ESIPT Fluorescent Chromism and Conformational Change of 3-(2-Benzothiazolyl)-4-hydroxy-benzenesulfonic acid by Amine Sorption

Sulfonic acid (−SO₃H)-substituted 2-(2′-hydroxyphenyl)­benzothiazole (<b>1</b>) was designed as a new solid-state ESIPT (excited-state intramolecular proton transfer) fluorescent chromic molecule that responds to various types of organic bases and amines as a sensing device of biologically important molecules such as ammonia and histamine. Crystal <b>1</b> exhibited a reversible adsorption–desorption behavior with pyridine, aniline, thiazole, quinoline, ammonia, propylamine, octylamine, diethylamine, 1,4-diaminobutane, histamine, and other compounds. The sorption behavior of these compounds induced the fluorescent chromism of crystal <b>1</b> from non-ESIPT weak blue, to ESIPT strong green, and finally to non-ESIPT strong green emissions, which applied to the solid-state sensing devices for biologically important organic bases and amines

Cation–Anion Dual Sensing of a Fluorescent Quinoxalinone Derivative Using Lactam–Lactim Tautomerism

A quinoxalinone derivative capable of lactam–lactim tautomerization was designed as a new fluorescence probe for sensing of cation (M⁺ = Li⁺ and Na⁺) and anion (X^– = F^–, Cl^–, Br^–, and CH₃COO^–) in organic solvents. In THF, the minor lactam tautomer exhibited a weak fluorescence band at 425 nm with a typical Stokes shift of ∼4400 cm^–1, whereas the major lactim tautomer exhibited an intense fluorescence band at 520 nm with large Stokes shift of ∼8900 cm^–1 due to excited-state intramolecular proton transfer (ESIPT). The presence of either cations or anions was found to promote lactim-to-lactam conversion, resulting in the lowering of the ESIPT fluorescence. The lone pairs on the alkylamide oxygen and the quinoxalinone ring nitrogen of the lactam were found to bind Li⁺ to form a 1:2 coordination complex, which was confirmed by single crystal X-ray structural analysis and fluorescent titrations. In addition, the N–H bond of the lactam was able to recognize anions via N–H···X hydrogen bonding interactions. Where X = F^– or CH₃COO^–, further addition of these anions caused deprotonation of the lactam to generate an anionic state, consistent with the crystal structure of the anion prepared by mixing tetrabutylammonium fluoride and the quinoxalinone derivative in THF. Dual cation–anion-sensing responses were found to depend on the ion-recognition procedure. The anionic quinoxalinone derivative and its Li⁺ complex, which are formed by the addition of CH₃COO^– and Li⁺, respectively, displayed different fluorescence enhancement behavior due to the two anions exchanging with each other

Cation–Anion Dual Sensing of a Fluorescent Quinoxalinone Derivative Using Lactam–Lactim Tautomerism

A quinoxalinone derivative capable of lactam–lactim tautomerization was designed as a new fluorescence probe for sensing of cation (M⁺ = Li⁺ and Na⁺) and anion (X^– = F^–, Cl^–, Br^–, and CH₃COO^–) in organic solvents. In THF, the minor lactam tautomer exhibited a weak fluorescence band at 425 nm with a typical Stokes shift of ∼4400 cm^–1, whereas the major lactim tautomer exhibited an intense fluorescence band at 520 nm with large Stokes shift of ∼8900 cm^–1 due to excited-state intramolecular proton transfer (ESIPT). The presence of either cations or anions was found to promote lactim-to-lactam conversion, resulting in the lowering of the ESIPT fluorescence. The lone pairs on the alkylamide oxygen and the quinoxalinone ring nitrogen of the lactam were found to bind Li⁺ to form a 1:2 coordination complex, which was confirmed by single crystal X-ray structural analysis and fluorescent titrations. In addition, the N–H bond of the lactam was able to recognize anions via N–H···X hydrogen bonding interactions. Where X = F^– or CH₃COO^–, further addition of these anions caused deprotonation of the lactam to generate an anionic state, consistent with the crystal structure of the anion prepared by mixing tetrabutylammonium fluoride and the quinoxalinone derivative in THF. Dual cation–anion-sensing responses were found to depend on the ion-recognition procedure. The anionic quinoxalinone derivative and its Li⁺ complex, which are formed by the addition of CH₃COO^– and Li⁺, respectively, displayed different fluorescence enhancement behavior due to the two anions exchanging with each other

Mesophases and Ionic Conductivities of Simple Organic Salts of M(<i>m</i>‑Iodobenzoate) (M = Li⁺, Na⁺, K⁺, Rb⁺, and Cs⁺)

Simple organic salts such as (Li⁺)­(<i>m</i>-IBA) (<b>1</b>), (Na⁺)­(<i>m</i>-IBA) (<b>2</b>), (K⁺)­(<i>m</i>-IBA) (<b>3</b>), (Rb⁺)­(<i>m</i>-IBA) (<b>4</b>), and (Cs⁺)­(<i>m</i>-IBA) (<b>5</b>) (<i>m</i>-IBA = <i>m</i>-iodobenzoate) were shown to form a mesophase before crystal melting or decomposition. The crystals were obtained in the hydrated form, e.g., <b>1</b>·(H₂O), <b>2</b>·(H₂O), <b>3</b>·0.5­(H₂O), <b>4</b>·(H₂O), and <b>5</b>·(H₂O); they were then converted into dehydrated forms by increasing the temperature to ∼450 K. Optically anisotropic-layered mesophases were observed in unhydrated crystals <b>2</b>, <b>3</b>, <b>4</b>, and <b>5</b>, whereas an optically isotropic mesophase (e.g., rotator phase) was found for crystal <b>1</b>. The single-crystal X-ray structural analysis of the hydrated crystals revealed an inorganic–organic alternate layer structure, which is consistent with the average molecular orientation in the layered mesophase. The <i>m</i>-IBA anions formed a π-stacking columnar structure in the hydrated crystals, while one- or two-dimensional M⁺∼O networks were observed in the inorganic layers. Our results showed that the M⁺∼O interactions and their connectivity are strongly influenced by the size of the cations. The reconstruction of the M⁺∼O networks by removing H₂O molecules was crucial for the formation of the mesophases. A strong response of both the real and imaginary parts of the dielectric constant was observed around the solid-mesophase phase-transition temperatures of crystals <b>1</b>–<b>5</b>, with the ionic conductions playing a critical role