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.

The synthesis and photochromic behavior of the fast photochromic polymers carrying [2.2]paracyclophane-bridged imidazole dimer are demonstrated. A significant feature of this synthetic strategy is that we can modify the photochromic properties such as coloration/decoloration rate, coloring, and photosensitivity via the stepwise synthetic approach of the imidazole dimer system. Notably, the photochromic behavior of the polymers is not affected by the environment around the photochromes and copolymerization with other monomers in both solution and film, which cannot be realized in any other conventional photochromic systems. The comparable photochromic behavior of the homopolymers and copolymers in solution and film indicates that the photochromic unit is independent from the local environment, which allows effective molecular design of the photochromic monomer unit to accomplish desired photochromic properties of the polymer.