Azobenzenes amphiphiles

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. 

In this paper, UV-visible absorption spectra and X-ray diffraction experiments of single crystals and solvent cast films of the azobenzene amphiphiles, CnAzoCmN+Br, were systematically investigated. Structural characterization of the cast bilayer films are discussed in comparison with aqueous solutions and single crystals. Some novel functional properties of the cast films are described, too. We also emphasize that the two-dimensional molecular assemblies, cast films and crystals of bilayer-forming amphiphiles, are suitable candidates for "crystal engineering" because of their simple structures compared with usual three-dimensional molecular crystals. 

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. 

X-ray diffraction studies strongly indicate that the films of the azobenzene amphiphiles simply cast on solid substrates are composed from highly oriented multiple stacked bilayer... 

Figure 9. Schematic models of bilayer structures of azobenzene amphiphiles in cast films.

In order to verify Okuyama s prediction on molecular orientation in bilayer assemblies, azobenzene amphiphiles having a viologen moiety as a hydrophilic head group, CnAzoCmV2+ 2Br, were newly prepared. Bathochromic shift to 390 nm in the visible absorption band of the... 

A typical photochemical isomerization of the azobenzene amphiphile was found in an ethanol solution. A trans isomer converted to a cis isomer with ultraviolet irradiation. Back reaction from cis to trans was accelerated when a weak n-n absorption band of the cis isomer at ca.450nm was excited (Figure 21a). An alternative irradiation of uv and visible light to the ethanol solution gave reversible changes of the ji-ji transition between 355nm and 325nm attributed to the trans and cis isomers, respectively. 

Figure 21. Spectral change of azobenzene amphiphile CgAzoCioN+ Br- on photoirradiation. (a) ethanol solution, (b) cast film at 50°C in dry condition.

Photoisomerization of the azobenzene amphiphile was found to be strongly affected by molecular packing and orientation in the aqueous bilayer solutions. A rate constant of trans to cis isomerization was extremely faster in the liquid crystalline state than in the crystalline bilayer membrane [33]. Photoreaction of the aqueous bilayer membrane of CgAzoCioN+ Br was... 

The mixed liposomal solutions were prepared by the ethanol-injection method(13) in order to obtain completely transparent solutions. It is interesting to note that miscibility of the photochromic amphiphiles with DPPC depend on the position of bulky azobenzene. If azobenzene is incorporated close to the end of long alkyl chain, a stable mixed bilayer state cannot be formed. On the other hand, when the azobenzene moiety is located near the head group or at the center of the hydrocarbon tail, the azobenzene amphiphiles are successfully incorporated into the bilayer membrane. No individual micelle formation nor phase separation in the bilayer was observed at 25 °C by absorption spectroscopy. However, the microstructure of the mixed liposomes depends on the type of azobenzene amphiphiles. 

FIG. 6.7 Spectral characterisacs and modes of aggregation of azobenzene amphiphiles (adapted from reference 27 with permission from Elsevier Science). 

The time necessary to reach the photostationary c/s-state for the first time in LBK films of this azobenzene amphiphile transferred in the trans-state is considerably longer than for the subsequent cycles. This is explained by the time it takes to break up some aggregates in the dense packing of the LBK film. The extent of isomerization, determined by Brode s method,is given to be 30%. Reestimation employing electrochemical methods showed that the irradiation yielded a c/s-isomer count of only 19% ... 

The extent of the isomerization hindrance depends on the structure of the azobenzene amphiphile. When the azobenzene is located in the middle of the alkyl chain (i.e., 7), the trans to cis photoisomerization in LBK films is severely hindered but not completely suppressed.However, when the azobenzene moiety is located directly at the head group, as in 12, the isomerization in the monolayer at the air/water interface is completely blocked. ... 

Furthermore, the means of binding the alkyl tail to the azobenzene influences the rigidity of the monolayers and the trans to cis photoisomerization. If the alkyl chain is bound to the azobenzene unit via an ether linkage (as in 13), the alkyl chain can adopt a better-ordered arrangement in which the chromophores are aggregated to an higher extent. So, in LBK films of 13, the hindrance of the isomerization is more pronounced than in LBK films of azobenzene amphiphiles having the alkyl chain directly bound to the chromophore (as in 7). ... 

A mixture that is stable in the cis-form was achieved by mixing an ionic azobenzene amphiphile having a long spacer between the head group and the... 

