The thermal isomerization of azobenzenes. III. Substituent, solvent, and pressure effects on the thermal isomerization of push-pull azobenzenes

The substituent, solvent, and pressure effects on the rate of the thermal cis-to-trans isomerization of 4-dimethylamino-4′-nitroazobenzene(NDAAB) derivatives have been studied. 2-Methyl- and 2′-chloro-NDAAB isomerize faster than NDAAB, and 2′-methyl- and 2-chloro-NDAAB isomerize slower than NDAAB. For 2,2′-dimethyl- and 2,2′-dichloro-NDAAB, the rates are between those of the monosubstituted ones. These findings are very similar to those for 4-(dimethylamino)azobenzene (DAAB) derivatives, and no fundamental difference in the kinetic-substituent effects was observed between NDAAB and DAAB. The volume of activation, which is of a negative value, varies remarkably from solvent to solvent and from substituent to substituent; its magnitude increases generally with an increase in the solvent polarity. Even for DAAB and 4,4′-bis(diethylamino)azobenzene (BDEAAB), the rate is appreciably accelerated by external pressures in polar solvents. It is concluded that the observed results are more consistent with the inversion mechanism than with the rotational mechanism.