The double closed-ring isomer of a diarylethene fused dimer was synthesized by a combination of photochemical and oxidative cyclization reaction. The double closed-ring isomer of a diarylethene fused dimer has fixed π-conjugation in a rigid planar framework so that this compound has been long-desired. However, with respect to the photocyclization of a diarylethene dimer with strong electronic interaction, the second-step cyclization is known to be suppressed due to fast energy transfer into the closed-ring isomer generated in the first-step photocyclization. In this work, instead of photoisomerization, oxidative isomerization was applied for the second-step cyclization, where the oxidized state spontaneously underwent thermal cyclization by radical coupling at room temperature. The resulting double closed-ring isomer showed NIR absorption, owing to a widely extended π-conjugation throughout the entire molecule. The double closed-ring isomer underwent a ring-opening reaction forming the closed-open-ring isomer when exposed to NIR light despite having a very low quantum yield of approximately 10. As a result, the fused dimer exhibits reversible stepwise switching induced by photo- and redox stimuli in the UV-vis-NIR region.
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