Pyrene-Based Co-Assembled Supramolecular Gel; Morphology Changes and Macroscale Mechanical Property

Two pyrene derivatives having the perylenediimide (1) or the alky chain (2) in the middle of molecules were synthesized. Co-assembled supramolecular gels were prepared at different molar ratios of 0.2, 0.5, and 0.8 equiv. of 2 to 1. By SEM observation, the morphology of co-assembled supramolecular gels changed from spherical nanoparticles to three-dimensional network nanofibers as the ratio of 2 increased. In addition, the pyrene-excimer emission of co-assembled gels increased with increasing concentration of 2, and was stronger when compared with the condition without 1 or 2, indicating the formation of pyrene interaction between 1 and 2. In addition, the sol-gel transition was found to be reversible over repeated measurement by tube inversion method. The rheological properties of co-assembled supramolecular gels were also improved by increasing the ratio of 2, due to the increased nanoscale flexibility of supramolecular packing by introducing alkyl chain groups through heterogeneous pyrene interaction. These findings suggest that macroscale mechanical strength of co-assembled supramolecular gel was strongly influenced by nanoscale flexibility of the supramolecular packing.

Recently, researchers have been investigating supramolecular gels composed of two or more molecule types, i.e., co-assembled supramolecular gels, to control the resulting physical properties [20,21]. As the two or more molecules co-assemble to form a three-dimensional network, their physical properties can be more easily controlled by varying the composition ratio of the components. In co-assembled supramolecular gels, the molecular arrangement of co-assembly system plays a vital role in optical, mechanical properties. Therefore, the design of building blocks that enable morph coassembled gels is important for developing functional soft materials.
Fluorescent gels are also useful as soft materials that be made of small molecular gelators though the supramolecular assembly of a well-defined complex of molecules [22][23][24]. Hydrogels, in particular, can potentially be a useful three-dimensional scaffold for tissue engineering due to their flexibility and water retention ability, similar to biological tissues [25,26]. Such gels differ from conventional hydrogels or organogels in their light emitting properties, but retain their extended networks that can be extensively swollen with water or organic solvents. In addition, fluorescent gels that possess several unique features are potential applicable in fields of sensors [11,12], optical and electrical devices, imaging agents [22], in biomedicine or for memory and display devices [4][5][6][7].
The various small molecular fluorescent dyes such as perylenediimide (PDI) [27][28][29], tetraphenylethene (TPE) [30], thienoviologen [31], lanthanides transition metal ions [32][33][34], and quantum dots [35,36], are excellent emitters with intense fluorescence and small molecular sizes that facilitate conjugation to the bulk gel network. Among fluorescent moieties, pyrene and its derivatives have been extensively applied as probes for molecular aggregation in polymer solutions, micelles, liquid crystals, and supramolecular gels [37,38]. Pyrene, representative of π-conjugated moieties with high fluorescence and unique emission properties, have been deemed as suitable candidates for investigating molecular arrangements and co-assembled nanomaterials [39], due to their ability of excimer formation in both solution and solid state [40]. Thus, we report control of morphology and mechanical property of co-assembled supramolecular gels by varying the ratio of pyrene derivatives having a perylenediimide (1) or an alkyl chain group (2).

