Magnesium and calcium chelation by a bis-spiropyran

A bis-benzospiropyranindoline was prepared by a simple two-step procedure. The magnesium and calcium chelating ability of this photochromic spiropyran was investigated and compared to simple mono-spiropyrans. Kinetic binding constants were measured. Moderately strong metal binding occurs in acetone solution (K=40 000 M−1 for Mg, K=13 000 M−1 for Ca) when the bis-spiropyran is irradiated with light at 365 nm. This binding is eight times higher than the binding of the mono-spiropyrans studied. The color of the merocyanine form of the bis-spiropyran (λmax=548 nm) is strongly influenced by the metal, blue-shifting the maximum absorbance 43 nm (Mg) and 22 nm (Ca). Strong fluorescence is observed when the bis-spiropyran complexed to either metal is irradiated at 365 nm, with emission maxima of 586 nm (Mg) and 606 nm (Ca). The strength of the binding is inversely correlated to the unimolecular decomposition rate constant of the spiropyran–metal complex. The fluorescence emission maxima become increasingly blue-shifted as the strength of the binding increases. The fluorescence is compared to the metal-free spiropyran, as well as to simple mono-spiropyrans coordinated to calcium. The mechanism of decoloration of the bis-spiropyran with and without metals present is discussed.