Tuning Gel State Properties of Supramolecular Gels by Functional Group Modification

The factors affecting the self-assembly process in low molecular weight gelators (LMWGs) were investigated by tuning the gelation properties of a well-known gelator N-(4-pyridyl)isonicotinamide (4PINA). The N―H∙∙∙N interactions responsible for gel formation in 4PINA were disrupted by altering the functional groups of 4PINA, which was achieved by modifying pyridyl moieties of the gelator to pyridyl N-oxides. We synthesized two mono-N-oxides (INO and PNO) and a di-N-oxide (diNO) and the gelation studies revealed selective gelation of diNO in water, but the two mono-N-oxides formed crystals. The mechanical strength and thermal stabilities of the gelators were evaluated by rheology and transition temperature (Tgel) experiments, respectively, and the analysis of the gel strength indicated that diNO formed weak gels compared to 4PINA. The SEM image of diNO xerogels showed fibrous microcrystalline networks compared to the efficient fibrous morphology in 4PINA. Single-crystal X-ray analysis of diNO gelator revealed that a hydrogen-bonded dimer interacts with adjacent dimers via C―H∙∙∙O interactions. The non-gelator with similar dimers interacted via C―H∙∙∙N interaction, which indicates the importance of specific non-bonding interactions in the formation of the gel network. The solvated forms of mono-N-oxides support the fact that these compounds prefer crystalline state rather than gelation due to the increased hydrophilic interactions. The reduced gelation ability (minimum gel concentration (MGC)) and thermal strength of diNO may be attributed to the weak intermolecular C―H∙∙∙O interaction compared to the strong and unidirectional N―H∙∙∙N interactions in 4PINA.


Gelation Experiments
The gelation experiments for diNO, INO, and PNO were tested in various solvents (Table S1, Supplementary Materials). In a typical experiment, a 10.0 mg portion of the compound was heated in 1.0 mL of solvent (1.0 wt %) to get a clear solution and was cooled to room temperature. The hydrogel formation of these amides tested at 1.0 wt % indicated no gelation, which prompted us to increase the concentration to 4.0 wt %. The mixture was heated (80.0-90.0 °C) and on cooling to room temperature, gelation was observed only for the di-N-oxide (diNO) after 1 h, which was confirmed via inversion test. Crystalline materials were obtained for both the mono-N-oxides (PNO and INO). The gelation experiment performed with an equimolar mixture of PNO and INO resulted in concomitant crystallization of the individual compounds, which was confirmed by single-crystal Xray diffraction (SCXRD). The gel strength of diNO was analyzed by sol-gel transition temperature (Tgel) and minimum gel concentration (MGC) experiments. The lowest concentration at which the gel was obtained was recorded as the minimum gel concentration (MGC). The MGC experiment of the hydrogel was performed in 1.0 mL of water by weighing different amounts of diNO gelator (30.0 to 50.0 mg) in a 7.0 mL vial. The mixture was heated and then cooled to room temperature, and the vial was left undisturbed for gel formation. After 24 h, the gel formation was confirmed by the inversion test and the MGC of the gelator was found to be 4.0 wt %.

Thermal Stability
The thermal stability of the gel network of diNO was evaluated using gel-to-solution transition temperature (Tgel) experiments, which is the critical temperature at which the gel converts to a solution. The experiment was performed by placing a small spherical glass ball (92.0 mg) on top of the diNO preformed hydrogel in a standard vial. The vial was heated and the temperature at which the ball touched the bottom of the vial was recorded as Tgel. The experiments were performed at 4.0 and 6.0 wt % and the Tgel did not vary much with the concentration of diNO, which was found to be 78.0 °C and 80.0 °C at 4.0 and 6.0 wt %, respectively.

Design and Synthesis
We synthesized three N-oxides compounds by modifying the pyridyl group of N-(4pyridyl)isonicotinamide (4PINA) [34]. This included a di-N-oxide (4-((1-oxidopyridin-4-yl)carbamoyl) pyridine-1-oxide, diNO) and two mono-N-oxides, namely 4-(isonicotinamido) pyridine-1-oxide (PNO) and 4-(pyridin-4-ylcarbamoyl)pyridine-1-oxide (INO) with the N-oxide group at the aminopyridine and isonicotinoyl end, respectively. Mono-N-oxide amides were synthesized by reacting acid chlorides with corresponding amines in anhydrous DMF in the presence of triethylamine (Scheme 1a,b). The precipitates obtained for INO and PNO were washed with aqueous sodium bicarbonate solution followed by water to ensure the removal of salts. The di-N-oxide (diNO) was synthesized by oxidizing 4PINA with m-chloroperoxybenzoic acid (Scheme 1c) and the product was washed with water and dried. All compounds were characterized by NMR, HRMS, IR, and X-ray diffraction (single-crystal and powder).

