A Strategy for Visible Light-Driven Bidirectional Switching of Spiropyrans and Fabrication of Photonic Logic Gate via the Modulation of Transmittance

The spiropyran-merocyanine (SP-MC)
system is one of the most widely
used photochromic systems in various light-responsive dynamic materials.
The 6-nitrospiropyran is the most frequently used spiropyran system
due to its efficient forward switching from the SP to MC form with
UV light and the reverse switching from MC to SP with visible light
or heat. Replacing UV with visible light for the SP to MC isomerization
remains a challenge. The substituent effect on the photochemical ring
opening, upon tuning the electronic properties at the 7-position,
has not been explored previously. These compounds were designed, thoroughly
characterized using single-crystal XRD structures, and compared with
control molecules that have substitutions at other positions. The
compounds with substitution at the 7-position with electron-donating
groups undergo the forward SP → MC transformation with 456
nm light, and the reverse MC → SP isomerization proceeds with
640 nm red light, offering bidirectional switching in the visible
range of wavelengths. Besides in solutions, these photoswitches can
be isomerized in polymer matrices, expanding the possibilities for
their broader applications, including an all-photonic logic gate in
thin films.