Azobenzene units

Irradiation with UV light isomerized the azobenzene units from the trans to the cis form, while the reverse isomerization occurred thermally in the dark. The cis to trans conversion is catalyzed by both protons and hydroxyl ions. Hence, the catalyzed dark process for tethered azobenzene is greatly modified in comparison with that for free azobenzene. For the tethered azobenzene, beginning at pH 6, the cis to trans return rate sharply decreased with increasing pH up to 10, whereas the rate for free azobenzene rapidly increased in the same pH range owing to OH- catalysis. These observations can be explained by the electrostatic repulsion which lowers the local OH concentration on the polyion surface below that in the bulk aqueous phase. 

Synthesis of Poly(p-phenylene) Containing Azobenzene Unit 60so... 

Autocatalytic formulations, 236 Autoclave cycle, 167 Automotive adhesives, 203 Azobenzene unit, 493... 

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. 

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. 

Recently, Aida and Jiang [96] synthesized a series of dendrimers similar to those reported by McGrath and coworkers and reported photoisomerization of their core azobenzene units using infrared radiation. This new strategy holds immense potential for using dendrimers as light-harvesting matrices. 

The bisbarium complex of azobis(berizo-18-crown-6) ether 28 exhibits catalytic properties that can be reversibly activated-deactivated by light-induced changes in molecular geometry [32]. The azobenzene unit is a well-known photochromic [33], often used in the construction of molecular switches [34]. 

Photochemical switching of the electrical properties of conductive Langmuir-Blodgett films results from photoinduced conformational changes produced by an azobenzene unit [8.253]. 

Many systems are described in which changing the cavity shape by allosteric effects allows the cation binding ability and the selectivity of the receptor to be modified and controlled. For instance a photo-responsive cis/trans isomerizable azobenzene unit has been introduced in macrocyclic structures in order to change the receptors cavity shape, leading to a photo-control of ion extraction. 

Photoisomerisations of azobenzene units have been found to proceed particularly clearly and to be well suited for experimental studies. The thermodynamically more stable E-isomer can be transformed photophysically into the Z-form, which can return to the E-form on photoexcitation or thermal treatment (Fig. 5.20). 

If the periphery of a POPAM dendrimer bears azobenzene units then dye molecules can be included as guests in the dendrimer scaffold (see also host/ guest chemistry in Section 6.2.3) [23]. Here the E- and Z-isomers (or their enriched versions) differ in their capacity for accommodating guest molecules. In principle, guest molecule inclusion can be controlled (switched) in this way (Fig. 5.21). 

Fig. 5.21 A POPAM dendrimer with peripheral azobenzene units, on the left in the E-configuration, on the right in the Z-configuration (schematic). The E- and Z-isomers interact differently with eosin as guest...

A light-controlled dendritic box was investigated in analogous fourth-generation POPAM dendrimers whose periphery alternately bore azobenzene and naphthyl moieties. Measurements of photoisomerisation at a wavelength of 365 nm in dichloromethane showed that not all azobenzene units in the E form are transformed into the Z form, but that, for example, four units remain in the E form and some 28 units isomerise to the Z form. At pH 7, six eosin molecules per host molecule were taken up by this mixture of isomers. On the other hand, the dendrimer having all azobenzene groups in the E form takes... 

In this rotaxane, a-CD exists at the trans-azobenzene part but it moves to the methylene part when the trans-azobenzene unit is converted into cts-azoben-zene. This light-driven locational change was regarded as a molecular shuttle system. 

The E-Z isomerization of an azobenzene unit was employed in an approach towards photocontrol of the chiral recognition event in a membrane.1581 To this end, (4-(phenyl-azo)phenyl carbamate residues were attached to carbamate-protected glucose units of cellulose and amylose. The photomodulation of the chiral recognition was explained by a change in the ordering of the polymer, leading to a change in solubility. 

Scheme 24 Chiral photoswitchable polyisocyanates A) schematic representation of the shift in equilibrium between P and M helices upon irradiation. B) illustration ofPto M helix transition in polyisocyanates upon photoisomerization ofthe azobenzene unit (adapted from references 75-78).

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