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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.
Kar et al. (Fri,) studied this question.