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Regulation of substituent groups on morphologies and self-assembly of organogels based on some azobenzene imide derivatives
Nanoscale Research Letters volume 8, Article number: 160 (2013)
In this paper, new azobenzene imide derivatives with different substituent groups were designed and synthesized. Their gelation behaviors in 21 solvents were tested as novel low-molecular-mass organic gelators. It was shown that the alkyl substituent chains and headgroups of azobenzene residues in gelators played a crucial role in the gelation behavior of all compounds in various organic solvents. More alkyl chains in molecular skeletons in present gelators are favorable for the gelation of organic solvents. Scanning electron microscopy and atomic force microscopy observations revealed that the gelator molecules self-assemble into different aggregates, from wrinkle, lamella, and belt to fiber with the change of solvents. Spectral studies indicated that there existed different H-bond formations between amide groups and conformations of methyl chains. The present work may give some insight to the design and character of new organogelators and soft materials with special molecular structures.
It is well known that organogels are one class of important soft materials, in which organic solvents are immobilized by gelators [1–6]. Although gels are widely found in polymer systems, there has recently been an increasing interest in low-molecular-mass organic gelators (LMOGs) [7, 8]. In recent years, physical gelation of organic solvents by LMOGs has become one of the hot areas in the soft matter research due to their scientific values and many potential applications in the biomedical field, including tissue engineering, controlled drug release, medical implants, and so on [9–14]. The gels based on LMOGs are usually considered as supramolecular gels, in which the gelator molecules self-assemble into three-dimensional networks in which the solvent is trapped via various non-covalent interactions, such as hydrogen bonding, π-π stacking, van der Waals interaction, dipole-dipole interaction, coordination, solvophobic interaction, and host-guest interaction [15–20]. Such organogels have some advantages over polymer gels: the molecular structure of the gelator is defined, and the gel process is usually reversible. Such properties make it possible to design various functional gel systems and produce more complicated and defined, as well as controllable, nanostructures [21–25].
In our reported work, the gelation properties of some cholesterol imide derivatives consisting of cholesteryl units and photoresponsive azobenzene substituent groups have been investigated . We found that a subtle change in the headgroup of azobenzene segment can produce a dramatic change in the gelation behavior of both compounds. In addition, the gelation properties of bolaform and trigonal cholesteryl derivatives with different aromatic spacers have been characterized . Therein, we have investigated the spacer effect on the microstructures of such organogels and found that various kinds of hydrogen bond interactions among the molecules play an important role in the formation of gels.
As a continuous work, herein, we have designed and synthesized new azobenzene imide derivatives with different substituent groups. In all compounds, the long alkyl chains were symmetrically attached to a benzene ring to form single or three substituent states, with the azobenzene as substituent headgroups. We have found that all compounds could form different organogels in various organic solvents. Characterization of the organogels by scanning electron microscopy (SEM) and atomic force microscopy (AFM) revealed different structures of the aggregates in the gels. We have investigated the effect of alkyl substituent chains and headgroups of azobenzene residues in gelators on the microstructures of such organogels in detail and found different kinds of hydrogen bond interactions between amide groups and conformations of methyl chains.
The starting materials, 4-aminoazobenzene and 2-aminoazotoluene were purchased from TCI Development Co., Ltd, Shanghai, China. Other used reagents were all for the analysis purity from either Alfa Aesar (Beijing, China) or Sigma-Aldrich (Shanghai, China) Chemicals. The solvents were obtained from Beijing Chemicals and were distilled before use. Deionized water was used in all cases. 4-Hexadecyloxybenzoic acid and 3,4,5-tris(hexadecyloxy)benzoic acid were synthesized in our laboratory according to a previous report  and confirmed by proton nuclear magnetic resonance (1H NMR). Then, these azobenzene imide derivatives were prepared by simple methods. Simply speaking, different benzoic acid chlorides were synthesized by heating acid compound solutions in sulfoxide chloride and a bit of dimethylformamide (DMF) for about 10 h at 70°C. Then, the prepared benzoic acid chlorides reacted with the corresponding azobenzene amines in dried dichloromethane at the presence of pyridine for 2 days at room temperature. After that, the mixtures were washed with diluted hydrochloric acid and pure water. The organic layer was evaporated to dryness. The residues were purified by recrystallization in ethanol solution as a yellow solid. The final products and their abbreviations are shown in Figure 1, which were confirmed by 1H NMR and elemental analysis.
