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(spirobenzopyran)s are highly selective for multivalent metal ions in the photochemical control of metal-ion complexation. Photochromic crown compounds such as crowned triphenylmethane dyes, when undergoing intermolecular photoionization, exhibit clear-cut photocontrol systems of metal-ion complexation, which are very attractive for applications, especially in materials chemistry. Polymerization of crowned spirobenzopyran and Malachite Green may make it easier to apply the photochromic crown compounds practically. Remarkable polymer effects also appear in the photochromic crown ether polymers; these include photoinduced switching between precipitation and solubilization in vinyl copolymers of spirobenzopyran and crown ether monomers. Several examples are described of the applications of photochromic crown compounds in materials and analytical chemistry, i.e., photo-responsive ionic conduction and photoenhanced ion sensing and separation.