Gelation Experiments
The gelation experiments for diNO, INO, and PNO were tested in various solvents (Table S1, Supplementary Materials). In a typical experiment, a 10.0 mg portion of the compound was heated in 1.0 mL of solvent (1.0 wt %) to get a clear solution and was cooled to room temperature. The hydrogel formation of these amides tested at 1.0 wt % indicated no gelation, which prompted us to increase the concentration to 4.0 wt %. The mixture was heated (80.0-90.0 • C) and on cooling to room temperature, gelation was observed only for the di-N-oxide (diNO) after 1 h, which was confirmed via inversion test. Crystalline materials were obtained for both the mono-N-oxides (PNO and INO). The gelation experiment performed with an equimolar mixture of PNO and INO resulted in concomitant crystallization of the individual compounds, which was confirmed by single-crystal X-ray diffraction (SCXRD). The gel strength of diNO was analyzed by sol-gel transition temperature (T gel ) and minimum gel concentration (MGC) experiments. The lowest concentration at which the gel was obtained was recorded as the minimum gel concentration (MGC). The MGC experiment of the hydrogel was performed in 1.0 mL of water by weighing different amounts of diNO gelator (30.0 to 50.0 mg) in a 7.0 mL vial. The mixture was heated and then cooled to room temperature, and the vial was left undisturbed for gel formation. After 24 h, the gel formation was confirmed by the inversion test and the MGC of the gelator was found to be 4.0 wt %.

Thermal Stability
The thermal stability of the gel network of diNO was evaluated using gel-to-solution transition temperature (T gel ) experiments, which is the critical temperature at which the gel converts to a solution. The experiment was performed by placing a small spherical glass ball (92.0 mg) on top of the diNO preformed hydrogel in a standard vial. The vial was heated and the temperature at which the ball touched the bottom of the vial was recorded as T gel . The experiments were performed at 4.0 and 6.0 wt % and the T gel did not vary much with the concentration of diNO, which was found to be 78.0 • C and 80.0 • C at 4.0 and 6.0 wt %, respectively.

Rheology
Rheological analysis was undertaken to determine the mechanical properties of the supramolecular hydrogels (4.0 wt %) [66,67]. The oscillatory amplitude sweeps, at a constant frequency (1.0 Hz), demonstrated a linear viscoelastic region (LVR) in which the storage (elastic) G' modulus was approximately an order of magnitude higher than the loss (viscous) G" modulus for both samples ( Figure 1a) [66,68]. Thus, both samples had a linear elastic response to low stress amplitudes. This behavior is indicative of a solid-like network throughout the sample gelling the aqueous solvent. At increased shear stresses, a stress softening behavior was observed, the viscous moduli (G") increased, and at a crossover point, there was a concomitant sharp decrease in the storage moduli (G') indicating a transition from an elastic gel to a viscous fluid [69,70]. At low shear stresses, 4PINA had an elastic modulus of approximately 140,000 Pa and a yield stress of approximately 500 Pa. diNO was a significantly softer gel with an elastic modulus of approximately 5000 Pa and was also weaker, with a yield stress of approximately 60 Pa. Rheological analysis was undertaken to determine the mechanical properties of the supramolecular hydrogels (4.0 wt %) [66,67]. The oscillatory amplitude sweeps, at a constant frequency (1.0 Hz), demonstrated a linear viscoelastic region (LVR) in which the storage (elastic) G' modulus was approximately an order of magnitude higher than the loss (viscous) G" modulus for both samples (Figure 1a) [66,68]. Thus, both samples had a linear elastic response to low stress amplitudes. This behavior is indicative of a solid-like network throughout the sample gelling the aqueous solvent. At increased shear stresses, a stress softening behavior was observed, the viscous moduli (G") increased, and at a crossover point, there was a concomitant sharp decrease in the storage moduli (G') indicating a transition from an elastic gel to a viscous fluid [69,70]. At low shear stresses, 4PINA had an elastic modulus of approximately 140,000 Pa and a yield stress of approximately 500 Pa. diNO was a significantly softer gel with an elastic modulus of approximately 5000 Pa and was also weaker, with a yield stress of approximately 60 Pa. Oscillatory frequency sweeps ( Figure 1b) at a fixed amplitude (0.05 Pa) demonstrated that G', arising from the elastic network, was higher than G" across all frequencies for 4PINA, which is characteristic of the presence of a solid-like network of filaments that are temporally persistent in the viscoelastic gel. These moduli were effectively indifferent to the frequency and this was true for diNO as well at low frequencies (<10 Hz). However, with increasing frequencies G' increased, the typical behavior of viscoelastic liquids rather than supramolecular gels [66], which further indicates that diNO formed weaker gels than 4PINA. Furthermore, the ratio of G':G'' is not at least an order of magnitude, the generally accepted ratio for supramolecular gels. Instead the ratio is roughly 7:1, further indicating that the modifications made to diNO severally affect its gelling ability. However, the phase angle, the lag between the applied shear stress and the measured strain, for diNO of approximately 15.0 δ is comparable to that of the original gelator, 4PINA, with a measured phase lag of approximately 7.0 δ. Moreover, the phase angle is not dependent on the frequency, a key Watson- Oscillatory frequency sweeps ( Figure 1b) at a fixed amplitude (0.05 Pa) demonstrated that G', arising from the elastic network, was higher than G" across all frequencies for 4PINA, which is characteristic of the presence of a solid-like network of filaments that are temporally persistent in the viscoelastic gel. These moduli were effectively indifferent to the frequency and this was true for diNO as well at low frequencies (<10 Hz). However, with increasing frequencies G' increased, the typical behavior of viscoelastic liquids rather than supramolecular gels [66], which further indicates that diNO formed weaker gels than 4PINA. Furthermore, the ratio of G':G" is not at least an order of magnitude, the generally accepted ratio for supramolecular gels. Instead the ratio is roughly 7:1, further indicating that the modifications made to diNO severally affect its gelling ability. However, the phase angle, the lag between the applied shear stress and the measured strain, for diNO of approximately 15.0 δ is comparable to that of the original gelator, 4PINA, with a measured phase lag of approximately 7.0 δ. Moreover, the phase angle is not dependent on the frequency, a key Watson-Chambon criterion for gels. Thus, as the diNO samples do not meet the moduli requirements but demonstrated other viscoelastic behavior typical of a physical gel, it can, with caveats, be considered a very weak gel.

