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A deprotonated 2,5-dihydroxyterephthalic acid (DHT) was synthesized to obtain a water-soluble, fluorescent molecule (DHT-K). The deprotonated DHT-K was converted to DHT upon exposure to acidic gas, changing fluorescence color from yellow to blue. DHT-K was incorporated to poly(vinyl alcohol) to form a film, and the film showed the same fluorescence change when exposed to HCl gas. The blue fluorescence of HCl-exposed DHT-K film was returned to yellow emission by simple treatment with NH3 gas, showing effective regeneration. Using detectable fluorescence changes by the naked eye, DHT-K can be a potential candidate for a fluorescent indicator toward HCl gas. [ABSTRACT FROM AUTHOR]

Though 2,5-dihydroxyterephthalic acid (DHT) is composed of a single benzene molecule, it is blue-emissive in common organic solvents and in the solid state. Like most organic fluorophores, DHT is not soluble in water, which limits its versatile use in metal ion detection in an aqueous medium. To improve the water solubility of DHT and its use as a molecular sensor in aqueous solutions, its deprotonated form, DHT-K, was synthesized through the simple one-pot reaction of DHT with KOH. Compared with DHT, DHT-K was highly soluble in water and emitted yellow fluorescence in the solution and the solid. In addition, DHT-K showed high selectivity for Al ions, exhibiting fluorescence wavelength changes from 540 to 495 nm depending on the Al ion concentration. A linear relationship between the fluorescence intensity of DHT-K and Al ion concentration was established ranging from 18.96 to 247 μM with a detection limit of 1.84 µM. The binding stoichiometry between DHT-K and Al ions was determined by Job's plot and found to be 1:2. Upon exposure to Al ions, DHT-K showed significant changes in fluorescence color and emission wavelength, whereas no fluorescence changes were observed by the addition of various metal ions such as Zn2+, Mg2+, Ca2+, Cu2+, Fe3+, and Co2+. Thus, DHT-K can be applied as a fluorescent sensor that can selectively detect Al ions in aqueous solutions. [ABSTRACT FROM AUTHOR]

• Fluorescence from simple benzene structure was used for sensing, patterning, and secret writing. • Cascade fluorescence color change was observed in esterase sensing. • Fluorescence manipulation was achieved by changing the intramolecular hydrogen bonding. Fluorescence from single-benzene-based compounds was controlled by changing the substituents that played a role in the electron push-pull structure and intramolecular hydrogen bonding. Diethyl 2,5-dihydroxyterephthalate (DDT) containing intramolecular hydrogen bonds with blue fluorescence, was turned into nonfluorescent compounds via simple blocking of the intramolecular hydrogen bonds, which was obtained via protection of the hydroxyl groups. In addition, deprotonation of the hydroxyl groups altered the fluorescence color of DDT from blue to yellow. The fluorescence manipulation was easily applied to related fields. Restoration of the original DDT fluorescence was observed via hydrolysis of DDT derivatives in the presence of an esterase (an enzyme that can hydrolyze the ester bond) and of a photoacid generator upon ultraviolet irradiation, which could be used in esterase sensing and fluorescence patterning, respectively. In addition, changes in fluorescence color by deprotonation/protonation of DDT were applied to an inkless paper, in which patterns could be fabricated using water as an ink. [ABSTRACT FROM AUTHOR]