Effect of photoirradiation on chromatographic separation of alkali metal ions using crowned-spirobenzopyran-immobilized silica

Abstract

We successfully developed a novel photocontrollable chromatographic system for separation of alkali metal ions. Based on the finding that the metal-ion complexation of spirobenzopyran derivatives bearing a crown ether moiety, which we call crowned spirobenzopyran, can be drastically changed by photoirradiation, crowned spirobenzopyrans were immobilized covalently onto silica gels for stationary phases of ion chromatography. Under dark or UV-light irradiation conditions, a marked separation between Li⁺ and K⁺ was attained by silica gels modified chemically with crowned spirobenzopyran bearing an 18-crown-6 ring. On the other hand, no significant separation between the two metal ions was found under visible-light irradiation conditions. This result originates from the fact that the K⁺-complexing ability of crowned-spirobenzopyran-immobilized silica is switched by photoisomerization of the spirobenzopyran moiety from its merocyanine form to spiropyran form. Furthermore, computer simulations of separation of alkali metal ions by crowned spirobenzopyrans in the silica phase were conducted using molecular dynamics calculations.