Scanning Electron Microscopy (SEM)
The morphology of gel fibers was studied by analyzing the SEM images of dried diNO gel. The gel was prepared at 4.0 wt % in water, filtered after 24 h, and dried under a fume hood. A small portion of the dried gel was placed on a carbon tab and was coated with gold for 2 min. The SEM images revealed that the xerogel displayed a rod-type architecture with needle-shaped microcrystalline materials in the network. The diameter of the small needle-shaped fibers ranged from 1.2 to 2.5 µm and the width of the larger microcrystalline rods ranged from 8.0 to 30.0 µm ( Figure 2).

Crystal Structure
The crystallization experiments of N-oxides and an equimolar mixture of two mono-N-oxides were performed in water and various organic solvents. The compounds were proved to be sparingly soluble in most of the organic solvents, hence the experiments were carried out in a water or aqueous solution of methanol, ethanol, acetonitrile, and tetrahydrofuran. About 20.0 mg of the material were dissolved by heating in 1.0 mL of water, and the solution was cooled to room temperature. Crystallization from mixed solvents (1:1, v/v) such as methanol/water, ethanol/water, acetonitrile/water, and tetrahydrofuran/water resulted in precipitates. In some cases, needle-shaped crystals were obtained with low yield and the crystal quality was not good for single-crystal X-ray diffraction. The crystallization experiments performed in water produced X-ray quality single crystals for all N-oxides. The crystals of diNO were found to be needle-shaped, whereas block-shaped crystals were observed for both mono-N-oxides. Interestingly, crystallization of a 1:1 mixture of PNO and INO also resulted in block-shaped crystals.
The gelator diNO crystallized in a triclinic space group (Pī) with the carbonyl moiety of the amide group slightly deviated from the amide plane (15.74(8)°) and the pyridyl N-oxide rings were twisted at an angle of 58.71(3)° ( Figure 3a). The N-H moiety was involved in hydrogen-bonding interaction (N-H···O) with the oxygen atom of the adjacent molecule to form a dimer. This dimer was further stabilized by a strong π-π interaction (3.6563(9) Å) between the isonicotinoyl N-oxide ring of diNO ( Figure 3b) and also N-H···O interactions (2.8178(16) Å) between the amide N-H moiety and isonicotinoyl N-oxide moieties ( Figure S9, Supplementary Materials). The aminopyridine N-oxide moieties of the diNO dimer interact with the isonicotinoyl N-oxide moieties of adjacent dimers via offset π-π interactions (3.8745(9) and 3.8219(9) Å) and also display bifurcated C-H···O interactions (3.1789(18) and 3.1621(19) Å) with adjacent dimers (Figure 3c).

