Rational design of multistimuli responsive organogels by alternation of hydrogen-bonding and amphiphilic properties

Abstract

A new low molecular-weight gelator G1 having hydrozone and azobenzene groups was designed and synthesized. The corresponding organogel from G1 exhibited multistimuli responsive capability by tuning the hydrogen-bonding and amphiphilic properties.

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Low molecular-weight gelators (LMWGs)^1–6 have received recently considerable attention due to their wide range of potential applications including sensors,⁷ three-dimensional cell-culture studies,⁸drug-delivery systems,⁹ and tissue engineering.¹⁰ Organogels, formed by assembly of LMWGs into entangled three-dimensional networks through supramolecular interactions, are generally sensitive to external stimuli, such as pH,¹¹ anions,^12,13 molecules,¹⁴ light,¹⁵ electricity¹⁶ and magnetism.¹⁷ Accordingly, organogels are very appealing in stimuli responsive materials because of the high versatility of such compounds towards synthetic modification. However, to the best of our knowledge, the work about multistimuli responsive organogels is still rare. Recently, Zhu and cowokers reported one example of multistimuli responsive organogels by tuning the π–π and electrostatic interactions.¹⁸ Besides, organogels by alternation of π–π interactions and hydrophobic interactions to realize a multistimuli response have also been explored.¹⁹

Herein, we designed and synthesized a new low molecular-weight gelator, G1, composed of a hydrazone group, a azobenzene unit, and a 3,4,5-trialkoxybenzoic acid moiety, which can respond to multistimuli by changing the hydrogen-bonding and amphiphilic properties. On one side, the hydrazone group introduced into the gelatorG1 offered a convenient way to modulate the sol–gel transformation by adding anions to disturb the hydrogen bonds. On the other hand, the azobenzene unit in the gelator's structure was also receptive to trigger the gel–sol transition, attributing to the alternation of the amphiphilic properties of the gelator through complexation/decomplexation with α-CD affected by photoisomerization of the azobenzene group. Besides, incorporation of 3,4,5-trialkoxybenzoic acid moiety into LMWG1 could increase its gelation ability, according to a previous report.²⁰

The structure of G1 is shown in Scheme 1 and its synthesis is easy to achieve (ESI)†. Firstly, its gelation ability was tested in several solvents according to a vial inversion test, and the results are summarized in Table 1. Among the solvents examined, G1 can readily gel in ethyl acetate, DMF and acetonitrile. The corresponding CGC (critical gelation concentration) were 5%, 5% and 2% (wt%) respectively. As an example, Fig. 1 illustrates the formation of an opaque yellow organogel of G1 in DMF at room temperature. This organogel could turn to transparent orange solution after heating up. The SEM image of the xerogel showed that the molecules of G1 were self-assembled into three-dimensional entangled network of thin slices (Fig. 2 and Figure S2†). In contrast, G1 was still insoluble in other solvents even though heating up the solution higher than the boiling temperature of solvent. This indicated that G1 could not gel in other solvents due to its poor solubility.

Fig. 1 (a) Organogel formed from a solution of G1 in DMF at room temperature; (b) after heating.

Fig. 2 EM image of the xerogel G1.

Scheme 1 Structure of G1 and schematic representation of the design rationale for multistimuli responsive organogels.

Table 1 Gelation properties of G1

Solvent	State	CGC ^a	Solvent	State	CGC
a CGC, critical gelation concentration (wt%), the minimum concentration necessary for gelation of solvents. b I, insoluble. c G, gel.
THF	I^b	—	Acetone	I	—
Ethanol	I	—	Hexane	I	—
Methanol	I	—	DMF	G	5%
Ethyl acetate	G^c	5%	Acetonitrile	G	2%
Dichloromethane	I	—	Water	I	—

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Besides the temperature, response towards other stimuli make organogels more attractive as potential smart materials. Recently, tuning the gel properties by adding anions is beginning to attract particular attention.^3,4,6 It is well known that the hydrazone group is normally utilized as anion binding site for development of novel anion chemosensors.⁶ Accordingly, the incorporation of a hydrazone group into G1 might offer a convenient way to tune its gel properties by anion binding to disturb the hydrogen bonds, which are essential to assemble G1 into a gel. Furthermore, the anion binding would deprotonate the hydroxyl group and induce an apparent color change of the azobenene moiety. In this sense, the organogels of G1 might have the ability to respond towards anions by transforming the gel–sol state with color change.

Just as we expected, the organogel of G1 displayed gel–sol transition with a clear color change due to the stimuli of anions. For example, the addition of H₂PO₄⁻ (1 equiv.) as its tetrabutyl ammonium (TBA) salt to the yellow gel at room temperature showed a gradual decomposition of the gelatinous state into an orange solution (Fig. 3). Anions, such as AcO⁻, F⁻, CN⁻, exhibited the same phenomena as that of H₂PO₄⁻ did, except for the difference in transition time. For the anions CN⁻, AcO⁻, F⁻ and H₂PO₄⁻, the time was 1 min, 2 min, 4 min and 10 min, respectively. While under the same conditions, the addition of Cl⁻, Br⁻, I⁻, NO₃⁻ could not make the organogels collapse owing to the weaker basicity of Cl⁻, Br⁻, I⁻ and NO₃⁻ compared to CN⁻, AcO⁻, F⁻, H₂PO₄⁻.

