Gelation And Liquid Crystalline Behaviour Of Azophenyl Hydrazide Derivatives

Self-assembled systems of molecules through noncovalent interactions, such as supramolecular gels and liquid crystals (LCs), are fascinating organized soft materials that can respond to external stimuli such as light, temperature, electrical pulses and chemicals. There are potential applications in template synthesis, controlled release, separations, and biomimetics. Among the noncovalent interactions, hydrogen bonds are one of the most important interactions in the self-assembly of molecules because of their strength, directionality, reversibility, and selectivity.In this context, we have designed and synthesized five new series of compounds derived from azophenyl dihydrazide: N-4-octanoxyphenyl-N'-4-[(4-hydroxy- phenyl)azophenyl] benzohydrazide (BNB-n, n=6, 8, 12, 14), N-4-(alkoxyphenyl)- N'-4-[(4-methoxyphenyl)azophenyl] benzohydrazide (BNBC-n, n=12, 14), N-(3,4,5-octanoxyphenyl)-N'-4-[(4-hydroxyphenyl)azophenyl] benzohydrazide (BNB-Dn, n=8, 12), N-(3,4,5-octanoxyphenyl)-N'-4-[(4-hydroxyphenyl)azophenyl] benzohydrazide (BNB-T8) and N-(3,4,5-octanoxyphenyl)-N'-4-[(4-methoxy- phenyl)azophenyl] benzohydrazide (BNBC-T8). The self-assembly behaviors of the BNB-n, BNBC-n, BNB-Dn, BNB-T8 and BNBC-T8 were systematically investigated, and the effect of molecular structures and noncovalent interactions on their liquid crystalline behaviors and gellation behaviors was discussed to understand the driving force for their self-assembly on molecular level. The gelling ability, the aggregation morphologies, packing structures and intermolecular H-bonding strength were significantly dependent on molecular structures (the type of substituent group, length and number of the terminal chains and intermolecular interactions, etc.). The photo-responsive organogel was obtained and the mechanism for it was proposed.1. Temperature-dependent ¹H NMR and FT-IR spectroscopic experiments were performed for BNBC-12 in 20% DMSO-d₆/CDCl₃ to confirm the primary involvement of N-H protons in lateral intermolecular hydrogen bonding. BNBC-n showed synclinic monolayer smectic C (SmCs) feature. It can be seen that the melting points decreased, whereas the Sm-N phase transition temperature (T_Sm-N) in BNBC-n series increased with the elongation of the flexible terminal chains. In contrast, the N-I phase transition temperature (T_N-I) of BNBC-n decreased with the elongation of the flexible terminal chains, suggesting that long flexible terminal chains are favourable to stabilize the smectic phase and unfavourable to the nematic phase.Based on the results of variable temperature ¹H NMR and FT-IR spectroscopy, supramolecular assembly motif of BNB-n through both axial intermolecular hydrogen bonds between hydroxyl groups and lateral ones between hydrazide groups was proposed. Interestingly, BNB-n (n=12, 14) with longer terminal chains displayed anticlinic bilayer smectic C mesophase (SmC_2a). The hydroxyl groups in BNB-n play an important role in the formation of anticlinic molecular packing in the smectic layers.2. The calamitic hydrazide derivatives BNB-n and BNBC-n can not form organogel. The swallow-tailed compound BNB-Dn and the tapered compounds BNB-T8 and BNBC-T8 form stable organogels in a variety of organic solvents. The gelling ability, the aggregation morphologies, packing structures and intermolecular H-bonding strength were different depending on the molecular structures (the type of substituent group, length and number of the terminal chains and intermolecular interactions, etc.). The increase of the length and number of the terminal chains and axial intermolecular hydrogen bonds between hydroxyl groups facilitate the gellation. BNB-T8 showed great ability to gel a variety of organic solvents to form stable organogels with the critical gelation concentration as low as 0.5 mg/mL. Xerogels from chloroform exhibited entangled and dense fibrous aggregates with the diameters of 50-60 nm, while both left- and right-hand helical structures in mixtures of DMSO and H₂O. Based on the results of variable temperature ¹H NMR spectroscopy, FT-IR spectroscopy and UV-vis absorption spectroscopy, intermolecular hydrogen bonding,Ï€-Ï€interactins between azobenzene and van der Waals force between alkoxy chains were demonstrated to be the driving force for the gelation.3. The organogel showed photoinduced gel-to-precipitate transition under the irradiation by 365 nm UV-light, which was attributed to trans-BNB-T8 to cis-BNB-T8 photoisomerization. The trans-cis isomerization in BNB-T8 gels further caused the morphological change from fibers to vesicles at supramolecular level. WAXD results showed that the xerogel exhibited rectangular columnar arrangement, while the precipitate exhibited a bilayer structure. Furthermore, the bilayers were stabilized by intermolecular hydrogen bonding, dipolar interactions arising from the cis-BNB-T8 as well as the microphase segregation between the rigid-rod core and the flexible chains. Reversible fiber–vesicle transition could be tuned by irradiation of gels through UV (365 nm) and visible light (455 nm) and the mechanism for it was proposed. Vesicles might find potential applications in biomimetic models, drug or gene carriers and further work is currently in progress.