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

The complexation of spiropyran-derived merocyanines having a 6-nitro group in the pyran moiety (6-NO2-BIPSs) or no nitro group and certain transition or rare-earth metal ions (Mn+, 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-Mn+) forms. For this two-step system two relaxation times are experimentally accessible which refer to fast (τI) and slow (τII) relaxation kinetics. Thermal or photochemical perturbation of the equilibria leads to the same τII value for a given 6-NO2-BIPS and metal ion in defined concentrations. With increasing [Mn+], τII 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-NO2-BIPSs are weak fluorescence and mainly radiationless decay of the excited singlet state, whereas intersystem crossing plays a role mainly for PbII and intramolecular energy transfer mainly for CoII or NiII. Photochemical reactions are trans→cis photoisomerization via the triplet state (PbII) and photosubstitution of the trans isomer by a solvent as another ligand via the singlet state (with spiropyran ring closure), e.g. for MnII or ZnII.