Tunable self-assembly of two-component gels from novel sorbitol-appended compounds

Abstract

A smart two-component gelator system was prepared by a combination of two good gelators based on sorbitol-appended compounds. The obtained two-component gels exhibit an effective thermal stabilization and dramatic gelation properties, which can be tuned by changing the proportion of gelators.

---

Supramolecular gels are nanostructured soft materials formed by low-molecular-weight organogelators by the sole action of non-covalent intermolecular interactions (solvophobic, ionic, H-bonding, π–π stacking, van der Waals, metal coordination, etc.).^1–4 Simple low-molecular-weight molecules are prepared by conventional solution synthesis and provided with assembling and functional groups.⁵ These molecules will self-assemble to form a fine fibrous structure (elemental fibers). Elemental fibers can be bundled together to form a micrometer scale cylinder that after physical cross-linking will form a spanned network that percolates the solvent, leading to the formation of a soft solid-like material called gel.^6–9 Gels have been classified in different ways based upon their origin, their constitution, the type of cross-linking, and the medium. Low molecular mass organogelators include carbohydrates, peptides, steroids, sugar, etc.¹⁰

Among the low molecular mass organogelators studied, the two-component gelators endow gel-phase materials with additional level of control over the self-assembly process.^11–14 Indeed this level of control is difficult to replicate in one-component gelation systems. In two-component systems, either one of the two components can be subtly modified to generate new morphologies and tune the materials behaviour. Furthermore, it is possible to tune the nanoscale architectures by altering the ratio of the two components.¹⁵

In two-component gels, one of the individual components can be present in an isotropic solution, and the addition of the second component will initially form a complex with the first component, and only then can further assembly into the network structure take place.^16,17 Moreover, it should be noted that gelation system containing a gelator and a second component that modifies the gelation process can also be regarded as two-component gels.¹⁸ Typically, in these two-component systems, self-assembly relies on hydrogen bonding, metal–ligand interactions and charge-transfer, etc. For example, Hanabusa¹⁹ presented the first report on the two-component gel based on the well-known hydrogen bonding interactions between barbituric acid and pyrimidine units. Maitra²⁰ reported systems in which a variety of pyrene derivatives were found to form strongly colored gels in the presence of an equivalent amount of 2,4,7-trinitrofluorenone. George²¹ presented the alternate coassembly of a noncovalent donor–acceptor amphiphilic pair in water resulting in very long nanofibers that form hydrogels. Dubey²² have investigated the A–π–D-chiral-D–π–A type L-tartaric acid-based nitrobenzylidenes with intent to achieve isomer specific metallogels with intramolecular charge transfer properties.

It is shown that the initial co-operation on a molecular scale between two components is important for a new self-assembling gelation system.^23–26 In this sense, even the use of two gelators can build a two-component gel. Recently there have been a number of publications focused on the two gelator component systems that can be achieved by the mixing of two gelators.^27–30 In such system the two gelators commonly involve similar binding motifs and differ only slightly in structure, where the similar binding motifs is assigned to control the mixed-assembly between the gelators, while the different binding motifs will tend to exhibit a preference for self-sorting.^31–33 Therefore, control over these two gelator component systems becomes difficult, with relatively few examples reported.^34–38 In the present work, we demonstrate novel two-component gels containing two good gelators based on sorbitol moieties, which can gel a range of organic solvents. Interestingly, the present study revealed that the gelation properties of the two-component gel can be significantly modulated by changing the proportion of gelators, and in some cases enhanced.

