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 2+ , Ca 2+ , Sr 2+ , Ba 2+ , Fe 2+ , Ni 2+ , Al 3+ , Sb 5+ ) changed 2o to be photochromic due to the strong attraction of the lone pair on the nitrogen atom. In the presence of excess Cu 2+ , 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 2+ by the irradiation of 365-nm light in the presence of Cu 2+ . ESI-MS measurements strongly suggested the generation of 2o ·+ by mixing 2o with Cu(ClO 4 ) 2 in acetonitrile, and the transformation of 2o ·+ to 2o 2+ by successive 365-nm light irradiation. Fe 3+ similarly worked as the oxidant, but the two-step oxidation of 2o to 2o 2+ occurred more easily.