Syntheses, cation complexation, isomerization and photochemical cation-binding control of spirobenzopyrans carrying a monoazacrown moiety at the 8-position

Abstract

Spirobenzopyran derivatives carrying a monoazacrown moiety, such as 12-crown-4, 15-crown-5, and 18-crown-6 moieties, and their acyclic analogue, at the 8-position have been synthesized. Alkali metal ion complexation by the crowned spirobenzopyrans, followed by isomerization to the corresponding merocyanine form, and their photoisomerization, have been studied by cation extraction, absorption spectroscopy, and NMR spectroscopy. Binding of alkali metal ions (Li⁺, Na⁺ and K⁺) by the crown moieties leads to isomerization of the crowned spirobenzopyrans even under dark conditions. ⁷Li and ²³Na NMR spectroscopy suggest that in the merocyanine isomer complexing an alkali metal ion, especially Li⁺, the crown-complexed metal ion interacts intramolecularly with the phenolate anion, thus being bound more powerfully than that in the corresponding spiropyran isomer, owing to anion, thus being bound more powerfully than that in the corresponding spiropyran isomer, owing to an additional-binding-site effect. UV-light irradiation in tetrahydrofuran further promotes the isomerization to the merocyanine form, the thermal stability of which depends significantly not only on the ion selectivities of their crown moieties but also on the ease of the intramolecular interaction between the crown-complexed metal ion and the phenolate anion. Under visible-light irradiation, the cation-bound merocyanine form readily reverts to the spiropyran form, releasing the metal ions to some extent. Alternating irradiation with UV and visible light or turning-on and -off of visible light, therefore, causes the isomerization of crowned spirobenzopyrans even in the presence of alkali metal ions, in turn affording control of their cation-complexing abilities.