Structural and Spectroscopic Study of 6,7-Dicyano-Substituted Lumazine with High Electron Affinity and Proton Acidity

The introduction of cyano groups into lumazine (pteridine-2,4-(1<i>H</i>,3<i>H</i>)­dione) at the C6 and C7 positions enhances its electron affinity, proton acidity, and solubility in solvents. As a result, 6,7-dicyanolumazine (DCNLH₂) forms charge transfer (CT) complexes with donors such as tetrathiafulvalene, 2,3,5,6-tetramethyl-1,4-phenylenediamine, and 3,3′,5,5′-tetramethylbenzidine and readily dissociates a proton from the N1 nitrogen to form a monoanionic salt with tetrabutylammonium (TBA⁺). Crystal structures of the CT complexes consist of mixed stacks in which DCNLH₂ interacts with donors in face-to-face configurations, but they form intermolecular hydrogen bonds differently depending on the donor type. In the TBA⁺ salt, two deprotonated DCNLH^– monoanions form a unique dianionic dimer connected by two centrosymmetric hydrogen bonds, N3–H···O–C2, which is electronically isolated by the presence of bulky TBA⁺ countercations and the absence of a proton at the N1 hydrogen-bonding site. This dimer fluoresces yellowish green (fluorescence quantum yield Φ = 0.04). Because the DCNLH^– anion only shows weak blue fluorescence in aqueous solution (Φ < 0.01), we suggest that the dimer formation is responsible for the fluorescence enhancement with a large emission band shift to the low-energy side

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

Structural and Spectroscopic Study of 6,7-Dicyano-Substituted Lumazine with High Electron Affinity and Proton Acidity

The introduction of cyano groups into lumazine (pteridine-2,4-(1<i>H</i>,3<i>H</i>)­dione) at the C6 and C7 positions enhances its electron affinity, proton acidity, and solubility in solvents. As a result, 6,7-dicyanolumazine (DCNLH₂) forms charge transfer (CT) complexes with donors such as tetrathiafulvalene, 2,3,5,6-tetramethyl-1,4-phenylenediamine, and 3,3′,5,5′-tetramethylbenzidine and readily dissociates a proton from the N1 nitrogen to form a monoanionic salt with tetrabutylammonium (TBA⁺). Crystal structures of the CT complexes consist of mixed stacks in which DCNLH₂ interacts with donors in face-to-face configurations, but they form intermolecular hydrogen bonds differently depending on the donor type. In the TBA⁺ salt, two deprotonated DCNLH^– monoanions form a unique dianionic dimer connected by two centrosymmetric hydrogen bonds, N3–H···O–C2, which is electronically isolated by the presence of bulky TBA⁺ countercations and the absence of a proton at the N1 hydrogen-bonding site. This dimer fluoresces yellowish green (fluorescence quantum yield Φ = 0.04). Because the DCNLH^– anion only shows weak blue fluorescence in aqueous solution (Φ < 0.01), we suggest that the dimer formation is responsible for the fluorescence enhancement with a large emission band shift to the low-energy side

Structural and Spectroscopic Study of 6,7-Dicyano-Substituted Lumazine with High Electron Affinity and Proton Acidity

The introduction of cyano groups into lumazine (pteridine-2,4-(1<i>H</i>,3<i>H</i>)­dione) at the C6 and C7 positions enhances its electron affinity, proton acidity, and solubility in solvents. As a result, 6,7-dicyanolumazine (DCNLH₂) forms charge transfer (CT) complexes with donors such as tetrathiafulvalene, 2,3,5,6-tetramethyl-1,4-phenylenediamine, and 3,3′,5,5′-tetramethylbenzidine and readily dissociates a proton from the N1 nitrogen to form a monoanionic salt with tetrabutylammonium (TBA⁺). Crystal structures of the CT complexes consist of mixed stacks in which DCNLH₂ interacts with donors in face-to-face configurations, but they form intermolecular hydrogen bonds differently depending on the donor type. In the TBA⁺ salt, two deprotonated DCNLH^– monoanions form a unique dianionic dimer connected by two centrosymmetric hydrogen bonds, N3–H···O–C2, which is electronically isolated by the presence of bulky TBA⁺ countercations and the absence of a proton at the N1 hydrogen-bonding site. This dimer fluoresces yellowish green (fluorescence quantum yield Φ = 0.04). Because the DCNLH^– anion only shows weak blue fluorescence in aqueous solution (Φ < 0.01), we suggest that the dimer formation is responsible for the fluorescence enhancement with a large emission band shift to the low-energy side

