Kinetics of the Z-E isomerization of monosubstituted azobenzenes in polar organic and aqueous micellar solvents

The rates of Z-E isomerization of azobenzene and seven 4-substituted derivatives (CH3, CF3, OCH3, n-Butyl, Ot-Butyl, Br, F) have been investigated at 25° C in ethanol, in methanol, in methanol/water mixtures and in aqueous solutions of ionic and non-ionic micelles by recording in the dark the spectral changes observed after exposure to UV light of the samples. The competitive contributions of the rotation vs. inversion mechanisms by which the isomerization of azobenzenes can occur have emerged from V-shaped Hammett plots. The obtained rate constants can be used as polarity indicators of the different micro-heterogeneous media with two main advantages with respect to other probes. Firstly they are not affected by hydrogen bonding between the substrate and the environment, as in the case of push-pull azobenzenes. Secondly, the derived substrate-independent Hammett rho values allow to separate the effects of orientation of the probe from the polarity of the supramolecular aggregate.