Synthesis and Study at a Solid/Liquid Interface of Porphyrin Dimers Linked by Metal Ions.

Several porphyrin dimers linked by metal ions were prepared. One trimeric compound was also isolated and one porphyrin dimer linked by palladium(II) could be structurally characterized. In solution, the size of the new compounds was estimated by DOSY NMR techniques. These compounds all contained long aliphatic chains (O-C12H25), which were used to assemble them at a highly oriented pyrolytic graphite (HOPG)/liquid interface. The highly ordered arrays were visualized by scanning tunneling microscopy (STM).

Trapping Nanostructures on Surfaces through Weak Interactions.

The assembly of imidazole-functionalized phenanthroline-strapped zinc porphyrins (ZnPorphen) with alkyl or polyethylene glycol (PEG) side chains was studied in solution and by AFM after casting on highly oriented pyrolytic graphite (HOPG) or mica. The nature of the solvent and its evaporation time influenced the morphology of the objects observed. On HOPG, short rods of about 100 nm were observed after fast evaporation of solutions of the alkyl derivatives in CHCl3 , THF, or pyridine, whereas islands of aligned rows of longer wires were obtained from methylcyclohexane (MCH). Slow evaporation of MCH led to a three-dimensional assembly. The PEG porphyrin assembled into short wires on HOPG or fibers on mica after slow evaporation of solutions in THF. This study shows the role of surface-molecule interactions in the interfacial assembly of ZnPorphen derivatives and contributes to understanding the parameters that control their noncovalent assembly into molecular wires on a surface.

Controlled stacking and unstacking of peripheral chlorophyll units drives the spring-like contraction and expansion of a semi-artificial helical polymer.

Developing new strategies for controlling polymer conformations through precise molecular recognition can potentially generate a machine-like motion that is dependent on molecular information-an important process for the preparation of new intelligent nanomaterials (e.g., polymer-based nanomachines) in the field bordering between polymer chemistry and conventional supramolecular sciences. Herein, we propose a strategy to endow a helical polymer chain with dynamic spring-like (contraction/expansion) motion through the one-dimensional self-assembly (aggregation/disaggregation) of peripheral amphiphilic molecules. In this developing system, we employed a semi-artificial helical polysaccharide presenting peripheral amphiphilic chlorophyll units as a power device that undergoes contractive motion in aqueous media, driven by strong π-π interactions of its chlorophyll units or by cooperative molecular recognition of bipyridyl-type ligands through pairs of chlorophyll units, thereby converting molecular information into the regulated motion of a spring. In addition, this system also undergoes expansive motion through coordination of pyridine. We anticipate that this strategy will be applicable (when combined with the established wrapping chemistry of the helical polysaccharide) to the development of, for example, drug carriers (e.g., nano-syringes), actuators (stimuli-responsive films), and directional transporters (nano-railways), thereby extending the frontiers of supramolecular science.

Nanoscopic imaging of meso-tetraalkylporphyrins prepared in high yields enabled by Montmorrilonite K10 and 3A†molecular sieves.

We have developed a high-yielding synthesis of meso-tetraalkylporphyrins, which previously have been obtained only in lower yields. By employing Montmorrilonite K10 as the acid catalyst and 3 Šmolecular sieves as the dehydrating agent, yields that reached 70 % could be achieved with some aliphatic aldehydes. The free-base porphyrins with decyl (C10) or longer chains were imaged at the single-molecule level at the solvent/surface interface. Highly oriented pyrolytic graphite (HOPG) was used as a π-stacking surface, whereas 1-phenyloctane and 1-phenylnonane were used as solvents. An odd-even effect was observed from C13 to C16. For C13 a single-crystal X-ray structure allowed an unprecedented insight into how packing from two dimensions is expanded into a three-dimensional crystal lattice.

Fluorescence properties of (E,E,E)-1,6-di(n-naphthyl)-1,3,5-hexatriene (n = 1, 2): effects of internal rotation.

