Dual-gated photochromism for both directions of azobenzene isomerization via metal complexation

Abstract

Photochromic compounds are attractive functional molecules because light-induced structural changes can regulate molecular properties and functions. Although gated photochromism, in which photoisomerization is switched ON or OFF by external stimuli, has attracted increasing interest, simultaneous control of both forward and reverse photoisomerization processes remains challenging. Herein, we report dual gated photochromism of azobenzene enabled by metal complexation of a macrocyclic ligand containing two azobenzene and two terpyridine units. The macrocyclic ligand underwent typical azobenzene photoisomerization in the metal-free state. Complexation of the trans-form ligand with Fe(II) ions and an appropriately sized spacer ligand afforded a dinuclear iron complex, in which the spacer acted as a structural “strut” inside the macrocycle. The resulting rigidified structure significantly suppressed trans-to-cis photoisomerization. The quantum yield of trans-to-cis isomerization decreased from 8.7% for the free ligand to 1.9% for the Fe(II) complex, while cis-to-trans isomerization became more favorable. In contrast, complexes without a spacer showed little suppression, highlighting the critical role of structural fixation. Complexation of the cis-ligand with Fe(II) afforded a stable cis complex in 92% isolated yield. The cis complex exhibited complete resistance toward photoirradiation and markedly enhanced thermal stability. This work establishes a metal-complexation strategy for controlling both directions of azobenzene photoisomerization within a single molecular platform.