4-Diethylamino-4 nitroazobenzene isomerization

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-diethylamino-4 -nitroazobenzene isomerizes with kz >E = 0.007 s" in hexane, but with k g = 600 s" in N-methylformamide. 4-Anilino-4 -nitroazobenzene in cyclohexane obeys the relation In kz, E = 22( 3) - 72( 9)-10 /(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... 

Schanze, K. S., Mattox, T. F., and Whitten, D. G. (1983). Solvent effects on llie thermal cis-trans isomerization and charge transfer absorption of 4-diethylamino-4 -nitroazobenzene /. Org. Ghem. 48, 2808-2813. 

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