NIR-Responsive Double Closed-Ring Isomer of a Diarylethene Fused Dimer Synthesized by Stepwise Photochemical and Oxidative Cyclization Reaction

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
10–8. As a result, the fused dimer exhibits reversible
stepwise switching induced by photo- and redox stimuli in the UV–vis–NIR
region.