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

Abstract

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.