The results of the gelation studies are summarized in Table 1. Gelator 1 can gel both alcoholic solvents, as well as aromatic solvents, and is also a very good gelator for nonalcoholic polar solvents, while it has a tendency to form precipitates in aliphatic solvents. Gelator 2, on the other hand, formed weak gel only in n-octanol and acetonitrile. However, mixtures of 1 and 2 can gelate several solvents, such as n-butanol, n-octanol, n-butyl acetate and acetonitrile. Interestingly, the individual gelator formed a colorless gel, whereas once they were mixed together, the color changes to yellow (as shown in Fig. 1). Such a behavior was still observed after several cycles of heating (gel melting) and cooling (gel formation), indicating the reversibility of this process. Moreover, ¹H NMR spectra of 1 and 2 also proved that the chemical structure of the two gelators had not changed in the heating and cooling cycles (no shown). To confirm that the benzene ring attached directly to the electron-withdrawing nitro group and the electron-donating amino group in D and A is indeed critical for the charge-transfer interaction, we tested the gelation of the 3 + 1 pair and observed gelation without appearance of any color (Fig. 1d), suggesting that the color change for the gelation of 2 + 1 can be ascribed to occurrence of co-stacking between the nitrobenzene group and the aminobenzene group in the gel fibers.

Table 1 Results of the gelation studies for different gelators in various solvents^a

Solvent	Test results
	1	2	3	2 + 1	3 + 1
a G, PG, S and P stand for gel, partial gel, solution and precipitation, respectively.b A yellow gel.
Methanol	G	P	P	P	G
Ethanol	G	P	G	P	G
n-Butanol	G	P	G	G^b	G
n-Octanol	G	G	G	G^b	G
Cyclohexane	P	P	P	P	P
n-Hexane	P	P	P	P	P
Ethyl acetate	G	P	P	P	P
n-Butyl acetate	G	P	P	G^b	G
Acetonitrile	G	G	P	G^b	G
Toluene	G	P	P	P	P
o-Xylene	G	P	P	G	G
Water	G	S	S	PG	PG
DMF	S	S	S	S	S
DMSO	S	S	S	S	S
THF	S	S	S	S	S
CH2Cl2	P	P	P	P	P

---

Fig. 1 Left: Structure of sorbitol-appended gelators. Right: Photographs of gels in n-octanol derived from 1, 2, 3, 2 + 1 (1 : 1 molar ratio), 3 + 1 (1 : 1 molar ratio). Total gelator concentration = 2 wt% in each case.

Unexpectedly, in several cases (3 + 1/methanol, 3 + 1/butyl acetate, 3 + 1/o-xylene, 3 + 1/acetonitrile), good gels were obtained. This inspired us to use gelator 1 as a modulator that can tune gelation properties of the two-component system. In order to prove this assumption, the samples of two-component gels formed by 2 + 1 and 3 + 1 were prepared and the thermal stability of these gels was measured using DSC (Fig. S1†). A plot of T_gel vs. the molar fraction of 1 is given in Fig. 2. It should be noted that maximum thermal stability of the two-component gel from 2 + 1 is achieved at a 1 : 1 molar ratio of the two gelators. This confirmed that the 1 : 1 stoichiometric requirement of the donor–acceptor interactions between 1 and 2 induced stabilization of the two-component gelation. The T_gel for 3 + 1 is strongly dependent on the concentration of 1 and can increase as the concentration increased. In addition, the T_gel became fairly rapid growth when the molar fraction of 1 was close to 0.5, which may suggest that such two-component gel was a more complicated gelation process than in simple one-gelator system.

Fig. 2 Plot of T_gel vs. molar fraction of 1.

To obtain spectroscopic evidence of the donor–acceptor interactions taking place between 1 and 2, the temperature-dependent UV-vis spectra obtained for the complex of 1 and 2 (0.5 wt%, 1 : 1 molar ratio) in n-octanol is shown in Fig. 3a. The simultaneous increase in signal at higher wavelengths (380–550 nm) were observed as the sample changed from sol to gel (from 60 °C to 15 °C), becoming a substantial intensification of the yellow color in the process. Such light absorption in the 380–550 nm range can be interpreted as intermolecular charge-transfer spectra. A plot of absorbance at 425 nm vs. temperature was also examined by UV-vis spectroscopy. The results presented in Fig. 3b clearly show that the intensity of the charge-transfer band increases substantially with decreasing temperature. However, for a solution of 1 and 2 (0.1 wt%, 1 : 1 molar ratio) below the minimum gel concentration, no significant change of intensity was observed. This observation indicates that the charge-transfer interaction between 1 and 2 is the major driving force for gelation.