Results and Discussion
Two pyrene derivatives having a perylenediimide (1) or an alkyl chain (2) were synthesized by a four-and one-step synthesis, respectively (Scheme S1 and Figure 1) The pyrene groups of compounds 1 and 2 were used as a linker to excimer formation in supramolecular gels. To increase flexibility in supramolecular gels, alkyl chain groups were introduced into compound 2. These compounds were characterized by 1 H NMR spectroscopy, high-resolution ESI mass spectroscopy, and IR spectroscopy (Figures S1-S5). The gelation ability of pyrene derivatives having a perylenediimide (1) at 0.5 equivalent of 2 was evaluated in various kinds of organic solvents. The gelation behavior was summarized in Table 1. A perylenediimide derivative 1 showed good solubility in chloroform resulting in no gel formation ( Figure S6). On the other hand, formation of gel 1 in DMSO was confirmed at different equivalents of 2. It was found out that compound 1 could be gelated DMSO at different molar ratios: (a) 1: 0.2, (b) 1: 0.5, and (c) 1: 0.8. This induced co-assembly by forming a pyrene-excimer between the pyrene groups, thus causing the transformation into a stable gel, as shown in Figure 2. The co-assembled supramolecular gels formed within 16 h, 30, and 80 min from the (a), (b), and (c) molar ratios, respectively. The gelation time was shortest when 0.5 equiv. of 2 was added to 1 solution, because the well-organized molecular arrangement to form excimer structure in co-assembled gel. In contrast, single-component solutions of either 1 or 2 did not form gel ( Figure S7). These findings indicate that the formation of supramolecular gels was strongly dependent to the molar ratio between 1 and 2. The gelation ability of pyrene derivatives having a perylenediimide (1) at 0.5 equivalent of 2 was evaluated in various kinds of organic solvents. The gelation behavior was summarized in Table 1. A perylenediimide derivative 1 showed good solubility in chloroform resulting in no gel formation ( Figure S6). On the other hand, formation of gel 1 in DMSO was confirmed at different equivalents of 2. It was found out that compound 1 could be gelated DMSO at different molar ratios: (a) 1: 0.2, (b) 1: 0.5, and (c) 1: 0.8. This induced co-assembly by forming a pyrene-excimer between the pyrene groups, thus causing the transformation into a stable gel, as shown in Figure 2. The co-assembled supramolecular gels formed within 16 h, 30, and 80 min from the (a), (b), and (c) molar ratios, respectively. The gelation time was shortest when 0.5 equiv. of 2 was added to 1 solution, because the well-organized molecular arrangement to form excimer structure in co-assembled gel. In contrast, single-component solutions of either 1 or 2 did not form gel ( Figure S7). These findings indicate that the formation of supramolecular gels was strongly dependent to the molar ratio between 1 and 2. The tube inversion method with heating and cooling of co-assembled gel provided confirmation of reversible sol-gel transition. Co-assembled gel 1 seemed to break down from 119 • C at molar ratios [2]/[1] = 0.8. The sol-gel transition temperature of co-assembled gel in the presence of 0.8 equiv. of 2 was highest than those presence of 0.5 equiv. of 2 ( Figure S8). Furthermore, the sol-gel transition was found to be reversible over repeated measurements. Gels 2020, 6, x FOR PEER REVIEW 3 of 9 The tube inversion method with heating and cooling of co-assembled gel provided confirmation of reversible sol-gel transition. Co-assembled gel 1 seemed to break down from 119 ℃ at molar ratios [2]/[1] = 0.8. The sol-gel transition temperature of co-assembled gel in the presence of 0.8 equiv. of 2 was highest than those presence of 0.5 equiv. of 2 ( Figure S8). Furthermore, the sol-gel transition was found to be reversible over repeated measurements. To demonstrate optical property, the co-assembled supramolecular gels of 1 + 2 in DMSO (1 wt%) were investigated by UV-vis and luminescence spectroscopy. The absorption spectrum of the co-assembled supramolecular gels centered at 345 nm, corresponding to the typical UV absorption band of pyrene ( Figure S9) [41,42]. As the molecular ratio of 2 increased, no significant changes occurred in the absorption spectra, including that of the pyrene absorption. In contrast, the absorbance at 491 and 530 nm decreased with an increase in the concentration of 2, those being the characteristic absorption bands of the PDI moiety of 1 [43,44].
The luminescence spectra of the co-assembled supramolecular gels with excitation at 345 nm were also observed; the results are shown in Figure 3. As the molecular ratio of 2 increased, the luminescence intensity of co-assembled gels increased at 480 nm, indicating the formation of the pyrene-excimer. A weaker luminescence intensity change at the wavelength of the pyrene-excimer emission was observed in the luminescence spectra of the single-component solutions than in the coassembled supramolecular gel under the same conditions, as shown in ( Figure S10). These results suggest that a pyrene-excimer emission was formed with interaction between pyrene (1) and pyrene (2) in co-assembly process [45][46][47]. To demonstrate optical property, the co-assembled supramolecular gels of 1 + 2 in DMSO (1 wt%) were investigated by UV-vis and luminescence spectroscopy. The absorption spectrum of the co-assembled supramolecular gels centered at 345 nm, corresponding to the typical UV absorption band of pyrene ( Figure S9) [41,42]. As the molecular ratio of 2 increased, no significant changes occurred in the absorption spectra, including that of the pyrene absorption. In contrast, the absorbance at 491 and 530 nm decreased with an increase in the concentration of 2, those being the characteristic absorption bands of the PDI moiety of 1 [43,44].