Structural and Spectroscopic Study of 6,7-Dicyano-Substituted Lumazine with High Electron Affinity and Proton Acidity

The introduction of cyano groups into lumazine (pteridine-2,4-(1<i>H</i>,3<i>H</i>)­dione) at the C6 and C7 positions enhances its electron affinity, proton acidity, and solubility in solvents. As a result, 6,7-dicyanolumazine (DCNLH₂) forms charge transfer (CT) complexes with donors such as tetrathiafulvalene, 2,3,5,6-tetramethyl-1,4-phenylenediamine, and 3,3′,5,5′-tetramethylbenzidine and readily dissociates a proton from the N1 nitrogen to form a monoanionic salt with tetrabutylammonium (TBA⁺). Crystal structures of the CT complexes consist of mixed stacks in which DCNLH₂ interacts with donors in face-to-face configurations, but they form intermolecular hydrogen bonds differently depending on the donor type. In the TBA⁺ salt, two deprotonated DCNLH^– monoanions form a unique dianionic dimer connected by two centrosymmetric hydrogen bonds, N3–H···O–C2, which is electronically isolated by the presence of bulky TBA⁺ countercations and the absence of a proton at the N1 hydrogen-bonding site. This dimer fluoresces yellowish green (fluorescence quantum yield Φ = 0.04). Because the DCNLH^– anion only shows weak blue fluorescence in aqueous solution (Φ < 0.01), we suggest that the dimer formation is responsible for the fluorescence enhancement with a large emission band shift to the low-energy side

Structural and Spectroscopic Study of 6,7-Dicyano-Substituted Lumazine with High Electron Affinity and Proton Acidity

The introduction of cyano groups into lumazine (pteridine-2,4-(1<i>H</i>,3<i>H</i>)­dione) at the C6 and C7 positions enhances its electron affinity, proton acidity, and solubility in solvents. As a result, 6,7-dicyanolumazine (DCNLH₂) forms charge transfer (CT) complexes with donors such as tetrathiafulvalene, 2,3,5,6-tetramethyl-1,4-phenylenediamine, and 3,3′,5,5′-tetramethylbenzidine and readily dissociates a proton from the N1 nitrogen to form a monoanionic salt with tetrabutylammonium (TBA⁺). Crystal structures of the CT complexes consist of mixed stacks in which DCNLH₂ interacts with donors in face-to-face configurations, but they form intermolecular hydrogen bonds differently depending on the donor type. In the TBA⁺ salt, two deprotonated DCNLH^– monoanions form a unique dianionic dimer connected by two centrosymmetric hydrogen bonds, N3–H···O–C2, which is electronically isolated by the presence of bulky TBA⁺ countercations and the absence of a proton at the N1 hydrogen-bonding site. This dimer fluoresces yellowish green (fluorescence quantum yield Φ = 0.04). Because the DCNLH^– anion only shows weak blue fluorescence in aqueous solution (Φ < 0.01), we suggest that the dimer formation is responsible for the fluorescence enhancement with a large emission band shift to the low-energy side

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

Optical and Structural Properties of ESIPT Inspired HBT–Fluorene Molecular Aggregates and Liquid Crystals

In bulk materials, positional isomers not only help in understanding how slight difference in molecular structure alters the crystal packing and optical properties, but also play a key role in developing new type of materials for functional applications. A detailed study on the photophysical properties of fluorene–HBT positional isomers in solution and in the solid state providing a molecular level understanding of the factors which influence fluorescence behavior is reported. Two molecules <b>Ia</b> and <b>IIa</b> were synthesized by Suzuki coupling reaction and their photophysical properties were compared to positional isomers <b>Ib</b> and <b>IIb</b>. Crystal structure analyses and density functional theory (DFT) computation studies were performed to understand structure–properties relation and the results reveal that changing substitution pattern has a marked influence on their packing modes and luminescence properties. Strong noncovalent interactions (π–π) in the solid state hamper the excited state intramolecular proton transfer (ESIPT) process which causes fluorescence quenching in the solid state (<b>Ia</b> and <b>IIa</b> = Φ_f, 28–40%; <b>Ib</b> and <b>IIb</b> = Φ_f, 55–67%). Compounds show solvent–responsive and aggregation induced emission (AIE) properties. Bent structures of <b>Ia</b> with double and symmetric substitution of ESIPT motifs exhibit particularly unique condensed phase upon heating, confirmed as a nematic liquid crystalline phase, and this is the first report on the ESIPT and AIE active liquid crystalline materials with a banana-shaped molecule