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

Preparation and Characterization of Carbazole-Based Luminogen with Efficient Emission in Solid and Solution States

Organic luminescent materials with high luminescence efficiency in both solution and solid states, namely dual-state emission (DSE), have attracted considerable attention due to their promising applications in various fields. In order to enrich the variety of DSE materials, carbazole, similar to triphenylamine (TPA), was utilized to construct a novel DSE luminogen named 2-(4-(9H-carbazol-9-yl)phenyl)benzo[d]thiazole (CZ-BT). CZ-BT exhibited DSE characteristics with fluorescence quantum yields of 70, 38 and 75% in solution, amorphous and crystalline states, respectively. CZ-BT shows thermochromic and mechanochromic properties in solution and solids, respectively. Theoretical calculations show that there is a small conformational difference between the ground state and the lowest singly excited state of CZ-BT and that it exhibits a low non-radiative transition characteristic. The oscillator strength during the transition from the single excited state to the ground state reaches 1.0442. CZ-BT adopts a distorted molecular conformation with intramolecular hindrance effects. The excellent DSE properties of CZ-BT can be explained well using theoretical calculations and experimental results. In terms of application, the CZ-BT has a detection limit for the hazardous substance picric acid of 2.81 × 10^-7 mol/L.

A. Water-based fluorescent flexo-ink for security applications

UV-readable fluorescent ink formulations find versatile applications in various fields including information encryption, automated identification systems, security markers and optical devices. In this context, a new bithiophene-based chalcone (BTCF) that exhibits good solution phase and solid-state fluorescence was synthesized as a colourant for formulating an eco-friendly UV fluorescent ink. The molecule demonstrated good thermal stability and photophysical features including intramolecular charge transfer, confirmed through emission studies in THF–hexane mixtures with varying hexane content. The intense greenish yellow solid-state fluorescence emission displayed by BTCF was exploited by using it as a colourant in a water-based fluorescent ink formulation. Further, the ink was used to print a fast-drying solid patch on an UV dull paper substrate using flexography technique. The analysis of colorimetric, densitometric and rub resistance properties of the printed paper samples demonstrated good fluorescence, moderate photostability and good rub resistance, and hence could be used for security printing applications.

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Multistimuli-Responsive Squaraine Dyad Exhibiting Concentration-Controlled Vapochromic Luminescence

Stimuli-responsive organic materials with controllable luminescence are of enormous importance because of their potential applications in sensing, data security, and display devices. In this study, a multistimuli-responsive squaraine dyad (SQ-d) composed of two rigid squaraine moieties and a flexible ethylene linker was rationally designed and synthesized. SQ-d exhibits polymorphic luminescence, which can be reversibly switched by various external stimuli, including solvent vapor exposure, heat, and shear force. Unexpectedly, the weakly luminescent phase (O1) of SQ-d exhibits concentration-controlled vapochromic behavior. Film O1 can convert to a highly green-emissive phase (G1) under a low concentration of CHCl₃ vapor and convert to a highly yellow-emissive phase (Y) under a high concentration of CHCl₃ vapor; these originate from two distinct crystallization-induced emission enhancement processes. To the best of our knowledge, this is the first investigation of the effect of vapor concentration on the phase transitions of organic vapochromic luminophores. By analyzing the single-crystal structures and photophysical properties of SQ-d, we concluded that the green and yellow emissions probably originated from a zigzag stacking mode and an H-type π-π stacking mode, respectively. Finally, two prototypes based on SQ-d for applications in information encryption and vapor sensing were successfully demonstrated.

Aggregation induced emission based active conjugated imidazole luminogens for visualization of latent fingerprints and multiple anticounterfeiting applications

Aggregation-induced emission based organic heterocyclic luminogens bearing conjugated electronic structures showed much attention due to its excellent fluorescence in aggregation state. In this communication, a novel conjugated blue light emitting imidazole molecule is synthesized by one pot multicomponent reaction route is reported for the first time. The prepared molecule exhibits a strong fluorescence in aggregation state with exceptional properties, such as high purity, inexpensive, eco-friendly, large scale production, high photostability, etc. By considering these advantages, a new fluorescence based platform has been setup for in-situ visualization of latent fingerprints and its preservation by spray method followed by Poly(vinyl alcohol) masking. A clear and well defined fluorescence fingerprint images are noticed on variety of surfaces by revealing level 1-3 ridge features upon ultraviolet 365 nm light exposure. The dual nature of binding specificity as well as excellent fluorescence properties permits the visualization of latent fingerprints for longer durations (up to 365 days) with superior contrast, high sensitivity, efficiency, selectivity and minimal background hindrance. We further fabricated unclonable invisible security ink for various printing modes on valuable goods for protection against forging. The developed labels are displaying uniform distribution of ink and exceptional stability under various atmospheric environments. The development of long preservative information using aggregation-induced emission based luminogen opens up a new avenue in advanced forensic and data security applications.

