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 (Mg, Ca, Sr, Ba, Fe, Ni, Al, Sb) changed 2o to be photochromic due to the strong attraction of the lone pair on the nitrogen atom. In the presence of excess Cu, 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 2o by the irradiation of 365-nm light in the presence of Cu. ESI-MS measurements strongly suggested the generation of 2o by mixing 2o with Cu(ClO) in acetonitrile, and the transformation of 2o to 2o by successive 365-nm light irradiation. Fe similarly worked as the oxidant, but the two-step oxidation of 2o to 2o occurred more easily.