Pseudo stilbenes

The spectroscopic and photochemical, especially isomerization, features of the azoaromatics warrant separation into three classes according to the relative energy of the lowest lying (n, c ) and (7C,7C ) states the azobenzene type, the aminoazobenzene type, and the pseudo-stilbene type. This determines the structure of this chapter. Section 1.2 provides basic information on... 

The prototype molecule for donor/acceptor substitution is 4-dimethyl-amino-4 -nitroazobenzene. Here, the n- n band is shifted far to the red due to the charge transfer character of the transition. The band has few vibrational features, and its energy is influenced by the polarity of the solvent. The weak n Tt band cannot be seen under the intense it —> it band (Figure 1.13). Most commercial azo dyes are pseudo-stilbenes rather than azobenzene-type molecules. 

Pseudo-stilbenes may emit fluorescence that is, contrary to true stilbenes, generally weak at room temperature and often weak even at low temperatures. Protonated azobenzene-type molecules and many protonated azo dye molecules emit strong fluorescence in sulfuric acid at 77 K with quantum yields of about 0.1. Inclusion of azobenzene in the channels of AIPO4-5 crystals provides complexation of the n-electrons and space confinement. This leads to emission by protonated azobenzene at room temperature. For their cyclopalladated azobenzenes, Ghedini et al. " report quantum yields of ca. 1T0 and lifetimes of ca. 1 ns. In contrast, donor/acceptor pseudo-stilbenes, if emitting at low temperatures or when adsorbed to surfaces, are weak emitters. In textile chemistry, it has long been known that azo dyes adsorbed to fibers may show fluorescence. ... 

There is a great difference between the two types of pseudo-stilbenes— donor/acceptor-substituted and protonated azo compounds— with regard to the stability of the Z-forms. The former isomerize quickly, whereas the latter may be relatively stable. 

Contrary to the Z-forms of protonated azobenzenes, the Z-forms of donor/acceptor di-substituted azobenzenes isomerize very quickly at room temperature. To investigate this reaction, the Z-form is produced by flash excitation. The isomerization of these pseudo-stilbenes is strongly dependent on the polarity of the solvent. " For instance, 4-diethyIamino-4 -nitroazobenzene isomerizes with k g = 0.007 s in hexane, but with kz, g = 600 s in N-methylformamide. 4-Anilino-4 -nitroazobenzene in cyclohexane obeys the relation In kz, E = 22( 3) - 72( 9)T0V(8.314 T). Schanze et al. " have established two linear relationships between the free-activation enthalpy of isomerization and the Kosower Z values of aprotic and protic solvents. Sanchez and de Rossi report a strong pH sensitivity of... 

One concludes from these facts that pseudo-stilbenes are not suitable for persistent switching of the molecular form. Any information based on E-Z isomerization is quickly lost. If, however, fast interconversion of E- and Z-forms is the aim, as it is in the alignment of the higher-order polarizability tensor of donor/acceptor azobenzenes, then thermal isomerization supports the photoisomerization process. 

For pseudo-stilbene-type molecules, the question of the mechanism of thermal isomerization was taken up again in the early eighties by Whitten et al. and later by Kobayashi et al., " who, on the basis of their isomerization experiments with donor/acceptor-substituted azobenzenes in polar solvents, postulate rotation. Asano and coworkers infer from the isomeriza-... 

Isomerization needs some extra sweep volume. The volumes for the two mechanisms of azobenzene should be quite different—ca. 0.25 nm for rotation and ca. 0.12 nm for in version. This bears out in restricted spaces. In some zeolites azobenzene can isomerize whereas stilbene does not. Kuriyama and Oishi found that there are two separate AH versus AS lines for azobenzenes isomerizing by inversion (azobenzene type) and rotation (pseudo-stilbene type). ... 

The photoisomerization mechanism in aminoazobenzene- and pseudo-stilbene-type compounds has attracted far less attention than the mechanism for azobenzenes. In pseudo-stilbenes, the (n,7t ) state is buried under the intense it —> Jt band and cannot be populated selectively. No state-specific quantum yields are available because the yields are independent of the exciting wavelength.There is only a very narrow experimental basis for a discussion of these two mechanisms. However, this may change when pseudo-stilbenes are subjected to ultrashort-time experiments. 

As in the azobenzene type systems, the triplet pathway seems to be decoupled from the singlet route in pseudo-stilbenes. Little is known about the mechanism in the triplet state. The only information comes from calculations, and these show that the triplet surfaces are frequently similar in shape to the singlet surfaces. Thus, both mechanisms may be operative in the triplet state, too. 

The photoisomerization of all types of azobenzenes is a very fast reaction on either the singlet or triplet excited-state surfaces according to the preparation of the excited state, with nearly no intersystem crossing. Bottleneck states have lifetimes on the order of 10 ps. The molecules either isomerize or return to their respective ground states with high efficiency. So photoisomerization is the predominant reactive channel, and the azobenKnes are photochemically stable. Only aminoazobenzene-type molecules and pseudo-stilbenes have small quantum yields of photodegradation. 

Rau, H., and Waldner, I. (2002). A non-rotatory isomerization path in ethene derivatives Investigation of a stilbenophane and protonated azobenzenophanes ( pseudo-stilbenes ). Phys. Chem. Chem. Phys. in press. 

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... 

Figure 1.11), and a pseudo-stilbene type with a (7t,7t ) state as the lowest-excited state (see also Figure 1.13). The assignment of an azo molecule or an azobenzene-containing macromolecule or system to one of these classes can be made by a simple inspection of the absorption spectrum. The spectroscopic properties of these types of azo compounds will be covered in Sections 1,3 to 1.5 of this chapter.

All azoaromatics, be they of the azobenzene, arainoazobenzene, or donor/acceptor pseudo-stilbene type, experience considerable spectral changes on protonation or complexation. Ortho metallation has the same effect.Usually the ic— ic band is red-shifted, possibly due to the localized charge at the N-atom. By the same token, the (n,Jt ) state is shifted to higher energies. Minor band shifts and intensity changes indicate double protonation of azobenzene. 

Pseudo-stilbenes may emit fluorescence that is, contrary to true stilbenes, generally weak at room temperature and often weak even at low temperatures. Protonated azobenzene-type molecules and many protonated azo dye molecules emit strong fluorescence in sulfuric acid at 77 with quantum... 

Pseudo-stilbene type azobenzenes are not as stable under UV irradiation as the azobenzenes. Irick and Pacifici found that in de-aererated alcoholic solvents, photoreduction gives the hydrazo compound with a yield of ca. 10 at 254 nm irradiation.Albini et aF found yields of one order higher, but at 313 nm and longer wavelength irradiation, the decomposition is virtually absent. Some caveat is necessary when triplet-sensitizing additives are present, because irradiation under air does not lead to photoreduction. Benzophenone-sensitized reduction in benzene proceeds with a yield of... 

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