Aminoazobenzenes isomerization

Table 2.4 shows a comparison of the experimental and PPP-MO calculated electronic spectral data for azobenzene and the three isomeric monoamino derivatives. It is noteworthy that the ortho isomer is observed to be most bathochromic, while the para isomer is least bathoch-romic. From a consideration of the principles of the application of the valence-bond approach to colour described in the previous section, it might have been expected that the ortho and para isomers would be most bathochromic with the meta isomer least bathochromic. In contrast, the data contained in Table 2.4 demonstrate that the PPP-MO method is capable of correctly accounting for the relative bathochromicities of the amino isomers. It is clear, at least in this case, that the valence-bond method is inferior to the molecular orbital approach. An explanation for the failure of the valence-bond method to predict the order of bathochromicities of the o-, m- and p-aminoazobenzenes emerges from a consideration of the changes in 7r-electron charge densities on excitation calculated by the PPP-MO method, as illustrated in Figure 2.14. 

Primary and secondary amines react with diazonium salts to form initially the N-azo derivatives, e.g., diazoaminobenzene, C,HjNHN= NC,Hs, from aniline and benzenediazonium chloride. In the presence of acids, these compounds isomerize to the corresponding p-aniinoazo derivatives for example, the above compound goes to p-aminoazobenzene, p-NH2CjH4N = NC6Hs. If the para position is not free, isomerization to the ortho position occurs. The isomerization may proceed by a fission of the diazoamino compound to the progenitors, which then undergo... 

Photochemically induced Z,E-isomerization has been observed in 4-diethyl-aminoazobenzene and 4-diethylamino-4 -methoxyazobenzene and in the mesoionic azo-compounds (6). The photoisomerization of aromatic azo-compounds on hydrated Si02 and AI2O3 has made available in the adsorbed state Z-isomers which were previously inaccessible. Z-2,3,4,5,6-Pentafluoro-phenylazobenzene, obtained by irradiation of the corresponding E-isomer, is unusually stable in the dark. ... 

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... 

Although the activation energies of aminoazobenzene-type compounds (Ea between 75 and 88 kj moH) are not very different from those of azobenzene-type molecules, therm Z -> E isomerization of aminoazobenzene-type molecules is in general much faster than that of the azobenzene-type compounds. Conventional flash experiments are necessary to monitor the changes. The half-life of the Z-form of dimethyl-aminoazobenzene in toluene at 298 K is 220 s. A Linear Free Energy Rehitionship and Hammett relation is established, which includes azobenzene- and aminoazobenzene type compounds.A linear In vs, n, the Taft parameter of solvent polarity, is also observed. The dependence of the isomerization rate on pressure is weak In most solvents, it increases less than 35% at 2100 bar, AV -1.65 ml moLL Methanol is exceptional, with AV = -17 ml... 

The photoisomerization of aminoazobenzene-type compounds is complicated by the presence of the rapid thermal Z -> E isomerization. Pure Z-isomers generally cannot be isolated, and their spectra thus cannot be... 

The method of variation of the irradiation intensity gives as well as Po " 4-dimethyl-aminoazobenzene in n-hexane irradiation at 366 nm or 405 nm, both of which are wavelengths of the 7i —> 7U band, the results are 0F.- z = 0.17 and (j)2 i = 0.25. Albini et al.,"" who calculated the extent of Z —> E isomerization by an extrapolation of this conversion back to zero time of irradiation, found somewhat higher values for 4-diethylamino-... 

Pressure dependence was thoroughly investigated by Asano and his group. It turns out that the partial volumes of the Z-forms of 4-dimethylamino-4 nitorazobenzene and related molecules are ca. 250 cm moP in all solvents. Those of the E-forms are smaller and solvent-dependent. Thermal isomerization rates are weakly dependent on pressure in nonpolar solvents, but contrary to azobenzene- and aminoazobenzene-type compounds, they are strongly dependent in polar solvents in hexane 10%, in acetone 475% for 2100 bar (AV = -0.7 and -25.3 em mol, respectively). This has implications for the discussion of the mechanism of isomerization (Section 1.6). 

