Azobenzene groups

Success of depositing compounds where an 18-carbon chain was attached to one end of an azobenzene group and various different hydrophilic groups attached to the other end has been reported in X and Z mode [52] and piezo-and pyroelectric effects were demonstrated. 

As just mentioned, phosphorus porphyrins have unique photochemical properties. Their photophysics is also interesting. Emitter-quencher assemblies based on porphyrin building blocks have attracted attention due to their potential to serve as models in photosynthetic research (see [90] for an example) or for the development of photoswitches that could be used for the fabrication of molecular electronic/optical devices. In this context, Maiya and coworkers constructed a P(VI) porphyrin system 59b with two switchable azobenzene groups positioned in the apical positions of the pseudo-octahedral phosphorus atom [92]. Photoswitch ability (luminescence on/off) was demonstrated as... 

Vogtle and co-workers first reported a photoswitchable dendrimer [33] with six peripheral azobenzene groups, which took advantage of the efficient and fully reversible photoisomerization reaction of azobenzene-type compounds (Scheme 7). In a follow-up study [34], polypropylene imine) dendrimers bearing azobenzene moieties (p-Im-Gn, n = 1-4) on the periphery were synthesized. These dendrimers displayed similar photoisomerization properties as the azobenzene monomers. Irradiation of the all-E azobenzene dendrimers at 313 nm led to the Z-form dendrimers, while irradiation at 254 nm or heating could convert the Z-form dendrimers back to the E-form dendrimers. The observation that the... 

This cross-sectional mismatch between the small anchor and the large dendritic component modified the intermolecular packing in the Langmuir-Blodgett mono-layer and film so that sufficient mobUity was present near the azobenzene group. This mobility greatly facihtated efficient molecular reorganization in response to E Z photoisomerization. 

For this puq)ose, the photoswitchable bis(crown ether)s 88 and 89 as well as the reference compound 90 have been synthesized. Compounds 88 and 89 are highly lipophilic derivatives of azobis(benzo-15-crown-5). The parent azobis crown ether was originally developed by Shinkai and its photoresponsive changes in complexation, extraction, and transport properties thoroughly examined. Compared to 87, more distinct structural difference between the cis and trans isomers can be expected for 88 and 89 because in the latter compounds the 15-crown-5 rings are directly attached to the azobenzene group. The photoequilibrium concentrations of the cis and trans forms and the photoinduced changes in the complexation constants for alkali metal ions are summarized in Table 7. 

Fig. 2. Transmittance changes at 750 nm of a 1 mass % aqueous solution of PNIPAM with pendant azobenzene groups (2.7 mole %) when heated at a rate of 2°C/min ( ) before photoirradiation and (O) in the photostationary state by irradiation with UV (350 < X < 410 nm) light...

Fig. 8. Volume changes of a PNIPAM gel containing pendant azobenzene groups (10.8 mole %) as a function of temperature ) before and (O) after UV irradiation (350 < X < 410 nm)...

In the case of compounds containing the azobenzene group, oxidation has been found to occur,16 yielding either symmetrical dibenzyls or stilbenes depending on the substitution of the azobenzene. It was, however, pointed out that this is a consequence of the oxidative ability of the azo group,17 since these reactions also take place under a nitrogen atmosphere. 

POPAM dendrimers bearing up to 32 photoswitchable azobenzene groups at the periphery were used as host compounds for eosin Y (2, 4, 5, 7 -tetrabromo-fluorescein dianion) (see also Section 5.1.2). One reason for the choice of this dye was that it shows strong fluorescence which should be influenced by being enveloped by the dendrimer. The other reason was that the energy of its lowest triplet state is higher than that of the lowest triplet state of azobenzene, leading... 

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... 

Dendritic molecules may possibly be of use in nanoscale impellers or valves . For this purpose azobenzene dendrons [80] with terminal hydroxyl groups (Fig. 8.27) were covalently fixed in the interior of well-defined silicon dioxide nanotubes. In the sol/gel process, the hydroxyl groups lead to incorporation of azobenzene groups into the Si02 nanotubes being formed (Fig. 8.26) [81]. 

Yamaguchi, I. Osakada, K. and Yamamoto, T. (2000) Pseudopolyrotaxane composed of an azobenzene polymer and y-cyclodextrin. Reversible and irreversible photoisomerization of the azobenzene groups in the polymer chain, Chem. Commun. 1335-1336. 

Polymers of poly(L-ornithine) possessing varying contents of azobenzene groups, from 20% up to almost 100% [Scheme 5, VIII (n= 3)], were found to be essentially a-helical in HFP when the samples were kept in the dark. The CD spectra also exhibited a couplet of bands centered at about 320 nm, attributed to electronic interactions between the azo side chains in the trans configuration. Irradiation at 360 nm and the consequent trans—xis photoisomerization, abolished the side chain CD... 

More recently, Ueno et al. have prepared and investigated a new series of copolymers containing p-phenylazobenzyl-L-aspartate and n-octadecyl-L-aspartate residues (Scheme 7, Structure XII). 55 561 In the case of copolymers containing less than 50 mol% azo residues, the CD spectra at 25 °C were consistent with the presence of right-handed helical conformations, which were not affected by irradiation at 320 nm. In contrast, in the case of copolymers containing 68 and 89 mol% azobenzene groups, irradiation caused the reversal of helix sense from the left-handed to the right-handed form. 

On the other hand, Tamada et al.44 have investigated stimulus-responsive gels utilizing the photochemical reaction of a polymeric azobenzene unit doped with IL (Fig. 23.5). Photoisomerization of the azobenzene group resulted in shrinkage of the irradiated site. It was also reported that ionic conductivity of the gel could be controlled by photoirradiation. The ionic conductivity of the gel decreased after UV light irradiation this effect was coupled with an increase in viscosity, in turn suppressing diffusion of the component ions within the gel. 

A very interesting behaviour is obtained for the azo-polysiloxane modified with adenine (Sample 2 - Table 1). In spite of the fact that the trans-cis isomerization process, as a result of the UV irradiation, is very fast in the solid state (similar to the behaviour in solution), the relaxation takes place in two steps, the recuperation rate in the first step being only 15 %. This behaviour can be explained by some associations that can take place between the cA-azobenzenic groups and adenine. The same curve profile, but less evident, is obtained for the azo-polysiloxane modified with cytosine... 

Sample 5 - Table 1) which has a chemical structure similar to that of adenine. A faster relaxation process is obtained in the case of the azo-polysiloxane modified with thymine, this behaviour being in agreement with the idea of the H-bonding formation that can create a 3D-physical network which forces the azobenzenic groups to relax faster. 

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