Crystal Structure
The crystallization experiments of N-oxides and an equimolar mixture of two mono-N-oxides were performed in water and various organic solvents. The compounds were proved to be sparingly soluble in most of the organic solvents, hence the experiments were carried out in a water or aqueous solution of methanol, ethanol, acetonitrile, and tetrahydrofuran. About 20.0 mg of the material were dissolved by heating in 1.0 mL of water, and the solution was cooled to room temperature. Crystallization from mixed solvents (1:1, v/v) such as methanol/water, ethanol/water, acetonitrile/water, and tetrahydrofuran/water resulted in precipitates. In some cases, needle-shaped crystals were obtained with low yield and the crystal quality was not good for single-crystal X-ray diffraction. The crystallization experiments performed in water produced X-ray quality single crystals for all N-oxides. The crystals of diNO were found to be needle-shaped, whereas block-shaped crystals were observed for both mono-N-oxides. Interestingly, crystallization of a 1:1 mixture of PNO and INO also resulted in block-shaped crystals.
The gelator diNO crystallized in a triclinic space group (Pī) with the carbonyl moiety of the amide group slightly deviated from the amide plane (15.74(8) • ) and the pyridyl N-oxide rings were twisted at an angle of 58.71(3) • (Figure 3a). The N-H moiety was involved in hydrogen-bonding interaction (N-H···O) with the oxygen atom of the adjacent molecule to form a dimer. This dimer was further stabilized by a strong π-π interaction (3.6563(9) Å) between the isonicotinoyl N-oxide ring of diNO ( Figure 3b) and also N-H···O interactions (2.8178(16) Å) between the amide N-H moiety and isonicotinoyl N-oxide moieties ( Figure S9, Supplementary Materials). The aminopyridine N-oxide moieties of the diNO dimer interact with the isonicotinoyl N-oxide moieties of adjacent dimers via offset π-π interactions (3.8745(9) and 3.8219(9) Å) and also display bifurcated C-H···O interactions (3.1789 (18)  Compound INO crystalized in a triclinic space group (Pī) and the twisting of the pyridyl Noxide ring (58.86°) was similar to diNO (Figure 4a). The hydrogen-bonding pattern was exactly the same as in diNO resulting in a dimer, which was stabilized by N-H···O and π-π interactions. The only difference between these two structures was the connection between adjacent dimers and the dimer in INO propagates via C-H···N interaction (3.4391 (16)    Compound INO crystalized in a triclinic space group (Pī) and the twisting of the pyridyl N-oxide ring (58.86 • ) was similar to diNO (Figure 4a). The hydrogen-bonding pattern was exactly the same as in diNO resulting in a dimer, which was stabilized by N-H···O and π-π interactions. The only difference between these two structures was the connection between adjacent dimers and the dimer in N-oxide was hydrogen-bonded to two water molecules via O-H···O interactions (2.793(3) and 2.796(2) Å), and the pyridyl nitrogen atom interacted with a water molecule through O-H···N interaction (2.771(2) Å). This resulted in the formation of a hydrogen-bonded macrocycle involving two INO molecules and four water molecules. The amide moieties of the macrocycle were hydrogen bonded to adjacent macrocycles via N-H···O (2.921(2) Å) interactions to form a hydrogen-bonded 3-D network.

X-ray Powder Diffraction (XRPD)
The phase purity of the compounds was further analyzed by comparing the XRPD pattern of the bulk crystals with the simulated pattern obtained from the single-crystal data. XRPD was performed on the bulk solid of recrystallized diNO from water and the xerogel obtained from water at 4.0 wt %. The powder X-ray pattern of the re-crystallized diNO matches perfectly well the simulated graph obtained from the single-crystal data ( Figure 6). However, the powder X-ray pattern of the xerogel did not match the calculated pattern from the crystal structure of diNO. The XRPD patterns of mono-N-oxides were performed with the recrystallized samples from water, which were filtered and dried in air. The XRPD pattern of PNO revealed that the pattern of bulk solid was virtually super-imposable on the simulated pattern of PNO ( Figure S11). The XRPD pattern of the bulk crystalline material of INO was also similar to the simulated pattern of INO ( Figure S12). We also performed the XRPD of the hydrated form of INO, but the pattern was different from the calculated pattern of the INO.2H2O

X-ray Powder Diffraction (XRPD)
The phase purity of the compounds was further analyzed by comparing the XRPD pattern of the bulk crystals with the simulated pattern obtained from the single-crystal data. XRPD was performed on the bulk solid of recrystallized diNO from water and the xerogel obtained from water at 4.0 wt %. The powder X-ray pattern of the re-crystallized diNO matches perfectly well the simulated graph obtained from the single-crystal data ( Figure 6). However, the powder X-ray pattern of the xerogel did not match the calculated pattern from the crystal structure of diNO. The XRPD patterns of mono-N-oxides were performed with the recrystallized samples from water, which were filtered and dried in air. The XRPD pattern of PNO revealed that the pattern of bulk solid was virtually super-imposable on the simulated pattern of PNO ( Figure S11). The XRPD pattern of the bulk crystalline material of INO was also similar to the simulated pattern of INO ( Figure S12). We also performed the XRPD of the hydrated form of INO, but the pattern was different from the calculated pattern of the INO.2H 2 O crystal structure ( Figure S13). Interestingly, the INO.2H 2 O bulk crystal pattern matched the simulated pattern of INO ( Figure S12).