The fluorescence spectroscopic properties of (E,E,E)-1,6-di(n-naphthyl)-1,3,5-hexatrienes (1, n = 1; 2, n = 2) have been investigated in solution and in the solid state. In solution, the absorption maxima (λ(a)) of the lowest-energy band (1, 374 nm; 2, 376 nm in methylcyclohexane) were similar for 1 and 2, whereas the fluorescence maxima (λ(f)) (1, 545 nm; 2, 453 nm) and quantum yields (φ(f)) (1, 0.046; 2, 0.68) were very different regardless of the solvent polarity. The fluorescence spectrum of 1 was independent of the excitation wavelength (λ(ex)), whereas the spectrum of 2 was weakly λ(ex)-dependent. In the solid state, the spectroscopic properties of 1 and 2 were similar (λ(a) = 437-438 nm, λ(f) = 496-505 nm, φ(f) = 0.04-0.07). The origins of emission are both considered to be mainly monomeric. With the help of single-crystal X-ray structure analysis and ab initio quantum chemical calculation, we conclude that the red-shifted and weak emission of 1 in solution originates from a planar excited state having small charge transfer character, reached from a twisted Franck-Condon state by the excited-state geometrical relaxation accompanied by the internal rotation around the naphthalene (Ar)-CH single bond. The similar fluorescence properties of 1 and 2 in the solid state can be attributed to the restriction of the geometrical relaxation. The effects of the Ar-CH rotational isomerism on the fluorescence properties in solution, for 2 in particular, are also discussed.

Effects of alkyl chain length, solvent and tandem Claisen rearrangement on two-dimensional structures of noncyclic isobutenyl compounds: scanning tunnelling microscopic study.

A series of isobutenyl compounds possessing various alkyl chain lengths (C(n)-1) with a carbon number of n = 14-21 were synthesized and their two-dimensional (2D) structures were systematically studied using scanning tunnelling microscopy (STM) at a highly oriented pyrolytic graphite (HOPG)/solvent interface. Two kinds of solvent, such as 1-phenyloctane (PO) and 1-phenylnonane (PN), were selected to examine the 2D structures by changing the alkyl chain length of the isobutenyl compounds. At the HOPG/PO interface, C(n)-1 molecules with shorter alkyl chains (n = 14-17) showed the same zig-zag shaped 2D structure regardless of the alkyl chain length, whereas an odd-even effect was recognized in C(n)-1 compounds with longer alkyl chains (n = 18-21) displaying the wavy and tripod structures, alternately. This odd-even effect was also observed at the HOPG/PN interface rather more distinctly. These results suggest that there is a specific alkyl chain length range that shows the odd-even effect in the present 2D system. After a tandem Claisen rearrangement (TCR), the 2D structures of all the C(n)-2 compounds formed were converged into the same linear structure, i.e. the odd-even effect was cancelled by the conformational limitation induced by the TCR.

Bipyridine derivatives at a solid/liquid interface: effects of the number and length of peripheral alkyl chains.

Bipyridine derivatives (bpys) with various number and length of peripheral alkyl chains (with carbon numbers of n = 11-17) were synthesized, and their self-assembled monolayers were observed by scanning tunneling microscopy (STM) at a 1-phenyloctane/highly oriented pyrolytic graphite (HOPG) interface. The effects of the number, the substitution position, and the length of alkyl chains on the two-dimensional structures were systematically studied. Bpys substituted by a single alkyl chain in the p-position on each side adopted an almost linear form with zigzag-type alignment of the pi-conjugated unit, whereas, in the case of m-substitution, the bpys showed Z-shaped morphology with interdigitated alkyl chains. In both cases, no odd-even alkyl chain length effects were observed. The bpys with double alkyl chains at m- and p-positions displayed odd-even alkyl chain effects, suggesting that the formation of two-dimensional structure is dominated by the interactions between alkyl chains. Bpys with triple alkyl chains at o-, m-, and p-positions also showed odd-even alkyl chain effects, but only for the higher number of carbon atoms in the alkyl chain unit (n = 14-17). These results indicate that concerted intermolecular interactions of the alkyl chain unit introduce the odd-even chain length effect on the self-assembled two-dimensional structure. After coordination of PdCl(2), odd-even effects were quenched, and bpys were converged into the same lamellar structure, in which the molecules are almost linear. All the structural differences due to the odd-even alkyl chain length effect were explained in terms of intermolecular and molecule-substrate interactions.