Fig. 3 (a) UV-vis absorption spectra of a mixture of 1 and 2 (0.5 wt%, 1 : 1 molar ratio, n-octanol) at different temperatures. (b) Plot of absorption at 425 nm for the gel (Δ) and for a solution (∇). Inset: photograph of the charge-transfer gel obtained by heating and cooling cycle.

Scanning electron microscopy (SEM) images of the dual-component gels in n-octanol (1 wt%, 1 : 1 molar ratio) provided further insight into the structure of these organogel systems (Fig. 4). SEM images of the single-component gels showed networks of entangled fibers (1, 0.1–0.3 μm in diameter), prismlike fibers (2, 0.8–1.5 μm in diameter) and entangled bundles of fibers (3, 1–2 μm in diameter). While the dual-component gels exhibit a straight fibrous network for 2 + 1 (0.5–1.5 μm in diameter) and 3 + 1 (0.5–2 μm in diameter), the fiber diameter being between that of the individuals, indicating that the interaction between two gelators brings about total morphological change in the presence of 1 in the gels 2 + 1 and 3 + 1.

Fig. 4 Scanning electron microscopy images of the xerogels of (a) 1, (b) 2, (c) 3, (d) 2 + 1 and (e) 3 + 1 from n-octanol.

The non-covalent interactions taking place between molecules of these sorbitol-appended compounds are principally hydrogen bonding between the sorbitol moieties, as revealed in the FTIR spectra (Fig. S2†). The spectra of the xerogels formed from 1, 2 and 3 show the broadening band at 3330, 3315 and 3332, respectively. In contrast, these bands in the gels 2 + 1 and 3 + 1 appear at 3259 and 3267, respectively. These results indicate that the hydrogen-bonding interactions are affected during the two-component gels formation because of the interaction between two gelators.

Dynamic rheological measurements were performed for all the gels and they showed a consistently higher value for the storage modulus (G′) over the loss modulus (G′′), indicating formation of a good stable gel (Fig. S3†). In contrast to the single-component gel of 2, we found that two-component mixed gel of 2 + 1 showed slightly higher G′ values, indicating higher elasticity, which correlate well with their thermal stability discussed earlier. As expected, the gel of 3 + 1 also showed higher G′ values compared to the gel of 3. These results would provide useful insight into co-assembly between two gelators in these two gelator component systems.^20,30

XRD was employed to provide the insight into the crystal structure of the two-component gelators and the results are shown in Fig. 5. For the two-component xerogel (2 + 1/n-octanol) the main peaks at 2θ = 5.36° (d = 1.64 nm) and 2θ = 10.63° (d = 0.83 nm) have a ratio of 1 : 1/2, suggesting that, in this sample, the two donor–acceptor gelators mainly assembles into a layered structure, and the interlayer distance is 1.64 nm. This is similar to the length of gelator 1 (1.63 nm, estimated by MM2 method). Based on the above results, a packing mechanism for 1 + 2 is proposed and is shown in Fig. 6. On the analogy of the previous gelator containing sorbitol moiety,^39–41 the hydrogen bonding of the sorbitol-to-sorbitol arrangement is considered to be organised into 1D fibrils that will further assemble into fibers that after physical cross-linking will form an organogel. Moreover, all experimental data indicate the alternative pile-up of the nitro and amino groups,⁴² thus the charge transfer interaction is formed in the fiber-axial direction. To accommodate the nitro-substituted aromatic ring and the amino-substituted aromatic ring more effectively, small cross-peaks were detected between the nitro-substituted benzene and the amino-substituted benzene protons (Fig. S4†), indicating that the two aromatic moieties dynamically approach each other, most probably in the folded charge-transfer conformation.⁴³

Fig. 5 XRD diffraction spectra of xerogels of 1, 2, 3, 2 + 1 (1 : 1 molar ratio), 3 + 1 (1 : 1 molar ratio) from n-octanol. Total gelator concentration = 2 wt% in each case.

