Enhancing the Versatility and Functionality of Fast Photochromic Bridged Imidazole Dimers by Flipping Imidazole Rings

The widely tunable optical properties and the visible sensitivity have been required for fast photochromic molecules whose coloration–decoloration cycle completes in μs to ms time scale not only for practical applications such as full-color holographic displays but also for fundamental researches in biochemistry. However, the so far developed [2.2]­paracyclophane-bridged imidazole dimers, which are one of the best candidates for fast photochromic molecules, have their weaknesses for these requirements. Herein, we overcome the issues with sustaining fast photochromism and high durability by flipping the two imidazole rings (the head-to-tail and tail-to-tail forms). The alteration in the relative configuration of the imidazole rings suppresses the broad absorption band resulting from the radical–radical interaction. The substitution to the 2-position of the imidazole ring of the tail-to-tail form gives the drastic changes in the steady-state and the transient absorption spectra. The pyrene-substituted tail-to-tail form demonstrates that the transient absorption spectrum is featured by the inherent spectrum of the imidazolyl radical. This molecular framework is easy to functionalize fast photochromic molecules such as sensitizations to the red light, chirality, and biological tagging, and therefore it is versatile for various fast photochromic applications.