Effect of Strain on the Photoisomerization and Stability of a Congested Azobenzenophane: a Combined Experimental and Computational Study

The stability and trans-cis photoisomerization properties of a macrocycle constituted of two para-aminoazobenzene units connected by two methylene bridges have been investigated by a combination of experimental and computational techniques. Irradiation at 365 nm leads to a photostationary state in which only 50% of the azobenzene units have isomerized, in contrast with para-aminoazobenzene, whose photoconversion is larger than 80%. In the case of the macrocycle, a faster cis-trans thermal back-reaction is observed. To assist the interpretation of the experimental results, molecular mechanics and quantum chemical calculations have been carried out. Of the possible conformers, the most stable trans-trans geometric isomer has been identified along with the more plausible trans-cis and cis-cis isomers. Excitation energies and intensities have been computed to identify the species at the photostationary state. The calculations point to a sequential photoisomerization mechanism, and suggest that the thermal and photochemical reactivity of the examined macrocycle is due to strain and substituent effects both concurring to favor the thermal cis-trans back-reaction.