A weighted amount of gelator and a measured volume of selected pure organic solvent were placed into a sealed glass bottle, and the solution was heated in a water bath until the solid was dissolved. Then, the solution was cooled to room temperature in air and the test bottle was inversed to see if a gel was formed. When the gelator formed a gel by immobilizing the solvent at this stage, it was denoted as ‘G’. For the systems in which only solution remained until the end of the tests, they were referred to as solution (S). The system in which the potential gelator could not be dissolved, even at the boiling point of the solvent, was designated as an insoluble system (I). Critical gelation concentration (CGC) refers to the minimum concentration of the gelator for gel formation.
Firstly, the xerogel was prepared by a vacuum pump for 12 to 24 h. The dried sample thus obtained was attached to mica, copper foil, glass, and CaF2 slice for morphological and spectral investigations. Before SEM measurement, the samples were coated with copper foil fixed by a conductive adhesive tape and shielded with gold. SEM pictures of the xerogel were taken using a Hitachi S-4800 field emission scanning electron microscope (Chiyoda-ku, Japan) with the accelerating voltage of 5 to 15 kV. AFM images were recorded using a multimode 8 scanning probe microscope (Veeco Instrument, Plainview, NY, USA) with silicon cantilever probes. All AFM images were shown in the height mode without any image processing except flattening. Transmission Fourier transform infrared (FT-IR) spectra of the xerogel were obtained using a Nicolet iS10 FT-IR spectrophotometer from Thermo Fisher Scientific Inc. (Waltham, MA, USA) with an average of 32 scans and at a resolution of 4 cm-1. The X-ray diffraction (XRD) measurement was conducted using a Rigaku D/max 2550PC diffractometer (Rigaku Inc., Tokyo, Japan). The XRD pattern was obtained using CuKα radiation with an incident wavelength of 0.1542 nm under a voltage of 40 kV and a current of 200 mA. The scan rate was 0.5° min-1. 1H NMR spectra were obtained using a Bruker ARX-400 NMR spectrometer (Bruker, Inc., Switzerland) in CDCl3 with tetramethylsilane (TMS) as an internal standard. The elemental analysis was carried out with the Flash EA Carlo-Erba-1106 Thermo-Quest (Milan, Italy).
Results and discussion
The gelation performances of all compounds in 21 solvents are listed in Table 1. Examination of the table reveals that all compounds are efficient gelators. Firstly, TC16-Azo can gel in 12 solvents, such as nitrobenzene, aniline, acetone, cyclopentanone, ethyl acetate, pyridine, and DMF. As for TC16-Azo-Me with additional methyl groups in azobenzene part, only eight kinds of organogels were formed. Secondly, as for the SC16-Azo and SC16-Azo-Me with single alkyl substituent chains in molecular skeletons, the numbers of formed organogels changed to 3 and 6, respectively. Their photographs of organogels of SC16-Azo and SC16-Azo-Me in different solvents were shown in Figure 2. The data shown in Table 1 indicate that change of substituent groups in azobenzene residue and benzoic acid derivatives can have a profound effect upon the gelation abilities of these studied compounds. It seemed that more alkyl chains in molecular skeletons in present gelators are favorable for the gelation of organic solvents. In addition, the space effect of methyl groups for intermolecular stacking in the gel formation process is also obvious for all cases. Moreover, in most cases, for a given solvent, the minimum concentration of the gelator for gel formation, named as CGC, is an important factor for the prepared gels [29, 30]. In the present case, all compounds can form organogels in DMF. And the CGC values for TC16-Azo and TC16-Azo-Me with three alkyl substituent chains in molecular skeletons seemed smaller than those of compounds with single alkyl substituent chains. The reasons for the strengthening of the gelation behaviors can be assigned to the change of the spatial conformation of the gelators due to the more alkyl substituent chains in molecular skeletons, which may increase the ability of the gelator molecules to self-assemble into ordered structures, a necessity for forming organized network structures.