Discussion
The ability of pyridyl amides to form a one-dimensional (1-D) hydrogen-bonded network is well established, which makes them ideal candidates in designing LMWGs with tunable properties. The availability of single-crystal X-ray structures of many organo/hydrogelators based on pyridyl amides enabled us to identify the key factors responsible for the formation of gel networks, for example, the formation of 1-D networks. The structural analysis of pyridyl amides indicates that these compounds self-assemble through mainly two types of hydrogen bonding, namely N-H···O synthon of the amide moiety and N-H···N synthon involving the amide and pyridyl nitrogen. These two types of interactions are predominant in the molecular aggregation of N-(4-pyridyl)isonicotinamide (4PINA) [34] along with various other non-bonding interactions. However, the impact of these types of interactions was not studied in detail. For example, what is the role of N-H···N synthon in gel network stabilization? What happens if these interactions are replaced by another synthon? We addressed this question by restricting the N-H···N synthon by modifying the pyridyl nitrogen to pyridyl-N-oxide moiety. Thus, we modified the pyridyl groups of 4PINA [34] to N-oxides; two mono-N-oxides were synthesized by oxidizing the nitrogen atoms at the pyridyl amine end (PNO) and the isonicotinic acid end (INO), and oxidation of both pyridyl nitrogen atoms resulted in diNO. This restricts N-H···N interactions between the gelators and hinders the one-dimensional growth of the gel fibril, which may facilitate the formation of either a two-dimensional (2-D) or 3-D network of gelator. Another advantage of pyridyl-N-oxides is the ease of crystallization in an aqueous medium, enabling us to compare the crystal structures of mono-N-oxides and di-N-oxide for structureproperty correlation.
The gelation properties of all the N-oxides in various solvents resulted in selective gelation of diNO in the water at higher concentration (4.0 wt %). The formation of needle-shaped crystals at lower concentration revealed that crystallization and gelation properties [71] were concentration dependent. The mono-N-oxides (PNO and INO) did not form gel in any solvents but formed crystals indicating the importance of similar groups at both ends in gel formation, which corroborates well with the gelation properties of 4PINA [34]. We also prepared a mixture of PNO and INO in different 10 20 30 40 2θ

Relative Intensity
Simulated Bulk crstyals Xerogels Figure 6. X-ray powder diffraction (XRPD) comparison of diNO: simulated pattern from single-crystal X-ray diffraction (SCXRD) data, bulk crystals, and xerogel obtained from water.

