Highly calcium-ion-accelerated coloration of bis(spirobenzopyran) bridged by diaza-18-crown-6 moiety at the 8-position

Complexation of metal ions, especially of polyvalent metal ions, by a spirobenzopyran dimer bridged by a diaza-18-crown-6 moiety at the 8-position induced remarkable Isomerization of its spiropyran moiety to the corresponding merocyanine even under dark conditions. Crowned bis(spirobenzopyran) can bind Ca2+ to form stable metal ion complexes of its merocyanine isomer, which in turn brings about the high rate constant of its thermal coloration after visible-light irradiation.