The photochromic, thermochromic and metallochromic behaviour of a series of three spiro[indoline-8-(benzothiazol-2-yl)-benzopyrans] has been investigated. The thermodynamic and kinetic parameters of their thermal equilibrium between the ring-closed (spiro) and ring-opened (merocyanine) isomeric forms have been determined using UV-Vis absorption and (1)H NMR spectroscopies. By adding Co(ii) and Ni(ii) ions in acetonitrile solution, 1 : 1 and 1 : 2 metal : merocyanine complexes are formed simultaneously. Using appropriate numerical methods, the kinetic analysis of the complexation allowed us to determine accurately key thermodynamic and spectroscopic parameters of the metal complexes. Results showed that the complexation strength is very sensitive to the size of the indoline nitrogen substituent. Complexation can be reversed by shining white light on the coloured complexes which regenerates the inactive spiropyran form, and releases the metallic ion; hence, these systems display fully reversible negative photochromism. The Zn(ii) complexes exhibit intense fluorescence in the 600-800 nm wavelength range. All these behaviours make these spiropyrans bearing benzothiazole heterocycles promising building blocks for the future construction of photodynamic chemosensors for transition metal ions.
This review focuses on the results of the studies of metal ion complexation by spiropyrans published mostly in recent years. Introduction of ionophore substituents in the molecules of photochromic spiropyrans induces a reasonably strong bonding in the complexes of these compounds with metal ions. The degree of complexation depends on the state (open or closed) of the photochromic spiropyran fragment. Directed synthesis of structures able to form com plexes with metal ion requires knowledge of structure property relationships based, among other, on the studies of the compounds synthesized earlier. This requires accurate determina tion of thermodynamic and kinetic parameters of complex formation. Difficulties of such quantitative analysis due to the presence of several interrelated equilibria in the system are discussed.
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