Calix[2]‐m‐benzo[4]phyrin with Aggregation‐Induced Enhanced‐Emission Characteristics: Application as a Hg^II Chemosensor

Abstract

In recent years, solid-state fluorescent materials have received much attention in diverse fields, such as fluorescent biological labels, sensors, and light-emitting diodes.[1] Many conjugated organic luminophores are highly emissive in dilute solutions, however, when fabricated into thin films, they suffer an aggregation-caused quenching (ACQ) effect, which limits their practical applications.[1] To solve this problem, recently, some organic molecules, which show an intense emission in the aggregated state, whereas they exhibit almost no or weak emission in dilute solutions, were reported. This unique phenomenon is widely researched as aggregation-induced emission (AIE) [2] and aggregation-induced enhanced emission (AIEE).[3] However, the potential utility of this phenomenon has not been explored in macrocycles. On the other hand, molecular sensors for Hg II ions have received considerable attention due to the extremely toxic impact exerted by Hg II on our environment. [4] Many efforts have been made to design various chemosensors specific to Hg II ions.[5] The major challenges involved in the creation of Hg II sensors are lack of selectivity and specificity. Calixphyrins, polypyrrolic macrocycles, are known for the coordination chemistry, [6] but reports on metal-ion sensing are very rare. Herein, we wish to report the AIEE characteristic of a calixbenzophyrin-derivative, M-1, and utilize this novel property for probing Hg II ions both in aqueous solution and in the solid state.The syntheses of calixbenzophyrins are outlined in Scheme 1. The synthetic methodology followed here is basically an acid-catalyzed condensation reaction. Stirring a solution of 1 [7] with pentafluorobenzaldehyde in dichloromethane in the presence of trifluoroacetic acid (TFA) followed by oxidation with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) resulted in calix[n]-m-benzo[m]phyrins (n = 2,3,4; m = 4,6,8) in 20 % (M-1), 10 % (M-2), and 5 % (M-3) yield, respectively. All the macrocycles were fully characterized by electronic, FAB-MS, and NMR spectral studies, and the structure of M-1 was finally confirmed by single-crystal Xray-diffraction analysis.[8] All the derivatives are soluble in common organic solvents, but insoluble in water.A solution of M-1 in dilute acetonitrile shows an absorption band at 434 nm, which arises from the p-p* transition of the dipyrrin moiety.[8] The emission spectrum shows a band at 539 nm. We observed an anomalous behavior in the emission spectrum of M-1 upon addition of various percentages of water (0-90 %), upon which the band at 539 nm is intensified with a bathochromic shift to 550 nm (Figure 1, left). Because water is not a good solvent for M-1, addition of water promotes efficient calixphyrin self-aggregation, which enhances the emission; in other words, M-1 is AIEE active. The trajectory of the intensity change suggests that the molecularly dissolved M-1 starts to congregate at a water fraction of 50 % and the population of the aggregate continues to increase as the water fraction increases fr...

pH-Regulated Reversible Transition Between Polyion Complexes (PIC) and Hydrogen-Bonding Complexes (HBC) with Tunable Aggregation-Induced Emission

“…This system principally involves two components: a fully quaternized poly­(ethylene oxide)- b -poly­(2-(dimethylamino)­ethyl methacrylate) (PEO- b -PQDMA) diblock polyelectrolyte and a pH-responsive tetrakis­(4-carboxylmethoxyphenyl)­ethene (TPE-4COOH) fluorophore. TPE chromophore is distinguished by the unique aggregation-induced emission (AIE) characteristic, which is nonfluorescent in a molecularly dissolved state yet highly emissive in an aggregated state due to the restriction of intramolecular rotation. − In a mildly basic aqueous solution (e.g., pH 8), the TPE-4COOH moieties exist in the deprotonation form carrying four negatively charged carboxylate anions, which is water-soluble and therefore nonemissive. However, upon mixing with cationic PEO- b -PQDMA, electrostatic association between the negatively charged carboxylic groups of TPE-4COOH and the positively charged quaternary ammonium pendent groups of the PQDMA blocks guides the formation of PIC micelles stabilized by hydrophilic PEO coronas.…”

Fluorine interaction controlled AIEE phenomenon in an expanded calixbenzophyrin and its vapoluminescent response: turn on emission with volatile ketones and esters

“…We have recently reported the synthesis and characterization of a group of meso-pentafluorophenyl incorporated calixbenzophyrins (M-I, M-II and M-III), intermolecular hydrogen bond driven AIEE phenomenon of M-I and its application as Hg II ion sensor. 7 Here, our attention is mainly focussed on the fluorine interactions for orchestrating the AIEE behavior of the expanded calixbenzophyrinderivative M-II (Fig. 1a).…”

A sensitive and selective “turn-on” fluorescent probe for Hg2+ based on thymine–Hg2+–thymine complex with an aggregation-induced emission feature

“…17,18 However, there are few reports on uorescent "turnon" sensors for Hg 2+ detection. 19 One kind of TPE derivative with thymine groups has been reported by D. Q. Zhang with a theoretical detection limit of 3.7 Â 10 À7 mol L À1 and a detection range from 2 Â 10 À5 mol L À1 to 2.14 Â 10 À4 mol L À1 for Hg 2+ detection. 20 Meanwhile another kind of TPE derivative with quaternary ammonium salt groups has been reported by B.…”