Fig. 1. Photochromic molecules encompassed in this review and their photoisomerization processes: (a) diarylethene, (b) spiropyran, and (c) azobenzene.

Fig. 2. (a) Images about information print-erase cycles on the same A4 size paper by employing the aqueous solution of compound 1 as cyan ink. Reproduced with permission from Ref. 62, © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (b) General strategy for covalent printing of photochromic compounds 4 and 5 (DBE). Reproduced with permission from Ref. 63, © 2019 American Chemical Society. (c) Fluorescence photographs of PMA film containing compound 6 under the irradiation of 302 nm (erasing), 621 nm (writing), and 720 nm (reading). Reproduced with permission from Ref. 64, © 2019 American Chemical Society. (d) Photographs of the four different solutions and gel states of the mixed organogel. Reproduced with permission from Ref. 65, © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Fig. 3. (a) Multidimensional anticounterfeiting demonstration based on compound 24. Reproduced with permission from Ref. 79, © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (b) Multilevel security information printing demonstration using zinc salts 40, 41, and 43 as the inks. Reproduced with permission from Ref. 80, © 2020 Science. (c) Information encryption and decryption demonstration by employing photoresponsive luminescent lanthanide-containing hydrogels 44 to 46. Reproduced with permission from Ref. 81, © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (d) Various patterns with different structural colors obtained using different photomasks. Reproduced with permission from Ref. 82, © 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Fig. 4. (a) Schematic illustrations of design strategy for full-colored photoswitchable fluorescent polymeric nanoparticles. Reproduced with permission from Ref. 60, © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (b) Full-color patterns realized using RGB Pdots as inks. Reproduced with permission from Ref. 105, © 2017 American Chemical Society. (c) Photopatterning and anticounterfeiting applications of SP-based latex nanoparticles. Reproduced with permission from Ref. 70, © 2018 American Chemical Society. (d) Schematic mechanism of the reversible dynamic regulating of the reflected wavelength of the light-driven chiral switches in CLC mixture. Reproduced with permission from Ref. 106, © 2010 Royal Society of Chemistry. (e) Reflection color images of the CLC mixture containing chiral switch 76 upon UV and visible light irradiation for different times. Reproduced with permission from Ref. 106, © 2010 Royal Society of Chemistry.

Fig. 5. (a) Pattern formation and color change in the R-mode and F-mode. Reproduced with permission from Ref. 117, © 2019 Royal Society of Chemistry. (b) Photographs of dynamic fluorescent anticounterfeiting using LCNPs-based ink. Reproduced with permission from Ref. 118, © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (c) Real cell images containing 104-(S,S) in R-mode (a-e) and F-mode (a’-e’) by irradiating the panda photomask with 365-nm light for different times. Reproduced with permission from Ref. 119, © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (d) Information encryption and decryption demonstration using CLC microtubes with and without dye 107. Reproduced with permission from Ref. 120, © 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.