Coordination-driven macrocyclization for locking of photo- and thermal cis→trans isomerization of azobenzene.

Both trans and cis isomers of azobenzene-linked bis-terpyridine ligand L1 were incorporated in rigid macrocycles linked by Fe(II) (tpy)2 (tpy: terpyridine) units. The complex of the longer trans-L1 is dinuclear [(trans-L1)2 ⋅Fe(II) 2 ], whereas the complex of the shorter cis-L1 is mononuclear [cis-L1⋅Fe(II) ]. The complex cis-L1⋅Fe(II) was not only thermally stable but also photochemically inactive. These results indicate a perfectly locked state of cis-azobenzene. The stable macrocyclic structure of cis-L1⋅Fe(II) causes locking of the isomerization. To the best of our knowledge, this is first example of dual locking of photo- and thermal isomerization of cis-azobenzene.