Norifumi Fujita

Spontaneous assembly of ten components into two interlocked, identical coordination cages

Supermolecules consisting of interlinked ring-like molecules (catenanes) are an interesting target for chemical synthesis both for their intrinsic interest as non-covalently bound but robust assemblies and because of the perspective they offer on materials chemistry. Catenanes have been prepared by metal-ion templating, and self-assembly through other non-covalent interactions,,,,,,,. Here we report the synthesis of a catenane composed not of two interlocking rings but of two cages. This structure is prepared by metal-mediated self-assembly,,. The framework of each cage is assembled from five components: two tridentate ligands held together with three metal ions. Because each cage framework can bind an aromatic ring, two cage units will bind one another during their assembly process through the formation of a quadruple aromatic stack, giving rise to the ten-component interlocked supermolecule,.

Post-polymerization of preorganized assemblies for creating shape-controlled functional materials.

Combination of supramolecular chemistry with molecular recognition has been successfully applied to creating large superstructures with a wide variety of morphologies. Control of shapes and patterns of ordered molecular assemblies in nano and micro scales has attracted considerable interest as promising bottom-up technology. It is known, however, that these molecular assembling superstructures are fragile, reflecting the characteristic of the non-covalent interaction, a driving force operating in these molecular systems. In fact, they easily collapse or change by small perturbation in the environmental conditions. Thus, over the last decade, researchers have been seeking possible methods for the immobilization these superstructures. This critical review focuses on recent advances in in situ post-modification under the influence of the molecular assemblies as templates and polymerization of ordered molecular assemblies such as organogel fibers and crystals to preserve their original superstructures and intensify their 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.

Manipulation of discrete nanostructures by selective modulation of noncovalent forces

Nanotube Engineering In general, the reversible self-assembly of nanotubes through noncovalent bonding either gives fully assembled or fully disassembled products. Fukino et al. (p. 499, published online 10 April; see the Perspective by Hudson and Manners) developed a system with an intermediate possibility. Hollow nanotube structures were assembled from ferrocene-based tetratopic pyridyl ligands mixed with AgBF4. Through oxidation of the ferrocene groups, the tubes could be cut into stable, large rings and then reversibly reassembled into nanotubes by reduction of the ferrocene groups. Self-assembled, large-diameter ferrocene-based nanotubes can be cleaved into stable rings and then reassembled into tubes. [Also see Perspective by Hudson and Manners] Covalent organic synthesis commonly uses the strategy of selective bond cleavage and formation. If a similar approach can be applied stepwisely to noncovalent synthesis, more exotic or challenging nanostructures might become achievable. Here, we report that ferrocene-based tetratopic pyridyl ligands, which can dynamically change their geometry by means of thermal rotation of their cyclopentadienyl rings in solution, assemble with AgBF4 into discrete metal-organic nanotubes with large and uniform diameters. The nanotubes can be cut into metal-organic nanorings through selective attenuation of the inter-nanoring interaction via ferrocene oxidation. The resultant nanorings can be transferred onto inorganic substrates electrostatically or allowed to reassemble to form the original nanotube by the reductive neutralization of their oxidized ferrocene units.

Proton-sensitive fluorescent organogels

A 1,10-phenanthroline-appended cholesterol-based gelator (1) and its nongelling reference compound (2) were synthesized. Among 19 solvents tested herein, gelator 1 could gelate 11 solvents including alcohols, dipolar aprotic solvents, organic acids and a base (triethylamine), indicating that 1 acts as a versatile gelator. The TEM observation gave a visual image showing that fibrillar aggregates are entangled in the three-dimensional network structure. In the fluorescence measurements, most gels afforded an emission maximum at 394 nm (purple emission), whereas only the acetic acid gel afforded an emission maximum at 522 nm (yellow emission). Thus, the influence of protonation of the 1,10-phenanthroline nitrogens (by trifluoroacetic acid) on the fluorescence properties in the gel phase was investigated in detail. The results have established that the fluorescence intensity of 1 x H+ becomes particularly strong in the gel phase, presumably because of the energy transfer from neutral 1* to protonated 1 x H+ and the restriction of the 1 x H+ molecular motion. The finding suggests the possibility that the gel system would be useful not only as a new proton-sensitive fluorescence system but also as a new medium for designing efficient energy transfer systems.

Discotic ionic liquid crystals of triphenylene as dispersants for orienting single-walled carbon nanotubes.

