矢貝 史樹

Hydrogen-bonded cyclic hexamer 3 composed of azobenzene-appended melamine and cyanurate was characterized by H NMR and dynamic light scattering measurements. In nonpolar solvents such as cyclohexane, 3 hierarchically organizes into higher level aggregates even in the diluted conditions less than 1 mM. At high concentration (over 10 mM), 3 formed supramolecular gel. Scanning electron microscopic images of the dried gel shows intertwined fibrous superstructures with average diameter of 500 nm. X-ray diffraction measurements revealed that the fibers are composed of 3 stacking in hexagonal columnar motif. When the supramolecular gel was irradiated with UV-light, gel phase turned into sol accompanying the trans-to-cis isomerization of the azobenzene side chains. The resulting sol phase restored original gel state when it was kept in the dark for 48 h. 1

Self-complementary hydrogen-bonding supramolecular components 1-3 possessing azobenzene functional side chains were synthesized and their self-aggregation behaviors in solution state as well as in solid state were investigated. Compound 1 and 2 showed poor solubility in various organic solvents whereas 3 is highly soluble even in aliphatic solvents. In alkane solvents such as n-hexane and n-heptane, compound 3 formed organogels at high concentration (20 mM). The dried gel is strongly birefringent and showed a clear diffraction peak corresponding to the molecular length of 3 (44 Å) in X-ray diffraction measurements. When a heptane gel was irradiated with UV-light (350 nm), the gel turned into complete fluid with the trans-to-cis isomerization of the azobenzene moiety.

Hydrogen-bonded cyclic hexamer (rosette) 2 composed of azobenzene-appended melamine and N-dodecyl cyanurate was characterized by H NMR and dynamic light scattering measurements. In nonpolar solvents such as cyclohexane, rosette 2 hierarchically organized into higher level aggregates even in the diluted conditions less than 1 mM. At higher concentration range (over 10 mM), rosette 2 formed supramolecular gel. Scanning electron microscopic and X-ray diffraction studies of the dried gel revealed that supramolecular gel are composed of rosette 2 stacking in hexagonal columnar packing motif. When the supramolecular gel was irradiated with UV-light, gel phase turned into sol accompanying the trans-to-cis isomerization of the azobenzene side chains. 1

The assemblies of melamine dimers (1-3) that have different linker length and dipolar dye 5 were investigated. The compound pair of 1+5 formed extended supramolecular polymers, and that of 2+5 and 3+5 formed cyclic or small aggregates as expected from the gelation behaviors in aliphatic solvents. The absorption spectrum of 1+5 aggregates in aliphatic solvents showed blue-shifted band of 5, and that of 2+5 and 3+5 aggregates showed red-shifted band of 5 in comparison to that of 5 alone. These results indicate that diverse aggregated species where the motif of excitonic interaction between dyes are considerably different are formed in these pairs, depending on the length of the alkyl linker of bismelamines. We further investigated this aspect in detail by fluorescence spectroscopic and dynamic light scattering methods.

Previously we reported supramolecular gels based on coaggregates between bismelamines 1 and various bisimide components in nonpolar organic solvents. Here is reported the influence of the bisimide merocyanines dyes 3 and 4 on the morphology and the physical properties of the organogel network. When the equimolar mixtures of 1 and 3 or 4 in cyclohexane were heated and then cooled, supramolecular gels were obtained. In the scanning microscopic observations of the freeze-dried gel, a sheet-like superstructures was observed for 1-3 whereas entangled fibrous network was observed for 1.4. Absorption spectra of the gel phase suggested that 3 forms J-aggregates whereas 4 forms H-aggregates in the gel phase. Therefore, difference in aggregation motif of the merocyanines dyes may affect the gel network. Dynamic rheological measurements were also conducted for the present gel, the results of which showed good correlation with the network structure of the gels observed by microscopy.

Zinc-chlorin 3 (see Fig, 2 in text) possessing a tertiary 3(1)-hydroxyl group and a 13-keto group was synthesized as a model for the antenna chlorophylls of green bacteria. Self-aggregation of 3 in nonpolar organic media was examined and compared to 1 and 2 possessing a primary and secondary 31-hydroxyl group, respectively. Zinc-chlorin 3 self-aggregated in 1 vol% CH2Cl2-hexane to form oligomers and showed a red-shifted Q(y) maximum at 704 nm compared to the monomer (648 nm in CH2Cl2), This red-shift is larger than that of 2S (648 --> 697 nm) and comparable to that of 2R (648 --> 705 rim), but smaller than that of 1 (648 --> 740 ram), indicating that while a single 3(1)-methyl group (prim-OH --> sec-OH) suppressed close and/or higher aggregation, the additional 31-methyl group (sec-OH --> tert-OH) did not further suppress aggregation. The relative stability of the aggregates was in the order 1 > 2R similar to 3 > 2S as determined by visible spectral analyses. Molecular modeling calculations on dodecamers of zinc-chlorins 1, 2R and 3 gave similar well-ordered energy-minimized structures, while 1 stacked more tightly than 2R and 3, In contrast, 2S gave a relatively disordered (twisted) structure. The calculated dodecameric structures could explain the visible spectral data of 1-3 in nonpolar organic media.