Masumi Asakawa

Ein chemisch und elektrochemisch schaltbares [2]Catenan mit Tetrathiafulvalen-Einheit

Ein [2]Catenan als mechanischer Schalter: In dieser Verbindung umschliest ein Cyclobis(paraquat-p-phenylen)-Tetrakation einen makrocyclischen Polyether, der eine redoxaktive Tetrathiafulvalen(TTF)-Einheit und einen 1,5-Dioxynaphthalinring enthalt. Dieser molekulare Schalter last sich chemisch oder elektrochemisch betatigen. Im reduzierten Zustand liegt die neutrale TTF-Einheit „innerhalb”, im oxidierten (TTF+/TTF2+) dagegen „langsseits” des tetrakationischen Cyclophans; der Wechsel zwischen reduziertem und oxidiertem Zustand, I4+ bzw. I5+(I6+), wird von einem deutlichen Farbwechsel begleitet.

Rotational libration of a double-decker porphyrin visualized.

Scanning tunneling microscopy has revealed the reorientation of one of the macrocyclic rings of the double-decker porphyrin complex [Ce(TPP-Fc)(C(22)OPP)] [TPP-Fc = 5-(4-(4-ferrocenylphenylethynyl)phenyl)-10,15,20-triphenylporphyrin; C(22)OPP = 5,10,15,20-tetrakis(4-docosyloxyphenyl)porphyrin] by 90 degrees between scans when the other ring is fixed on a surface. This libration was evidenced by monitoring the location of the appended ferrocene unit, which functioned as a molecular beacon signaling its position.

Aggregation of self-assembling branched [n]rotaxanes

The so-called slippage methodology has been employed to self-assemble novel [2]-, [3]-, and [4]-rotaxanes incorporating, respectively, one, two, and three bis-p-phenylene-34-crown-10 macrocyclic components and a branched ‘dumbbell’ component, consisting of three arms containing bipyridinium units attached covalently to a 1,3,5-trisubstituted benzene central core and each bearing at its other end a substituted tetraarylmethane stopper. The absorption spectra, luminescence properties, and electrochemical behaviour of the branched component and its [2]-, [3]-, and [4]-rotaxanes have been investigated and discussed on the basis of the properties of their chromophoric and electroactive units. Charge- and energy-transfer processes between specific chromophoric units and the correlations between the unusual redox patterns of the various compounds have been evidenced and interpreted. The 1H-NMR spectroscopic investigation of the ‘free’ triply-branched hexacationic core, containing three bipyridinium units, one in each arm and terminated by bulky hydrophobic tetraarylmethane-based stoppers revealed, in chloroform solution, the formation of aggregates—a phenomenon which has been modeled using force field calculations. In addition, the formation of a gel was observed after the slow liquid–liquid diffusion of hexane into a chloroform solution of the triply-branched compound. Field-emission scanning electron microscopic investigation of this gel revealed the presence of domains of regular size. Surface-pressure–area measurements demonstrated the formation of stable monolayers by the ‘free’ backbone and the rotaxanes at an air–water interface: two distinct aggregates are formed by each compound. Interestingly, for the rotaxanes, the measured limiting area per molecule of both aggregates increases with the number of macrocyclic components which are incorporated within the rotaxane molecule. Atomic force microscopic analyses of the monolayers transferred onto mica revealed significant differences in their shapes when the two distinct aggregates formed by the same compound at different pressures were compared. In particular, the section analyses of the monolayers showed nanosized domains possessing diameters ranging from approximately 10 to 56 nm.

Alkyl Chain Length Dependence of the Self-Organized Structure of Alkyl-Substituted Phthalocyanines

The alkyl chain length on alkyl-substituted phthalocyanines (C(n)OPc) dependence of their self-organized structures was examined in this study. STM results indicated that the symmetry of ordered structures decreased as the alkyl chain became longer, with the exception of C(6)OPc, which preferentially formed a quasi-3-fold symmetrical structure. This could be explained by the fact that the C(n)OPc molecules are most likely to form densely packed structures. With C(n)OPc, when n = 4 to 10, the self-organized structures were dependent on the competition between how densely the molecules were arranged and how loose the intermolecular interaction energy was, caused by the formation of the densely packed structure. However, with C(n)OPc, when n = 10-18, the molecules tended to form densely packed structures by reducing the symmetry, even though the C(n)OPc molecules were distorted. When C(12)OPc and cobalt phthalocyanine were coadsorbed, the mixed system exhibited a four-fold symmetrical structure, which is rarely observed in C(12)OPc.

Pseudorotaxanes and Catenanes Containing a Redox-Active Unit Derived from Tetrathiafulvalene

