Dynamic photochromism in cocrystals and tri-state fluorescence switching in films for multilevel optical encryption

Organic crystalline systems combining dynamic photosalient effect with static photochromism represent a promising class of intelligent responsive materials. In this work, we construct a 3Abf-Ofn cocrystal (3Abf = 3-aminodibenzofuran, Ofn=Octafluoronaphthalene) through molecular co-assembly strategy. Upon UV irradiation, the 3Abf-Ofn undergoes a solid-state transformation involving volatile Ofn release and N-N coupling of 3Abf to bis(benzofuran-3-yl) azo compound (Bfa), triggering significant photochromism and photosalient behavior. This reaction pathway differs from conventional mechanisms such as photoisomerization or cycloaddition, representing an example of such photochemical process in organic cocrystals. Furthermore, the 3Abf-Ofn@PMMA composite film exhibits dynamically tunable fluorescence under UV irradiation: an initial enhancement of fluorescence upon photoactivation, followed by excitation wavelength-dependent emission (EWDE) and progressive fluorescence quenching, showing great potential for advanced anti-counterfeiting and information encryption applications. This work not only provides valuable insights for designing multifunctional, light-controlled organic luminescent materials but also offers complementary synthetic approaches to azo-aromatic compounds. Photosalient effects in organic crystals have potential for responsive materials, but design of such materials is challenging. Here, the authors report the development of cocrystals that undergo a solid state transformation on exposure to UV, resulting in photochromism and photosalient behaviour.