Effect of metal cations on the photochromic properties of spironaphthoxazines conjugated with aza-15(18)-crown-5(6) ethers

Abstract

Spironaphthoxazines conjugated with aza-15(18)-crown-5(6)-ether moieties at the 6′-position of the naphthalene fragment (Crown-containing Spironaphthoxazines, CSN) were synthesised and studied for the first time. The addition of Li⁺ and alkaline earth (Mg²⁺, Ca²⁺, Sr²⁺ and Ba²⁺) metal cations to CSN solutions results in a hypsochromic shift of the UV absorption band of the spiro form and a bathochromic shift of the absorption band of the merocyanine form in the visible region. In addition, the equilibrium shifts to the merocyanine form, and the lifetime of the photoinduced merocyanine form increases. Analysis of the spectral and kinetic data allows a complexation scheme to be proposed and the stability constants of the resulting complexes to be calculated. According to the results obtained, the complexation with Li⁺ and alkaline earth metal cations in acetonitrile initially involves the crown ether moiety; the participation of the merocyanine oxygen atom in the complexation process occurs at a high metal cation concentration. The UV-induced isomerisation of CSN into the merocyanine form causes a decrease of the cation binding ability.