Thermal isomerization aminoazobenzenes

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 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... 

Although the activation energies of aminoazobenzene-type compounds (Eg between 75 and 88 kj mol ) are not very different from those of azobenzene-type molecules, thermal 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 Relationship and Hammett relation is established, which includes azobenzene- and aminoazobenzene type compounds. A linear In kis n, vs, it, 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 moT . Methanol is exceptional, with AV = -17 ml... 

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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