In addition, in order to obtain a visual insight into the gel microstructures, the typical nanostructures of the xerogels were studied using the SEM technique, as shown in Figures 3 and 4. From the present diverse images, it can be easily investigated that the microstructures of the xerogels of all compounds in different solvents are significantly different from each other, and the morphologies of the aggregates change, from wrinkle, lamella, and belt to fiber with the change of solvents. In addition, more regular lamella-like or fiber-like aggregates with different aspect ratios were prepared in the gels of SC16-Azo and SC16-Azo-Me with single alkyl substituent chains in molecular skeletons. As for the two other compounds with multialkyl substituent chains, most of the aggregates tended to have wrinkled or deformed films. Furthermore, the xerogels in DMF of all compounds were characterized by AFM, as shown in Figure 5. From the images, it is interesting to note that these big belt or lamella aggregates were composed of many little domains by the stacking of the present imide derivatives. The morphologies of the aggregates shown in the SEM and AFM images may be rationalized by considering a commonly accepted idea that highly directional intermolecular interactions, such as hydrogen bonding or π-π interactions, favor formation of belt or fiber structures [31–34]. The difference of morphologies between molecules with single alkyl substituent chains and multichains can be mainly due to the different strengths of the intermolecular hydrophobic force between alkyl substituent chains, which have played an important role in regulating the intermolecular orderly staking and formation of special aggregates.
It is well known that hydrogen bonding plays an important role in the formation of organogels [35, 36]. At present, in order to further clarify this and investigate the effect of substituent groups on assembly, we have measured the FT-IR spectra of all compounds in chloroform solution and xerogel forms. Firstly, TC16-Azo-Me was taken as an example, as shown in Figure 6A. As for the spectrum of TC16-Azo-Me in chloroform solution, some main peaks were observed at 3,412, 2,926, 2,854, and 1,676 cm-1. These bands can be assigned to the N-H stretching, methylene stretching, and the amide I band [37, 38]. As far as the spectra of these xerogels, these bands shifted to 3,252, 2,918, 2,848, and 1,651 cm-1, respectively. The shift of these bands indicates H-bond formation between amide groups and conformational distortion of methyl chains in the gel state. In addition, the spectra of xerogels of all compounds in DMF were compared, as shown in Figure 6B. One obvious change is the decrement of methylene stretching for SC16-Azo and SC16-Azo-Me in comparison with the other two compounds, which can be attributed to the number difference of alkyl substituent chains in molecular skeletons. Another change is that the peaks assigned to N-H stretching and amide I band for SC16-Azo and SC16-Azo-Me shifted to 3,365, 3,310, and 1,645 cm-1, respectively. This implied that there were differences in the strength of the intermolecular hydrogen-bond interactions in these xerogels, even though they were from the same solvent system. The present data further verified that the substituent groups in molecular skeletons can regulate the stacking of the gelator molecules to self-assemble into ordered structures by distinct intermolecular hydrogen bonding .
Furthermore, in order to investigate the orderly stacking of xerogel nanostructures, XRD of all compound xerogels from gels were measured. Firstly, TC16-Azo-Me samples were taken as example, as shown in shown in Figure 7A. The curves for TC16-Azo-Me xerogel samples show similar main peaks in the angle region (2θ values: 5.26°, 7.74°, 21.38°, and 23.12°) corresponding to the d values of 1.68, 1.14, 0.42, and 0.38 nm, respectively. The corresponding d values of 1.68 and 0.42 nm follow a ratio of 1:1/4, suggesting a lamellar-like structure of the aggregates in the gel . In addition, the XRD data of xerogels of all compounds in DMF were compared, as shown in Figure 7B. Firstly, the curve for TC16-Azo xerogel in DMF shows one weak peak at a 2θ value of 4.36° corresponding to the d value of 2.03 nm. As for the curve of SC16-Azo, many peaks were obtained, suggesting a polycrystalline structure. In addition, only a little bit peaks in the low angle range observed in the curve of SC16-Azo-Me, indicating an amorphous state. The XRD results described above demonstrated again that the substituent groups had a great effect on the assembly modes of these compounds.
Four azobenzene imide derivatives with different substituent groups have been synthesized. Their gelation behaviors in various organic solvents can be regulated by changing alkyl substituent chains and headgroups of azobenzene segment. The substituent groups in azobenzene residue and benzoic acid derivatives can have a profound effect upon the gelation abilities of these studied compounds. More alkyl chains in molecular skeletons in present gelators are favorable for the gelation of organic solvents. Morphological studies revealed that the gelator molecules self-assemble into different aggregates, from wrinkle, lamella, and belt to fiber with the change of solvents. Spectral studies indicated that there existed different H-bond formations between imide groups and conformations of methyl chains, depending on the substituent groups in molecular skeletons. These results afford useful information for the design and development of new versatile low molecular mass organogelators and soft matter.
