Predetermined Chiroptical Properties Expressed on Rosette Nanotubes
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Transferring the film from the liquid substrate to a solid substrate has proven difficult. The researchers tried two techniques. For the investigation of the upper side of the film, a microscope slide can be slid into the liquid just under the monocrystalline film to lift it slowly. With this method, the film broke, but crystal fragments of more than 2 mm2 area were preserved. To investigate the underside of a crystal, tape affixed to a sample holder was drawn down until it gently touched the monocrystalline film. Then the film was lifted from the liquid. With this method, 7-mm-diameter crystals could be separated without breaking. The researchers concluded that this is a potentially useful approach for producing large photonic single crystals, which are the prerequisite for the preparation of resonators or, more generally, for wave guiding in photonic structures. ANDREI A. ELISEEV
Predetermined Chiroptical Properties Expressed on Rosette Nanotubes Chirotechnology is an emerging field with the potential for application in the design of sensors, chiral cholesteric phases, catalysts, and materials with electromagnetic, optoelectronic, information-storage, display-system, nonlinear optical, and novel chiral light-emitting applications. Induction of nonrandom symmetry breaking in supramolecular systems by external means, such as chiral vortex forces, electron transfer, and photoswitches, have the advantages of predictability and reproducibility. As a step toward a better understanding of nature’s approach to generating supramolecular structures with predefined stereochemistry and architecture, researchers at the H.C. Brown Chemistry Laboratory, Purdue University, have extended the functionality of self-assembled helical rosette nanotubes by imparting on them adjustable chiroptical properties. As reported in the August 24 issue of the Journal of the American Chemical Society, a team of Purdue University researchers led by H. Fenniri induced supramolecular chirality in rosette nanotubes (see figure) using two methods: by inducting symmetry breaking in a preexisting racemic mixture of M- and P-helical rosette nanotubes; and by employing a chiral promoter on a prochiral molecular module to trigger the hierarchical self-assembly of chiroptical rosette nanotubes. Both methods employ promoters that transfer their molecular chirality to the supramolecular ensemble as well as stabilize the nanotube assembly. Rosette nanotubes are a class of organic MRS BULLETIN/DECEMBER 2002
nanostructures whose aqueous selfassembly is entropically driven. The single building block in these assemblies is a bicyclic molecule—compound 1—with a hydrogen-bond donor–donor–acceptor array on one side, an acceptor–acceptor–donor array on the other, and a crown ether substituent (see figure). Rosettes, which consist of a six-membered macrocycle held together by 18 hydrogen bonds, stack to form a nantobube with a central pore and peripheral channels formed by the crown ethers. As expected by Fenniri and co-workers, induced circular dichroism (ICD) resu
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