Efficient linking of two epoxides using potassium thioacetate in water and its use in polymerization.

Here, we show that two epoxides can be efficiently linked using potassium thioacetate (AcSK) in water even at their imbalanced stoichiometric ratios. We found that the first reaction between epoxide and AcSK gave rise to an intermediate that underwent the second reaction with another epoxide with a reactivity much higher than that of AcSK. Time-dependent 1H NMR measurements revealed that the rate constant of the second reaction was 31 times larger than that of the first reaction. Using this reaction, we succeeded in nonstoichiometric polymerization of a bifunctional epoxide. Furthermore, in the presence of a multifunctional epoxide, we obtained hydrogels and self-standing films. We expect that this straightforward and efficient reaction of versatile reagents, epoxide and AcSK, in water would lead to new applications of epoxides.

Creation of polynucleotide-assisted molecular assemblies in organic solvents: general strategy toward the creation of artificial DNA-like nanoarchitectures.

The influence of added polynucleotide on the gelation ability of nucleobase-appended organogelators was investigated. Uracil-appended cholesterol gelator formed a stable organogel in polar organic solvents such as n-butanol. It was found that the addition of the complementary polyadenylic acid (poly(A)) not only stabilizes the gel but also creates the helical structure in the original gel phase. Thymidine and thymine-appended gelators can form stable gel in apolar solvents, such as benzene, where poly(A)-lipid complex can act as a complementary template for the gelator molecules to create the fibrous composites. Based on these findings, we can conclude that self-assembling modes and gelation properties of nucleobase-appended organogelators are controllable by the addition of their complementary polynucleotide in organic solvents. We believe, therefore, that the present system can open the new paths to accelerate development of well-controlled one-dimensional molecular assembly systems, which would be indispensable for the creation of novel nanomaterials based on organic compounds.

Axial coordination changes the morphology of porphyrin assemblies in an organogel system.

The gelation properties of a zinc porphyrin bearing peripheral urea groups (1 x Zn) were evaluated in the absence and the presence of several diamines. In aromatic solvents such as benzene, toluene and p-xylene, 1 x Zn only provided the precipitate. In contrast, 1 x Zn with 0.5 and 1.0 equiv. of piperazine formed gels, and the gel with 0.5 equiv. of piperazine showed a unique physical property called 'thixotropy'. On the other hand, upon addition of similar diamines such as DABCO, ethylenediamine and N,N'-dimethylethylenediamine, 1 x Zn did not gelate these solvents. When the critical gelation concentration was plotted against the ratio of piperazine versus 1 x Zn, it afforded a minimum breakpoint at 0.5 equiv. and the critical concentration increased with further increase in the fraction of piperazine, indicating that the stable gel is formed from the 1 x Zn + piperazine 2:1 complex and the subsequent transformation to the 1:1 complex rather destabilizes the gel. Very interestingly, it was clearly shown by SEM and TEM observations that such structural changes of the unit complex induced by the ratio of piperazine versus 1 x Zn can lead to gradual morphological transitions: that is, spherical structure at 0 equiv., 1-D fibrous structure at 0.5 equiv. and 2-D sheet-like structure at 1.0 equiv. In addition, UV-VIS spectra revealed that 1 x Zn itself adopts a J-aggregation mode, whereas 1 x Zn + piperazine 2:1 and 1:1 complexes adopt an H-like aggregation mode. On the other hand, upon addition of 0.5 equiv. of other diamines, 1 x Zn + diamine complexes result in different morphologies other than the 1-D fibrous structure. To explore a reasonable rationale for these results, we conducted computational studies. As a result, we found that the complex symmetry of the unit complex plays an important role in determining the final ordered structure.

Porphyrin gels reinforced by sol-gel reaction via the organogel phase.

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.

Sol-gel reaction of porphyrin-based superstructures in the organogel phase: creation of mechanically reinforced porphyrin hybrids.

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.

Binary organogelators which show light and temperature responsiveness.

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.