Notably, UV irradiation for 5 min with a flashlight (≤ 5 W) is sufficient for photochromic fluorescence with highcontrast (Figure ). We then attempted to reverse the reaction by photocleaving the dimer with 254-nm UV light or with heat. However, no obvious change of the PL spectra of both polymers was observed. This might be due to irreversible side reactions of anthracene cycloaddition in aggregate state (Figure ) . Information encryption studies were subsequently conducted using the photochromic NDI-coAntMA and lightinsensitive NDI-coPyMA, which were formulated as security inks. As show in Figure , a group of English alphabet letters were painted with polymer inks on a glass substrate, exhibiting indistinguishable red fluorescent colors. After 365-nm UV irradiation on the substrate for 30 min, the letters "ETHZ" fluoresced blue, while the color of other letters remained red. This observation is because of the light-activated color change of the "ETHZ" letters painted with NDI-coAntMA, while the remaining letters painted with NDI-coPyMA were not sensitive to UV light. Therefore, the "ETHZ" information, which was initially encoded in random letters, can be deciphered using UV light. Similarly, a test involving the information "2022" was also demonstrated (Figure ).