Fig. 3 (a) Organogel formed from a solution of G1 in DMF; (b) immediately after addition of solid tetrabutylammonium phosphate salt (1 eqv.); (c) after 1 min; (d) after 3 min; (e) after 5 min; (f) and (g) after 10 min.

In order to elucidate the mechanism of this gel–sol transition with color change, UV-Vis experiments of G1 in solution were performed (Figure S3†). From the spectra, it was found that a new absorption band at 480 nm instead of 310 nm appeared upon addition of CN⁻, ACO⁻, F⁻, H₂PO₄⁻, responsible for a vivid color change of the solution from yellow to orange-red. The appearance of this new peak at 480 nm was ascribed to the deprotonation of hydroxyl group. In contrast, for other anions investigated, the spectra showed negligible changes. Furthermore, ¹H NMR titration experiments were also carried out. As shown in Fig. 4, addition of F⁻ in CDCl₃ resulted in the disappearance of the signals of hydroxyl proton H_a and the hydrazone proton H_c indicating the deprotonation processes occurred.

Fig. 4 Partial ¹H NMR spectra of compound G1 in CDCl₃ upon the addition of F⁻. (a) Free, (b) 1 equiv of F⁻.

With the above results in hands, the mechanism of anion-induced gel–sol transition with color change could be presumed. Firstly, the gelatorG1 was assembled to yellow gel with the aid of intermolecular hydrogen bonds coming mainly from hydroxyl protons and hydrozon protons. Then, addition of anions with relative stronger basicity deprotonated those protons and transformed the gel to solution. The color change was mainly due to the charge transfer of azobenzene group before and after deprotonation of hydroxyl group.^21,22 Accordingly, through this process, an anion-stimuli responsive organogel was achieved successfully.

To realize multistimuli response, an azobenzen moiety was introduced into G1 not only as signalling part for anion binding. It is known that the combination of azobenzene moiety with α-CD can be regulated by UV and visible light irradiation^23,24,25 due to the light-triggered cis/transisomerization of the azobenzene moiety.²⁶ Upon irradiation of UV light, the trans-state of azobenzene is readily isomerized to its corresponding cis-form, whereas the cis-azobenzene is isomerized to the trans-azobenzene by irradiating with visible light or in the dark. Furthermore, the trans-azobenzene is suitable for the combination with α-CD, and the cis-azobenzene is not. As a result, the amphiphilicity of G1 combined with α-CD should be stronger than that without α-CD due to the hydrophilic surface of α-CD, which is helpful to gel.¹⁵ Accordingly, this phenomenon could be utilized to tune the gel properties.

First, the photoisomerization of G1 in solution was explored (Fig. 5). Before irradiation, G1 displayed only one absorption peak at 324 nm due to the π–π* transition of trans-azobenzene. Upon UV light irradiation, a new absorption band at 455 nm ascribed to n–π* transition of cis-azobenzene appeared.²⁶ With the extension of irradiation time, this peak gradually increased with the expense of the peak at 324 nm, indicating the isomerization between cis-azobenzene and trans-azobenzene in G1. Then, the UV-Vis spectra of G1 in the presence of α-CD illustrated (figure S5†) the successfully combination of G1 with α-CD. Addition of α-CD in solution of G1 induced a clear increase of the absorption band at 324 nm, because of the stabilization effect of α-CD on trans-form of azobenzene moiety.

Fig. 5 UV-Vis spectra of G1 under UV irradiation in DMSO.

With all these results in hand, the gelation of G1 in the presence of α-CD and its photoirradiation were investigated. Interestingly, with α-CD, G1 still could gel in DMF, however, the CGC reduced from 5% without α-CD to 1% (wt%) with α-CD. The titration between α-CD and G1 indicated the formation of a 1 : 1 complex between them (figure S6†). Besides, the increased amphiphilicity of G1 after combined with α-CD made it easier to gelation and this provided us a new route to tune the transition between sol and gel by changing the amphiphilicity. This expectation was successfully realized as shown in Fig. 6. The organogel formed by staying the solution of G1 in the dark or visible light would become solution in UV irradiation and vice versa. Such a gel-sol transition could be repeated several times.

Fig. 6 (a) Organogel formed from a solution of 1% G1 with α-CD in DMF in the dark or vis irradiation; (b) after UV.

In summary, a new gelatorG1 appended with hydrozone and azobenzene groups was rationally designed and synthesized. By tuning the hydrogen-bonding and amphiphilicity, a multistimuli responsive capability of the gel was successfully achieved. This work provides a new efficient way to design and produce multistimuli responsive organogels. Further work is ongoing in our lab.

Acknowledgements

This research has been supported by the National Natural Science Foundation of China (20923006) and the Fundamental Research Funds for the Central Universities (DUT11LK13).

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Footnote

† Electronic Supplementary Information (ESI) available: details of experimental procedures, characterisation of G1, UV-vis spectra of G1 with various anions, metal ions and α-CD, the gelation properties of G1 on adding α-CD. See DOI: 10.1039/c1ra00722j/

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