Orient and conduct: Triphenylene-based discotic ionic liquid crystals (ILCs) with six imidazolium ion pendants can disperse pristine single-walled carbon nanotubes (SWNTs). When the ILC is columnarly assembled, doping with SWNTs results in macroscopic homeotropic columnar orientation. Combination of shear and annealing treatments gives rise to three different orientation states, which determine the anisotropy of electrical conduction.

Organogel of an 8-quinolinol platinum(II) chelate derivative and its efficient phosphorescence emission effected by inhibition of dioxygen quenching.

A newly synthesized 8-quinolinol platinum(II) chelate derivative gelates various organic solvents, and the gel formed shows unique thermo- and solvatochromism of visible and phosphorescent colour in response to a sol-gel phase transition and possesses an attractive ability to inhibit dioxygen quenching of excited triplet states.

Novel host–guest organogels as stabilized by the formation of crown–ammonium pseudo-rotaxane complexes

A dibenzo-24-crown-8 derivative bearing two cholesterol groups is either insoluble in or precipitates from most organic solvents, but its pseudo-rotaxane complex with a diammonium guest acts as a good gelator of aromatic solvents.

Fluorescent organogels as templates for sol–gel transcription toward creation of optical nanofibers

1,10-Phenanthroline-appended cholesterol-based gelators (1 and 2) and their non-gelling reference compounds (1′ and 2′) were synthesized. It was shown that the gelation abilities of 1 and 2 are quite different, in spite of their structural similarity. Compound 1 can form gels with various kinds of organic solvents, whereas 2 is capable of gelation only in acidic solvents. This difference in gelation abilities seems to be due to the difference in the stacking mode of the phenanthroline moieties, as confirmed by CD spectra. TEM and AFM observations showed that fibrillar aggregates are entangled in a three-dimensional network structure. The gels also showed beautiful fluorescence, characteristic of their molecular structures and gelation conditions (in the presence and the absence of acid). Organic assemblies thus characterized were subjected as templates for sol–gel polycondensation of metal alkoxide such as tetraethyl orthosilicate (TEOS) and tetra-n-butyl titanate. The resultant organic–inorganic composite materials showed the same fluorescence properties as the organic gels. After calcination, it was revealed by TEM observation that the silica and titania materials have a hollow structure, indicating that the sol–gel polycondensation reaction proceeds selectively at the surface of the organic assemblies. The CD spectrum of the organic–inorganic composite was similar to that of the organogel, indicating that the aggregation stacking mode of the original phenanthroline moieties is still retained even in the silica-gel phase. The organic–inorganic hybrids obtained are expected to lead to various nanostructured optoelectronic devices.

Sol–gel transcription of silica-based hybrid nanostructures using poly(N-vinylpyrrolidone)-coated [60]fullerene, single-walled carbon nanotube and block copolymer templates

This review concentrates on our recent achievements that have enabled us to construct the novel organic/inorganic-hybridized materials. In a natural system, biomineralization processes are initiated where organic elements meet inorganic counterparts. This situation implies that both elements of organic and inorganic species should interact selectively with each other. In other words, one cannot join the organic and inorganic elements if there is no strong interaction in between. Our approach to solve this dilemma includes the use of a ‘chemical glue’. The glue is the third component for hybridizing organic/inorganic materials. The glue should be selected so as to interact with both organic and inorganic components. The inorganic precursors contact with the organic template and react with the help of catalysts, which leads to the accumulation of the inorganic material selectively on the surface of the organic template. This process, especially its initiation step resembles the natural processes except for the use of glue. To apply this idea to sol–gel reactions, we adopted three templates different in a hierarchical sense: that is, [60]fullerene as a zero-dimensional template, single-walled carbon nanotube as a one-dimensional template and a cast film of styrene/4-vinylpyridine block copolymer as a two-dimensional template. Firstly, we successfully obtained the [60]fullerene aggregate-silica spherical clusters and SWNTs-silica hybridized nanorods even though no strong interaction exists between silica and carbonclusters. Secondly, this concept was applied to a thin film system having periodic surface structures. The films were prepared from polystyrene-poly(4-vinylpyridine) diblock copolymer. Once AcOH is extended to the film, the protonated phase attracts the metallic components and also acts as a catalyst to mediate the sol–gel reaction. These methods are feasible for hybridizing organic and inorganic materials with the aid of the amphiphilic nature of the glue.