Yoshihiro Kikkawa

Toroidal Nanoobjects from Rosette Assemblies of Melamine-Linked Oligo(p-phenyleneethynylene)s and Cyanurates†

Nanodonuts: A hydrogen-bonded cyclic assembly (rosette, left) of a melamine featuring π-conjugated oligo(p-phenyleneethynylene) hierarchically self-organizes with a cyanurate in decane under dilute conditions to form toroidal nanostructures (right) with a diameter of 40 nm.

Supramolecularly engineered perylene bisimide assemblies exhibiting thermal transition from columnar to multilamellar structures

Perylene 3,4:9,10-tetracarboxylic acid bisimide (PBI) was functionalized with ditopic cyanuric acid to organize it into complex columnar architectures through the formation of hydrogen-bonded supermacrocycles (rosette) by complexing with ditopic melamines possessing solubilizing alkoxyphenyl substituents. The aggregation study in solution using UV-vis and NMR spectroscopies showed the formation of extended aggregates through hydrogen-bonding and π-π stacking interactions. The cylindrical fibrillar nanostructures were visualized by microscopic techniques (AFM, TEM), and the formation of lyotropic mesophase was confirmed by polarized optical microscopy and SEM. X-ray diffraction study revealed that a well-defined hexagonal columnar (Col(h)) structure was formed by solution-casting of fibrillar assemblies. All of these results are consistent with the formation of hydrogen-bonded PBI rosettes that spontaneously organize into the Col(h) structure. Upon heating the Col(h) structure in the bulk state, a structural transition to a highly ordered lamellar (Lam) structure was observed by variable-temperature X-ray diffraction, differential scanning calorimetry, and AFM studies. IR study showed that the rearrangement of the hydrogen-bonding motifs occurs during the structural transition. These results suggest that such a striking structural transition is aided by the reorganization in the lowest level of self-organization, i.e., the rearrangement of hydrogen-bonded motifs from rosette to linear tape. A remarkable increase in the transient photoconductivity was observed by the flash-photolysis time-resolved microwave conductivity (FP-TRMC) measurements upon converting the Col(h) structure to the Lam structure. Transient absorption spectroscopy revealed that electron transfer from electron-donating alkoxyphenyl groups of melamine components to electron-deficient PBI moieties takes place, resulting in a higher probability of charge carrier generation in the Lam structure compared to the Col(h) structure.

Self‐Organization of Hydrogen‐Bonding Naphthalene Chromophores into J‐type Nanorings and H‐type Nanorods: Impact of Regioisomerism

Nature has evolved complex self-organized architectures of pigment assemblages with rational nanoscale topologies and chromophore orientation. Circular architectures of the chlorophyll–protein complexes found in the light-harvesting systems of purple photosynthetic bacteria could be regarded as perfect supramolecular assemblies of pigments in view of not only their topological features but also a functional standpoint. 2] Sunlight is absorbed uniformly by circularly organized chlorophyll pigments, and the resulting exciton states are delocalized over the closed arrays of chlorophylls. Such exciton delocalization is achieved by partially overlapped arrays of chlorophyll p systems, which could be referred to as J-type (offset) stacking. In contrast, onedimensional stacks of largely overlapped p systems, which can be referred to as H-type (face-to-face) stacking, are promising as quasi one-dimensional pathways of mobile charge carriers for organic electronics. Thus, these natural and artificial assemblies of p systems show the importance of simultaneous control over dimensionality of nanostructures and local stacking arrangements to optimize the functionality in the systems. Herein we report that a regioisomerism in specifically designed self-assembling small molecules can offer the aforementioned two extreme nanoarchitectures with favorable stacking arrangements of their p systems. We have recently reported the self-assembly of p-conjugated molecules substituted asymmetrically with barbituric acid (BAR) and a wedge-shaped aliphatic tail (“minidendron”). An unique feature of these BAR–p-wedge molecules in nonpolar media is that their ability to form dramatically different nanostructures, namely nanorings and nanorods, depending on the structure of the p moiety. X-ray diffraction analysis in the mesomorphic state suggested that the BAR–p-wedge molecules form columnar stacks of hydrogen-bonded hexamers (rosettes), which are in nonpolar media solvated to form curved (for nanorings) or straight nanostructures (for nanorods). Although this molecular design is attractive for the construction of well-defined nanostructures consisting of functional p systems, it remains difficult to clarify the structural relationship between monomers and assemblies. We now report a striking impact of the regioisomerism of BAR–naphthalene-wedge molecules (Figure 1) on their self-assembled nanostructures, which