TJ and QZ are associate professors. YW is an MD student. FG is a professor and the Dean of the School of Environmental and Chemical Engineering. JZ is a laboratory assistant in Yanshan University.
Oh H, Jung BM, Lee HP, Chang JY: Dispersion of single walled carbon nanotubes in organogels by incorporation into organogel fibers. J Colloid Interf Sci 2010, 352: 121–127. 10.1016/j.jcis.2010.08.025
Delbecq F, Kaneko N, Endo H, Kawai T: Solvation effects with a photoresponsive two-component 12-hydroxystearic acid-azobenzene additive organogel. J Colloid Interf Sci 2012, 384: 94–98. 10.1016/j.jcis.2012.06.045
Wang X, Zhou L, Wang H, Luo Q, Xu J, Liu J: Reversible organogels triggered by dynamic K + binding and release. J Colloid Interf Sci 2011, 353: 412–419. 10.1016/j.jcis.2010.09.089
Wang C, Li Z, Wang X, Wei W, Chen S, Sui Z: Gelation mechanism and microstructure of organogels formed with L-Valine dihydrazide derivatives. Colloid Surf A-Physicochem Eng Asp 2011, 384: 490–495. 10.1016/j.colsurfa.2011.05.027
Xing P, Sun T, Li S, Hao A, Su J, Hou Y: An instant-formative heat-set organogel induced by small organic molecules at a high temperature. Colloid Surf A-Physicochem Eng Asp 2013, 421: 44–50.
Xin F, Zhang H, Hao B, Sun T, Kong L, Li Y, Hou Y, Li S, Zhang Y, Hao A: Controllable transformation from sensitive and reversible heat-set organogel to stable gel induced by sodium acetate. Colloid Surf A-Physicochem Eng Asp 2012, 410: 18–22.
Roy S, Chakraborty A, Chattopadhyay B, Bhattacharya A, Mukherjee AK, Ghosh R: Tailor-made chiral pyranopyrans based on glucose and galactose and studies on self-assembly of some crystals and low molecular weight organogel (LMOG). Tetrahedron 2010, 66: 8512–8521. 10.1016/j.tet.2010.08.054
Lofman M, Koivukorpi J, Noponen V, Salo H, Sievanen E: Bile acid alkylamide derivatives as low molecular weight organogelators: Systematic gelation studies and qualitative structural analysis of the systems. J Colloid Interf Sci 2011, 360: 633–644. 10.1016/j.jcis.2011.04.112
Bastiat G, Plourde F, Motulsky A, Furtos A, Dumont Y, Quirion R, Fuhrmann G, Leroux JC: Tyrosine-based rivastigmine-loaded organogels in the treatment of Alzheimer’s disease. Biomaterials 2010, 31: 6031–6038. 10.1016/j.biomaterials.2010.04.009
Tao ZG, Zhao X, Jiang XK, Li ZT: A hexaazatriphenylene-based organogel that responds to silver(I) with high selectivity under aqueous condition. Tetrahedron Lett 2012, 53: 1840–1842. 10.1016/j.tetlet.2012.01.137
Miyamoto K, Jintoku H, Sawada T, Takafuji M, Sagawa T, Ihara H: Informative secondary chiroptics in binary molecular organogel systems for donor-acceptor energy transfer. Tetrahedron Lett 2011, 52: 4030–4035. 10.1016/j.tetlet.2011.05.131
Pieve SD, Calligaris S, Panozzo A, Arrighetti G, Nicoli MC: Effect of monoglyceride organogel structure on cod liver oil stability. Food Res Int 2011, 44: 2978–2983. 10.1016/j.foodres.2011.07.011
Iwanaga K, Sumizawa T, Miyazaki M, Kakemi M: Characterization of organogel as a novel oral controlled release formulation for lipophilic compounds. Int J Pharm 2010, 388: 123–128. 10.1016/j.ijpharm.2009.12.045
Bhatia A, Singh B, Raza K, Wadhwa S, Katare OP: Tamoxifen-loaded lecithin organogel (LO) for topical application: Development, optimization and characterization. Int J Pharm 2013, 444: 47–59. 10.1016/j.ijpharm.2013.01.029
