Azobenzenes push-pull derivatives

Functionalisation of PO PAM-dendrimers with the push/pull azobenzene derivative methyl orange (cf. Fig. 5.21) [27] offers further photoswitching opportunities. The change in colour of this pH indicator from red (pH 3.1) to yellow (pH 4.4) can be explained by protonahon of the azo function to form the me-somerically stabilised azonium ion (Fig. 5.22). 

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. 

As concerns photochromes in a solid matrix, a question that immediately arises is to what extent the nature of the matrix impedes the photochromic reaction. This problem has been studied in detaih but it is beyond the scope of this review. There is a general rule that states photochromic reactions are sluggish in polymer matrices compared to fluid solutions. This statement is true for some stilbene derivatives, but it is not true for azo derivatives, especially for push-pull azobenzene derivatives like DRl, for which the trans->cis quantum yield equals 0.11 in PMMA at 20°C compared to 0.24 in a liquid hydrocarbon mixture at -110°C. Photochromism of spiropyrans shows an important matrix effect as the quantum yield for the conversion between the spiropyran and the photomerocyanin is equal to 0.8 in ethyl acetate and decreases to 0.102 in PMMA at room temperature. The same decrease is observed for the back photochemical reaction efficiency 0.6 in ethyl acetate, compared with 0.02 in PMMA at room temperature. Conversely, the matrix effect is much less for furylfulgides the quantum yields are almost the same in solutions as in polymer matrices. Although most of photochromic molecules exhibit photochromism in polymers and sol-gels, few of them exhibit this property in the crystalline state, due to topochemical reasons. However, some anils and dithienylethenes are known to be photochromic in the crystalline state. 

The photoisomerization of the push-pull azobenzene derivative disperse red 1 (DRl) has also been shown recently to occur efficiently in poly-(methyl metacrylate) (PMMA) thin films at room temperature [23,24]. DRl is a pseudo-stilbene type azobenzene molecule, as classified by Rau. This means that the high-energy n-n transition is overlapping the low energy n-n transition, which leads to a large structureless band in the trans isomer and... 

Importantly, not all azo dyes are useful for optical oscillation. In fact, for high values to be achieved, the light-sensitive azo compound should revert very quickly to its thermodynamically stable trans form in the dark, most desirably within the micro- or nanosecond temporal domain. Two different families of azo dyes have been described so far that could be suitable for this task 4-donor-4 -acceptor azobenzenes, also known as push-pull azo dyes, and azophenols. Indeed, both azo derivatives are well known to exhibit fast thermal isomerization rates under ambient conditions. Among all push-pull azo dyes, 4-A/ ,A/ -dimethylamino-4 -nitroazobenzene, has been considered, until a few years ago, as the fastest thermally isomerizing azo dye, with a relaxation time for its thermal back reaction ranging from 21 to 122 ms in alcoholic solutions. On the other hand, and as it has been described previously in this chapter, the... 

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