Two-dimensional organization of mono- and bisurea supramolecular polymers studied by scanning tunneling microscopy.

Scanning tunneling microscopy (STM) of mono- and bisurea-functionalized oligo(p-phenylenevinylene)s at solid/liquid interface visualized two-dimensionally-ordered double columnar structures of pi-conjugated segments scaffolded by one-dimensional supramolecular polymerization of urea hydrogen-bonding units. In contrast to a persistent alignment of the bisurea compound supported by twofold intermolecular urea-urea hydrogen-bonding, the building blocks in the monourea double columns shows dynamic fluctuation and defects because of their rotational motion around urea-urea hydrogen-bonding axis and/or adsorption-desorption of the individual molecules from the surface. Self-assembled structures of mono- and bisurea compounds at solid/liquid interface revealed by STM can be related to their gelation abilities in organic solvents.

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.

Effect of phase structure on enzymatic degradation in poly(L-lactide)/atactic poly(3-hydroxybutyrate) blends with different miscibility.

Thin films of poly(L-lactide) (PLLA)/atactic poly(3-hydroxybutyrate) (ataPHB) blends with different miscibility were prepared and characterized by using differential scanning calorimetry (DSC) and atomic force microscopy (AFM). The DSC analysis suggested that the blend thin films exhibited different phase structures, such as miscible, partially miscible, and immiscible depending on the blending ratio as well as molecular weight of ataPHB component. The different miscibility was further confirmed by the surface morphological observation by AFM. Both the immiscible and partially miscible blends of PLLA/ataPHB revealed the formation of phase-separated morphology of PLLA and ataPHB components, whereas the homogeneous surface morphology was observed for the miscible blend. On the basis of the changes in the depth profile from the surface level of the thin films, the enzymatic degradation rates of the PLLA and ataPHB domains were determined in the presence of either PHB depolymerase or proteinase K, respectively. The erosion rate of PLLA/ataPHB blends was strongly dependent on the blend composition and the degree of dispersion of the two components. The enzymatic degradation behaviors were discussed in terms of phase structure, molecular mobility, and retardation effect of the components in the blends.

Interaction force of chitin-binding domains onto chitin surface.

Interaction force of chitin-binding domains (ChBD1 and ChBD2) from a thermostable chitinase onto chitin surface was directly measured by atomic force microscopy (AFM) in a buffer solution. In the force curve measurement, multiple pull-off events were observed for the AFM tips functionalized with either ChBD1 or ChBD2, whereas the AFM tips terminated with nitrilotriacetic acid groups without ChBD showed no interaction peak, suggesting that the detected forces are derived from the binding functions of ChBDs onto the chitin surface. The force curve analyses indicate that the binding force of ChBD2 is stronger than that of ChBD1. This result suggests that ChBD1 and ChBD2 play different roles in adsorption onto chitin surface.

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.

Surface potential switching by metal ion complexation/decomplexation using bipyridinethiolate monolayers on gold.