Iwanaga K, Kawai M, Miyazaki M, Kakemi M: Application of organogels as oral controlled release formulations of hydrophilic drugs. Int J Pharm 2012, 436: 869–872. 10.1016/j.ijpharm.2012.06.041
Yu X, Li Y, Yin Y, Yu D: A simple and colorimetric fluoride receptor and its fluoride-responsive organogel. Mater Sci Eng C 2012, 32: 1695–1698. 10.1016/j.msec.2012.04.070
Takizawa M, Kimoto A, Abe J: Photochromic organogel based on [2.2]paracyclophane-bridged imidazole dimer with tetrapodal urea moieties. Dyes Pigments 2011, 89: 254–259. 10.1016/j.dyepig.2010.03.019
Xue M, Gao D, Chen X, Liu K, Fang Y: New dimeric cholesteryl-based A(LS)2 gelators with remarkable gelling abilities: Organogel formation at room temperature. J Colloid Interf Sci 2011, 361: 556–564. 10.1016/j.jcis.2011.05.074
Delbecq F, Tsujimoto K, Ogue Y, Endo H, Kawai T: N-stearoyl amino acid derivatives: Potent biomimetic hydro/organogelators as templates for preparation of gold nanoparticles. J Colloid Interf Sci 2013, 390: 17–24. 10.1016/j.jcis.2012.09.029
Svobodova H, Nonappa , Wimmer Z, Kolehmainen E: Design, synthesis and stimuli responsive gelation of novel stigmasterol-amino acid conjugates. J Colloid Interf Sci 2011, 361: 587–593. 10.1016/j.jcis.2011.05.084
Kim JU, Schollmeyer D, Brehmer M, Zentel R: Simple chiral urea gelators, (R)- and (S)-2-heptylurea: Their gelling ability enhanced by chirality. J Colloid Interf Sci 2011, 357: 428–433. 10.1016/j.jcis.2011.02.006
Huang Y, Ge J, Cai Z, Hu Z, Hong X: The correlation of microstructure morphology with gelation mechanism for sodium soaps in organic solvents. Colloid Surf A-Physicochem Eng Asp 2012, 414: 88–97.
Ren X, Yu W, Zhang Z, Xia N, Fu G, Lu X, Wang W: Gelation and fluorescent organogels of a complex of perylenetetracarboxylic tetraacid with cationic surfactants. Colloid Surf A-Physicochem Eng Asp 2011, 375: 156–162. 10.1016/j.colsurfa.2010.11.080
He P, Liu J, Liu K, Ding L, Yan J, Gao D, Fang Y: Preparation of novel organometallic derivatives of cholesterol and their gel-formation properties. Colloid Surf A-Physicochem Eng Asp 2010, 362: 127–134. 10.1016/j.colsurfa.2010.04.009
Zhao W, Li Y, Sun T, Yan H, Hao A, Xin F, Zhang H, An W, Kong L, Li Y: Heat-set supramolecular organogels composed of β-cyclodextrin and substituted aniline in N, N-dimethylformamide. Colloid Surf A-Physicochem Eng Asp 2011, 374: 115–120. 10.1016/j.colsurfa.2010.11.021
Jiao TF, Wang YJ, Gao FQ, Zhou JX, Gao FM: Photoresponsive organogel and organized nanostructures of cholesterol imide derivatives with azobenzene substituent groups. Prog Nat Sci 2012, 22: 64–70. 10.1016/j.pnsc.2011.12.011
Jiao TF, Gao FQ, Wang YJ, Zhou JX, Gao FM, Luo XZ: Supramolecular gel and nanostructures of bolaform and trigonal cholesteryl derivatives with different aromatic spacers. Curr Nanosci 2012, 8: 111–116. 10.2174/1573413711208010111
Yang H, Yi T, Zhou Z, Zhou Y, Wu J, Xu M, Li F, Huang C: Switchable fluorescent organogels and mesomorphic superstructure based on naphthalene derivatives. Langmuir 2007, 23: 8224–8230. 10.1021/la7005919
Zhao C, Bai B, Wang H, Qu S, Xiao G, Tian T, Li M: Self-assemblies, helical ribbons and gelation tuned by solvent–gelator interaction in a bi-1,3,4-oxadiazole gelator. J Mol Struct 2013, 1037: 130–135.