Control over Hierarchy Levels in the Self-Assembly of Stackable Nanotoroids

We report a precise control over the hierarchy levels in the outstanding self-organization process shown by chiral azobenzene dimer 1. This compound forms uniform toroidal nanostructures that can hierarchically organize into chiral nanotubes under the control by temperature, concentration, or light. The nanotubes further organized into supercoiled fibrils, which finally intertwined to form double helices with one-handed helical sense.

Rational Design of Nanofibers and Nanorings through Complementary Hydrogen‐Bonding Interactions of Functional π Systems

A simple protocol to create nanofibers and -rings through a rational self-assembly approach is described. Whereas the melamine-oligo(p-phenylenevinylene) conjugate 1a self-aggregates to form ill-defined nanostructures, conjugate 1b, which possesses an amide group as an additional interactive site, self-aggregates to form 1D nanofibers that induce gelation of the solvent. AFM and XRD studies have shown that dimerization through the melamine-melamine hydrogen-bonding interaction occurs only for 1b. Upon complexation with 1/3 equivalents of cyanuric acid (CA), conjugate 1a provides well-defined, ring-shaped nanostructures at micromolar concentrations, which open to form fibrous assemblies at submillimolar concentrations and organogels in the millimolar concentration range. Apparently, the enhanced aggregation ability of 1a by CA is a consequence of columnar organization of the resulting discotic complex 1a(3).CA. In contrast, coaggregation of 1b with CA does not provide well-defined nanostructures, probably due to the interference of complementary hydrogen-bonding interactions by the amide group.

Reversible transformation between rings and coils in a dynamic hydrogen-bonded self-assembly.

Several proteins, such as tobacco mosaic virus coat protein and the beta protein of the bacteriophage lambda, are known to exhibit unique dynamic self-organization processes involving ring-shaped and extended helical nanostructures triggered by chemical stimuli. However, transformation of rings into coils as observed in biological assemblies has never been realized with synthetic molecular building blocks. Oligo(p-phenylenevinylene) functionalized on one end with barbituric acid and on the other end with aliphatic tails self-organizes in aliphatic solvents to form nanorings through hydrogen-bonding and pi-stacking interactions. Upon an increase in concentration, the nanorings transform into rodlike nanostructures, which are considered to be formed through helically coiled objects consisting of quasi-one-dimensional fibers.

Interconvertible Oligothiophene Nanorods and Nanotapes with High Charge‐Carrier Mobilities

