Photochromic Phenoxyl-Imidazolyl Radical Complexes with Decoloration Rates from Tens of Nanoseconds to Seconds

We
report a novel photochromic molecular system, phenoxyl-imidazolyl
radical complex (PIC), in which both a phenoxyl radical site and an
imidazolyl radical site are reversibly and simultaneously generated
upon UV light irradiation. PIC consists of the three parts: an aromatic
linker, a diarylimidazole moiety, and a 4H-cyclohexadienone
ring. Upon UV light irradiation, the C–N bond between the 4H-cyclohexadienone ring and the imidazole ring in the colorless
closed-ring isomer of PIC undergoes a homolytic cleavage, leading
to the formation of the transient colored open-ring isomer. Based
on the substituents on the imidazoyl/4H-cyclohexadienone
rings and the nature of the aromatic linker, the half-life of the
colored open-ring isomer can be varied between tens of nanoseconds
and seconds. PIC derivatives containing a 1,2-phenylene linker exhibit
high fatigue resistance toward repeated photochromic reactions. Analysis
using laser flash photolysis reveals that the absorption spectra of
the open-ring isomers are not readily rationalized by a straightforward
superposition of the spectra of the two component radical fragments
and the photogenerated radicals are electronically coupled through
the aromatic linker. Furthermore, the open-ring isomer can be treated
as a hybrid of the pure open-shell biradical and closed-shell quinoid
resonance structures.