A Dendron Based on Natural Amino Acids: Synthesis and Behavior as an Organogelator and Lyotropic Liquid Crystal

Rapid and reversible gel-sol transition of self-assembled gels induced by photoisomerization of dendritic azobenzenes

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.

Hydrogen-Bonded Dendronized Polymers and Their Self-Assembly in Solution

Frechet-type benzyl ether dendrons of second and third generations with a carboxyl group (G2, G3) at the apex site could attach to poly(4-vinylpyridine) (PVP), forming hydrogen-bonded dendronized polymers (HB denpols) in their common solvent, chloroform. The HB denpols show unique self-assembly behavior, forming vesicles in the common solvent under ultrasonic treatment. The structure and morphology of the vesicles were characterized by dynamic light scattering (DLS), static light scattering (SLS), SEM, TEM, and AFM. The size of the vesicles decreases and the thickness of the vascular membrane increases as the molar ratio of Gx/PVP increases. The hydrogen bonding, pi-pi aromatic stacking of the dendrons, and the considerable difference in architecture between the dendron Gx and PVP are the main factors facilitating the assembly of the HB denpols in the common solvent.

Synthesis and self-assembly of dendritic-helical block copolypeptides

Dendritic-helical diblock copolypeptides, dendritic poly(-lysine)--poly(γ-benzyl--glutamate) (PBLG-Lys) were synthesized up to 4th generation of dendritic poly(-lysine). PBLG was synthesized by conventional ring-opening polymerization of γ-benzyl--glutamate--carboxyanhydride with heptyl amine as an initiator. The -terminus of this PBLG was used for further coupling reactions with ,-bis(-butoxycarbonyl)--lysine pentafluorophenylester. These block copolypeptides possess well-defined 3-D structures in solution, and they self-assemble into a fibrous structure a nanoribbon mechanism in toluene. Amphiphilic block copolypeptides were obtained after deprotecting BOC groups at the periphery of the lysine dendrimer block. Due to the well-defined size and structure of the dendritic lysine block and well-defined α-helical conformation of the PBLG block, the block copolypeptides showed a generation-dependent self-assembly behavior in aqueous solution.