The photochromic and fluorescence properties of two diarylethene derivatives having benzo[b] thiophene S, S-dioxide groups were studied in the single-crystalline phase. The derivatives showed reversible photochromic reactions and turn-on mode photoswitching of fluorescence in solution as well as in the single-crystalline phase. Upon irradiation with UV light, the open-ring isomers in the crystals underwent cyclization reactions to produce fluorescent closed-ring isomers. The UV-irradiated crystals emitted green or yellow-green fluorescence and exhibited dichroism of their absorption and fluorescence spectra under linearly polarized light. Upon irradiation with visible light, the closed-ring isomers reverted to the open-ring isomers and the fluorescence of the crystals disappeared. The switching cycles could be repeated more than 50 times without detectable deterioration. Area-selective switching of the fluorescence in the single crystal was demonstrated by patterned light irradiation, suggesting potential applications of the photo-switchable fluorescent molecular crystals in optical memory and display devices.
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