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.