FIG. 6.10 Schematic representation of the molecular packing arrangement in the nrth monolayer of an ionic azobenzene amphiphile and an oppositely charged spacer amphiphile (reproduced with permission from reference 50 Copyright (1995) American Chemical Society). 

Azobenzene amphiphiles can be isolated from each other and equipped with sufficient free volume for reversible photoisomerization by inclusion into amphiphilic P-cyclodextrin. The inclusion complex can be transferred to give LBK films in which the azobenzene moieties photoisomerize as they would in solution (see Figure 6.11). ... 

The free volume around the azobenzene that is necessary for an unrestricted photolsomerization can be established by the introduction of bulky head groups or additional alkyl chains in the azobenzene amphiphile. An example of this approach has been published by Markava et al. The authors compared azobenzene amphiphiles with one (14) and two (15) alkyl chains as well as amphiphiles with small (16) and large (17) head groups. They found that photoisomerization takes place in LBK films of 15 and 17, where the areal requirements of the amphiphile are high, but not in the LBK films of 14 or 16, which can be packed more densely. 

Coupling between Ionic Azobenzene Amphiphiles and Polyelectrolytes... 

The first example of an azobenzene amphiphile polyelectrolyte complex was reported by Shimomura and Kunitake. They used poly(vinylsulfate) 23 to stabilize an ammonium amphiphile (21, n = 5). Because the tertiary ammonium head group is rather large and the distance of the ionic sites in poly(vinylsulfate) is rather small, the packing of the amphiphiles is not sipti-ficantly loosened by the complexation. In this case, the influence of the poylelectrolyte on the spectral properties and the photoisomerization is small. 

A small but significant influence on the packit in monolayers at the air/water interface was found when dextran sulfate 24, which has a much larger distance between the ionic sites, is used with azobenzene amphiphile 21 (n = 10). A large effect was found for complexes of an azobenzeme fatty acid 22 (n = 3) complexed with polylallylamine) 26 however. In this case, the area per amphiphile increased from 0.28 nm to 0.39 nm. This increase... 

A detailed investigation into the influence of polyelectrolytes was conducted by Nishiyama et al. The authors spread an azobenzene amphiphile 22 (n = 5) on subphases containing the polyelectrolyte 28 at different pH values. The degree of ionization does not change for this polyelectrolyte, but it does for the amphiphile. They found that the area per amphiphile measured at 30 mN/m depends very much on the pH (see Figure 6.13). It was rather low at pH = 6.0, where not all of the carboxyl group are deprotonated, but it reached a plateau at approximately pH = 8. 

The increased area per amphiphile in LBK films in which azobenzene amphiphiles have been complexed with a polymeric counterion results in... 

FIG. 6.14 Schematic representation of the pH-dependent change in the monolayer structure of azobenzene amphiphile 11 spread on (A) a pure water subphase, (B) a subphase containing 18 at pH = 6.0, and (C) at pH = 8.0 (adapted from reference 61 with permission from Elsevier Science). 

FIG. 6.15 Fraction of the as-isomer at the photostationary state in LBK films as a function of the area per molecule at 25 mN/m for azobenzene amphiphile 30 (reproduced from reference 62 with permission from Elsevier Science). 

The corresponding fatty acid could not be photoisomerized in the LBK film. By attaching the azobenzene chromophore to the hydrophilic backbone, however, the free volume in LBK films was increased and photoisomerization was possible (i.e., 50 to 70% cts-isomer compared to 0% for the nontethered azobenzene amphiphile and 90% cis-isomer in solution). However, concomitant with the increased free volume, there is a decrease in the orientational order of the chromophores. These polymers have been widely used as command surfaces to control the orientation of liquid crystals and to investigate the photomechanical effect. ... 

The structural changes within LBK films upon irradiation can cause morphological changes, too. Irradiation of LBK films of azobenzene amphiphile 43 results in an increase of the surface roughness, as shown by AFM measurements. The roughness is most likely caused by a recrystalliza-tion of the azobenzene amphiphiles in the irradiated area. 

Employing UV-vis spectroscopy in combination with electrochemical methods, Wang et al. have shown that for an LBK film of azobenzene amphiphile, indeed only those chromophores that are parallel to the electric field vector of the incident light are isomerized by polarized irradiation. Fmthermore, the authors have shown that the steric requirements of the ds-isomer favor a back reaction of the cis-isomer into a new orientation of the trans-isomer ... 

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