Photo- and thermal cis–trans isomerization of [23](4,4′)azobenzenophane

Abstract

Four isomers, EEE, EEZ, EZZ and ZZZ, exist for [2₃](4,4′)azobenzenophane depending on the degree of the isomerization of three azobenzene units. These isomers are stable enough to be separated by reversed-phase liquid chromatography. The compound in methanol exists as the EEE form in the dark. Upon irradiation at 366 nm the EEE form isomerizes via the EEZ form and the EZZ form to the ZZZ form. At the photostationary state at 366 nm, 85% of the compound exists as the ZZZ form. From the photostationary state the compound thermally isomerizes gradually to the EEE form. The thermal reaction from the ZZZ form to the EEE form also proceeds in a stepwise fashion. The relative values of the quantum yields for the photoisomerization and the absolute value of the rate for the thermal isomerization have been determined for each step. Both the photo- and thermal processes in the Z→E direction show the different efficiencies (or rates) depending on the processes. These experimental results can be explained by supposing that the different degree of ring strain works for ground and (or) excited states of each isomer. In this compound, a certain isomer can be enriched by the combination of external stimuli, such as light or heat. This raises the possibility of synthesizing molecules with more than two functional states.