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Updated: May 28, 2026

Determination of the Photoisomerization Quantum Yield of a Hydrazone Photoswitch
Published on: February 7, 2022
Dual-gated photochromism for both directions of azobenzene isomerization via metal complexation
Koji Yamakawa1, Masaki Yamamura2, Tatsuya Nabeshima1,3
1Graduate School of Science and Technology, University of Tsukuba, 1-1-1, Tennodai, Tsukuba, Ibaraki 305-8571, Japan.
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.
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Stereoisomerism
Isomers are different chemical species that have the same chemical formula.
Transition metal complexes often exist as geometric isomers, in which the same atoms are connected through the same types of bonds but with differences in their orientation in space. Coordination complexes with two different ligands in the cis and trans positions from a ligand of interest form isomers. For example, the octahedral [Co(NH3)4Cl2]+ ion has two isomers (Figure 1) In the cis...
Colors and Magnetism
When atoms or molecules absorb light at the proper frequency, their electrons are excited to higher-energy orbitals. For many main group atoms and molecules, the absorbed photons are in the ultraviolet range of the electromagnetic spectrum, which cannot be detected by the human eye. For coordination compounds, the energy difference between the d orbitals often allows photons in the visible range to be absorbed and emitted, which is seen as colors by the human eye.
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