Stabilization of the mesomorphic phase in a self-assembled two-component system

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Abstract

We reported here the two-component self-assembling building blocks capable of forming lyotropic liquid crystal and liquid-crystalline physical gel. One of the components has a molecular characteristic of C3-symmetrical trisureas containing three azobenzene groups, which can form liquid-crystal phase in a temperature range of 133–215 °C. Another one has a trisamide core, which can self-aggregate to fibrous network through hydrogen bonds of amide moieties. The mixture of these two components performs lyotropic liquid crystal as well as liquid-crystalline physical gel in a temperature range larger than that of sole compound, suggesting that the cooperation of hydrogen bonds between these components stabilizes the mesophase of the assembly. The mechanism of formation of the mesophase was investigated by infrared spectra and small-angle X-ray scatterings.

Graphical abstract

The cooperation of intermolecular interaction between C3-symmetrical trisurea compounds and trisamide gelator stabilizes the mesophases of the assembly; thus performs LCs and LC physical gel in a large temperature range.

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Introduction

Self-assembly of small functional molecules into supramolecular structures becomes a powerful approach toward the development of new materials and nanoscale devices [1], [2], [3]. The control of these self-organizing processes to realize the expected functionality by chemical and physical means is a major challenge all the while [4], [5]. Liquid crystals (LCs), one of the important self-assembly mesophase, attracted much attention in recent years due to their potential applications as new functional materials for information and mass transport, sensing, catalysis, templates, and electro-optical displays as well as in bio-related fields [6], [7], [8]. Supramolecular liquid-crystalline phases with well-defined structures have been formed by exploiting intermolecular hydrogen bonding, which give us another tool to build LCs except covalent bonding [9], [10], [11], [12], [13], [14].

Recently, the combination of LCs and fibrous aggregate of gelators has been found to form liquid-crystalline physical gels with unique optical properties and dynamic functions [5], [6], [15], [16], [17], [18], [19], [20]. Those LCs containing azobenzene linkage have attracted tremendous attention because of their photoswitchable properties induced by light [20], [21], [22], [23], [24], [25], [26]. In the previous works, the compounds containing azobenzene moieties are generally used as gelators to gelate commercial LCs, thus form a physical mixed phase-separating structure without a specific interaction between the fibrous structure and the liquid-crystalline phase [20], [21], [22], [23]. Therefore the concentration of azobenzene gelator dopants is low (⩽5 wt%) due to their limited solubility in LCs [27], [28]. Very recently, Y. Zhao et al. developed a novel system with a high concentration of liquid-crystalline azo dopant (⩾10 wt%) [23]. However, the control of the functional mesophase by cooperation of intermolecular interactions of the assembly is still a challenge for developing novel soft materials.

It was known that C3-symmetrical molecules were highly suitable building blocks for the formation of mesophase as well as cylindrically shaped fibers [29], [30], [31], [32], [33]. In the previous research, we found that the two-component gels could be formed by mixing trisurea compound L1 or L2 with a C3-symmetrical trisamide gelator (G1) [34] (Scheme 1), with the dopant of L1 (or L2) up to 20 wt% [35]. In order to get more insight in structural compatibility of H-bonds between ureas and amides, another C3-symmetrical trisamide compound L3 (Scheme 1) was synthesized here for comparison. Though the mixture of G1/L3 in a proper proportion (such as G1/L3 = 4:1, w/w) can gel 1,4-dioxane, the gel was not so stable as G1/L1 and can be destroyed even by a slight shake. Interestingly, here we found that the mixtures of L (L1–L3) and G1 can also self-assemble through hydrogen bonds to form LCs as well as liquid-crystalline physical gel in a large temperature range. The thermotropic properties and mesophase structures of these self-assembled systems are investigated in this report.

Section snippets

General

All starting materials were obtained from commercial supplies and used as received. Moisture sensitive reactions were performed under an atmosphere of dry argon. 1H NMR and 13C NMR spectra were recorded on a Mercuryplus, at 400 and 100 MHz, respectively. Proton chemical shifts are reported in parts per million downfield from tetramethylsilane. Matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF-MS) was recorded on an AXIMA-CFRPLVS mass spectrometer (Shimadzu). Element analysis

Behavior of the mesophase state in the precursors and the two-component system

All three compounds L1L3 were found to exhibit liquid-crystalline properties. L1 forms liquid-crystalline phase in a comparatively larger temperature range (133–215 °C) than that of L2 (134–158 °C) (Figs. 1a and 1b). On the first heating of L1 from room temperature, two endothermic peaks at 133 and 215 °C were observed in DSC curve (Fig. 1a), indicating transitions from the crystalline solid phase to the liquid-crystalline mesophase and the mesophase to isotropic state, respectively. An

Summary

In conclusion, a new system of liquid-crystalline phases through intermolecular hydrogen bonds has been achieved by combination of C3-symmetrical azobenzene compounds with a gelator of trisamide core. The mixture of G1/L1 can self-assemble to form lyotropic LCs as well as liquid-crystalline physical gel in a temperature range larger than that of sole L1 and G1, which suggests that the cooperation of intermolecular interaction between urea and amide in these components stabilizes the mesophase

Acknowledgments

We wish to acknowledge National Science Foundation of China (20771027, 20571016 and 20490210), Shanghai Sci. Tech. Comm. (06PJ14016) and Shanghai Leading Academic Discipline Project (B108) for financial support. The authors greatly appreciate Prof. Peiyi Wu, Prof. Feng Qiu and Dr. Yi Shen (Department of macromolecular science, Fudan University) for their kindly help on IR analysis and POM observation.

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