Azobenzene chromophores derivatives

To date, two types of chromophores have been successfully incorporated in molecular junctions diarylethenes (DAE) [100] and azobenzene (AZO) derivatives [101]. 

Organic compounds which show reversible color change by a photochemical reaction are potentially applicable to optical switching and/or memory materials. Azobenzenes and its derivatives are one of the most suitable candidates of photochemical switching molecular devices because of their well characterized photochromic behavior attributed to trans-cis photoisomerization reaction. Many works on photochromism of azobenzenes in monolayers LB films, and bilayer membranes, have been reported. Photochemical isomerization reaction of the azobenzene chromophore is well known to trigger phase transitions of liquid crystals [29-31]. Recently we have found the isothermal phase transition from the state VI to the state I of the cast film of CgAzoCioN+ Br induced by photoirradiation [32]. 

Amphiphilic cationic azobenzene derivatives (scheme Fig. 14) have been used as the guest species for intercalation into the layered silicates magadiite and montmorillonite (182-185). The azobenzene chromophore photoisomaized effectively in the interlayer space of silicates, despite the fact that the azobenzene... 

Side-chain homopolymers based on poly(meth)acrylic derivatives (Fig. 16.14a) have been also widely studied (Andruzzief al., 1999 Rodriguez et al, 2005, 2006). When a suitable flexible spacer mediates between the main chain and the chromophore, these side chain polymers tend to exhibit thermotropic hquid crystalline properties, which favor cooperative chromophores motion during photoorientation and lead to more stable anisotropic optical properties. The azobenzene chromophore strongly influences a material s photoresponse. As a representative example, Ikeda... 

Nakahara H and Fukuda K 1983 Orientation of chromophores in monolayers and multilayers of azobenzene derivatives with long alkyl chains J. Colloid Interface Sol. 93 530-9... 

Studies by Nishiyama and Fujihara [149] utilizing azobenzene derivative (27) as isomerizable chromophores have demonstrated the importance of reaction cavity free volume in L-B films. The L-B films of amphiphilic derivative 4-octyl-4 -(3-carboxytrimethyleneoxy)-azobenzene (27) upon irradiation was found to be stable, no geometric isomerization of the azobennzene moiety occurred. This compound forms L-B films with water soluble polyallylamine 28 at an air-water interface. Reversible cis-trans photoisomerization occurs in the film containing 28. The reversible photoisomerization reaction in polyion complexed films is thought to occur because of the increased area per molecule provided in the film. The cross sections of molecule 27 in the pure film and in film containing 28 were estimated to be 0.28 and 0.39 nm2. Such an increased area per molecule... 

Figure 23. (a) Schematic representation of an anionic surfactant azobenzene derivative monolayer film at the air-water interface. (i>) Schematic representation of the stable monolayer film formed from the polyion complex of anionic surfactant azobenzene derivatives with a cationic polymer. Note the difference in free volume around the reactant chromophores in the two monolayers. 

The theoretical models discussed above indicate that the sulfonyl group, although slightly weaker in electron acceptor strength, is indeed a viable alternative to the nitro group. In particular, sulfonyl derivatives of stilbene and azobenzene display large molecular hyperpolarizabilities and can be used as bifunctional chromophores for the construction of materials with nonlinear optical properties. 

The optical spectra of the stilbene and azobenzene derivatives were studied in different solvents to establish the solvatochromic shifts of the new sulfonyl-containing chromophores, and to compare them to those found for the nitro analogues. Figure 3 shows the spectra in toluene and in DMF of 4-dibutylamino-4 -nitrostilbene, and of 4-dibutylamino-4 -methylsulfonylstilbene, and Figure 4 shows the spectra in the same solvents, but of 4-dibutylamino-4 -nitroazobenzene, and 4-dibutylamino-4 -methylsulfonylazobenzene. Table II summarizes the results. 

Section 3.2 of this chapter recalls the pure photochemical point of view of photoisomerization of azobenzene derivatives. Section 3.3 discusses the theory of photo-orientation by photoisomerization and gives analytical expressions for the measurement of coupled photoisomerization and photo-orientation parameters. Sections 3.4 and 3.5 review observations of photo-orientation in azobenzene and push-pull azobenzene derivatives, respectively. Among other things, these sections address photo-orientation in both cis and trans isomers and discuss the effect of trans<->cis cycling, i.e., the photochemical quantum yields, on photo-orientation. Section 3.6 discusses the effect of the symmetry of photochemical transitions on photo-orientation in spiropyran and diarylethene-type chromophores. Finally, I make some concluding observations in Section 3.7. 

For all Azo-PURs, the quantum yields of the forth, i.e., trans—>cis, are small compared to those of the back, i.e., cis—>trans, isomerization—a feature that shows that the azo-chromophore is often in the trans form during trans<->cis cycling. For PUR-1, trans isomerizes to cis about 4 times for every 1000 photons absorbed, and once in the cis, it isomerizes back to the trans for about 2 absorbed photons. In addition, the rate of cis—>trans thermal isomerization is quite high 0.45 s Q 1 shows that upon isomerization, the azo-chromophore rotates in a manner that maximizes molecular nonpolar orientation during isomerization in other words, it maximizes the second-order Legendre polynomial, i.e., the second moment, of the distribution of the isomeric reorientation. Q 1 also shows that the chromophore retains full memory of its orientation before isomerization and does not shake indiscriminately before it relaxes otherwise, it would be Q 0. The fact that the azo-chromophore moves, i.e., rotates, and retains full orientational memory after isomerization dictates that it reorients only by a well-defined, discrete angle upon isomerization. Next, I discuss photo-orientation processes in chromophores that isomerize by cyclization, a process that differs from the isomeric shape change of azobenzene derivatives. 

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