Discussion
The ability of pyridyl amides to form a one-dimensional (1-D) hydrogen-bonded network is well established, which makes them ideal candidates in designing LMWGs with tunable properties. The availability of single-crystal X-ray structures of many organo/hydrogelators based on pyridyl amides enabled us to identify the key factors responsible for the formation of gel networks, for example, the formation of 1-D networks. The structural analysis of pyridyl amides indicates that these compounds self-assemble through mainly two types of hydrogen bonding, namely N-H···O synthon of the amide moiety and N-H···N synthon involving the amide and pyridyl nitrogen. These two types of interactions are predominant in the molecular aggregation of N-(4-pyridyl)isonicotinamide (4PINA) [34] along with various other non-bonding interactions. However, the impact of these types of interactions was not studied in detail. For example, what is the role of N-H···N synthon in gel network stabilization? What happens if these interactions are replaced by another synthon? We addressed this question by restricting the N-H···N synthon by modifying the pyridyl nitrogen to pyridyl-N-oxide moiety. Thus, we modified the pyridyl groups of 4PINA [34] to N-oxides; two mono-N-oxides were synthesized by oxidizing the nitrogen atoms at the pyridyl amine end (PNO) and the isonicotinic acid end (INO), and oxidation of both pyridyl nitrogen atoms resulted in diNO. This restricts N-H···N interactions between the gelators and hinders the one-dimensional growth of the gel fibril, which may facilitate the formation of either a two-dimensional (2-D) or 3-D network of gelator. Another advantage of pyridyl-N-oxides is the ease of crystallization in an aqueous medium, enabling us to compare the crystal structures of mono-N-oxides and di-N-oxide for structure-property correlation.
The gelation properties of all the N-oxides in various solvents resulted in selective gelation of diNO in the water at higher concentration (4.0 wt %). The formation of needle-shaped crystals at lower concentration revealed that crystallization and gelation properties [71] were concentration dependent. The mono-N-oxides (PNO and INO) did not form gel in any solvents but formed crystals indicating the importance of similar groups at both ends in gel formation, which corroborates well with the gelation properties of 4PINA [34]. We also prepared a mixture of PNO and INO in different ratios to test the ability of these compounds to form multi-component gels. Such gels are obtained by mixing two or more compounds (gelator/non-gelator) where individual molecules interact either constructively or destructively to form well-ordered fibers containing individual components (self-sorting), both components (specific co-assembly), or a mixture of both (random co-assembly) [72,73]. We showed that mixing enantiomeric supramolecular gels based on bis(urea) compounds tagged with a phenylalanine methyl ester leads multi-component gels with enhanced thermal and mechanical strength [74]. However, the experiments performed with the mixtures of PNO and INO resulted in concomitant crystallization of individual compounds. INO was proved to be more soluble between the two mono-N-oxides leading to slow crystallization. The crystallization of the individual compounds suggests self-sorting of the components in the mixture.
The minimum gel concentration (MGC) of diNO (4.0 wt %) indicates that the gel network formed was weaker compared to 4PINA (MGC 0.37 wt %) [34]. This may be attributed to the fact that converting the pyridyl group of 4PINA to pyridyl-N-oxide increased the hydrophilic interactions preferring crystalline form, which hindered the gel formation at low concentration [75]. The gel-to-solution transition temperature (T gel ) experiments performed at MGC of diNO to evaluate the thermal stability of the gel revealed that the gel network collapsed at 78.0 • C. This indicates that the gel network of diNO is slightly weaker compared to 4PINA and the T gel of diNO network did not change drastically with concentration. This is in agreement with the rheological measurements that revealed that diNO gels were weaker compared to the 4PINA gelator. The rheology experiments enabled us to elucidate the information regarding factors controlling the gelation, gel strength, and the solid-like properties of pure gels, and the mechanical strength of the 4PINA and diNO gels was evaluated using rheology. The rheological measurements performed for both 4PINA and diNO gels revealed that diNO gels were significantly softer than 4PINA gels. At a low strain of 1.0%, both diNO and 4PINA samples had an effectively infinite shear stress relaxation time, typical of a temporally persistent supramolecular elastic network. On the other hand, at a higher strain of 10.0% the relaxation time for the 4PINA sample, which had sheared, now displayed a finite relaxation time and behaved like a viscous liquid ( Figures S14 and S15). The morphologies of the gel fibers were analyzed using SEM images of the dried gel, and the results indicated that a microcrystalline network was observed in diNO gels. The xerogels of 4PINA at a similar concentration displayed fibrous morphology with tape-type architecture with widths ranging from 0.57 to 5.7 µm [34], which was different from diNO gels. This is quite interesting because changing the pyridyl groups to N-oxide resulted in a drastic change in the morphology of the gel fibers. The stronger gelation properties of 4PINA may be attributed to the ability of 4PINA to immobilize the solvent molecules more efficiently in the fibrous network compared to the N-oxide network.
The crystallization experiments of the N-oxides performed in aqueous solutions of polar solvents resulted in either needle-shaped crystals or precipitate, which prompted us to explore the crystallization in pure water. The crystallization of INO and PNO in water resulted in blocked-shaped crystals in 3-5 h, but needle-shaped diNO crystals were formed in 1 to 2 days. The solid-state structures of the N-oxides were analyzed by single-crystal X-ray diffraction. The structure of N-oxides with both ends modified (diNO) was compared to the parent 4PINA structure and these two structures differ mainly in the type of non-bonding interactions. The crystal structure of 4PINA showed that the pyridyl nitrogen atoms and the amide moieties were hydrogen bonded via N-H···N interaction to form a 1-D hydrogen-bonded network. However, discrete dimers were formed in diNO by N-H···O hydrogen bonding, which was stabilized by π-π interactions. These dimers interact with each other via bifurcated C-H···O interactions and π-π interactions to form a 2-D supramolecular structure. The comparison of the intermolecular interactions in both the structures indicated that the N-H···N interaction in 4PINA is replaced by bifurcated C-H···O interactions in diNO. This explains why 4PINA is a good gelator, which indicates the importance of N-H···N interaction in the formation of supergelators. The phase purity of the N-oxide compounds was analyzed by comparing the diffraction pattern of the crystal structures with the powder X-ray pattern of the bulk materials. The analysis of the pattern revealed that the recrystallized form of diNO matched the single-crystal structure of diNO, indicating an identical structure. However, the XRPD pattern of the xerogels of diNO did not match the pattern obtained from the crystal structure of diNO. This is presumably due to the presence of hydrogen-bonded solvent molecules in the xerogels. The XRPD data of the bulk materials of The structure analysis of PNO revealed the presence of two water molecules in the crystal structure, which helped the molecule to retain the molecular planarity. The water molecules play a crucial role in overall packing of the crystal, and a hexagonal architecture of oxygen atoms was formed by the hydrogen bonding between PNO and the water molecules. The structural analysis of 4PINA and the N-oxides confirm the importance of the non-bonding interactions and the presence of identical groups at both ends of the compounds in gel formation.

