Thermal cis → trans isomerization in the vapour phase of 4-dimethylamino-4′-nitroazobenzene: A flash spectroscopy study

Abstract

The kinetics of the thermal cis → trans isomerizaton of 4-dimethylamino-4′-nitroazobenzene in the vapour phase was studied by conventional microsecond flash photolysis as a function of temperature, concentration and buffer gas pressure. The Arrhenius parameters determined for this first-order reaction in the interval 185 – 275 °C are 102 ± 3 kJ mol −1 and (3 ± 2) × 10 13 s −1. Comparison with the azobenzene system suggests that the cis → trans isomerization in the gas phase proceeds by inversion. According to evidence in the literature the solution isomerization of these highly bipolar azobenzenes occurs by a rotational mechanism. The results obtained in this investigation indicate, however, that the inversion mechanism is also involved in the solution isomerization.