Drastically Accelerated Radical Recombination Kinetics of a Hexaarylbiimidazole Derivative

More than 60 years have passed since the discovery of
hexaarylbiimidazole
(HABI), which exhibits a characteristic photochromism that produces
colored lophyl radicals through a radical dissociation reaction induced
by light irradiation and reverts to its original state through a radical
recombination reaction in the dark. Lophyl radicals are relatively
stable among organic radicals, have low reactivity with oxygen, and
have a very slow radical recombination reaction rate. HABI has been
used industrially as a photoinitiator to date. However, the guidelines
for molecular design to accelerate the thermal reverse reaction of
HABI are still unknown and remain a challenge. We found that suppressing
the rotation of the phenyl groups attached to the 4- and 5-positions
of the imidazole ring of HABI is effective in accelerating the radical
recombination reaction. The simple molecular design strategy to accelerate
the thermal reverse reaction of HABI is expected to improve the performance
of photoinitiators and photoresponsive materials that utilize HABI
as a photoresponsive unit.