Conventional security inks, generally directly printed on the data page surface, are vulnerable to counterfeiters, thereby raising the risk of chemical structural deciphering. In fact, polymer film-based data pages with customized patterns embedded within polymer matrix, rather than printed on the surface, emerge as a promising solution. Therefore, the key lies in developing fluorophores offering light dose-controlled fluorescent color inside polymer matrices. Though conventional fluorophores often suffer from photobleaching and uncontrolled photoreactions, disqualifying them for this purpose. Herein a diphenanthridinylfumaronitrile-based phototransformers (trans-D5) that undergoes photoisomerization and subsequent photocyclization during photopolymerization of the precursor, successively producing cis- and cyclo-D5 with stepwise redshifted solid-state emissions is developed. The resulting cyclo-D5 exhibits up to 172 nm emission redshift in rigidifying polymer matrices, while trans-D5 experiences a slightly blueshifted emission (≈28 nm), cis-D5 undergoes a modest redshift (≈14 nm). The markedly different rigidochromic behaviors of three D5 molecules within polymer matrices enable multicolor photochemical printing with a broad hue ranging from 38 to 10 via an anticlockwise direction in Munsell color space, yielding indecipherable fluorescent patterns in polymer films. This work provides a new method for document protection and implements advanced security features that are unattainable with conventional inks.
Keywords: document protection; indecipherable fluorescent patterns; inside polymer matrices; photochemical printing; phototransformers.
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