Synthesis, Self-Assembly and Photoresponsive Behavior of Liquid Crystals Based on Azobenzene

Abstract

A new amphiphilic surfactant (C4-Azo-C5-HDA) was formed by liquid crystals (LCs) based on azobenzene, whose structures were characterized by ¹H-NMR spectroscopy. The reversible hydrogelation upon changes in temperature and light exposure was also studied. Under the irradiation of UV light, the trans-isomer of C4-Azo-C5-HDA rapidly photoisomerized to the cis-isomer, resulting in rapid disruption of the gel. The thermotropic liquid crystal behavior of the gelator was investigated via Differential Scanning Calorimetry (DSC) and Polarizing Optical Microscopy (POM). The biocompatibility experiment of multi-stimulus response of the liquid crystal provides a potential driving force for the development of biomaterials.

1. Introduction

Gels are topical soft materials that are very suitable for advanced applications in many fields such as the sol-gel process, drug delivery, sensors, etc. [1,2,3]. The gel may swell due to the solvent binding together through covalent bonds or non-covalent interactions. According to the solvents contained, the gels have been categorized as organogels or hydrogels. Unlike traditional polymer gels that form a three-dimensional network structure through covalent cross-linking, low molecular weight gelators (LMWGs) are a low concentration of low-molecular-mass molecule, which are prepared by gelation to form three-dimensional networks based on non-covalent interactions’ aggregation, such as hydrogen bonds, electrostatic interactions, π–π interactions and van der Waals forces. These gels produced by non-covalent interactions are also called supramolecular gels. Especially the smart responsive supramolecular gel for the environment has aroused great interest of researchers. The development of such gels may lead to novel smart materials [4,5,6].

For these smart response systems, the structure conversion is carried out through the photochemical process of the photochromic unit. Azobenzene has been successfully introduced into gelators for reversible trans-cis photochemical isomerization upon exposure to different wavelengths of light, resulting in changes in the self-assembly structure [7,8,9,10,11,12]. Smart liquid crystal has developed rapidly in recent years [13,14,15]. The liquid crystal containing azobenzene attracts much attention due to the combined properties of liquid crystallinity, photoresponsivity and reactivity. These make them promising in designing intelligent photo-driven soft materials [16]. Peng designed and studied a novel azobenzene compound, which can be assembled into highly ordered and viscous lyotropic liquid crystal (LLC) phases at a certain water content [17]. Azobenzene is a hydrophobic group, which means it is not easily introduced into water. In addition to its solubility, its ability to self-assemble into a three-dimensional structure should also be carefully considered [18]. Thus, designing and implementing multiple responses in liquid crystals is of significance not only for scientific research, but also for potential applications.

Therefore, in this work, we use azobenzene units, alkyl chains and amine moieties to design and synthesize a supramolecular gelator (C4-Azo-C5-HDA). This liquid crystal shows property changes in response to external multi-stimuli, such as photo response and thermo response. The molecular design is based on the following considerations. (1) Light irradiation can trigger the cis-trans isomerization of the azobenzene group, thereby changing the molecular conformation. Accordingly, the intermolecular interactions will be changed [19,20]. (2) The amine unit is used as the hydrophilic part, and the spacer alkyl and alkyl tail (C₄H₉) on the side of the azobenzene unit are used as the hydrophobic part. These amphiphilic molecular structures are conducive to the formation of molecular self-assembly structures [21]. (3) Incorporating alkanes into the molecule will increase its gel capacity [22].

2. Materials and Methods

Hexamethylene diamine (HDA) was purchased from Sigma-Aldrich (St. Louis, MO, USA) and purified by vacuum distillation. All other chemicals were purchased from China National Pharmaceutical Group Co., Ltd. (Shanghai, China) and used without further purification.

¹H NMR spectroscopy was characterized on a Bruker Ultrashiled Spectrometer (400 MHz, Bruker, Germany). Photo isomerization of the surfactant was measured on a Cary 50-Bio UV-vis spectrophotometer (Varian, Santa Clara, USA) against a background of water in a quartz cuvette. Isomerization was tested on an EXFO Acticure 4000 light source (working wavelength: 365 nm). The UV light intensity was 3.8 mW/cm². The visible light intensity was 12.1 mW/cm². POM was conducted using a Nikon Eclipse 80i (Nikon, Tokyo, Japan). Phase transition temperatures and enthalpies of discogens were investigated using a TA-DSC Q100 instrument (TA Instruments, New Castle, DE, USA) under N₂ atmosphere with heating and cooling rates of 5 °C min⁻¹. Rheology was conducted using an ARES photo-rheometer (TA Instruments, New Castle, DE, USA).

