A Highly Selective Fluorescent Chemosensor for K+ from a Bis-15-Crown-5 Derivative

A class of bis-crown ether fluorescent chemosensors for detection of trace amounts of Na+, K+, and Cs+ and Pb2+ was investigated. A distyrylbenzene derivative with crown ether rings of varying size attached to each benzene (12-crown-4 (1), 15-crown-5 (2), and 18-crown-6 (3)) can form intermolecular sandwich structures with Na+, K+, and Cs+ ions, respectively. In addition, bis-crown 2 forms unique sandwich complexes with Pb2+; the stoichiometry depends on the counterion present. Crystal structure data for the complex of Pb(CF3COO)2 with 2 indicate a unique sandwich structure having dilead tetrakis(trifluoroacetate) bridging two crowns on separate ligands. The bis-crown/alkali metal ion complexes have 1:1 stoichiometry in each case. 1H NMR spectra and MALDI-TOF mass spectra clearly demonstrate that formation of the self-assembled sandwich structure is associated with the fluorescence enhancement for sensor molecules 2 and 3 with K+ and Cs+, respectively. Sensor molecule 1, however, has an additional fluores...

Chemosensors for Lead(II) and Alkali Metal Ions Based on Self-Assembling Fluorescence Enhancement (SAFE)

This paper focuses on various chromophores that have been covalently linked to two azacrown or benzocrown ether units. These compounds can interact with near-UV or visible light. By comparison with monocrown dyes, they display particular behaviour in the presence of cations. The crowns may have the possibility to act in concert, thus forming sandwich complexes around the guest. This often leads to increased selectivity towards large cations. This is the case for photochromic crown ethers, which allow the photochemical control of cation binding, as well as for some optical sensors, designed for cation recognition. Biscrown dyes can also be rigid structures in which the two spatially separated crowns cannot co-operate intramolecularly. However, intermolecular sandwich complexes can be detected. They range from discrete face-to-face arrangements to extended supramolecular architectures, which can generate nanosized aggregates and materials. The variety of possible combinations with cations bestows a particular interest on biscrown dyes.

Bis(azacrown ether) and bis(benzocrown ether) dyes: butterflies, tweezers and rods in cation binding

A prototype for the chemosensing of Ba2+ based on self-assembling fluorescence enhancement.

It was shown by 1H NMR spectroscopy that symmetrical bis(crown)stilbenes (L) and small alkali and alkaline-earth metal cations form 1(L) ∶ 1(Mm+) and 1(L) ∶ 2(Mm+) complexes in MeCN solutions. In the case of large or hydrated metal cations such as Cs+, Rb+, K+, Ba2+, Sr2+, [Ca(H2O)x]2+ and stilbenes with a small crown-ether cavity as compared with the metal cation size, stable bis-sandwich complexes 2(L) ∶ 2(Mm+) can also be formed. A stable bis-pseudosandwich 2 ∶ 2 complex is also produced from bis(18-crown-6)stilbene with the propanediammonium ion. The effect of the crown-ether size and the cation size and nature on the route of stilbene phototransformation and product composition was elucidated. The bis-(pseudo)sandwich complexes undergo effective stereoselective [2+2] photocycloaddition giving mainly rctt isomers of new 1,2,3,4-tetracrown cyclobutanes. The structures of complexes of bis(crown)stilbenes and obtained cyclobutanes were confirmed by X-ray diffraction.

Yuming Liu

Papers

Bis crown ethers—VI. The first single crystal structure of the complex of schiff base type bis benzo-15-crown-5 and potassium picrate