Chemicals and Reagents
All starting materials and solvents were purchased from Sigma-Aldrich (MEDOR ehf, Reykjavik, Iceland) and were used as supplied. Deionized water was used for all the experiments and anhydrous methanol was obtained by distilling the solvent over Mg turnings and iodine. 4-aminopyridine-1-oxide [76] and N-(4-pyridyl)isonicotinamide (4PINA) [34] were synthesized following the reported procedures. 1 H NMR and IR spectra were recorded on a Bruker Advance 400 spectrometer (Rheinstetten, Germany) and a Nicolet iN10 (Thermo Fisher Scientific, Hvidovre, Denmark), respectively. Single-crystal X-ray diffraction (SCXRD) was performed on a Bruker D8 VENTURE (Karlsruhe, Germany), and X-ray powder diffraction (XRPD) was carried out using a Bruker D8 Focus instrument (Karlsruhe, Germany). The morphology of the xerogel was analyzed by scanning electron microscopy (SEM) using a Leo Supra 25 Microscope (Carl Zeiss, Oberkochen, Germany).

Synthesis
General procedure for INO and PNO: The acid chloride was prepared by stirring the carboxylic acid (10.0 mmol) with 5.0 mL of thionyl chloride overnight in a round-bottom flask at 65.0 • C. The pale-yellow solution obtained was cooled and the excess thionyl chloride was removed by distillation. The resulting white solid/crystal was transferred to a 100 mL two-neck RB flask containing 4-aminopyridine/4-aminopyridine-1-oxide (10.0 mmol). A pale yellowish solution was obtained by adding 20.0 mL anhydrous DMF to the flask, the solution was cooled to 0 • C, and triethylamine (1.5 mL, 10.8 mmol) was added dropwise to this solution. The resulting mixture was stirred overnight at room temperature and the solution was added to a beaker containing 200 mL diethyl ether resulting in a white suspension. The mixture was filtered, and the residue was stirred with 4.0% NaHCO 3 for 5 h, filtered, and washed with cold water. The resulting white solid was recrystallized from water to obtain the desired product.

Gelation Studies
Gelation experiments were initially carried out by weighing 10.0 mg of the N-oxide ligands (INO, PNO, diNO, and an equimolar mixture of INO and PNO) in a 7.0 mL vial and 1.0 mL of distilled water was added to the compounds. The mixtures were heated until a clear solution was obtained; cooling to room temperature resulted in no gelation in any case after 24 h. Since the compounds were soluble only in water, gelation experiments were carried out in aqueous solution of organic solvents. An aqueous solution (1:1, v/v) was prepared by mixing 500 µL of organic solvents (methanol, ethanol, tetrahydrofuran, acetonitrile, or nitrobenzene) with 500 µL of water. These solutions were added to 10.0 mg of the N-oxide compounds and the mixtures were heated; cooling to room temperature resulted in a white precipitate after 24 h in all cases. The higher solubility of the compounds in water compared to other solvents prompted us to check the gelation experiments at higher concentration. Crystals were obtained when the gelation experiments were performed in 1.0 mL water with 20.0 mg compounds (2.0 wt %). The gelation ability tested in mixed solvents at 2.0 wt % mostly produced precipitate, whereas some crystalline materials were observed in a few experiments (in methanol/water).
The gelation experiments at higher concentration (4.0 wt %) also resulted in crystals for INO, PNO, and their equimolar mixture but an opaque gel was formed under identical conditions (confirmed by inversion test) for diNO. The gelation experiments at higher concentration in a mixed solvent were avoided due to insolubility of these materials. The solubility of diNO was found to be approximately 60.0 mg/mL in water, and the solution formed gel at concentrations between 4.0 and 6.0 wt %. The two mono-N-oxides and their equimolar mixture at high concentration (>4.0 wt %) were dissolved in boiling water, which readily formed crystals on cooling.

Minimum Gel Concentration (MGC)
The MGC was performed by taking various amounts (30.0 to 50.0 mg) of diNO in a standard 7.0 mL vial and the vial was sealed after adding 1.0 mL of distilled water. The mixtures were heated to dissolve the compound and left undisturbed to form gel. The minimum concentration at which gel was obtained was noted as MGC. The MGC of diNO was found to be 4.0 wt % in pure water.

T gel Experiments
The gel-to-solution transition temperature (T gel ) experiment was performed at two different concentrations (4.0 and 6.0 wt %) of diNO. The required amount (40.0 or 60.0 mg) of diNO was placed in a 7.0 mL vial and 1.0 mL of water was added. The vial was sealed, and the mixture was heated to obtain a clear solution and cooled to room temperature to form the gel. After 24 h, a small spherical glass ball (92.0 mg) was carefully placed over the gel. The vial was gradually heated in an oil bath equipped with a magnetic stirrer and a thermometer. The temperature at which the glass ball touched the bottom of the vial was recorded as T gel . The T gel of diNO was found to be 78.0 • C and 80.0 • C at 4.0 and 6.0 wt %, respectively.

