Photochemical and thermal cis/trans isomerization of cyclic and noncyclic azobenzene dimers:: Effect of a cyclic structure on isomerization

The effect of ring strain on the photochemical and thermal isomerization of azobenzene dimers was investigated by comparison of macrocyclic (1) and noncyclic (2) azobenzene dimers. The macrocycle 1 comprises two azobenzene moieties connected at their meta positions through two methylene linkers, whereas the noncyclic dimer 2 has only one methylene linker bridging two azobenzenes at the meta positions. Upon irradiation of 1 or 2 with UV light, each (E/E) (transl/ trans) isomer exhibited a stepwise photoisomerization to give the corresponding (E/Z) (trans/cis) isomer, followed by isomerization to yield the (Z/Z) (cis/cis) isomer. Quantum yields for the isomerization indicated that the photochemical isomerizations of 1 were altered by its cyclic structure. The effect on the photochemical isomerization process can be explained in terms of the ring strain, causing shifts in the absorption spectra of the azobenzene chromophore and perturbations in molecular motion in the excited state. Thermal (Z) -> (E) isomerization was observed in each I and 2, taking place in stepwise manner as (Z/Z) -> (E/Z) -> (E/E). The cyclic structure gave higher activation barriers (E-a and Delta H-double dagger) and activation entropies (Delta S-double dagger). HF/6-31G** calculations were carried out for 1 and 2 in their ground electronic states in order to estimate the ring strain of each isomer of 1. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006).