Synthesis of C4-Azo-C5-HDA, preparation and characterization are described in detail in the following information (Scheme 1).

Compound 1. Sodium nitrite (4.45 g, 64.0 mmol) was dissolved in water (30 mL). 4-butylaniline (9.0 g, 53.4 mmol) was dissolved in water, and HCl mixture solvent was added drop wise under the temperature 0–5 °C. After 30 min, 6.1 g phenol (64.0 mmol) was added. After 1 h, the solution was made basic using a saturated solution of NaHCO₃. A brown solid precipitated. Filtered off and washed with distilled water, the product was isolated by recrystallization with ethanol. Mp: 68–69 °C, yield: 56%.

Compound 2. Anhydrous K₂CO₃ (10.0 g), KI (0.200 g) and compound 1(10.0 g) were dissolved in acetone (100 mL) under N₂ protection. After stirring for 1/2 h, ethyl 6-bromohexanoate (11.0 g) was added drop wise. Then, the mixture was stirred overnight under reflux. After cooling down to 10 °C, the salt solid was formed and filtered off. The filtrate was evaporated. The product was recrystallized with ethanol. Mp: 60–62 °C, yield: 68% [23,24,25].

Compound 3. 3.0 g HAD was dissolved in 10 mL dichloromethane and sodium ethylate (0.05 g) were added. Then compound 2 (1.0 g, dissolved in 10 mL dichloromethane) was added. The reaction mixture was stirred overnight under the temperature 40 °C. Then, the mixture was cooled to 10 °C. The solid was filtered off and washed with cooled dichloromethane. Finally, the product was dried in the vacuum oven at 30 °C, yield: 42%.

¹H NMR spectrums of compound 1 to compound 3 are shown in Figure 1, Figure 2 and Figure 3. The corresponding integral value is marked in Figures S1–S3. The chemical structures of compounds 1, compound 2 and C4-Azo-C5-HDA were also confirmed by the FTIR results. As shown in Figure S4, the absorption occurs at 1600 cm⁻¹ corresponding to the benzene skeleton vibration in the spectrum of azobenzene (compounds 1). The absorption occurs at 1730 cm⁻¹, which is attributed to the carbonyl group of the compounds (compounds 2). The FTIR bands at 1550 cm⁻¹ represent the absorption spectra of amide group (C4-Azo-C5-HDA). The presence of the above bands in the compounds gives strong evidence of successful synthesis.

3. Results

3.1. Liquid Crystals’ Characterization

A POM with a heating stage and DSC were used for measuring the melting temperature (Tm) and isotropization temperature (Ti) of the compound (Figure 4 and Figure 5). The peak temperatures obtained in DSC traces were found to be in accordance with POM observations except for the heating scan (Figure 5). They underwent crystal-to-isotropic transitions in heating scan without showing any liquid crystal property. However, POM showed a time of liquid crystal phase (Figure 4b). Therefore, it was speculated that the temperature between Tm and Ti was very close. The DSC thermogram showed that these two peaks partially overlapped.

Upon cooling cycles, the DSC thermograms of the C4-Azo-C5-HDA compound are shown in Figure 5. During the cooling process, it exhibited phase transition behavior at different temperatures. Obvious phase transition peaks were observed during cooling process. An exothermic peak appeared at 134 °C corresponded the transition temperatures from the Iso to the nematic mesophase on cooling. Another very small change at 126 °C was assigned to the nematic to smectic phase transitions (Figure 4e). Finally, a solid crystal phase was formed at 122 °C (Figure 4f). Through POM analysis, the packing structure of this crystal was different from the self-assembly structure of the gelator obtained from the hydrogel, which is shown in Figure 4a.

3.2. Light Response

Under alternating UV and visible light irradiation, azobenzene compounds can reversible photoresponsive between the trans and cis forms. The UV/Vis spectroscopic test of the C4-Azo-C5-HDA was performed to investigate the light-responsive behavior. Figure 6 shows a broad absorption around 350 nm mainly due to the transformation of the azobenzene group. When the UV light was on, the trans-cis isomerization indicated by the decay of the maximum absorption band was at around 350 nm and the generation of a new absorption was at 446 nm (Figure 6a). When the visible light was switched on, the profiles of the peaks increased, which shows that this photo-induced isomerization is reversible (Figure 6b) [26].