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. 

One example, which exceptionally exhibited a real photochemical contraction effect in the film state, is poly(ethyl acrylate) cross-linked with 4,4 -dimethacryloyl-aminoazobenzene studied by Eisenbach [35]. Figure 14 shows the data. The film contracts upon irradiation with ultraviolet light, which causes the tram to cis isomerization of the azobenzene chromophores, while it expands by irradiation with visible light, which induces the cis to tram isomerization. Both contraction and expansion are induced by photoirradiation. This finding indicates that the structural change of the cross-linking azobenzene chromophores in the polymer network is responsible for the contraction/expansion behavior. However, the observed contraction was very small, being only about 0.15-0.25%. 

We recognize many of these azo species as classical dyes and so expect thermochemical data, if available, to be rather old and for the solids. Our expectations are fulfilled from the nearly century old Reference 134, we find the data for solid 4-aminoazobenzene, 2,4-diaminoazobenzene and 4-dimethylaminoazobenzene. From this early source we find the enthalpy of formation of azobenzene itself of 374 kJmol-1 suggesting that reaction 26 is nearly thermoneutral, an altogether reasonable result if it is assumed that the isomeric 2- and 4-amino azobenzenes have very nearly the same enthalpy of formation. Likewise, reaction 27 is roughly thermoneutral. 

According to the spectral features and isomerization behavior, the aromatic azo compounds have been classified as azobenzene type, aminoazobenzene type, and pseudostilbene type (Rau, 1990). For the azobenzene-type molecules, the 71-71 transition band (320 nm) appears at shorter wavelength than n-n transition band (430 nm) (Kumar and Neckers, 1989). The cis-to-trans isomerization is relatively slow at room temperature, and the existence of cis isomers can be easily identified by the spectroscopic method. For aminoazobenzene-type and pseudos-tilbene-type molecules, the ti-ti and n-n transition bands are overlapped. The cis state of the molecules is unstable, which relaxes back to the trans state quickly. According to the definition, the azo polymers given in Figs. 5.1 and 5.2 can be assigned to contain pseudostilbene-type and azobenzene-type chromophores, respectively. 

One compound is formed from the other by the migration of a hydrogen atom, one of the first examples of tautomerism. The conversion of diazoaminobenzene into the isomeric aminoazobenzene (see p. 792) was discovered by Kekule. By diazotising o-diaminobenzene derivatives, e.g. o-phenylene-diamine, Griess obtained azimino-compounds one NHg group is diazotised... 

The diazo-compounds form the starting point of numerous organic compounds, including the azo-dyes, the first representative of which (amino-azobenzene hydrochloride, aniline yellow ) was prepared and shown to be a dye by Mene. What was called aniline yellow is the oxalate of aminoazo-benzene (CeH5 N N-CeH4 NH2), introduced as a dye by Simpson, Maule, and Nicholson in 1863 its constitution was established by C. A. Martius and Griess, and the isomeric change of diazoaminobenzene into aminoazobenzene was explained by Kekule. ... 

Figure 2. Thermal cis-trans isomerization of methyl methacrylate copolymer with 0.9 mole % of p-(N-methacrylyl)aminoazobenzene at 60 C after photochemical trans-cis isomerization at the same temperature (zr. )bulk polymer, (O)polymer plasticized with 30% dioctyl phthalate,( )dilute solution in butyl acetate.

Because thermal Z—>E isomerization can occur, determining quantum yields of aminoazobenzenes is more difficult. They can be determined if the photostationary state shows a different absorbance compared to the starting pure E-isomer and if can be measured. 4-Diethylaminoazobenzene and 4-(diethylamino)-4 -methoxyazobenzene gave 4>g > 0.7 on excitation at 436 nm but lower values of <0.4 were obtained using 366 nm and shorter wavelengths when these were determined at room temperature. ... 

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