Shinobu Uemura

Thermodynamically Controlled Self-Assembly of Covalent Nanoarchitectures in Aqueous Solution

The pursuit of methods for design and preparation of robust nanoarchitectonic systems with integrated functionality through bottom-up methodologies remains a driving force in molecular nanotechnology. Through the use of π-conjugated covalent bonds, we demonstrate a general substrate-mediated, soft solution methodology for the preparation of extended π-conjugated polymeric nanoarchitectures in low-dimensions. Based on thermodynamic control over equilibrium polymerization at the solid-liquid interface whereby aromatic building blocks spontaneously and selectively link, close-packed arrays composed of one-dimensional (1-D) aromatic polymers and two-dimensional (2-D) macromolecular frameworks have been prepared and characterized by in situ scanning tunneling microscopy. This methodology eliminates the necessity for severe conditions and sophisticated equipment common to most current fabrication techniques and imparts almost infinite possibilities for the preparation of robust materials with designer molecular architectures.

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.

Biosupramolecular Nanowires from Chlorophyll Dyes with Exceptional Charge‐Transport Properties

Conductive tubes: Self-assembled nanotubes of a bacteriochlorophyll derivative are reminiscent of natural chlorosomal light-harvesting assemblies. After deposition on a substrate that consists of a non-conductive silicon oxide surface (see picture, brown) and contacting the chlorin nanowires to a conductive polymer (yellow), they show exceptional charge-transport properties.

Reversible photoinduced change in molecular ordering of diarylethene derivatives at a solution-HOPG interface.

A diarylethene-pyrene diad and a diarylethene-pyrene-diarylethene triad were synthesized to investigate the photoinduced two-dimensional ordering change at a solution-HOPG interface. The molecular arrangements were detected by STM. The different photochromic isomers showed different orderings reflecting the differences in their molecular structures. For the diarylethene-pyrene-diarylethene triad, a new ordering appeared upon irradiation with UV light and returned to the original ordering upon subsequent irradiation with visible light. The new arrangement was assigned to the ordering of the closed-closed isomers based on the images of the isolated open- and closed-isomers. The relationship between the nature of the two-dimensional ordering and the molecular structure is discussed.

Photocontrol Over Self‐Assembled Nanostructures of π–π Stacked Dyes Supported by the Parallel Conformer of Diarylethene

Diarylethenes (DAEs) have rarely been used in the design of photoresponsive supramolecular assemblies with a well-defined morphology transition owing to rather small structural changes upon photoisomerization. A supramolecular design based on the parallel conformation of DAEs enables the construction of photoresponsive dye assemblies that undergo remarkable nanomorphology transitions. The cooperative stacking of perylene bisimide (PBI) dyes was used to stabilize the parallel conformer of DAE through complementary hydrogen bonds. Atomic force microscopy, UV/Vis spectroscopy, and molecular modeling revealed that our DAE and PBI building blocks coassembled in nonpolar solvent to form well-defined helical nanofibers featuring J-type dimers of PBI dyes. Upon irradiating the coassembly solution with UV and visible light in turn, a reversible morphology change between nanofibers and nanoparticles was observed. This system involves the generation of a new self-assembly pathway by means of photocontrol.

Two-Dimensional Self-Assembled Structures of Melamine and Melem at the Aqueous Solution−Au(111) Interface†

Self-assembled structures of melamine and the condensed melamine derivative melem were investigated at aqueous solution-Au(111) interfaces by cyclic voltammetry and in situ scanning tunneling microscopy (STM) observation. The adsorption/desorption behaviors of both molecules on Au(111) surfaces could be controlled by varying the electrochemical potential and solution concentration. In the negative potential region, self-assembled structures of melem and melamine were constructed by double hydrogen bonding systems between nitrogen atoms of triazine rings and amine groups. In addition, melem formed a closely packed structure at potentials of between -0.3 and -0.15 V or in solutions at higher concentrations.

Molecular Dynamics in Two-Dimensional Supramolecular Systems Observed by STM

Since the invention of scanning tunneling microscopy (STM), 2D supramolecular architectures have been observed under various experimental conditions. The construction of these architectures arises from the balance between interactions at the medium-solid interface. This review summarizes molecular motion observed in 2D-supramolecular structures on surfaces using nanospace resolution STM. The observation of molecular motion on surfaces provides a visual understanding of intermolecular interactions, which are the major driving force behind supramolecular arrangement.

In situ observation of coronene epitaxial adlayers on Au(111) surfaces prepared by the transfer of Langmuir films

Abstract A highly ordered epitaxial adlayer of coronene on an Au(111) surface was prepared by a ‘wet process’ technique, which consisted of the simple transfer of Langmuir films. The structure and dependence on potential of the adlayer were then investigated by in situ scanning tunneling microscopy (STM). The adlayer possessed a (4×4)-Au(111) superlattice structure, and each coronene molecule was visualized as an individual hexagon on high-resolution STM images. The multilayer portion was present without any potential control, probably due to the excess generated by the transfer. The adsorption behavior of coronene, including the formation of the multilayers, showed dependence on the potential. A flawless adlayer without the presence of any multilayers was achieved by the application of a negative potential, in which the multilayer–substrate interaction was adequately weakened.

Columnar Mesophases Based on Zinc Chlorophyll Derivatives Functionalized with Peripheral Dendron Wedges

A chlorophyll derivative with a central zinc ion, a methoxy functionality at its 3(1)-position, and functionalized with a second-generation dendron (3,4-3,4,5)12G2-CH(2)OH at its 17(2)-position was synthesized starting from natural chlorophyll a (Chl a). This compound exhibits liquid crystalline (LC) behavior and its mesomorphic properties have been characterized by differential scanning calorimetry (DSC), polarisation optical microscopy (POM), powder X-ray diffraction (XRD), and scanning probe microscopy (SPM). A combination of powder XRD, high resolution scanning tunneling microscopy (STM), and atomic force microscopy (AFM) experiments revealed the formation of nano-segregated well-ordered columnar tubular superstructures consisting of about five molecules in the column stratum. These self-assembled columns are further self-organized into a two-dimensional oblique unit cell lattice. Semiconducting behavior of this compound has been studied by pulse-radiolysis time-resolved microwave conductivity (PR-TRMC) method and charge carrier mobility values of ∼10(-2) cm(2) V(-1) s(-1) are observed. Such organized columnar superstructures constructed from semisynthetic zinc chlorins are reminiscent of the tubular organization of the bacteriochlorophyll dyes in the light-harvesting chlorosomal antennae of green sulphur bacteria.

Solvent- and Guest-Responsive Self-Assembly of Hamilton Receptor Tethered Bis(merocyanine) Dyes

A novel supramolecular building block (8) that consists of a Hamilton receptor and two merocyanine dyes has been synthesized, and the self-assembly based on orthogonal hydrogen bonding and dipolar interactions has been studied in detail. Different self-assembled species, including oligomers, polymers, and inverted micelles could be observed upon variation of the solvent polarity and the concentration. Moreover, this system is highly responsive toward molecular stimuli such as merocyanine molecules with the barbituric acid motif that are bound by the Hamilton receptors. Detailed UV/Vis absorption studies provided insight into isodesmic or cooperative steps during the self-assembly of 8 into different species. The size of the aggregates in solution and the morphology on substrates have been explored by a combination of dynamic light scattering (DLS), atomic force microscopy (AFM), and TEM investigations.