Solvent and substituent effects on thermal cis–trans-isomerization of some 4-diethylaminoazobenzenes

Thermal cis–trans-isomerization of some 4′-substituted 4-diethylaminoazobenzenes has been studied. Substitution in the 4′-position leads invariably to an acceleration of the reaction regardless of the nature of the substituent. Logarithms of kinetic constants in various solvents were found to be satisfactorily correlated by the π* scale of solvent polarities both for 4′-nitro-4-diethylaminoazobenzene (I) and for 4′-methoxy-4-diethylaminoazobenzene (II), excluding amphiprotic solvents. Activation parameters (ΔH‡ and ΔS‡) were also measured in NN-dimethylformamide. The results support an inversion mechanism rather than a rotational one; in the case of (I), the highly dipolar nature of the transition state can be justified by its stabilization through coplanar resonance structures (impossible in pure rotation).