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

Abstract

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 atriplet state of the zwitterionic merocyanine form, which converts into a mixture of trans- and cis-isomers. Eventually, bidentatemetal complexes (trans-Mⁿ⁺) were observed, with rate constants for 3 from 3 × 10⁷ (with Ni^II) to 1.2 × 10⁹ d 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.