Carbazole-Anthranyl π-Conjugates as Small and Stable Aggregation-Induced Emission-Active Fluorogens: Serving as a Reusable and Efficient Platform for Anticounterfeiting Applications with an Acid Key and Multicolor Ink for a Quill Pen

[Image: see text] Being a unique, simple, and inexpensive approach, continuous development on the fluorescence-based technologies remains active in fluorescent anticounterfeiting. A number of polymeric, nano-, carbon dot, and rare-earth oxide materials were preferably explored for such applications, but the complex synthesis, purity, and high cost are the major concerns to make these materials accessible for commercial applications. To address these difficulties, we herein report simple mono-carbazole-linked anthranyl π-conjugates that are synthesized in a gram scale via an inexpensive and convenient route. These unsymmetrically substituted new π-conjugates are found to be promising blue-shifted aggregation-induced emission-active fluorogens (AIEgens) having a distinct color on varying substituents with electron-rich (−NEt(2)) and electron-poor (−CN) functionalities. The direct link of a single carbazole unit with an anthracenyl π-conjugate possibly enforces the achievement of a highly twisted molecular structure, accountable for the AIE characteristics. The π-conjugate with −NEt(2) substituents is established to be highly sensitive under protonation–deprotonation stimuli by a sharp and rapid fluorescence color change [yellow (Φ(f) = 37%) to green (Φ(f) = 39.4%)] in the solid state (no fluorescence on/off). Upon the exposure of the base vapors (deprotonation), the original emission color (yellow) comes back. Such reversible and also repeatable acidchromism is demonstrated to be perfectly suitable for anticounterfeiting applications by marking the AIEgen on the paper that shows the bright image of the AIEgen under the UV torch (365 nm). Almost equal efficiencies by fabricating on different surfaces such as polythene paper and a fresh leaf are observed. While these spots can be duplicated with typical yellow fluorescent dyes, our AIEgen can easily be differentiated with the acid key. The emission color change of this AIEgen from yellow to green under acid stimuli is distinctly defined compared to other dyes and vividly recognized by naked eyes. Thus, one can combat the counterfeiters with the acid key. The reversible color-changing behavior on the paper remains intact even after six consecutive days of exposure to sunlight, and the AIEgen is thermally stable up to 445 °C. Further, this compound is also utilized as ink (10 μM 1,4-dioxane solution) where a pigeon feather is used as a quill pen. The mechanistic insights behind these facts have also been proposed and validated wherever possible.