C4-Azo-C5-HDA is also an amphiphilic surfactant, which can be used as an emulsifier. C4-Azo-C5-HDA dissolved in water and n-hexane solvent was added drop wise under the siring (water: n-hexane = 5:1). After few minutes, a stable emulsion can form. POM observed that latex particles can exist stably, and the interface of latex particles has obvious polarized light display due to the regular arrangement of surfactants (Figure 7a). When UV light was shone on the emulsion, the adsorbed trans isomers converted into their cis form that rapidly desorbs from the interface, which caused the amphipathy to disappear. As illustrated in Figure 7b, no emulsion could be maintained. The emulsion was broken, and the water-oil phases were separated.

3.3. Self-Assembly Morphology Analysis

The morphology of the self-assembled liquid crystal into the hydrogelator was studied by SEM. The gelation ability was measured in water by inversion of the vials. C4-Azo-C5-HDA formed a gel with critical gelation concentrations (CGCs) as low as 1.5 wt%. For the in situ observation of the three-dimensional fibrous structure, we took the hydrogel out from bottle, and then rapidly freeze-dried it. Three-dimensional entangled fibers could be clearly seen under SEM analysis (Figure 8b). These fibrous networks containing fibers with widths of several micrometers and lengths in the hundreds of micrometers were observed for the gel state. (Figure 8c) [27].

C4-Azo-C5-HDA can promote aggregation of the gelator into the high-order assemblies for gelation. XRD spectra were used to investigate the molecular packing and orientation structure of the gel fiber detail. Figure 9 gave a peak at 2θ = 19.8 (d spacing = 4.473 Å) and a peak at 2θ = 9.5 (d spacing 9.396 Å), accompanied by another peak at 2θ = 5.9 (d spacing 14.712 Å). C4-Azo-C5-HDA exhibits complete self-assembly behavior. As shown in Figure 9, three scattering peaks with the ratios of 1:2:3 in the d spacing were observed, demonstrating the presence of a long-range, ordered lamellar structure. This is probably because the hydrophobic domains’ phase is separate from the hydrophilic units, and this nanophase separation process promotes the bilayer packing of the molecules.

3.4. Self-Assembly Morphology Analysis

Dynamic changes in rheological properties of materials upon exposure to light can be readily measured using photorheology. As shown in Figure 10, the dynamic changes of the storage modulus(G’) and loss modulus (G”), with time were clearly observed under UV light exposure. In the beginning, the G’ was greater than the G”. During exposure of the gel to UV light, a dramatic decrease of moduli was observed and a new plateau values were reached within 2 min. A corresponding change in viscosity was also observed (Figure 11). The initial highly viscous gel changed to a low-viscosity fluid under UV irradiation within 2 min. After the UV exposure, the UV light was turned off. At the same time, visible light was turned on. The reversibility of the system was demonstrated by the recovery of the rheological properties upon exposure of visible light. However, the rate of recovery was relatively slow comparing to exposure with UV light. For the solution to completely return to the initial state took more than 12 h.

3.5. Live-Dead Assay of the Hydrogel

Hydrogels formed by liquid crystals have received significant attention due to their high-tech applications, such as drug delivery, template materials, etc. [28,29,30]. In order to verify the biocompatibility of C4-Azo-C5-HDA hydrogels, live-dead assays were used to study the in vitro cell viability of human fibroblasts L929-supportive properties of C4-Azo-C5-HDA hydrogel. Figure 10 showed that the cells survived images under 5 wt% of the gelators after one day of culture. Figure 12b showed that the cells were perforated into the intermediate fibrous network, which caused the cells to show a different size and clarity under the fluorescence microscope. We concluded that, due to the existing release rate of small molecules from the hydrogel to the medium, the sol-gel has a dynamic equilibrium in the aqueous solution [31]. Molecular gelators are easily swallowed by cells in a solution with hundreds of molecular weights, resulting in cytotoxicity and preventing cells from dividing and growing. Therefore, in order to improve the biocompatibility of C4-Azo-C5-HDA and use it as a cell growth-supporting material, it is necessary to develop and incorporate a supramolecular hydrogel with lower solubility. These will be in our future work.