Atsuko Kosaka

Redox-responsive molecular helices with highly condensed π -clouds

Helices have long attracted the attention of chemists, both for their inherent chiral structure and their potential for applications such as the separation of chiral compounds or the construction of molecular machines. As a result of steric forces, polymeric o-phenylenes adopt a tight helical conformation in which the densely packed phenylene units create a highly condensed π-cloud. Here, we show an oligomeric o-phenylene that undergoes a redox-responsive dynamic motion. In solution, the helices undergo a rapid inversion. During crystallization, however, a chiral symmetry-breaking phenomenon is observed in which each crystal contains only one enantiomeric form. Crystals of both handedness are obtained, but in a non-racemic mixture. Furthermore, in solution, the dynamic motion of the helical oligomer is dramatically suppressed by one-electron oxidation. X-ray crystallography of both the neutral and oxidized forms indicated that a hole, generated upon oxidation, is shared by the repeating o-phenylene units. This enables conformational locking of the helix, and represents a long-lasting chiroptical memory.

Conductive One‐Handed Nanocoils by Coassembly of Hexabenzocoronenes: Control of Morphology and Helical Chirality

Electroconductive one-handed helical nanofibers are attractive in view of their potential for the realization of nanoscale solenoids. While there are many reported examples of the construction of helical nanofibers by self-assembly of pelectronic molecules, most of these involve twisted ribbons, which do not provide the coiled pathways essential for electromagnetic properties. Some coiled assemblies of aromatic molecules have been reported, although they are still very rare and their conducting properties have not been investigated, mainly because of their insufficient morphological robustness for electrochemical doping. Thus, the design of nanostructures that satisfy the three requisites for electromagnetic properties—coiled pathways, one-handedness, and electroconductivity—is still a highly challenging research topic. A few years ago we found that a Gemini-shaped hexabenzocoronene (HBC) bearing triethylene glycol and dodecyl chains self-assembles into a graphitic nanotube. More recently, we have also found that the incorporation of pendant norbornene groups into the amphiphilic HBC (1) gives rise to a nanocoiled assembly with uniform diameter and helical pitch. The metastable coiled structure exists for a sufficiently long time, probably because of a steric effect of the pendant norbornene groups, and therefore allows post ring-opening metathesis polymerization (ROMP) of the norbornene groups to covalently stabilize the kinetically selected assembly against a thermodynamic coil-to-tube transformation. The polymerized nanocoil consists of a pstacked HBC array that exhibits conductivity upon oxidative doping. However, the resultant nanocoils are a mixture of rightand left-handed ones, which means that further structural elaboration is needed to realize the one-handed helical graphitic array that is essential for the exploration of electromagnetic properties. Herein we report the selective formation of covalently stabilized conductive nanocoils with a one-handed helical chirality by coassembly of norbornene-appended HBC derivatives 1 and 4 (Figure 1). This achievement is the result of a

Radially Diblock Nanotube: Site-Selective Functionalization of a Tubularly Assembled Hexabenzocoronene

An amphiphilic Gemini-shaped hexa-peri-hexabenzocoronene (1) with azide functionalities self-assembles to form graphitic nanotubes whose interior and exterior surfaces can be post-functionalized by “click chemistry”. A click reaction of the nanotubes with bulky macromolecules such as a dendronized alkyne (G2-alkyne) takes place site-selectively on the exterior surface of the nanotubes, leaving the azide functionalities on the interior surface mostly intact. A subsequent click reaction of the resulting nanotubes with alkynes having a much smaller steric bulk allows the remaining azide groups to be functionalized, affording radially diblock graphitic nanotubes.

Remarkable effects of terminal groups and solvents on helical folding of o-phenylene oligomers.

Although o-phenylene oligomers (OP(n)R) made of dimethoxyphenylene units are thought to be intrinsically dynamic due to π-electronic repulsion, we show that they fold into a regular helical geometry in CH(3)CN when they carry terminal groups such as CH(3), CH(2)OH, Br, CO(2)Bn, and NO(2). We evaluated their helical inversion kinetics via optical resolution of long-chain oligomers (e.g. 16- and 24-mers) by chiral HPLC. OP(24)Br at 298 K shows a half-life for the optical activity of 5.5 h in CH(3)OH/water (7/3 v/v) and requires 34 h for complete racemization. The perfectly folded helical conformers of OP(n)R, unlike their imperfectly folded ones, are devoid of extended π-conjugation and show a cyclic voltammogram featuring reversible multistep oxidation waves.

