Photochromic Reaction by Red Light via Triplet Fusion Upconversion

Red
or near-infrared (NIR) light responsive molecules have received
much attention for biological and material applications because potentially
harmful UV light for materials and cells is not required for the photochemical
reactions. Although some molecular designs for photochromic molecules
to increase the photosensitivity to red or NIR light have been reported,
the strategies are limited to the extension of π-conjugation
length and the utilization of charge transfer transition or energy
and electron transfers. Triplet fusion is an attractive tool to cause
chemical reactions by converting low-energy excitation light to high-energy
upconversion light. However, the efficient use of the high energy
of upconversion light is difficult because almost all reported triplet
fusion systems rely on reabsorption of upconversion light. Here, we
demonstrated red-light-driven photochromism via the triplet fusion
of a phenoxyl–imidazolyl radical complex, Pery-RPIC, that has
a covalently bonded perylene as an annihilator unit. The femtosecond
time-resolved absorption and fluorescence spectroscopy revealed that
this photochromic reaction proceeds by the highly efficient singlet
energy transfer from the annihilator unit to the photochromic unit.
This strategy can be applied not only to the development of visible
and NIR light responsive photochromic system but also to various photochemical
reactions.