Surface potential switching on gold(111) surfaces is induced by complexation/decomplexation reactions of a bipyridine (BP) derivative and palladium(II) chloride, as observed by Kelvin probe force microscopy (KFM). On the basis of the theoretical predictions, a 4-(5-phenylethynyl-2,2'-bipyridine-5'-yl-ethynyl)benzenethiol (PhBP) derivative was synthesized and used as an active monolayer to catch transition metal ions. By using the microcontact printing (CP) technique, micron-size patterned PhBP monolayers, which act as effective hosts to coordinate palladium(II) chloride, were prepared on gold(111) surfaces. The KFM signal decreases by complexation of the Pd(II) chloride in PhBP monolayers and is recovered by removal of Pd ions using an ethylenediamine solution, as confirmed by X-ray photoelectron spectroscopy. This process is reversible, indicating that the surface potential switching is realized by complexation/decomplexation of Pd(II). A CP PhBP monolayer, when it detects the target palladium ion, shows sensitivity for the picomolar level detection judged from surface potential changes in KFM measurements. The dipole moment estimated by the surface potentials is much smaller than the calculated value, indicating that mechanisms for the reduction of the surface dipole moment exist in real monolayers prepared by the CP method.

[3]rotaxane synthesized via covalent bond formation can recognize cations forming a sandwich structure.

A novel [3]rotaxane composed of two 25-membered crownophanes and one axle molecule having two anthryl end groups was successfully synthesized via covalent bond formation followed by aminolysis, and can incorporate caesium ion into the space between the two macrocycles as a 1 : 1 sandwich-type complex, whereas it makes a 1 : 2 complex with lithium ion.

Organization of polyhydroxyalkanoate synthase for in vitro polymerization as revealed by atomic force microscopy.

Individual polyhydroxyalkanoate synthase molecules from Ralstonia eutropha (PhaCRe) were directly visualized on highly oriented pyrolytic graphite (HOPG) by atomic force microscopy (AFM). PhaCRe molecule was observed as a spherical particle of 2.9 +/- 0.4 nm in height and 28 +/- 4 nm in width. In vitro polymerization reaction on HOPG was carried out for 5 min by reacting the PhaCRe molecules with (R)-3-hydroxybutyryl-CoA monomers. The reaction product was then observed after the removal of water solution. Several PhaCRe molecules associated with each other to form an assembly, which was attached to a fibrillar structure of ca. 0.2-0.3 nm in height. The fibrillar structure that elongated from the PhaCRe assembly was interpreted as the poly[(R)-3-hydroxybutyrate] polymer chain. High resolution AFM suggested that the PhaCRe assembly was composed of 3-4 subunits of PhaCRe molecules. This was further supported by SDS-PAGE analysis of the cross-linked PhaCRe enzyme. These results suggest that more than two subunits of PhaCRe are necessary for the in vitro polymerization of PHB molecular chains.

First helical zinc(II) complex with a salen ligand.

The structure of a tetra-coordinated zinc(II) complex with a salen ligand was determined for the first time; unexpectedly, the complex was an interesting 2:2 metal-to-ligand complex.

STM-based molecular detection of "catch-and-release" of protons for bipyridine bound to phenylene-ethynylene thiol.

The protonation/deprotonation response of a novel bipyridine containing (phenylene-ethynylene) thiol adsorbed to a Au surface was investigated with scanning tunneling microscopy (STM), showing reversible changes in the average heights (approximately 50 spots) and the height distribution arising from protonation/deprotonation.

Dynamic assembly of porphyrin wires trapped on a highly oriented pyrolitic graphite surface.

Carefully designed porphyrin building blocks assemble through selective imidazole binding in various solvents to form linear multiporphyrin objects. From a dynamic mixture of monomers, dimers, and oligomers, linear objects were observed on a highly oriented pyrolitic graphite (HOPG) surface. On the surface, the objects' morphology clearly depended on the solvent used for deposition and was modified upon heating.

Fabrication and transformation of novel two-dimensional tripod structures: structural modulation by alkyl chain length and tandem Claisen rearrangement.

Novel two-dimensional C3 symmetric (tripod) structures are fabricated from isobutenyl compounds possessing long alkyl chains. Alteration from tripod to wavy structures is accomplished by odd-even effect, and tandem Claisen rearrangement allows the transformation to the linear structures.