Fig. 6 Molecular packing model and a schematic representation for the two-component gel fiber formed by 2 + 1.

However, the 3/n-octanol XRD pattern showed the peak at 2θ = 5.06° (d = 1.74 nm) that can also be observed for 3 + 1/n-octanol, but the peak at 2θ = 7.65° (d = 1.15 nm) was disappeared. This confirmed that 3 + 1/n-octanol gel is a genuine two-component gel in which 1 and 3 would undergo a co-assembly process as a result of the interactions between two gelators.²³ In such two-component gel, the thermal stability of the gel based on poor gelator 3 was modified by good gelator 1. Conversely, the gelation behaviour of 1 was also modified by 3.

In conclusion, we have synthesized and described novel sorbitol-appended two-component gelators. The presence of sorbitol moieties makes the dual-component gelators a more efficient gelator in varieties of organic solvents. In addition, UV-vis spectroscopy studies revealed that the charge-transfer interaction takes place in 2 + 1 system. In the two-component gel from 3 + 1, the gelation properties can be continuously changed just by gently changing the concentration of one compound. Furthermore, the smart thermochromic supramolecular gelation in dual-component systems, where both donor and acceptor moieties are confined to separate gelators, may find applications in sensor devices, novel electronic or optical materials, etc.

Acknowledgements

This research was supported by the National Natural Science Foundation of China (No. 21276188), Foundation of He'nan Educational Committee (16A530010), Science and Technique Foundation of He'nan Province of China (152102210358), College Innovation Program (201510462065) and Research Fund of Zhengzhou University of Light Industry (2015XJJZ032).

