It′s in the I′s: Three suitably oriented electron‐deficient iodoaryl groups form the basis for the first anion receptor (see structure; white H, gray C, red O, blue F, purple I, green Cl) that employs the halogen‐bonding interaction alone to achieve high‐affinity molecular recognition in dilute solution. The anion selectivity of this tridentate host differs from those of similar receptors based on hydrogen bonding.

Fluorescein is used extensively for visualization and diagnostics in biological and medical applications. The popularity of fluorescein, which has been studied for over a century, [1] arises from its bright fluorescence and its ease of conjugation to biomolecules.[2] Fluorescein exists in up to seven different pHdependent states: [3a] three neutral forms and four charged forms (Scheme 1). These forms each have different excitation and fluorescence emission properties, some of which are strongly solvent-dependent. To better understand the effect of the microenvironment on the spectroscopic properties of fluorescein, knowledge of its intrinsic (solvent-free) properties is crucial. Herein, we use the isolation capabilities of trapping mass spectrometry to individually probe the spectroscopy of the three fluorescein charge states. An unexpected result is that the brightest form of fluorescein in solution, the dianion, does not fluoresce significantly in the gas phase.The absorbance and the quantum yield of fluorescein in solution vary significantly with the protonation state. The fluorescein dianion ([FlÀ2 H] 2À ; Scheme 1) has the highest molar absorptivity (ca. 10 5 m À1 cm À1 at l ab max ¼ 490 nm in water) and fluorescence quantum yield (0.92).[3] The monoanion ([FlÀH] À ) is also fluorescent, but has a lower absorptivity (two maxima of ca. 30 000 m À1 cm À1 at l ab max ¼ 450 and 470 nm in water) and fluorescence quantum yield (0.37).[3] Fluorescence upon excitation of cationic (and neutral) fluorescein is observed; however, this fluorescence is believed to occur through deprotonation in the excited state, thus forming the fluorescent excited monoanionic species. The effective fluorescence quantum yield for the fluorescein cation is 0.18, which reflects both the efficiency of the excited state proton transfer reactions and the quantum yield of the monoanion.[3a]The fluorescein dianion exhibits significant solvatochromism.[4] This observation was first reported by Martin, [4a] who showed that as the solvent was changed from H 2 O to dimethyl sulfoxide (DMSO), the absorption maximum for the dianion shifted from 490 nm to 520 nm. The observed solvatochromism was attributed to the hydrogen bonds between the fluorescein dianion and the solvent being stronger in the ground state than in the excited state, thus increasing the gap between the S 0 and S 1 electronic energy levels as the hydrogen-bonding ability of the solvent increases.Whereas there is a breadth of information on the behavior of fluorescein in solution, studies of the properties of fluorescein in the gas phase have been limited to computational work.[5] Jang et al.[5b] have performed electronic structure theory calculations for nine different fluorescein tautomers in vacuo and in DMSO and water. Computations at the B3LYP/6-31 + + G** level of theory with a Poisson-Boltzmann continuous solvation approach showed that the most stable conformers of cationic and dianionic fluorescein in solution are similar to the most stable gas-phase forms. However, dep...

No abstract

Drei passend orientierte elektronenarme Iodarylgruppen bilden die Grundlage des ersten Anionenrezeptors (siehe Bild; weiß H, grau C, rot O, blau F, lila I, grün Cl), der ausschließlich Halogenbrücken verwendet, um eine hochaffine molekulare Erkennung in verdünnter Lösung zu erzielen. In seiner Anionenselektivität unterscheidet sich der Rezeptor von strukturverwandten Wasserstoffbrückenrezeptoren.

Fluorescein is used extensively for visualization and diagnostics in biological and medical applications. The popularity of fluorescein, which has been studied for over a century, [1] arises from its bright fluorescence and its ease of conjugation to biomolecules. [2] Fluorescein exists in up to seven different pHdependent states: [3a] three neutral forms and four charged forms (Scheme 1). These forms each have different excitation and fluorescence emission properties, some of which are strongly solvent-dependent. To better understand the effect of the microenvironment on the spectroscopic properties of fluorescein, knowledge of its intrinsic (solvent-free) properties is crucial. Herein, we use the isolation capabilities of trapping mass spectrometry to individually probe the spectroscopy of the three fluorescein charge states. An unexpected result is that the brightest form of fluorescein in solution, the dianion, does not fluoresce significantly in the gas phase.The absorbance and the quantum yield of fluorescein in solution vary significantly with the protonation state. The fluorescein dianion ([FlÀ2 H] 2À ; Scheme 1) has the highest molar absorptivity (ca. 10 5 m À1 cm À1 at l ab max ¼ 490 nm in water) and fluorescence quantum yield (0.92). [3] The monoanion ([FlÀH] À ) is also fluorescent, but has a lower absorptivity (two maxima of ca. 30 000 m À1 cm À1 at l ab max ¼ 450 and 470 nm in water) and fluorescence quantum yield (0.37). [3] Fluorescence upon excitation of cationic (and neutral) fluorescein is observed; however, this fluorescence is believed to occur through deprotonation in the excited state, thus forming the fluorescent excited monoanionic species. The effective fluorescence quantum yield for the fluorescein cation is 0.18, which reflects both the efficiency of the excited state proton transfer reactions and the quantum yield of the monoanion. [3a] The fluorescein dianion exhibits significant solvatochromism. [4] This observation was first reported by Martin, [4a] who showed that as the solvent was changed from H 2 O to dimethyl sulfoxide (DMSO), the absorption maximum for the dianion shifted from 490 nm to 520 nm. The observed solvatochromism was attributed to the hydrogen bonds between the fluorescein dianion and the solvent being stronger in the ground state than in the excited state, thus increasing the gap between the S 0 and S 1 electronic energy levels as the hydrogen-bonding ability of the solvent increases.Whereas there is a breadth of information on the behavior of fluorescein in solution, studies of the properties of fluorescein in the gas phase have been limited to computational work. [5] Jang et al. [5b] have performed electronic structure theory calculations for nine different fluorescein tautomers in vacuo and in DMSO and water. Computations at the B3LYP/6-31 + + G** level of theory with a Poisson-Boltzmann continuous solvation approach showed that the most stable conformers of cationic and dianionic fluorescein in solution are similar to the most stable gas-phase forms. Howe...