Selective Expression of a Carbazole-Phenothiazine Derivative Leads to Dual-mode AIEE, TADF and Distinctive Mechanochromism

In this article, we report a newly designed D-A-D' derivative (CNCzPTZ), which displays selective expression of chromophores. This enables CNCzPTZ with solvatochromism, rare dual-mode AIEE properties, solid-state dual-emissions with phosphorescence and distinctive mechanochromism.CNCzPTZ exhibits dual-mode AIEE properties, since the emission band abruptly shifts from 550 nm to 500 nm as the water fraction increases. In the crystalline state, CNCzPTZ demonstrated dual emission bands of 478 nm and 538 nm.CNCzPTZ shows distinctive mechanochromic property in the solid state due to the planarization.

Sensitive detection of iron (II) sulfate with a novel reagent using spectrophotometry

In this study, a novel reagent was developed for sensitive detection of iron (II) sulfate, spectrophotometrically. A novel thio-anthraquinone derivative, 1-(Dodecylthio)anthracene-9,10-dione (3), was synthesized from the chemical reaction of 1-Chloroanthraquinone (1) and 1-Dodecanethiol (2) by an original reaction method and was used in the preparation of the novel reagent called Catal's reagent. A synthesized thio-anthraquinone analogue (3) was purified by column chromatography, and its chemical structure was characterized by spectroscopic methods such as Fourier-transform infrared spectroscopy (FT-IR), nuclear magnetic resonance (NMR) spectroscopy, mass spectrometry (MS), and ultraviolet (UV)-visible spectrophotometry. The chemical and molecular structure of the developed thio-antraquinone derivative (3) was illuminated using computational methods with the GaussView5 and Gaussian09 programs. Various solvents including ethanol, methanol, and acetonitrile were examined in the preparation of the reagent. A concentration range from 0.2 mg mL^-1 up to 10 mg mL^-1 of iron (II) sulfate heptahydrate solution in distilled water was prepared. The absorption spectra of Catal's reagent (0.816 mM) showed three peaks between 185 nm-700 nm of wavelength. However, after the reaction with H₂O₂ and the 30 mM trisodium citrate dihydrate mixture in the presence of an iron sulfate (II) solution, a single peak was observed, producing a stable and reddish/brownish homogenous solution (λ max = 304 nm). The following concentrations of iron (II) sulfate heptahydrate was examined using developed protocol and the reagent, and the concentrations were measured spectrophotometrically at 304 nm, 0.2-1 mg mL^-1. Absorbances of reaction mixtures of iron (II) sulfate remained stable up to 48 h. The results indicated that the novel Catal's reagent can be used for sensitive spectrophotometric detection of iron (II) sulfate in aqueous solutions.

Multicolored fluorescence variation of a new carbazole-based AIEE molecule by external stimuli

In this article, we design and synthesize a new carbazole-based molecule, Cz2CN, with a twisted D-A structure, using the carbazole group as the donor and a dicyanoethylene fragment as the acceptor. Such a twisted D-A structure endows Cz2CN with two characteristic emission bands, LE emission and ICT emission. Cz2CN exhibits AIEE, solvatochromism, and different responses to anisotropic shearing force and isotropic hydrostatic pressure, due to the sensitivity of ICT emission to aggregation, solvent polarity and mechanical force. Aggregation benefits the ICT emission, leading to the AIEE of Cz2CN in the THF/water system. Polar solvents can help stabilize the ICT-excited states and make the ICT emission red shift. The original crystalline powder is strongly fluorescent with a high quantum yield of 40.4%. A single crystal of Cz2CN is obtained and dimers without π-π interactions among carbazole groups contribute to the strong emission. Anisotropic shearing alters the emission of Cz2CN powder from sky blue (474 nm) to green (520 nm). The single crystal of Cz2CN undergoes a distinct multicolored variation from sky blue (476 nm) to green (510 nm) and further to orange (590 nm) upon isotropic hydrostatic pressure. The emission of the original powder/crystal of Cz2CN is dominated by LE (locally excited state) emission in HLCT (intercrossed hybridized local and charge transfer excited state). To the best of our knowledge, Cz2CN is an interesting carbazole-based molecule that shows simultaneous AIEE, solvatochromism and force-induced multicolored variation.