Azobenzene chains solubility

These unexacting requirements make the simplest unsulphonated azo structures, often monoazo types, quite acceptable [80]. Typical of the least polar members of this class are Cl Solvent Yellow 2 (4-68), Cl Solvent Orange 1 (4.69) and Cl Solvent Red 17 (4.70). Simple azo structures carrying sulphonamide, sulphone or carboxylate ester groups are used where a somewhat more polar, less soluble dye is needed. Simple disazo compounds (4-amino-azobenzene— 2-naphthol, for example) are used as red solvent dyes. Probably the only structural feature worthy of note in this class is the occasional adoption of structures carrying long alkyl chains to enhance solubility, as in the case of the disazo dye Cl Solvent Yellow 107 (4.71). 

Poly(L-lysine) containing azobenzene units linked to the side chains by means of a sulfonamide function (Scheme 4, Structure VI), was obtained by treating poly(L-lysine) with p-phenylazobenzenesulfonyl chloride. The poly(a-amino acid) was modified quantitatively conversion to the azo-lysine units of VI was effectively 100%. The azo-modified polypeptide was soluble in HFP, in which it exhibited an intense photochromism attributed to the trans-cis photoisomerization of the azobenzene units. Like other sulfonated azobenzene compounds, 33 azosulfonyl-modified polymers of L-lysine were found to be very stable in their tis form, and no thermal decay was observed at room temperature over periods of times as long as several weeks. Interconversion between the two forms at room temperature could only be effected by irradiation at appropriate wavelengths. This behavior allowed the authors to purify the trans and cis forms of the model compound NE-azobenzenesulfonyl-L-lysine (VII) by chromatography, and to measure the absorption spectra of the two pure photoisomers. 

A polymer of I-a,y-diaminobutanoic acid almost quantitatively substituted with azobenzene units in the side chains [Scheme 5, VIII(n= 2)] was not completely soluble in HFP when the sample was kept in the dark. The initial, slightly turbid solution became clear on irradiation at 360 nm and the consequent photoconversion of the azo moieties from their trans to the cis configuration (for photosolubility effects see Section 13.2.3). The cis polymer was found to adopt an essentially random coil conformation. Exposure to 460 nm light and the consequent back-isomerization of the azo units to about 70/30 trans-cis isomeric composition gave rise to a reversible photoinduced change from random coil to a-helical structure (helix content, about 60%).1411... 

Fig. 16 Poly(L-glutamic acid) containing 85 mol % azobenzene units (III). Change in solubility in HFP/water= 85/15 as a function ofthe trans-cis isomeric composition of the azo side chains.

Figure 7.2. Photoswitch of the solubility of chains, (a) Schematic drawing of the phototriggered coiiapse and aggregation of azobenzene-containing polymers in poor soivent conditions or ciose to iow critical solubility temperature (LCST). (b) Typical variation of the radius of the chains as a function of solvent parameter, or temperature in the case of chains having a LCST in water. Bold line parent chain with no azobenzene dashed and dot-dashed lines azo-modified chains, respectively, exposed to UV and dark-adapted.

Most of the work on photoresponsive azobenzene-containing materials is based on polymer matrices. Numerous chemistries have been utilized to graft azobenzene ligands to various polymer chains. The azobenzene chromophores transfer light energy into conformational changes upon photoirradiation, which can be used to control chemical and physical properties of the materials, such as viscosity, conductivity, pH, solubility, wettability, permeability, transport properties, mechanical properties, and structural properties. 

Gold NPs with azobenzene terminated alkane thiol chains chemisorbed on the surface and complexed with a-CD gave a photoresponsive suspension in water, in which the azobenzene photosomerization was as efficient as that of the free molecule. Preferential complexation of the allgrl chain in both trans- and ds-azobenzene configurations favoured the water solubility of the alkyl-aromatic moiety and the chemisorption process and reduced the interaction between the azobenzene units on the particle surface with positive effects on the efficiency of the photoisomerization. 

Water-soluble pillar[6]arenes with 12 carboxylate anions (3.24) have been synthesized by etherification of alltyl-halide with per-hydrotylated pillar[6]-arene (Scheme 3.3). Pillar[6]arenes with 12 mono- (3.25) or tri(ethylene oxide) moieties (3.26) were also synthesized by etherification. " As with the pillar[5]arene with 10 tri(ethylene oxide) chains, 3.19, tri(ethylene oxide)-substituted pillar[6]arene 3.26 also showed LCST behavior. The irons form of an azobenzene-guest derivative formed a host-guest complex with 3.26, but the cis form of the azobenzene guest did not. Based on the photo-responsive host-guest system between 3.26 and the azobenzene guest, photo-switching of LCST behavior was demonstrated. 

Liquid crystal polymers containing azobenzene groups in the side chains have also been reported. Polymers 18 and 19 are signihcant examples in which light-triggered conformational changes of the azobenzene units resulted in nematic/smectic-isotropic phase transitions. These systems will be discussed in detail in Section 4.3. Isomerization of spiropyran moieties (20 in Fig. 4.4) introduced in the side chain have also been used to modulate polymer solubility in an aqueous environment and we will further discuss this below. 

Poly(azobenzene) and its derivatives have applications in optical devices [17]. A novel polyaniline containing azo groups was synthesized by the HRP-catalyzed oxidative coupling of 4,4 -diaminoazobenzene (Scheme 1). The polymerization was carried out at pH 6.0 in tris buffer with 70% yield. The polymer analyzed by GPC (80 000, PD = 4.8) was soluble in DMSO and DMF. Azo groups were detected in the main chains as well as in the side chains. Photoexcitation studies indicated that cis-trans isomerization of the chromophore may be the result of structmal constraints in the polymer [ 18). Photodynamic properties of the azo functionahzed polyaniline in their relaxed or constrained conformations were quite different. 

---