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An asymmetric bis-dendritic gelator (1) consisting of an azobenzene dendron and an aliphatic amide dendron was synthesized to achieve a photoresponsive self-assembly. The compound gelled in a wide range of organic solvents, even at concentrations as low as 0.02% (w/v) in cyclohexane. The self-assembled fibrillar network structure was confirmed by field-emission scanning electron microscopy (FE-SEM), atomic force microscopy (AFM), and X-ray diffraction (XRD) analyses. The rapid and reversible gel-sol transition by irradiation with UV and visible light was investigated by UV-vis and Fourier transform infrared (FT-IR) spectroscopy, FE-SEM, and XRD analyses. Upon irradiation of the gel with UV, trans-to-cis isomerization of the azobenzene groups occurred, and the gel turned into a sol state. The gel was recovered immediately by the reverse cis-to-trans isomerization after the exposure to visible light. The trans-to-cis isomerization of the azobenzenes disrupted the hydrogen bonding of azobenzene amide groups, together with the hydrogen bonding in the aliphatic amide dendron. This facile communication between the two amide dendrons leads to the dissociation of the gel fibers and collapse of the gel.

A series of bis-dendritic gelators consisting of a benzamide dendron and an alkyl dendron were synthesized to investigate the dendritic effect on self-assembly. The gelators with a first-generation benzamide (benzamide-G1) dendron or a first-generation alkyl (alkyl-G1) dendron formed stable gels in most aromatic solvents, and their self-assembled fibrillar networks were imaged by electron microscopy. The unbranched molecule (G0-G0) or the molecule possessing a second-generation benzamide (benzamide-G2) dendron did not form gels. Differential scanning calorimetry, powder X-ray diffraction, and Fourier transform IR studies revealed that introduction of a dendritic branch strongly affected the molecular packing as well as the strength of intermolecular interactions. Furthermore, concentration-dependent diffusion coefficient measurements and the evaluation of association constants by (1)H NMR spectroscopy indicated that bis-dendritic gelators with a benzamide-G1 dendron possessed high association constants and formed large aggregates, whereas molecules with a single benzamide formed dimers in chloroform. The formation of self-assembled fibrillar networks was driven by the multivalent and cooperative hydrogen bonding observed in the benzamide-G1 dendrons. Pi-pi stacking of aromatic groups and van der Waals interactions between alkyl chains also played roles in the self-assembly process, thus indicating that a spatial balance between two dendrons is important.

Analogues of 4-dodecyloxy-2-trifluoromethylbenzamide (12FH2) consisting of a hydrophobic alkyl chain, a trifluoromethylated aromatic ring, and a self-complementary hydrogen-bonding amido group were synthesized, and the structural effect of each component on the formation of parallelogrammatic pipes was investigated. Differential scanning calorimetry and powder XRD analyses revealed that all-trans L and gauche-rich S polymorphic forms appeared for the analogues with more than eight carbon atoms in the alkyl chain, that is, the polymorphism originates in the conformation of the alkyl groups and hydrogen-bonding patterns of the benzamide group. Also, the trifluoromethyl substituent is crucial in that it provides an appropriate molecular balance between the benzamide and alkyl groups. Scanning electron microscopy and powder XRD analyses of solids obtained by a drying-mediated assembly process revealed that production of the L polymorph by polymorphic transition from the S polymorph resulted in evolution of a three-dimensional structure when the alkyl group has more than 12 carbon atoms. Among the series of compounds, 12FH2 and 4-tetradecyloxy-2-trifluoromethylbenzamide (14FH2) formed parallelogrammatic pipes with micrometer dimensions. An atomic force microscopy study of 12FH2 suggested that a single pipe may be composed of platelike crystallites of L polymorph. From a mercury-intrusion porosimetry study, it was determined that macroporous materials with average pore diameters of about 40 microm and porosity of about 80% were obtained. The previously proposed self-templation mechanism by polymorphic transition from S to L polymorph was further discussed in view of polymorphism and the crystallization rate. An appropriate molecular balance between the benzamide and alkyl groups is necessary to induce a proper polymorphic transition for the development of a three-dimensional hollow structure in the evaporation process.

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