A volumetric study on the thermal cis-to-trans isomerization of 4-(dimethylamino)-4′-nitroazobenzene and 4,4′-bis(dialkylamino) azobenzenes: Evidence of an inversion mechanism

The reaction volume (ΔV) for the thermal cis-to-trans isomerization of 4-(dimethylamino)-4′-nitroazobenzene in various solvents has been obtained by means of the photostationary state and capillary method. It has been shown that the activation volume (ΔV^‡) is closely related to the reaction volume in each solvent. Substantial kinetic pressure and solvent effects have been observed for 4,4′-bis(dimethylamino)azobenzene, 4,4′-bis(diethylamino)azobenzene, and 4,4′-(dipyrrolidin-1-yl) azobenzene. The π-π* conjugation band of the trans-isomers of these compounds shows a remarkable solvatochromic shift, and logarithms of kinetic constants in various solvents are satisfactorily correlated with the Taft π* scale, which represents solvent polarity-polarizability interactions. From McRae's formula, it has been deduced that the interaction between the solvent dipole and the solvent-induced dipole has an important influence on the behaviour of these 'push-pull' and 'push-push' azobenzenes. The results afford unequivocal evidence that the isomerization proceeds via a coplanar inversion transition state which is electronically similar to the trans-isomer.