Photo-control of the thermal radical recombination reaction: photochromism of an azobenzene-bridged imidazole dimer

Abstract

Abstract Among various kinds of photochromic compounds, bridged imidazole dimers have been known as fast photo-switch molecules. Bridged imidazole dimers have opened up various potential applications to photochromic lenses and real-time holographic displays. The optical properties of bridged imidazole dimers strongly depend on the bridging moiety to tether two imidazole rings. Therefore, the control of the bridging structure by introducing another photochromic moiety would increase the versatility of bridged imidazole dimers. In this study, we designed and synthesized a new type of the bridged imidazole dimer 1 which has the azobenzene moiety as the photo-responsive linker. The cis–trans isomerization of the azobenzene moiety enables to change the distance between the photogenerated radical pairs. The two structural isomers, cis–1 and trans–1, are observed and both compounds undergo the photochromism to produce the imidazolyl radicals. We found that the two imidazolyl radicals generated from cis–1 are close enough to form the intramolecular C–N bond, whereas the imidazolyl radicals of trans–1 undergo the intermolecular recombination reaction due to the long distance between the radicals. Our results demonstrate the control of intra-/intermolecular radical recombination reactions by the combination of the two photochromic compounds.