Molecular Design and Applications of Photochromic Crown Compounds —How Can We Manipulate Metal Ions Photochemically?

The design and applications of photochromic crown compounds are reviewed here. Such compounds exhibit the possibility of distinct photochemical control of metal-ion complexation, based on the ionic interactions of their photoionized photochromic moieties. Crowned spirobenzopyran derivatives which the authors designed, when isomerized to their corresponding merocyanine form by UV irradiation, can afford an additional binding site for a metal ion complexed by their crown ether moiety in a sophisticated way, i.e., by the six-membered chelate formation with the phenolate oxygen atom and the crown-ether-ring nitrogen atom. The crowned spirobenzopyrans, therefore, have their metal-ion complexing properties modified remarkably by photoirradiation, like the crowned spironaphthoxazines that are more photolabile than the spirobenzopyran derivatives. Crowned spirobenzothiopyrans possess an affinity to softer metal ions such as silver ion, which promotes their photoisomerization to the open form. Crowned bis(spiroben...