Carbazole-Anthranylπ-Conjugates as Small and Stable Aggregation-InducedEmission-Active Fluorogens: Serving as a Reusable and Efficient Platformfor Anticounterfeiting Applications with an Acid Key and MulticolorInk for a Quill Pen

摘要

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 (−NEt2) 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 −NEt2 substituents isestablished to be highly sensitive under protonation–deprotonationstimuli by a sharp and rapid fluorescence color change [yellow (Φf = 37%) to green (Φf = 39.4%)] in the solidstate (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 tobe perfectly suitable for anticounterfeiting applications by markingthe AIEgen on the paper that shows the bright image of the AIEgenunder the UV torch (365 nm). Almost equal efficiencies by fabricatingon different surfaces such as polythene paper and a fresh leaf areobserved. While these spots can be duplicated with typical yellowfluorescent dyes, our AIEgen can easily be differentiated with theacid key. The emission color change of this AIEgen from yellow togreen under acid stimuli is distinctly defined compared to other dyesand vividly recognized by naked eyes. Thus, one can combat the counterfeiterswith the acid key. The reversible color-changing behavior on the paperremains intact even after six consecutive days of exposure to sunlight,and the AIEgen is thermally stable up to 445 °C. Further, thiscompound is also utilized as ink (10 μM 1,4-dioxane solution)where a pigeon feather is used as a quill pen. The mechanistic insightsbehind these facts have also been proposed and validated whereverpossible.