The luminescence spectra of the co-assembled supramolecular gels with excitation at 345 nm were also observed; the results are shown in Figure 3. As the molecular ratio of 2 increased, the luminescence intensity of co-assembled gels increased at 480 nm, indicating the formation of the pyrene-excimer. A weaker luminescence intensity change at the wavelength of the pyrene-excimer emission was observed in the luminescence spectra of the single-component solutions than in the co-assembled supramolecular gel under the same conditions, as shown in ( Figure S10). These results suggest that a pyrene-excimer emission was formed with interaction between pyrene (1) and pyrene (2) in co-assembly process [45][46][47]. The morphologies of the co-assembled supramolecular gels were investigated using SEM and TEM; resulting images are shown in Figures 4A and S11. The morphology of the co-assembled supramolecular gel at molar ratio = 0.2 exhibited a small spherical nanoparticle with a diameter of ca. 60 nm ( Figure 4A (a)). At the middle 2:1 molar ratio studied, 0.5, spherical particles connected by a three-dimensional network were formed (Figures 4A (b) and S11b). At 0.8 equiv. of 2, the resulting gel was a 3D network of nano-fibrous structures, as shown in Figures 4A (c) and S11b. Distinct different morphologies were observed in the single-component solutions under the same conditions as supramolecular gels (Figures S12 and S13). Thus, an increase in the ratio of compound 2 would improve the flexibility of molecules in the supramolecular gel, thus leading to a 3D network of nanofiber forms of supramolecular nanostructures. The rheological properties of the co-assembled supramolecular gels were then examined via rheometer measurements to determine the storage (G′) and loss (G″) moduli. Strain sweep, frequency sweep, and continuous step strain tests were conducted for three of the co-assembled supramolecular gels at 0.2, 0.5, and 0.8 equiv. of 2 ( Figures 4B, S14, S15 and S16). The results indicated that the moduli values gradually increased with an increasing concentration of 2. G′ was greater than G″ when γ The morphologies of the co-assembled supramolecular gels were investigated using SEM and TEM; resulting images are shown in Figure 4A and Figure S11. The morphology of the co-assembled supramolecular gel at molar ratio = 0.2 exhibited a small spherical nanoparticle with a diameter of ca. 60 nm ( Figure 4A (a)). At the middle 2:1 molar ratio studied, 0.5, spherical particles connected by a three-dimensional network were formed ( Figure 4A (b) and Figure S11b). At 0.8 equiv. of 2, the resulting gel was a 3D network of nano-fibrous structures, as shown in Figure 4A (c) and Figure  S11b. Distinct different morphologies were observed in the single-component solutions under the same conditions as supramolecular gels (Figures S12 and S13). Thus, an increase in the ratio of compound 2 would improve the flexibility of molecules in the supramolecular gel, thus leading to a 3D network of nanofiber forms of supramolecular nanostructures. The morphologies of the co-assembled supramolecular gels were investigated using SEM and TEM; resulting images are shown in Figures 4A and S11. The morphology of the co-assembled supramolecular gel at molar ratio = 0.2 exhibited a small spherical nanoparticle with a diameter of ca. 60 nm ( Figure 4A (a)). At the middle 2:1 molar ratio studied, 0.5, spherical particles connected by a three-dimensional network were formed (Figures 4A (b) and S11b). At 0.8 equiv. of 2, the resulting gel was a 3D network of nano-fibrous structures, as shown in Figures 4A (c) and S11b. Distinct different morphologies were observed in the single-component solutions under the same conditions as supramolecular gels (Figures S12 and S13). Thus, an increase in the ratio of compound 2 would improve the flexibility of molecules in the supramolecular gel, thus leading to a 3D network of nanofiber forms of supramolecular nanostructures. The rheological properties of the co-assembled supramolecular gels were then examined via rheometer measurements to determine the storage (G′) and loss (G″) moduli. Strain sweep, frequency sweep, and continuous step strain tests were conducted for three of the co-assembled supramolecular gels at 0.2, 0.5, and 0.8 equiv. of 2 (Figures 4B, S14, S15 and S16). The results indicated that the moduli values gradually increased with an increasing concentration of 2. G′ was greater than G″ when γ The rheological properties of the co-assembled supramolecular gels were then examined via rheometer measurements to determine the storage (G ) and loss (G") moduli. Strain sweep, frequency sweep, and continuous step strain tests were conducted for three of the co-assembled supramolecular gels at 0.2, 0.5, and 0.8 equiv. of 2 ( Figure 4B, Figures S14, S15 and S16). The results indicated that the moduli values gradually increased with an increasing concentration of 2. G was greater than G" when γ (G":G ) was less than 0.1 % ( Figure 4B). In contrast, when γ exceeded 0.1%, G decreased more than G", indicating the breakdown of the gel network. This tendency suggested that the co-assembled supramolecular gels slid before breakdown. The performed frequency sweeps revealed a predominantly elastic character (G > G") with almost constant G and G" over the entire tested frequency range (0.6283-6283 rad s −1 ), demonstrating the quasi-solid property of the gels (Figures S14, S15, and S16A). A gel-to-quasi-liquid transition was reversible because G and G" recovered quickly within 30 s when γ was reduced from 100% to 0.1%. When the amplitude oscillatory force was decreased at the same frequency (γ = 0.1% at 1.0 Hz), G rapidly increased, and the system returned to a quasi-solid state (tan δ = G"/G ≈ 0.26). Thus, the co-assembled supramolecular gel exhibited a rapid thixotropic response ( Figure S16B). The recovery process involved the regeneration of the gel network. Significantly larger storage and loss moduli were observed in the resulting gel at higher molar ratios of 2. These results suggest that introduction of nanoscale flexibility inside supramolecular gel enhanced macroscale mechanical strengths in co-assembly process.

