MAGNESIUM AND CALCIUM CHELATION BY A BIS-SPIROPYRAN

A bis-benzospiropyranindoline was prepared by a simple two-step proced ure. The magnesium and calcium chelating ability of this photochromic spiropyran was investigated and compared to simple mono-spiropyrans. K inetic binding constants were measured. Moderately strong metal bindin g occurs in acetone solution (K=40000 M-1 for Mg, K=13000 M-1 for Ca) when the bis-spiropyran is irradiated with light at 365 nm. This bindi ng is eight times higher than the binding of the mono-spiropyrans stud ied. The color of the merocyanine form of the bis-spiropyran (lambda(m ax)=548 nm) is strongly influenced by the metal, blue-shifting the max imum absorbance 43 nm (Mg) and 22 nm (Ca). Strong fluorescence is obse rved when the bis-spiropyran complexed to either metal is irradiated a t 365 nm, with emission maxima of 586 nm (Mg) and 606 nm (Ca). The str ength of the binding is inversely correlated to the unimolecular decom position rate constant of the spiropyran-metal complex. The fluorescen ce 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 calc ium. The mechanism of decoloration of the bis-spiropyran with and with out metals present is discussed. (C) 1998 Elsevier Science S.A. All ri ghts reserved.