Molecular designs leading functional p-conjugated oligomers into well-defined nanoarchitectures through noncovalent interactions are crucial for the realization of miniaturized organic electronic devices based on the bottom-up approach. In particular, the bottom-up construction of electronically addressable 1D nanostructures has received a great deal of attention, owing to their potential as effective pathways for charge carriers. If such nanostructures have stimuli-responsive properties in their morphologies and chromophore packing motifs, they could be used as smart nanomaterials in which their electronic properties can be controlled at will. Several fibrillar nanostructures have been reported to exhibit morphological change induced by guest and solvent. On the other hand, temperature is one of the most convenient variables to control, but its use to regulate well-defined nanostructures remains challenging, because most enthalpy-driven noncovalent assemblies in solution dissociate into monomers upon increasing temperature. Here we report thermally interconvertible semiconductive nanorods and nanotapes comprising p-conjugated oligothiophenes, displaying high charge-carrier mobilities in their condensed states (Scheme 1). Recently, we have shown that merocyanine dyes equipped with barbituric acid can be loaded into diverse self-organized nanoarchitectures upon complexation with flexible bismelamine receptors (BM, see Scheme 1) through complementary hydrogen-bonding interactions. Our next challenge is to corroborate the validity of our strategy to extended p-conjugated systems and to offer more practical nanoto microarchitectures, as functional modules for miniaturizing electronic devices. We prepared quaterthiophene (1 a), functionalized on one end by a barbituric acid (BA) headgroups and the other end by a tridodecyloxyphenyl (TDP) tail (Scheme 1). Compound 1 a was synthesized according to Scheme S1 (in the Supporting Information) and fully characterized. Barbituric acid group is a well-known ditopic triple-hydrogen-bonding module that complementarily binds with diaminopyridine or melamine modules. However, the examples that demonstrate its capability to lead p-conjugated molecules into well-defined nanostructures without assistance are scant. The UV/Vis spectra of 1 a (1 10 6 m<c<5 10 3 m) in methylcyclohexane (MCH) showed the absorption maximum at l=439 nm, which was strongly blue-shifted (n=3474 cm ) from that in dichloromethane (l= 518 nm). Furthermore, a significant drop in the fluorescence intensity was observed. These optical changes are typical for the H-type aggregation of oligothiophenes in nonpolar and aqueous conditions, but not observed for N,N’-dimethylated derivative 1 b. This demonstrates that the BA group of 1 a plays a crucial role in the aggregation. The H-aggregate showed a pronounced thermal stability: it remained intact upon heating to 80 8C in MCH, above which reversible transition to monomeric state was observed. Dynamic light scattering of 1 a in MCH (c= 5 10 5 m) showed the presence of large aggregates with hydrodynamic diameters around 400 nm. Atomic force microscopy (AFM) of H-aggregated 1 a exhibited uniform rodlike nanostructures with hundreds of nanometers in length, which are [a] Dr. S. Yagai, T. Kinoshita, Prof. Dr. T. Karatsu, Prof. Dr. A. Kitamura Department of Applied Chemistry and Biotechnology Graduate School of Engineering, Chiba University 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522 (Japan) Fax: (+81) 43-290-3039 E-mail : yagai@faculty.chiba-u.jp [b] Dr. S. Yagai, Prof. Dr. S. Seki PRESTO (Japan) Science and Technology Agency (JST) 4-1-8 Honcho Kawaguchi, Saitama (Japan) [c] Dr. Y. Kikkawa Photonics Research Institute National Institute of Advanced Industrial Science and Technology (AIST) 1-1-1 Higashi, Tsukuba, Ibaraki 305-8562 (Japan) [d] Y. Honsho, Prof. Dr. S. Seki Department of Applied Chemistry Graduate School of Engineering, Osaka University 2-1, Yamadaoka, Suita, Osaka 565-0871 (Japan) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.200901336.

Photoreversible Supramolecular Polymerisation and Hierarchical Organization of Hydrogen‐Bonded Supramolecular Co‐polymers Composed of Diarylethenes and Oligothiophenes

Diarylethene 1 equipped with two monotopic melamine hydrogen-bonding sites and oligothiophene-functionalized ditopic cyanurate (OTCA) were mixed in a nonpolar solvent to form AA-BB-type supramolecular co-polymers (SCPs) bearing photoswitchable moieties in their main chains and extended π systems as side chains. UV/Vis, fluorescence, dynamic light scattering (DLS), TEM, and AFM studies revealed that the two functional co-monomers formed flexible quasi-one-dimensional SCPs in solution that hierarchically self-organized into helical nanofibers through H-aggregation of the oligothiophene side chains. Upon irradiating the SCPs with UV light, a transition occurred from the H-aggregated state to non-aggregated monomeric oligothiophene side chains, as shown by spectroscopic studies, which indicates the formation of small oligomeric species held together only by hydrogen-bonding interactions. TEM and AFM visualized unfolded fibrils corresponding to elongated single SCP chains formed upon removal of solvent. The helical nanofibers were regenerated upon irradiating the UV-irradiated solution with visible light. These results demonstrated that the supramolecular polymerisation followed by hierarchical organization can be effectively controlled by proper supramolecular designs using diarylethenes and π-conjugated oligomers.

Odd–even effect and metal induced structural convergence in self-assembled monolayers of bipyridine derivatives

Scanning tunneling microscopy (STM) observations reveal that bipyridine derivatives which exhibit various two-dimensional structures due to the odd-even chain length effect are converged into a lamellar structure upon metal coordination.

Unconventional hydrogen-bond-directed hierarchical co-assembly between perylene bisimide and azobenzene-functionalized melamine

Co-assembly of ditopic perylene bisimide and azobenzene-functionalized melamine occurs with an unconventional stoichiometric ratio, providing well-defined nanostructures with a helically-coiled architecture where perylene chromophores are packed in desirable J-type arrangements.