Lupi FR, Gabriele D, Greco V, Baldino N, Seta L, de Cindio B: A rheological characterisation of an olive oil/fatty alcohols organogel. Food Res Int 2013, 51: 510–517. 10.1016/j.foodres.2013.01.013
Zhu GY, Dordick JS: Solvent effect on organogel formation by low molecular weight molecules. Chem Mater 2006, 18: 5988–5995. 10.1021/cm0619297
Xin H, Zhou X, Zhao C, Wang H, Lib M: Low molecular weight organogel from the cubic mesogens containing dihydrazide group. J Mol Liq 2011, 160: 17–21. 10.1016/j.molliq.2011.02.002
Nayak MK: Functional organogel based on a hydroxyl naphthanilide derivative and aggregation induced enhanced fluorescence emission. J Photochem Photobiol A: Chem 2011, 217: 40–48. 10.1016/j.jphotochem.2010.09.014
Atsbeha T, Bussotti L, Cicchi S, Foggi P, Ghini G, Lascialfari L, Marcelli A: Photophysical characterization of low-molecular weight organogels for energy transfer and light harvesting. J Mol Struct 2011, 993: 459–463. 10.1016/j.molstruc.2010.12.046
Shimizu T, Masuda M: Stereochemical effect of even-odd connecting links on supramolecular assemblies made of 1-glucosamide bolaamphiphiles. J Am Chem Soc 1997, 119: 2812–2818. 10.1021/ja961226y
Kogiso M, Ohnishi S, Yase K, Masuda M, Shimizu T: Dicarboxylic oligopeptide bola-amphiphiles: proton-triggered self-assembly of microtubes with loose solid surfaces. Langmuir 1998, 14: 4978–4986. 10.1021/la9802419
Wang TY, Li YG, Liu MH: Gelation and self-assembly of glutamate bolaamphiphiles with hybrid linkers: effect of the aromatic ring and alkyl linkers. Soft Matter 2009, 5: 1066–1073. 10.1039/b813932f
Li YG, Wang TY, Liu MH: Ultrasound induced formation of organogel from a glutamic dendron. Tetrahedron 2007, 63: 7468–7473. 10.1016/j.tet.2007.02.070
Suarez M, Fernandez A, Menendez JL, Torrecillas R: Transparent yttrium aluminium garnet obtained by spark plasma sintering of lyophilized gels. J Nanomater 2009, 2009: 138490.
Wu JC, Yi T, Xia Q, Zou Y, Liu F, Dong J, Shu TM, Li FY, Huang CH: Tunable gel formation by both sonication and thermal processing in a cholesterol-based self-assembly system. Chem Eur J 2009, 15: 6234–6243. 10.1002/chem.200900409
This work was financially supported by the National Natural Science Foundation of China (Grant nos. 20903078, 21207112), the Natural Science Foundation of Hebei Province (Grant nos. B2012203060, B2013203108), the China Postdoctoral Science Foundation (Grant nos. 2011M500540, 2012M510770), the Support Program for Hundred Excellent Innovation Talents from Universities and Colleges of Hebei Province (Grant no. CPRC020), the Science Foundation for the Excellent Youth Scholars from Universities and Colleges of Hebei Province (Grant no. Y2011113), the Scientific Research Foundation for Returned Overseas Chinese Scholars of Hebei Province (Grant no. 2011052), and the Open Foundation of State Key Laboratory of Solid Lubrication (Lanzhou Institute of Chemical Physics, CAS) (Grant no. 1002).
The authors declare that they have no competing interests.
TJ carried out the synthesis of compounds and characterization of organogels. YW participated in the analysis and the testing of the nanostructures. QZ and FG supervised this work, helped in the analysis and interpretation of data, and, together with JZ, worked on the drafting and revisions of the manuscript. TJ and QZ conceived of the study and participated in its design and coordination. JZ participated in the design of the study and provided analysis instruments. All authors read and approved the final manuscript.
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Jiao, T., Wang, Y., Zhang, Q. et al. Regulation of substituent groups on morphologies and self-assembly of organogels based on some azobenzene imide derivatives. Nanoscale Res Lett 8, 160 (2013). https://doi.org/10.1186/1556-276X-8-160
- Substituent groups
- Imide derivative