Dendrimers with azobenzene, isomerization

Junge and MacGrath synthesized benzyl aryl ether dendrimers with azobenzene central linkers.The irradiation of the -dendrimers in dichloromethane with 350-nm light resulted in clean isomerization to Z-isomers, which reverted to E-isomers upon exposure to bright sunlight for a period of only several seconds (Scheme 61). 

Jiang and Aida reported on the synthesis of even more highly branched aryl ether dendrimers trans-LnAZO, where n denotes the number of aromatic layers, 1, 3-5) with azobenzene in the center of the molecules. Upon UV irradiation at 21°C, these -dendrimers isomerized to Z-isomers, which slowly reverted to the -form in the dark Remarkably, IR irradiation appreciably accelerated reversion to the -form, and this reversion depended on the size of the dendrimers the rate of the isomerization of Z-L5AZO to the -form 16 was 260 times faster than that of thermal isomerization at 21°C in the dark, whQe, in sharp contrast, no effects were observed upon IR irradiation for the isomerization of smaller homologues, such as Z-LlAZO,17 and Z-L3AZO. 

According to the authors, intramolecular energy transfer from the aromatic rings of the dendrimer matrix, which absorbed the infrared energy, excites the central azobenzene chromophore and accelerates the Z,E-isomerization. Thus, low-energy photons may serve for the chemical transformation of molecules such as dendrimers with the azobenzene unit in the center. 

Vogtle et al. reported the first example of a photoswitching dendrimer [94] with six azobenzene moieties attached to a derivative of 1,3,5-trisubstituted benzene as the central core. Irradiation of the all (F)-isomer at 313 nm led to a photostationary equilibrium where most of the azobenzene units were switched to the (Z)-configuration. Conversely, irradiation of this species again at a lower energy frequency (436 nm) led to a second photostationary equilibrium where the (F)-form was dominant however, it was not proven as to how many azobenzene units isomerized after irradiation. 

The thermal cis- trans isomerization of the azobenzene ligands confined in the nanopores has a constant rate constant and exhibits faster isomerization than in solution with the exception of the bulky AzoG3 dendrimer. This result is consistent with Brinker and coworkers observation of azobenzene-modified nanoporous silica films and supports their two-rate-constant physical model. The nanoporous silica materials prepared by Brinker and coworkers have a cubic (BCC) pore structure. The azobenzene ligands positioned on the pore coimections have different local environments from those positioned on the spherical pore surfaces. Thus the azobenzene ligands isomerize at two different rates—fast and slow. The MCM-41 nanoporous materials prepared by Zink and coworkers have a hexagonal array pore structure in which all the azobenzene ligands positioned on the channel surfaces have the same local environment. Thus simple one-rate-constant first-order kinetics is sufficient to describe the isomerization process. 

Junge and MeGrath reported studying photoresponse of dendrimers eontaining three interior azobenzene moieties that are radially configured about the eore. With each azobenzene capable of cis-trans isomerization, the... 

The formation of vesicles can also be induced by electrostatic interaction or electrostatic self-assembly. Among others, it was shown that LUVs are obtained when a polycation is mixed with a polyanion-fc-poly(ethylene glycol). The vesicle size can be controlled by changing the polymer concentration. Vesicles are also obtained when a polyanion- -poly(ethylene glycol) is mixed with a cationic azobenzene surfactant. In that case, the vesicles are photoresponsive, that is, they dissemble if the azobenzene is isomerized from the irans form to the cis form. Finally, vesicles can also be prepared by mixing a cationic dendrimer and an anionic sulfonate dye. ... 

Ramamurthy and Jayaraman employed the cavity of poly(aIkyl aryl ether) dendrimers of the third generation with various n-alkyl linker (ethyl to n-pentyl) connecting the branch points (C2G3-C5G3) for photochemical isomerizations of azobenzene and stilbene. Isomerization was carried out in aqueous dendrimer solutions. Irradiation of solutions for 6 h furnished products. The results of isomerization of stilbene in the presence of various dendrimer solutions and in... 

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