Regulating the photoisomerization pathway to achieve efficient E → Z isomerization of azobenzene-embedded molecular cages

Achieving efficient E -> Z photoconversion of Azobenzene is now still a challenge. Based on our previously reported cage C1, which exhibits an E -> Z photoisomerization efficiency of up to 94%, two additional cages C2 and C3 were synthesized with more rigid and longer units replacing the flexible diphenyl ether motifs in C1. It was found that, when the diphenyl ether rims were changed to biphenyl groups (C2), or to diphenylacetylene motifs (C3), the dihedral angles of the two benzene rings of the azobenzene rim in these cages decreased correspondingly from 48.86 degrees to 42.96 degrees, and finally to 30.13 degrees. The E -> Z photoisomerization quantum yields measurements indicate that, with the decreasing of the dihedral angles, the role of the inversion pathway in the E -> Z processes of these cages gradually decreases, while the role of the rotation pathway becomes increasingly greater, accompanied by the decreasing in the E -> Z photoconversions from 94% to 87%, and to 72%. These results validate our new strategy of promoting inversion pathway/inhibiting rotation pathway to achieve efficient E -> Z photoconversion of azobenzene.