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.

2-(1,3-Benzothiazol-2-yl)-4-methoxy-6-(1,4,5-triphenyl-1H-imidazol-2-yl)phenol (BTImP) is an excited-state intramolecular proton transfer (ESIPT) fluorophore, containing an acid-stimuli-responsive intramolecular hydrogen bond (H-bond) that can switch from the central phenolic proton to the imidazole (Im) or benzothiazole (BT) nitrogen atoms. Here, we demonstrate that BTImP shows full-color (red, green, blue, and white) emission upon the addition of different concentrations of HClO or, with time, after the addition of HBF . It also shows thermally dependent color changes from pink through white to blue in a narrow temperature range of 25-60 °C. H and N NMR measurements suggest that, after the green fluorescent BTImP is protonated at its Im nitrogen atom, a conjugate base anion coordinates to the imidazolium (HIm ) proton, forming two types of complexes with different coordination states. One state shows a significantly Stokes-shifted red emission resulting from ESIPT at the BT side, whereas the other shows a typical Stokes-shifted blue emission, probably caused by interaction of the anion with the phenolic proton, which breaks the H-bond on the BT side. BF and ClO are effective in forming such a blue emitter, whereas Cl and PF are not; this behavior depends on whether the anion can fit into the bidentate binding site consisting of HIm and the phenolic hydroxy group.

Sulfonic acid (−SO3H)-substituted 2-(2′-hydroxyphenyl)­benzothiazole (1) 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 1 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 1 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.

Both K+ and AcO ions became the key ions to lock the fluorescent organogel state of a crown-ether-fused quinoxalinone derivative.