Notes and references

1. P. Terech and R. G. Weiss, Chem. Rev., 1997, 97, 3133.
2. S. S. Babu, V. K. Praveen and A. Ajayaghosh, Chem. Rev., 2014, 114, 1973.
3. L. A. Estroff and A. D. Hamilton, Chem. Rev., 2004, 104, 1201.
4. C. Tomasini and N. Castellucci, Chem. Soc. Rev., 2013, 42, 156.
5. B. Escuder, F. Rodriguez-Llansola and J. F. Miravet, New J. Chem., 2010, 34, 1044.
6. A. M. Vibhute, V. Muvvala and K. M. Sureshan, Angew. Chem., 2016, 128, 7913.
7. G. Cravotto and P. Cintas, Chem. Soc. Rev., 2009, 38, 2684.
8. P. Dastidar, Chem. Soc. Rev., 2008, 37, 2699.
9. A. Doering, W. Birnbaum and D. Kuckling, Chem. Soc. Rev., 2013, 42, 7391.
10. M. Dolores Segarra-Maset, V. J. Nebot, J. F. Miravet and B. Escuder, Chem. Soc. Rev., 2013, 42, 7086.
11. M.-O. M. Piepenbrock, G. O. Lloyd and N. Clarke, Chem. Rev., 2010, 110, 1960.
12. R. Amemiya, M. Mizutani and M. Yamaguchi, Angew. Chem., Int. Ed., 2010, 49, 1995.
13. E. Ressouche, S. Pensec, B. Isare, G. Ducouret and L. Bouteiller, ACS Macro Lett., 2016, 5, 244.
14. H. Kar and S. Ghosh, Nat. Chem., 2015, 7, 765.
15. H. Kar, M. R. Molla and S. Ghosh, Chem. Commun., 2013, 49, 4220.
16. A. Das, M. R. Molla, A. Banerjee, A. Paul and S. Ghosh, Chem.–Eur. J., 2011, 17, 6061.
17. U. Maitra, P. Vijay Kumar, N. Chandra, L. J. D'Souza, M. D. Prasanna and A. R. Raju, Chem. Commun., 1999, 595.
18. A. R. Hirst and D. K. Smith, Chem.–Eur. J., 2005, 11, 5496.
19. K. Hanabusa, T. Miki, Y. Taguchi, T. Koyama and H. Shirai, Chem. Commun., 1993, 1382.
20. R. K. Das, S. Banerjee, G. Raffy, A. Del Guerzo, J. P. Desvergne and U. Maitra, J. Mater. Chem., 2010, 20, 7227.
21. K. V. Rao, K. Jayaramulu, T. K. Maji and S. J. George, Angew. Chem., Int. Ed., 2010, 49, 4218.
22. M. K. Dixit, V. K. Pandey and M. Dubey, Soft Matter, 2016, 12, 3622.
23. L. E. Buerkle and S. J. Rowan, Chem. Soc. Rev., 2012, 41, 6089.
24. D. M. Ryan, T. D. Doran and B. L. Nilsson, Chem. Commun., 2011, 47, 475.
25. J. G. Hardy, A. R. Hirst, D. K. Smith, C. Brennan and I. Ashworth, Chem. Commun., 2005, 385.
26. J. R. Moffat and D. K. Smith, Chem. Commun., 2009, 316.
27. A. R. Hirst, B. Huang, V. Castelletto, I. W. Hamley and D. K. Smith, Chem.–Eur. J., 2007, 13, 2180.
28. M. M. Smith and D. K. Smith, Soft Matter, 2011, 7, 4856.
29. M. Suzuki, H. Saito and K. Hanabusa, Langmuir, 2009, 25, 8579.
30. R. K. Das, R. Kandanelli, J. Linnanto, K. Bose and U. Maitra, Langmuir, 2010, 26, 16141.
31. R. Luboradzki and Z. Pakulski, Supramol. Chem., 2009, 21, 379.
32. B. Isare, L. Bouteiller, G. Ducouret and F. Lequeux, Supramol. Chem., 2009, 21, 416.
33. J.-H. Fuhrhop and C. Boettcher, J. Am. Chem. Soc., 1990, 112, 1768.
34. D. G. Velazquez and R. Luque, Chem.–Eur. J., 2011, 17, 3847.
35. L. Boutellier, O. Colombani, F. Lortie and P. Terech, J. Am. Chem. Soc., 2005, 127, 8893.
36. Z. Dzolic, K. Wolsperger and M. Zinic, New J. Chem., 2006, 30, 1411.
37. C. Colquhoun, E. R. Draper, E. G. B. Eden, B. N. Cattoz, K. L. Morris, L. Chen, T. O. McDonald, A. E. Terry, P. C. Griffiths, L. C. Serpell and D. J. Adams, Nanoscale, 2014, 6, 13719.
38. E. R. Draper and D. J. Adams, Nat. Chem., 2016, 8, 737.
39. K. Fan, J. Song, J. Li, X. Guan, N. Tao, C. Tong, H. Shen and L. Niu, J. Mater. Chem. C, 2013, 1, 7479.
40. J. Li, K. Fan, L. Niu, Y. Li and J. Song, J. Phys. Chem. B, 2013, 117, 5989.
41. B. O. Okesola, V. M. P. Vieira, D. J. COrnwell, N. K. Whitelaw and D. K. Smith, Soft Matter, 2015, 11, 4768.
42. J. Landauer and H. McConnell, J. Am. Chem. Soc., 1952, 74, 1221.
43. M. Kumar, K. V. Rao and S. J. George, Phys. Chem. Chem. Phys., 2014, 16, 1300.

---

Footnote

† Electronic supplementary information (ESI) available: Experimental section, synthesis and figures (S1). See DOI: 10.1039/c6ra19492c

---