Ion valence-gated photochromism of an aza-crowned diarylethene

A non-photochromic diarylethene 2o with an N-phenylaza-15-crown-5 was synthesized. When the nitrogen atom in the aza-crown ring was protonated, it became photochromic due to the prevention of a twisted intramolecular charge transfer (TICT). Although addition of a monovalent metal cation (Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, Cu⁺, Ag⁺) in acetonitrile could not stop the TICT so that it was not photochromic, the addition of a multivalent metal cation (Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, Fe²⁺, Ni²⁺, Al³⁺, Sb⁵⁺) changed 2o to be photochromic due to the strong attraction of the lone pair on the nitrogen atom. In the presence of excess Cu²⁺, 2o was oxidized to be EPR-detectable 2o^·+, which was thermally unstable as well as inert towards visible-light irradiation. However, 2o^·+ was further oxidized to be fairly stable 2o²⁺ by the irradiation of 365-nm light in the presence of Cu²⁺. ESI-MS measurements strongly suggested the generation of 2o^·+ by mixing 2o with Cu(ClO₄)₂ in acetonitrile, and the transformation of 2o^·+ to 2o²⁺ by successive 365-nm light irradiation. Fe³⁺ similarly worked as the oxidant, but the two-step oxidation of 2o to 2o²⁺ occurred more easily.