title: Hierarchical organization of photoresponsive hydrogen-bonded rosettes.
abstract: Hydrogen-bonded disk-shaped aggregates (rosettes) composed of azobenzene-appended melamine and barbiturate or cyanurate are investigated in view of their hierarchical organization and photoresponsive behavior by (1)H NMR and UV/vis spectroscopies, dynamic light scattering, and gelation behavior in aliphatic solvents and liquid crystalline behavior in bulk state. In the bulk state the rosette possessing a sterically bulky tridodecyloxyphenyl substituent in the barbiturate component stacks in an offset arrangement to form a rectangular columnar mesophase, whereas in aliphatic solvents it does not hierarchically organize into higher-order columnar aggregates. This drawback is improved by exchanging the barbiturate component into a more sterically nondemanding N-dodecylcyanurate component. The resulting new rosette stacks in a face-to-face arrangement to form a hexagonal columnar mesophase in the bulk state and hierarchically organizes into elongated fibrous aggregates in cyclohexane, which eventually leads to the formation of organogel. Dynamic light scattering and UV-vis experiments upon UV-irradiation of the columnar aggregates in cyclohexane revealed that the dissociation and the reformation of columnar aggregates can be controlled by the trans-cis isomerization of the azobenzene moiety. Molecular modeling indicates that the rosette possessing cis-azobenzene side chains loses its planarity. Using this photoinduced morphological change of the rosette, photoresponsive organogel is created by the use of a disk-shaped supramolecule the first time.
authors.affiliation: Department of Applied Chemistry and Biotechnology, Faculty of Engineering, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan. yagai@faculty.chiba-u.jp
authors.forename: Shiki
authors.initials: S
authors.lastname: Yagai
authors.forename: Toshiharu
authors.initials: T
authors.lastname: Nakajima
authors.forename: Keiki
authors.initials: K
authors.lastname: Kishikawa
authors.forename: Shigeo
authors.initials: S
authors.lastname: Kohmoto
authors.forename: Takashi
authors.initials: T
authors.lastname: Karatsu
authors.forename: Akihide
authors.initials: A
authors.lastname: Kitamura