New benzimidazole/benzothiazole imide derivatives with different alkyl substituent chains were designed and synthesized. Their gelation behaviors in 22 solvents were tested as novel low-molecular-mass organic gelators. The test showed that the alkyl substituent chains and headgroups of benzimidazole/benzothiazole 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. SEM and AFM observations revealed that the gelator molecules self-assemble into different aggregates from wrinkle, lamella and belt to dot with change of solvents. Spectral studies indicated that there existed different H-bond formation between imide groups and hydrophobic force of alkyl substituent chains in molecular skeletons. The present work may give some insights into design and character of new organogelators and soft materials with special molecular structures.
In recent years, self-assembly and gelation properties of low-molecular-mass organic gelators have become one of the hot areas in soft matter research due to their scientific values and many potential applications in biomedical field, including tissue engineering, controlled drug release, and medical implants [
In addition, as versatile units, the covalently bound benzimidazole/benzothiazole groups, of which one type of rings is benzene rings and the other type is a five-member ring with N or S elements, have been widely chosen for designing new amphiphiles because of their unique directional self-association through
In this paper, as a continuous work, we have designed and synthesized new benzimidazole/benzothiazole imide derivatives with different alkyl substituent chains. We have found that all compounds could form different organogels in various organic solvents. Characterization of the organogels by SEM and AFM revealed different structures of the aggregates in the gels. We have investigated the effect of alkyl substituent chains and headgroups residues in gelators on the microstructures of such organogels in detail and found different kinds of hydrogen bond interactions between intermolecular imide groups.
The starting materials, 2-aminobenzimidazole and 2-aminobenzothiazole, were purchased from Alfa Aesar Tianjin Chemicals. Other used reagents were all for analysis purity from TCI Shanghai Chemicals or Aldrich Chemicals, respectively. 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 previous report [
Structures and abbreviations of these imide derivatives with different substituent groups.
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 12–24 h. The dried sample thus obtained was attached to mica, copper foil, glass, and CaF2 slice for morphological and spectral investigation, respectively. Before SEM measurement, the samples were coated on copper foil fixed by conductive adhesive tape and shielded by gold. SEM pictures of the xerogel were taken on a Hitachi S-4800 field emission scanning electron microscopy with the accelerating voltage of 5–15 kV. AFM images were recorded using Nanoscope VIII Multimode Scanning Probe Microscope (Veeco Instrument, USA) with silicon cantilever probes. All AFM images were shown in the height mode without any image processing except flattening. Transmission FT-IR spectra of the xerogel were obtained by Nicolet is/10 FT-IR spectrophotometer from Thermo Fisher Scientific Inc. by average 32 scans and at a resolution of 4 cm−1. The XRD measurement was conducted using a Rigaku D/max 2550PC diffractometer (Rigaku Inc., Tokyo, Japan). The XRD pattern was obtained using CuK
The gelation performances of all compounds in 22 solvents are listed in Table
Gelation behaviors of all compounds at room temperature.a
Solvents | SC16-S | TC16-S | SC16-N | TC16-N |
---|---|---|---|---|
Acetonitrile | I | I | I | I |
1,4-Dioxane | S | I | I |
|
Isopentanol | I |
|
I |
|
n-Butyl acrylate | I | I |
|
S |
Benzene | S | S | S | S |
Formaldehyde | I | I | I | I |
n-Butanol |
|
|
I |
|
Petroleum ether | I | S |
|
I |
Nitrobenzene | S |
|
|
|
Tetrachloromethane | S | S | S | S |
Acetone | S | I |
|
I |
Cyclohexanone | S | I |
|
S |
Cyclopentanone | I | I |
|
I |
n-Propanol |
|
|
I |
|
Pyridine | S | S | I | S |
THF | S | S | S | S |
DMF | I |
|
I |
|
Ethyl acetate | I |
|
I |
|
Ethanolamine |
|
I |
|
I |
Isopropanol | I |
|
I |
|
n-Hexane | I | S | I | S |
Aniline | I |
|
I |
|
aS: solution; G: gel; I: insoluble.
Photographs of organogels in different solvents: (a) SC16-S; (b) TC16-S; (c) SC16-N; (d) TC16-N.
In addition, in order to obtain a visual insight into the gel microstructures, the typical nanostructures of the xerogels were studied by SEM technique, as shown in Figures
SEM images of xerogels from SC16-S gels ((a) n-butanol; (b) n-propanol; (c) ethanolamine, resp.) and TC16-S gels ((d) isopentanol; (e) n-butanol; (f) nitrobenzene; (g) n-propanol; (h) DMF; (i) ethyl acetate; (j) isopropanol; (k) aniline, resp.).
SEM images of xerogels from SC16-N gels ((a) n-butyl acrylate; (b) petroleum ether; (c) nitrobenzene; (d) acetone; (e) cyclohexanone; (f) cyclopentanone; (g) ethanolamine, resp.) and TC16-N gels ((h) 1,4-dioxane; (i) isopentanol; (j) n-butanol; (k) nitrobenzene; (l) n-propanol; (m) DMF; (n) ethyl acetate; (o) isopropanol; (p) aniline, resp.).
AFM images of xerogels: SC16-S (a) and TC16-S (b) gels in n-butanol, respectively; SC16-N (c) and TC16-N (d) gels in nitrobenzene, respectively.
In addition, in order to further investigate the orderly stacking of xerogels nanostructures, XRD of all compounds xerogels from gels were measured. Firstly, the data of TC16-S was taken as an example, as shown in Figure
X-ray diffraction patterns of xerogels: (a) TC16-S (a: isopentanol; b: n-butanol; c: nitrobenzene; d: n-propanol; e: DMF; f: ethyl acetate; g: isopropanol; h: aniline, resp.); (b) SC16-S (a) and TC16-S (b) gels in n-butanol, respectively; SC16-N (c) and TC16-N (d) gels in nitrobenzene, respectively.
It is well-known that hydrogen bonding plays an important role in the formation of organogels [
FT-IR spectra of xerogels: (a) TC16-S (a: isopentanol; b: n-butanol; c: nitrobenzene; d: n-propanol; e: DMF; f: ethyl acetate; g: isopropanol; h: aniline, resp.); (b) SC16-S (a) and TC16-S (b) gels in n-butanol, respectively; SC16-N (c) and TC16-N (d) gels in nitrobenzene, respectively.
In summary, some benzimidazole/benzothiazole imide derivatives with different alkyl substituent chains have been synthesized. Their gelation behaviors in various organic solvents can be regulated by changing alkyl substituent chains and headgroups of benzimidazole/benzothiazole segment. The experimental data demonstrated that the numbers of alkyl substituent chains linked to benzene ring in these imide 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 dot with change of solvents. Spectral studies indicated that there existed different H-bond formation and hydrophobic force, depending on benzimidazole/benzothiazole segment and alkyl substituent chains in molecular skeletons. The present research work affords new useful exploration for the design and development of new versatile low-molecular-mass organogelators and soft matter.
The authors declare that they have no any direct financial relation with the commercial identities mentioned in this paper that might lead to a conflict of interest for any of the authors.
This work was financially supported by the National Natural Science Foundation of China (Grant nos. 20903078 and 21207112), the Natural Science Foundation of Hebei Province (Grant nos. B2012203060 and B2013203108), the China Postdoctoral Science Foundation (Grant nos. 2011M500540 and 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).