Two bis(2-oxy-1,3-propylenedithio)tetrathiafulvalene-containing acyclic polyethers and two macrocyclic polyethers, each incorporating one bis(2-oxy-1,3-propylenedithio)tetrathiafulvalene unit and one p-phenylene ring, have been synthesized. The two acyclic polyethers are bound by cyclobis(paraquat-p-phenylene) with pseudorotaxane geometries in solution. The two macrocyclic polyethers have been mechanically interlocked with this tetracationic cyclophane to form [2]catenanes in a kinetically controlled self-assembly process. The X-ray crystallographic analysis of one of the two [2]catenanes and 1H-NMR-spectroscopic studies of both compounds showed that the p-phenylene ring of the macrocyclic polyether is located inside the cavity of the tetracationic cyclophane, while the bis(2-oxy-1,3-propylenedithio)tetrathiafulvalene unit resides alongside. The [2]pseudorotaxanes and [2]catenanes show broad bands around 780 nm, arising from the charge-transfer (CT) interaction between the electron-donor tetrathiafulvalene-(TTF-)type unit and the electron-acceptor units of the tetracationic cyclophane. 1H-NMR-spectroscopic studies have shown that the [2]pseudorotaxanes dissociate into their separate components upon oxidation of the TTF-type unit, as a result of disruption of the CT interaction and electrostatic repulsion between the tetracationic host and the newly formed monocationic guest. The subsequent reduction of the guest to its neutral state affords back the pseudorotaxane-type complex restoring the original equilibrium. The results obtained from electrochemical experiments are consistent with the reversible, redox-driven dethreading/rethreading process observed by 1H-NMR spectroscopy. Variable-temperature 1H-NMR-spectroscopic investigations have revealed two dynamic processes, both involving the relative movements of the mechanically interlocked components in the [2]catenanes. The two consecutive oxidation processes involving the TTF-type unit, observed electrochemically, are displaced toward more positive potentials compared with the free cyclic polyethers. The two reversible two-electron reduction processes, characteristic of free cyclobis(paraquat-p-phenylene), separate into four reversible one-electron processes because of the topological difference between the “inside” and “alongside” electron-acceptor units in the [2]catenane.

Molecular motion of surface-immobilized double-decker phthalocyanine complexes.

The molecular motion of surface-immobilized double-decker phthalocyanine complexes was examined using STM. (C(8)OPc)(2)Ce (1), (C(12)OPc)(2)Ce (2), and (C(8)OPc)Ce(Pc) (3) double-decker complexes, of which two ligands contained Pc nuclei, formed well-ordered self-organized structures on their own. Square-shaped top Pc ligands were clearly observed for complexes 1, 2, and 3 even though free space presented around the top ligands caused by mixing the complexes with template molecules. However, the details of the shapes of the top ligands of complexes 1, 2, and 3 were changed by the surrounding environment. The surrounding environment was considered to have influenced the mobility of the top ligands. Another complex, (C(8)OPc)Ce(TPP) (4), had difficulty forming a self-organized structure by itself. Complex 4 could have been immobilized by coadsorbing on the substrate with the C(8)OPc template, but the intramolecular structure of the top ligands of complex 4 was difficult to observe. The results strongly suggested that combinations of molecules composed of double-decker complexes as well as the free space presented around a top ligand are important factors that control the molecular motion of immobilized double-decker complexes on solid surfaces.

Organic nanotubes for drug loading and cellular delivery

Organic nanotubes made of synthetic amphiphilic molecules are novel materials that form by self-assembly. In this study, organic nanotubes with a carboxyl group (ONTs) at the surface were used as a carrier for the anticancer drug doxorubicin, which has a weak amine group. The IC(50) values of ONT for cells were higher than that of conventional liposomes, suggesting that ONTs are safe. The results showed that the drug loading of ONTs was susceptible to the effect of ionic strength and H(+) concentration in the medium, and drug release from ONTs was promoted at lower pH, which is favorable for the release of drugs in the endosome after cellular uptake. ONTs loaded with the drug were internalized, and the drug was released quickly in the cells, as demonstrated on transmission electron microscopy images of ONTs and the detection of a 0.05% dose of ONT chelating gadolinium in the cells. Moreover, ONT could be modified chemically with folate by simply mixing with a folate-conjugate lipid. Therefore, these novel, biodegradable organic nanotubes have the potential to be used as drug carriers for controlled and targeting drug delivery.

A Molecular Chameleon: Chromophoric Sensing by a Self-Complexing Molecular Assembly

A color change from purple to green takes place on addition of tetrathiafulvalene (TTF) to the macrobicyclic receptor 14+ , which is composed of a cyclobis(paraquat-p-phenylene) tetracation that shares one of its paraphenylene rings with a 1,5-naphthoparaphenylene-[36]crown-10 macrocycle. The TTF molecule forces the macrobicycle to turn inside out (see schematic drawing below) and displaces the self-complexed 1,5-dioxynaphthalene ring system from the center of the tetracationic cyclophane.

Highly efficient production of various organic nanotubes with different surfaces and their application to an adsorbent

Self-assembly of simple glycyl–glycine-containing peptide lipids, both in water and alcohol, gave various organic nanotubes covered with different functional surfaces in a facile production manner. Especially, the self-assembly in alcohol media increased the yield of nanotube by more than 5 times and decreased the production time by less than one-tenth when compared with conventional self-assembly in water. The glycyl–glycine residue proved to be critical to afford a tubular morphology stabilized by the polyglycine-II-type hydrogen-bond networks. Therefore, the glycyl–glycine-containing peptide lipids can give nanotubes with desired surfaces by connecting an appropriate functional moiety to the glycyl–glycine residue. In addition, the adsorption ability of gold nanoparticles to the specific nanotube surface showed the high potentiality of the nanotubes as an adsorbent.

Measuring the length distribution of self-assembled lipid nanotubes by orientation control with a high-frequency alternating current electric field in aqueous solutions.

The present work addresses the length distribution of self-assembled lipid nanotubes (LNTs) by controlling the orientation of the LNTs using an alternating current (ac) electric field in aqueous solutions. The effect of the ac field on the orientation and rotation of individual LNTs was examined to evaluate the optimum orientation frequency by visualizing the individual LNTs in real time. By using the high-frequency ac field, we have successfully measured the length distribution for two different types of LNTs and have quantitatively analyzed the maximum occurrences of the length distribution as well as the extension of the longer length region.