Rheology
The rheology experiments were performed at Durham University using a TA Instruments AR 2000 (New Castle, DE, USA) fitted with a rough Peltier plate and a 25 mm rough plate geometry (gap width of 2500 µm) at 25 • C. Supramolecular gels were prepared as discussed previously at a concentration of 4.0 wt %. The top plate was lowered and the normal force was allowed to reach equilibrium. Oscillatory amplitude sweeps were performed at a constant frequency (1.0 Hz) and the oscillatory frequency sweeps were performed at a constant shear stress (0.5 Pa). These experiments were performed in triplicate and the mean moduli and standard deviation were calculated.

Scanning Electron Microscopy (SEM)
The gelator diNO (40.0 mg) was dissolved in 1.0 mL of water by heating and cooled to room temperature to form the gel. After 24 h, the yellowish opaque gel was filtered through a filter paper and the residue was air dried in a fume hood. The xerogels were gold-coated for 2 min and SEM was performed using a Leo Supra 25 Microscope.

Crystallography
X-ray quality single-crystals were isolated from water, immediately immersed in cryogenic oil, and mounted. The diffractions were collected using MoK α radiation (λ = 0.71073 Å) on a Bruker D8 VENTURE (Photon100 CMOS detector) diffractometer equipped with a Cryostream (Oxford Cryosystems, Oxford, UK) open-flow nitrogen cryostat at room temperature. The unit cell determination, data collection, data reduction, structure solution/refinement, and empirical absorption correction (SADABS) were carried out using Apex III (Bruker AXS, Madison, WI, USA). The structure was solved by a direct method and refined by the full-matrix least-squares on F 2 for all data using SHELXTL [77] (Version 2017/1, University of Göttingen, Göttingen, Germany) and Olex2 [78] (Version 1.2, OlexSys Ltd., Durham, U.K.) software. All non-disordered non-hydrogen atoms were refined anisotropically, and the hydrogen atoms were placed in the calculated positions and refined using a riding model.

X-ray Powder Diffraction (XRPD)
The N-oxide compounds (20.0 mg) were dissolved in 1.0 mL of hot water and left undisturbed for crystallization. After crystallization, the mixture was filtered and the residue was dried in air. The crystals were ground to fine powder and XRPD was carried out using a Bruker D8 Focus instrument. The xerogel of diNO was prepared by heating 40.0 mg of the compound in 1.0 mL of water (4.0 wt %), and the gel formed was filtered after 24 h followed by drying the residue overnight in a fume hood.

Conclusions
We synthesized two mono-N-oxides (INO and PNO) and a di-N-oxide (diNO) by altering the pyridyl moieties of a known hydrogelator (4PINA) to N-oxide moieties. The gelation ability of these compounds tested in various solvent/solvent mixtures indicated that selective gelation of diNO was observed in water. The mechanical strength and the thermal stability of diNO were evaluated using rheology and gel-to-solution transition temperature (T gel ) experiments, respectively. The gelation properties of diNO were compared with the parent 4PINA gelator, which indicated that diNO is a weaker gelator. SEM images were used to visualize the changes in the morphology of the gel fibers due to the alteration in the functional groups. The effect of various non-bonding interactions in the crystalline state and dried gel was studied using X-ray diffraction. Single-crystal X-ray analysis of the di-N-oxide structure revealed the formation of a hydrogen-bonded dimer, which interacted with adjacent dimers via C-H···O interactions to form the extended network, and similar dimers observed in non-gelator INO were interconnected through C-H···N interaction. The strong interaction of the mono-N-oxide with water molecules indicates the increased hydrophilic nature of these compounds to form solvated crystals. The reduced gelation ability (MGC) and thermal strength of diNO may be attributed to the weak intermolecular C-H···O interaction. The absence of strong N-H···N interactions in the N-oxide compounds compared to 4PINA clearly indicate the role of these interactions in gel formation. The tuning of gelation ability (MGC) and thermal/mechanical strength of 4PINA by alerting the functional group proves the importance of non-bonding interactions and functional groups in designing LMWGs with tunable properties, which contributes to the ongoing efforts to identify the key structural features of gel network formation.  Figure S16: Amplitude and frequency sweeps, Scheme S1: Chemical structure of 4PINA, Table S1: Gelation table of N-oxide compounds, Table S2: Determination of MGC of diNO, Table S3: Determination of T gel of diNO, Table S4: Crystal data for the N-oxide compounds.