Color Changes of a Full-Color Emissive ESIPT Fluorophore in Response to Recognition of Certain Acids and Their Conjugate Base Anions.

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 HClO4 or, with time, after the addition of HBF4 . It also shows thermally dependent color changes from pink through white to blue in a narrow temperature range of 25-60 °C. 1 H and 15 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. BF4- and ClO4- are effective in forming such a blue emitter, whereas Cl- and PF6- are not; this behavior depends on whether the anion can fit into the bidentate binding site consisting of HIm+ and the phenolic hydroxy group.

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.

Crystal structures and redox responses coupled with ion recognition of p-benzoquinone- and hydroquinone-fused [18]crown-6.

The crystal structures and redox properties of p-benzoquinone (BQ)-fused [18]crown-6 1 and bis-BQ-fused [18]crown-6 2 were examined. The anion radicals of these BQ molecules were stabilized by addition of metal ions, through effective electrostatic interactions between the negatively charged BQ moiety and positively charged ion-capturing [18]crown-6 unit. The electrostatic interactions and solvation energy played important roles in determining the magnitudes of anodic redox shifts in the reduction potentials. Regular π-stacking of BQ units and regular arrays of [18]crown-6 units were observed in crystal 2, in which one-dimensional π-electron columns were separated by ionic channels. The hydroquinone-fused [18]crown-6 molecule 3 and a new BQ- and phenol-fused [18]crown-6 derivative 4 were obtained as single crystals. The molecular conformations of [18]crown-6 in crystal 3 and hydrated crystal 3⋅H2 O were different from each other.

Mesophases and ionic conductivities of simple organic salts of M(m-iodobenzoate) (M = Li(+), Na(+), K(+), Rb(+), and Cs(+)).

Simple organic salts such as (Li(+))(m-IBA) (1), (Na(+))(m-IBA) (2), (K(+))(m-IBA) (3), (Rb(+))(m-IBA) (4), and (Cs(+))(m-IBA) (5) (m-IBA = m-iodobenzoate) were shown to form a mesophase before crystal melting or decomposition. The crystals were obtained in the hydrated form, e.g., 1·(H2O), 2·(H2O), 3·0.5(H2O), 4·(H2O), and 5·(H2O); they were then converted into dehydrated forms by increasing the temperature to ∼450 K. Optically anisotropic-layered mesophases were observed in unhydrated crystals 2, 3, 4, and 5, whereas an optically isotropic mesophase (e.g., rotator phase) was found for crystal 1. 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 m-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 1-5, with the ionic conductions playing a critical role.