Rational synthesis of organic thin films with exceptional long-range structural integrity

Standing at order Thin films of organic molecules on solid substrates tend to nucleate at many sites and grow multiple domains. However, one large uniform film would be much more desirable in device applications. Seiki et al. designed organic molecules that filled space in a hexagonal tiling; a propeller-like triptycene base adhered to crystalline surfaces and alkyl tails extended away from it. The authors could make well-ordered multilayer films up to centimeter length scales. Science, this issue p. 1122 Tripodal triptycene building blocks that fill space form large-area organic thin films free of domain boundaries. Highly oriented, domain-boundary–free organic thin films could find use in various high-performance organic materials and devices. However, even with state-of-the-art supramolecular chemistry, it is difficult to construct organic thin films with structural integrity in a size regime beyond the micrometer length scale. We show that a space-filling design, relying on the two-dimensional (2D) nested hexagonal packing of a particular type of triptycene, enables the formation of large-area molecular films with long-range 2D structural integrity up to the centimeter length scale by vacuum evaporation, spin-coating, and cooling from the isotropic liquid of the triptycene. X-ray diffraction analysis and microscopic observations reveal that triptycene molecules form a completely oriented 2D (hexagonal triptycene array) + 1D (layer stacking) structure, which is key for the long-range propagation of structural order.

The pH dependence of the anisotropy factors of essential amino acids

In order to examine the pH dependence of the anisotropy factor (g) of all the essential amino acids, at pHs 1, 2, 7 and 11, the molar absorption coefficient (e) and circular dichroism (Δe) were determined in aqueous solution. The g factors were high, and except for Asn and the aromatic and sulfur-containing compounds (Tyr, His, Trp, Cys and Met) the gmax values are larger than 0.01 in the pH range 1–7. Large g factors of aliphatic amino acids are observed even at pH 11. On the whole the magnitudes of the g factors at pH 1 are 2–3 times those at pH 7. The n,π* transition of the carboxylic group is the major contributor to the large g factors in amino acids such as Ala and Val. It is thought that in connection with the possible origin of amino acid homochirality generated by circularly polarised light irradiation, the % ee (≡ % op) of the amino acid generated will also be pH dependent in a manner similar to that previously reported for Leu.

Unusual Side‐Chain Effects on Charge‐Carrier Lifetime in Discotic Liquid Crystals

When hexa-peri-hexabenzocoronene (HBC) carries paraffinic side chains with ester-ether termini (1a-1c), a hexagonal columnar liquid-crystalline (LC) mesophase forms, which is characterized by a very wide temperature range (ca. 0-300 degrees C). The LC materials from these HBC derivatives show characteristic electronic properties with an extremely long photocarrier lifetime. For example, by means of a flash-photolysis time-resolved microwave conductivity technique, the photocarrier lifetime, observed for liquid-crystalline 1a, is as long as 14 ms, which is four-orders of magnitude longer than that of fully paraffinic HBC 4 (0.0011 ms). The maximum transient photoconductivities of liquid-crystalline 1a-1c are comparable to one another and also to those of HBCs without ester-ether terminals. Liquid-crystalline 1a exhibits an obvious photocurrent generation in response to light illumination.

Possible One-Dimensional Helical Conductor: Hexa-peri-hexabenzocoronene Nanotube

ESR and 1 H NMR investigations were performed on iodine-doped HBC nanotubes to understand the low-temperature electronic states and carrier dynamics. Using ESR measurements, we clarified that the charged carriers induced by iodine-doping possess spins, and we found two kinds of spin species with different doping levels. The ESR results indicate the itinerant features down to 170 K. The 1 H T 1 -1 for the heavy-iodine-doped HBC nanotubes follows T 0.5 and exhibits a characteristic frequency dependence. We propose that the spin excitation propagates along the one-dimensional helical chain.

A design principle of polymers processable into 2D homeotropic order.

How to orient polymers homeotropically in thin films has been a long-standing issue in polymer science because polymers intrinsically prefer to lie down. Here we provide a design principle for polymers that are processable into a 2D homeotropic order. The key to this achievement was a recognition that cylindrical polymers can be designed to possess oppositely directed local dipoles in their cross-section, which possibly force polymers to tightly connect bilaterally, affording a 2D rectangular assembly. With a physical assistance of the surface grooves on Teflon sheets that sandwich polymer samples, homeotropic ordering is likely nucleated and gradually propagates upon hot-pressing towards the interior of the film. Consequently, the 2D rectangular lattice is constructed such that its b axis (side chains) aligns along the surface grooves, while its c axis (polymer backbone) aligns homeotropically on a Teflon sheet. This finding paves the way to molecularly engineered 2D polymers with anomalous functions.