Complexes of spiropyran-derived merocyanines with metal ions - Thermally activated and light-induced processes

The properties of complexes of five spiropyran-derived merocyanines with Mg-II, Pb-II, transition and rare-earth metal ions (Mn+, n = 2, 3) have been studied in acetone by steady-state and time-resolved techniques. Within nanoseconds, excitation of the spiropyran (Sp) form of 1',3',3'-trimethyl-6-nitro-8-methoxy-spiro[2H-1-benzopyran-2,2'-indoline] (3) and derivatives leads to a triplet state of the zwitterionic merocyanine form, which converts into a mixture of trans- and cis-isomers. Eventually, bidentate metal complexes (trans-Mn+) were observed, with rate constants for 3 from 3 x 10(7) (with Ni-II) to 1.2 x 10(9) dm(3) mol(-1) s(-1) (with Pb-II). This reaction also occurs with similar rate constants for 1 (where only unidentate complexes are formed) and 2 (derivatives without the methoxy group). For 5, where the methyl group on N(1') is replaced by a phenyl group, the yield of complex is small. The kinetics were studied for equilibria of the Sp form with both trans-merocyanine and trans-Mn+, being perturbed either thermally or photochemically. Upon excitation of complexes with 3 and 4 [where the methyl group on N(1') is replaced by C2H4OH], triplet formation, trans --> cis photoisomerization (into cis-Mn+) and photosubstitution (into the Sp form) were observed. The yields depend on the nature of the metal ion; they are negligible for Co-II, Ni-II or Cu-II, substantial for Cd-II or Zn-II and largest for Pb-II. The lifetime of the observed cis-Mn+ complex is limited by interaction with the metal ion and water.