Unusual Negative Photochromism via a Short-Lived Imidazolyl Radical of 1,1′-Binaphthyl-Bridged Imidazole Dimer

We have synthesized a new photochromic
compound that exhibits unusual
negative photo­chromism, in which the stable colored species
photo­chemically converts into the metastable colorless species
via a short-lived radical. This compound has a 1,1′-bi­naphthyl
moiety bridging the two diphenyl­imid­azole units. Its photo­chemical
properties were investigated by nanosecond laser flash photolysis.
The colored species isomerizes to the colorless species upon exposure
to visible light and thermally returns to the original colored species
within 20 min at room temperature. Moreover, the photodecoloration
reaction proceeds via a short-lived radical with a half-life of 9.4
μs in benzene at room temperature. Both the colored and colorless
species show the photo­induced homolytic bond cleavage reaction
of the C–N bond between the nitrogen atom of the imid­azole
ring and the carbon atom of the 1-position of the 1,1′-bi­naphthyl
moiety and that of the C–C bond between each of the carbon
atoms of the 2-position of the imid­azole ring, respectively,
followed by their formation by rapid radical coupling.