Fast Photochromism Involving Thermally-Activated Valence Isomerization of Phenoxyl-Imidazolyl Radical Complex Derivatives

Open-shell
biradicals have received considerable attention in material
science because of their high two-photon absorption cross sections
and broad and high absorptive features over the visible region. However,
the instability of the biradical caused by the open-shell nature was
one of the drawbacks; therefore, novel radical compounds which can
suppress unwanted reactions by tuning the open-shell features are
desired to expand the versatility of the radical compounds. Here,
we report a novel radical-dissociation-type photochromic compound
whose photochromic reaction involves a valence isomerization from
the open-shell biradical to closed-shell quinoidal forms by using
a phenoxyl-imidazolyl radical complex framework. The valence isomerization
from the biradical to quinoid forms effectively tunes the open-shell
feature in time and drastically changes the spectral features, which
were revealed by time-resolved Fourier transform infrared spectroscopy.
This novel fast photochromic property not only is important for fundamental
spin chemistry but also expands the versatility of the radical compounds
for novel advanced photofunctional materials.