Fast Negative Photochromism of 1,1′-Binaphthyl-Bridged Phenoxyl–Imidazolyl Radical Complex

Negative
photochromism, in which a thermally stable colored form
isomerizes to the transient colorless form by light irradiation and
the back reaction occurs thermally, is advantageous in its applications
for photoswitching materials because visible light can cause the photochromic
color change of the materials. Moreover, the photochromic color change
can be induced even on the inside of the materials due to the absence
of the reabsorption of the visible excitation light by the photogenerated
colorless species. While several negative photochromic compounds have
been reported, the time scales of the back reaction are still slower
than minutes, and no available fast responsive negative photochromic
compounds have been reported. Here, we developed a negative photochromic
1,1′-binaphthyl-bridged phenoxyl–imidazolyl radical
complex (BN-PIC) which enables fast photoswitching by visible light.
The stable colored BN-PIC shows instantaneous decoloration by visible
light irradiation, and the photogenerated colorless form thermally
reverts to the initial colored form with a half-life of 1.9 s at room
temperature. BN-PIC can also cause the drastic change in the chiroptical
properties by the photochromic reaction, and the rate of the thermal
back reaction is affected by the chirality of the solvent. Since the
negative photochromic reaction can occur on the inside of the materials,
the fast negative photochromism is expected to have an impact in the
fields of photoresponsive materials of solid states and molecular
aggregates.