrogel and the contribution of blue emitting DPPQA molecules would further broaden the emission spectrum of photo-irradiated Rh6G 0.08% @MOC-G5 (Fig. 6b, c). Interestingly, photo-irradiation of Rh6G 0.08% @MOC-G5 hydrogel for 5 min appreciably broadens the 474 nm band which increases the spectral overlap of donor emission with acceptor (Rh6G) absorption and eventually an increase in extent of energy transfer is observed (Fig. 6b). Excitation of photoirradiated (5 min) Rh6G 0.08% @MOC-G5 at 350 nm indeed shows an enhancement in the emission intensity at 580 nm due to enhanced energy transfer efficiency (Fig. 6b). Remarkably, emission colour of the The emission spectrum of Rh6G 0.08% @MOC-G5 before photoirradiation (black), after 5 min (red) and 9 min (blue) irradiation with 365 nm light source. c Schematic showing the energy transfer pathways before and after photoirradiation. The corresponding CIE coordinates of Rh6G 0.08% @MOC-G5 and white-light-emitting gel (Rh6G-DPPQA@MOC-G5) are mentioned below the respective hydrogel images. d Chromaticity diagram showing corresponding CIE coordinates of MOC-G5 (black), Rh6G 0.08% @MOC-G5(pink), white-light-emitting hydrogel (red, form after 5 min of photoirradiation on Rh6G 0.08% @MOC-G5), hydrogel (navy blue, form after 9 min of photoirradiation on Rh6G 0.08% @MOC-G5) and Rh6G (wine). e A glass slide coated with MOC-G5 hydrogel is irradiated with 365 nm light to write 'JNC'. Processability of white-light-emitting hydrogel: f image of glass substrate coated with white-light-emitting hydrogel under UV lamp, g writing and h painting using same hydrogel on 365 nm UV lamp. i, j Images of a flexible plastic substrate (5 × 3 cm) coated with white-light-emitting-hydrogel under UV light and k under day light gel changes from pink to white as seen under UV lamp. The CIE coordinates of photo-irradiated gel (Rh6G-DPPQA@MOC-G5) are found to be (0.33, 0.32) indicating a pure white-lightemission from the hybrid gel (Fig. 6d). Positive ion acquisition mode HRMS analysis of the photo-irradiated Rh6G 0.08% @MOC-G5 shows nearly 8% conversions of DATPE molecules to DPPQA on 5 min photoirradiation (Supplementary Figure 61). The stability of MOC in the photo-irradiated Rh6G 0.08% @MOC-G5 is confirmed by negative ion acquisition mode HRMS analysis which shows peak at m/z = 1233.26 (z = 2 − ), corresponding to ((Ga 8 (ImDC) 12 )(2Na + )(8 H + )(H 2 O)) 2− moiety (Supplementary Figure 62). The nanocube morphology is retained in Rh6G 0.08% @MOC-G5 after photo-irradiation (Supplementary Figure 63). Energy transfer in white-light-emitting hybrid is further evident from direct excitation of the Rh6G at 530 nm, which shows less emission intensity compared to the indirect excitation at 350 nm, clearly suggesting efficient excitation energy transfer through FRET mechanism (Supplementary Figure 64). Energy transfer is further evident from excitation spectrum of photoirradiated Rh6G 0.08% @MOC-G5 collected at 580 nm, which shows maximum intensity at 350 nm confirming the contribution of DATPE to the observed emission at 580 nm (Supplementary Figure 65). Fluorescence decay profiles of white-light-emitting gel monitored at DATPE emission (474 nm) shows shorter life time value (1.5 ns) than MOC-G5 hybrid gel (4.5 ns) clearly indicating excitation energy transfer from DATPE to Rh6G in the hybrid gel (Supplementary Figure 66). The energy transfer efficiency (Φ e ) and rate constant (k e ) are calculated to be 66.7% and 4.446 × 10 8 s −1 , respectively in white-light-emitting hydrogel. This prove that photo-irradiation indeed enhances the energy transfer efficiency. The absolute quantum yield of white-light-emitting gel is found to be 16%.
Easy reversible sol-gel transformation of white-light-emitting hydrogel prompted us to study large area coating for device fabrication (Fig. 6f). This hybrid white-light-emitting hydrogel can be easily painted on a commercially available UV lamp (Fig. 6g, h). Upon turning on the UV lamp the uncoated portion remains dull blue while coated part strongly illuminates white light. Similarly, letters written on the UV lamp with the softhybrid become readable when power source is on, as they emit the white light upon excitation. Also, the hybrid hydrogel can be coated on flexible plastic substrates that not only exhibits strong white emission under UV lamp but also remain highly transparent under day light (Fig. 6i-k).

