Azobenzenes aggregates

The structure with the deformed hairy rods is preserved when multilayers are deposited on solid supports, as evidenced by X ray reflectometry revealing a marked bilayered structure (see Figure 6.16). Photoisomerization of the aggregated azobenzene chromophores in LBK films is possible and leads to significant structural changes. The bilayered structure is completely... 

The UV-visible spectra of the H- and nifro-azobenzene dendrimers in chloroform solution showed strong absorption bands within the visible region due to the transitions of azobenzene chromophores (Table 2). Because of the stronger delocalization of n-electrons in nitro-azobenzene, the maximum absorption band is at a longer wavelength compared with that for H-azoben-zene. There was little spectral shift of the absorption maximum for dendrimers with different numbers of azobenzene units, indicating that dendrimers did not form any special intermolecular aggregates. 

A series of aggregation structures of bilayer forming azobenzene amphiphiles, CnAzoCmN+Br, both in single crystals and cast films was determined by the X-ray diffraction method and uv-visible absorption spectroscopy. From the relationship between chemical structures and their two-dimensional supramolecular structure, factors determining the molecular orientation in bilayer structure were discussed. Some unique properties based on two-dimensional molecular ordering were also discussed. 

Recently, Okuyama et al. succeeded to prepare single crystals of some azobenzene amphiphiles and decided molecular and aggregation structure of single crystals [14-19], The spectral prediction of the chromophore orientation in the bilayer assemblies were very consistent with the X-ray structural analyses of the single crystals. 

The amphipathic compounds shown in Scheme 2 can form a disc-like micelle(7). The shape of a molecular aggregate depends on the shape of the constituent molecules(8). For instance, conical molecules with large polar head groups prefer to form spherical micelles while cylindrical molecules tend to give flat aggregates. Trans-azobenzene is a rod-like molecule whereas the cis-form is bent. 

Photochemical response of these liposomes is different from each other. With progression of trans - cis photoisomerization of azobenzene, ICD at the absorption band of the trans isomer decreases. As shown in Figure 4, depression in ICD is almost proportional to the amount of photoisomerization for the phase separated system. Photoisomerization in the domain of azobenzene aggregate proceeds independently from the rest of DPPC aggregate so that the depression in ICD corresponds to the concentration of remaining transazobenzene. When the two components are molecularly mixed, change of... 

Figure 16.7 TEM picture (uranyl acetate staining) of vesicles reported by Schenning etal. [44] (A) schematic representation of the bilayer, (B) palmitoyl-and (C) azobenzene-modified poly(propylene imine) dendrimers used in the construction of the aggregates...

Similar generation-dependent aggregation behavior was reported for a series of dendrons having a phosphate focal group and functionalized at the periphery with azobenzene chromophores in water (Zhang et al. 2007). In this system, the stability... 

Figure 11.17 Aggregation behavior and photoresponsive properties of an amphiphilic azobenzene dendrimer.

Because of some aggregation (H aggregation) of the azobenzene chromophores, the collapse pressure increased with higher azobenzene/palmitoyl ratios. However, only the palmitoyl functionalized dendrimer exhibited a change in surface area upon irradiation with 365-nm light. The lack of photoinduced surface area changes... 

Zhang W, Xie J, Yang Z, Shi W. Aggregation behaviors and photoresponsive properties of azobenzene constructed phosphate dendrimers. Polymer 2007 48 4466-4481. 

Transform-limited (TL) visible pulses with as short as sub-5-fs duration have been generated from a noncollinear optical parametric amplifier and applied to the study of polyacetylene, polydiacetylene, azobenzene, and J-aggregates of porphyrin for optical -devices. 

Irradiation at X>460 nm gave a fast cis-trans isomerization, with a concomitant increase in Tg. The sol-gel phase transition could be controlled in this way by light, while read-out of the states could be performed by measuring the modulation of the transmission or CD. For instance, the traras-azobenzene gelators displayed a CD effect, presumably due to the formation of helical aggregates, whereas the cis-isomers did not. 

The compound XXIII, consisting of two amphiphilic helical rods linked by an azobenzene moiety, was found to form micelles and ordered aggregates in aqueous solution in the dark, when the azo moiety is in the trans configuration. Photoisomerization of the azo linkage into the cis configuration, and the consequent bending in the structure of the molecules, induced disaggregation and disruption of the micelles.1118,1191... 

The cac values were estimated as the first inflexion point from the curves and they are equal to 2xlO g/L for sample 9 and lO g/L for sample 10, respectively. These relatively low values can be explained by the presence on the polysiloxane side-chain of the azobenzenic hydrophobic groups with a high aggregation tendency (H-type or J-type - Fig. 18) [33, 34]. 

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