The gelation ability of 10 alkylammonium (CnH(2n+1)NH3+ where n=4-11, 12 and 16) anthracene-9-carboxylates (1n) has been evaluated. In cyclohexane, 1(4), 1(5), 1(6) and 1(7) only provided precipitates whereas 1(11), 1(12) and 1(16) provided very viscous solutions. In contrast, 1(8) 1(9) and 1(10) resulted in gels. The critical gelation concentration of 1(10) was very low (5.0 x 10(-4) mol dm(-3)). SEM observations showed that in the gel phase the morphology changes from straight fibrils to frizzy fibrils with the increase in n, whereas in the sol phase the formation of the sheet-like, two-dimensional aggregate is recognized. When the cyclohexane 1(10) gel was photoirradiated (lambda > 300 nm), the UV-VIS absorption bands assignable to monomeric anthracene were decreased and the gel was changed into the sol. It was confirmed by dark-field optical microscopy that the fibrillar bundles supporting the gel formation gradually disappear with photoirradiation time. When this sol was warmed at 30 degrees C in the dark, the gel was not regenerated but the precipitation of 1(10) resulted. When this sol was heated once at the bp of cyclohexane and cooled to 15 degrees C, the solution was changed into the gel again. This finding indicates that the fibrillar structure required for the gel formation is not reconstructed at 30 degrees C but obtained only when the hot cyclohexane solution is cooled.
Porphyrins bearing four urea-linked dodecyl groups (3a) or four urea-linked triethoxysilylpropyl groups (3TEOS) at their peripheral positions were synthesized. 3a tends to assemble into a sheetlike two-dimensional structure due to the predominant hydrogen-bonding interaction among the urea groups and acts as a moderate gelator of organic solvents. On the other hand, its Cu(II) compelx (3a.Cu) tends to assemble into a fibrous one-dimensional structure due to the predominant porphyrin-porphyrin pi-pi stacking interaction and acts as an excellent gelator of many organic solvents. 3TEOS and 3TEOS.Cu, which also act as gelators, afforded similar superstructures as those of 3a and 3a.Cu, respectively, and as evidenced by SEM and TEM observations and XRD measurements, the original superstructures could be precisely immobilized by in situ sol-gel polycondensation of the triethoxysilyl groups. The TEM images of 3a gels and 3TEOS gels after sol-gel polycondensation showed a fine striped structure, the periodical distance of which was either 2 or 4 nm. X-ray crystallographic analysis of a single crystal obtained from a reference porphyrin bearing four urea-linked butyl groups revealed that there are two different porphyrin-stacked columns in the crystal and both the 2 nm distance and the 4 nm distance can appear, depending on the observation tilting angle. The hybrid gel prepared from 3TEOS.Cu by sol-gel polycondensation showed unique physicochemical properties such as a high sol-gel phase-transition temperature (>160 degrees C), sufficient elasticity, high mechanical strength, etc. Thus, the present study has established new concepts for molecular design of porphyrin-based gelators on the basis of cooperative and/or competitive actions of hydrogen-bonding and pi-pi stacking interactions and for immobilization of their superstructures leading to development of new functional organic/inorganic hybrid materials.
We have demonstrated that a one-dimensional molecular assembly created by an H-aggregated porphyrin.Cu(II) stack can be immobilized, without a morphological change, by sol-gel polycondensation of the peripheral triethoxysilyl groups. The resultant gel prepared according to this flowchart has gained a very high thermal stability as well as a unique mechanical strength.
Materials that change their physical properties on mechanical agitation are known as thixotropic materials. In the human body, for example, the functions of protoplasm, red blood cells, synovial fluid, and muscular activities are regulated by thixotropy. [1][2][3] Similarly, the properties of materials such as paints, bentonite clay, cosmetics, and foodstuffs arise from thixotropy. Despite the significant potential of this dynamic phenomenon, there is a lack of materials that can act as a model system to investigate vital natural processes such as muscle thixotropy and nerve fiber regeneration. In this respect, low-molecular-weight organogels [4] (LMOGs), which generally consists of 1D fibrous molecular assemblies, are envisioned to be attractive candidates when provided with thixotropic properties. However, not only are examples of LMOGs that exhibit thixotropy rare, [5][6][7][8][9] but there is also little information related to their aggregation and stability. These aspects circumvent the use of LMOGs for the study of this unique phenomenon.Of the hundreds of functional LMOGs reported to date, only very few exhibit the unique property of thixotropy. In general, LMOGs are extremely sensitive to mechanical stress and these materials irreversibly expel solvent molecules from their network when subjected to flow. On removal of the external force, these materials behave as a solid suspension that loses its original elastic properties. On the other hand, thixotropic gels can disintegrate in solution under an external mechanical stress and can regain their elastic properties upon removal of the stress. This operation can normally be carried out for an infinite number of cycles.Thixotropic LMOGs represent an intriguing and unique class of truly dynamic self-assembled supramolecular systems. Discernible visual insights into the process would be required in order to understand how such self-assembled entities evolve under mechanical stress followed by a resting time. To obtain such "snapshots", the thixotropic gel has to operate under a strict real concentration domain. However, the LMOGs reported to date show thixotopic properties only at concentrations of 10 wt % or more. At these high concentrations, the molecules are closely associated and therefore the reaggregation to the gel state from the disaggregated solution state occurs within seconds, thus rendering these systems unviable for real-time imaging. Hence, from the perspective of a supramolecular chemist, a successful thixotropic LMOG candidate has to fulfill three stringent requirements: firstly, the LMOG has to be an extremely efficient gelator (close to a super-gelator); secondly, it must exhibit the thixotropic property at such low concentrations; and thirdly, the gel has to maintain its original state without undergoing aging or forming crystalline domains during the cycles of breaking and regeneration processes. It is therefore difficult to obtain a supramolecular system that comprises all of these favorable functional features.Herein we report the discove...
All line up together! A new concept for the alignment and assembly of conjugated polymers through the action of supramolecular bundling (“aligner”) molecules is inspired by actin‐filament bundling proteins. The approach provides a general means of preparing complex, ordered assemblies of conjugated polymers.
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