Conclusions
In this work, we have demonstrated that morphology and mechanical property of co-assembled gels were dependent on the composition ratio of building blocks. The largest luminescence intensity of co-assembled gel was observed in a mixed solution when 0.8 equiv. of 2 was added to solution 1, due to the well-organized molecular arrangement of excimer. The morphology of co-assembled supramolecular gels changed from spherical nanoparticles to three-dimensional network nanofibers as the ratio of 2 increased. In addition, the mechanical properties of co-assembled supramolecular gels was controllable by modification of the ratio of compound [2]: [1], due to the formation of pyrene-excimer and increased flexibility of supramolecular packing structure. Thus, further development of co-assembly supramolecular gels using the formation of pyrene-excimer can offer materials for optical applications and dual-functional gels by implementing functional derivatives.

Reagents and Instruments
Unless otherwise noted, chemical reagents and solvents were purchased from commercial suppliers (Tokyo Chemical Industry, Tokyo, Japan, and Sigma-Aldrich, St. Louis, MO, USA), and used without further purification. The NMR spectra for 1 H and 13 C were taken on a Bruker DRX 300, and mass spectroscopy samples were observed using a JEOL (JMS-700, JEOL, Tokyo, Japan) mass spectrometer. A UV-visible spectrophotometer (Evolution 600, Thermo scientific, Waltham, MA, USA) was used to obtain the absorption spectra. IR spectra were observed over the range 500-4000 cm −1 using a Thermo scientific Nicolet iS10 infrared spectrometer. The fluorescence spectra were obtained using a RF-5301PC spectrophotometer (Shimadzu, Kyoto, Japan).

FE-SEM Observation
FE-SEM imaging was performed with a JEOL (JSM-7610F, JEOL, Tokyo, Japan) using an accelerating voltage 5 kV and an emission current of 8 µA. Samples were prepared by dropping dilute solutions of xero gel on a glass, then drying, and coating it with a thin layer of Pt to increase the contrast.

Rheological Properties
Rheological testing of the prepared gels was carried out using an AR-2000ex (TA Instruments Ltd, Newcastle, DE, USA) implemented with a 40-mm diameter parallel plate attached to a transducer. The gap in the setup for rheological testing of the gels was 1.0 mm and the experiments were conducted at 298 K. Strain sweep tests were performed with increasing amplitude oscillation up to 10% apparent strain on shear. Frequency sweeps were performed from 5 to 1000 Hz. The recovery properties of the gels in response to the applied shear force were investigated over 1500 s: 0.01% (300 s), then 1% (300-600 s), then 0.01% (600-900 s), then 1% (900-1200 s), and finally 0.01% (1200-1500 s).

Preparation of Co-Assembled Supramolecular Gel
Co-assembled supramolecular gels (16.54 mM, 1 wt%) were prepared at different molar ratio of 0.2, 0.5, and 0.8 equiv. of 2 to 1. Compound 1 was placed in a 5 mL vial, where 2 (0.2, 0.5, and 0.8 equiv.) was added and then dissolved in DMSO (300 uL). The mixture was sonicated in a bath for a few seconds and heated until a clear solution was obtained. The samples were then maintained at room temperature to form the co-assembled supramolecular gel.