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

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 Zn 2+ strongly and reversibly. The thermal equilibrium between open and closed 2 is slow. Thus, binding of Zn 2+ to 2 is slow in the dark. Under illumination with 306−416-nm light, Zn 2+ 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 × 10 9 is estimated from a Job plot, which also indicates formation of a 1:1 Zn 2+ /open 2 complex.