Optical Control of Divalent Metal Ion Binding to a Photochromic Catechol: Photoreversal of Tightly Bound Zn²⁺

Abstract

Photochromic molecules with functional groups that bind metal ions strongly make possible the development of photoreversible sensors and preconcentrators for detection of metal ions at trace concentrations. While several photochromic metal chelators have been reported, none have been shown to bind strongly to divalent metal ions. A novel photochromic benzopyran 2, which is a photochromic catechol, binds Zn2+ strongly and reversibly. The thermal equilibrium between open and closed 2 is slow. Thus, binding of Zn2+ to 2 is slow in the dark. Under illumination with 306−416-nm light, Zn2+ binds rapidly to open 2. The stable complex dissociates upon illumination with visible light, showing that both formation and dissociation of the complex are controllable with light. A binding constant of 1.1 × 109 is estimated from a Job plot, which also indicates formation of a 1:1 Zn2+/open 2 complex.