Discussion
In summary, we have prepared water soluble, Ga 3+ bases anionic MOC which self-assemble to hydrogel in presence of different molecular binders through charge-assisted H-bonding interaction. Depending upon shape and geometry of the molecular binders, the hydrogels show different morphologies, such as nanotube, nano-bouquet, nanosheet and nanocube. Moreover, the properties of the hydrogels are tuned by selecting the suitable binders. Here we have exploited two different properties of the MOC-based hydrogels. In one hand, the surface negative charge of the nanotubes of MOC-G1 is exploited for gelchromatographic separation of cationic dyes from anionic dye. On the other hand, we have documented a photo-responsive luminescent hydrogel based on AIE active chromophoric binder that is further exploited for light harvesting application. In addition, we also prepared a white-light-emitting hydrogel by tuning the donor-acceptor energy transfer efficiency using photocyclization of TPE as a tool. In short, MOC-hydrogels provide a platform to integrate different type of molecular binders with MOC to form self-assembled nanostructures, where the properties and corresponding functionalities can be deliberately tuned.

Methods
Synthesis of ((Me 2 NH 2 ) 12 (Ga 8 (ImDC) 12 )·DMF·29H 2 O) (1). H 3 ImDC (0.5 mmol, 78 mg), Ga(NO 3 ) 3 .H 2 O (0.5 mmol, 128 mg) and 10 ml DMF were mixed in a 20 ml Teflon container and stirred for 30 minutes at room temperature. A volume of 15 µl NEt 3 was added into the reaction mixture and stirred for additional 30 minute. After that the Teflon container was kept inside a stainless steel autoclave which was heated at 120°C for 24 h. After the reaction was over, the autoclave was slowly cooled down to room temperature. The white product formed was centrifuged and washed repeatedly by methanol and dried in air. The air dried powder was dissolved in 10 ml water and kept for recrystallization at room temperature. Within 4 days colourless, block shaped crystals were formed. The crystal structure determination reveals the molecular formula of the compound as ((Me 2 NH 2 ) 12 (Ga 8 (ImDC) 12  Preparation of MOC-hydrogels. For preparing MOC-G1 hydrogel, 15 mg 1 was dissolved in 1 ml water and 100 µl aq NH 3 is added into in the solution. The mixture was sonicated for few minutes and kept at room temperature. MOC-G1 hydrogel was formed after 8 h. For preparing MOC-G2 hydrogel, 20 mg 1 was dissolved in 500 μl water and 500 µl N-(2-aminoethyl)-1,3-propanediamine solution (0.126 M) was added dropwise into it. The mixture was sonicated for few minutes and kept undisturbed at room temperature. The mixture became viscous after 4-5 h and formed stable MOC-G2 hydrogel after one day. For preparing MOC-G3 hydrogel, 20 mg 1 was dissolved in 300 µl water and 600 µl solution of guanidine hydrochloride (0.1 M) was added into the solution. The mixture was sonicated for few minutes and kept undisturbed at room temperature for one day to form MOC-G3 hydrogel. For preparing MOC-G4 hydrogel, 20 mg 1 was dissolved in 500 µl water and 500 µl solution of β-alanine (0.1 M) was added into the solution. The mixture was sonicated for few minutes and kept undisturbed. The mixture converted to stable gel after 1 day. For preparing MOC-G5 hydrogel, 12 mg 1 was dissolved in 500 µl water. 30 µl DATPE was dissolved in 500 µl water and the solution was drop-wise added into the solution of 1. The mixture was sonicated for few minutes. An opaque gel was formed instantaneously. In all cases formation of hydrogels was confirmed by inversion test method. More detail synthesis and characterization of MOC-based hydrogels containing mixture of binders have been discussed in Supplementary Methods.
Preparation of hydrogel column for separation. A glass column (diameter = 0.5 cm and length = 10 cm) having silica base was packed with MOC-G1 hydrogel to prepare the gel-column (length of the stationary phase = 2.5 cm). The mixture of NB (1 × 10 −6 M) and SHG (1 × 10 −6 M) in methanol was layered on the hydrogel column and eluted through the gel-column. The solution coming out from the column was collected in a vial and monitored by UV-Vis spectra.

Data availability
The X-ray crystallographic data that support the finding of this study are available in Cambridge Crystallographic Data Centre (CCDC) under deposition number CCDC 1587189. All other data supporting the findings are available in the article as well as the supplementary information files and from the authors on reasonable request.