Thermal degradation of azobenzene dyes

Ten azobenzene dyes were tested for their thermal degradation products from ambient temperature to 800 °C using a thermogravimetric technique. Degradation products were analyzed at each 100 °C increment using gas chromatography-mass spectrometry. Products were identified based on their mass spectral data and their retention times. Thermal degradation from all dyes resulted in loss of the azo group as nitrogen gas and aromatic free radicals. Electron withdrawing groups enhance the dissociation of the phenyl nitrogen bond at the benzene moiety carrying the electron withdrawing group. On the other hand, electron donating groups inhibited the dissociation of the phenyl-N bond of the benzene moiety carrying the electron donating groups. The loss of nitrogen molecule from the dyes appeared to be taken place in a two-step rather than a one-step reaction in which the phenyl nitrogen bond of the phenyl group carrying the electron withdrawing group splits to form phenyl and azo phenyl radicals following by the loss of a nitrogen molecule and a new phenyl radical. This was also supported by calculating degradation reaction enthalpies and bond dissociation energies using density functional theory quantum mechanics.