Complexes of spiropyran-derived merocyanines with metal ions: relaxation kinetics, photochemistry and solvent effects

Abstract

The complexation of spiropyran-derived merocyanines having a 6-nitro group in the pyran moiety (6-NO 2-BIPSs) or no nitro group and certain transition or rare-earth metal ions (M n+ , n=2, 3) were studied by steady-state and time-resolved techniques in acetonitrile, butyronitrile, acetone and dioxane at room temperature. The overall kinetics of complex formation are governed by equilibria between the spiropyran (Sp), trans-merocyanine (trans) and complex (trans-M n+ ) forms. For this two-step system two relaxation times are experimentally accessible which refer to fast ( τ I) and slow ( τ II) relaxation kinetics. Thermal or photochemical perturbation of the equilibria leads to the same τ II value for a given 6-NO 2-BIPS and metal ion in defined concentrations. With increasing [M n+ ], τ II strongly increases and finally approaches the time needed for the thermal Sp→trans conversion. Water in small concentration (<0.3 M), which is necessary for dissolving the salt, does not affect the properties of complexes significantly, but above 1 M the complex concentration decreases steeply. The photophysical processes of complexes with 6-NO 2-BIPSs are weak fluorescence and mainly radiationless decay of the excited singlet state, whereas intersystem crossing plays a role mainly for Pb II and intramolecular energy transfer mainly for Co II or Ni II. Photochemical reactions are trans→cis photoisomerization via the triplet state (Pb II) and photosubstitution of the trans isomer by a solvent as another ligand via the singlet state (with spiropyran ring closure), e.g. for Mn II or Zn II.