Recent advances in the chemistry of organic chemosensors containing crown-ether groups are reviewed.Organic chemosensors are molecules of abiotic nature capable of interacting selectively and irreversibly with a specific substrate (ion, molecule) with corresponding changes in one or more characteristics of the system. A series of reviews have described methods for the synthesis of chemosensors [1] and their use in the determination of metal cations [2-8], anions, and molecules [4, 9, 10]. Some of them treat the various types of fluorescent chemosensors [5][6][7][8] or photoswitching compounds [11][12][13][14] selectively. In this review, based exclusively on the results of recent years, sensors containing crown-ether groups are examined. Special attention is paid to the various mechanisms of action of crown-containing chemosensors, which are classified according to their electronic and photochemical characteristics.In the general case a chemosensor molecule consists of signal and receptor parts with a bridge between them although the latter may be absent. As a rule chemosensors can be subdivided into chromogenic, fluorescent, and photoswitchable.During the interaction of a chromogenic sensor with the substrate (in particular with a metal ion) a hypsochromic or bathochromic shift of the long-wave absorption band of the initial compound is observed. If the shift of the bands is significant and takes place in the visible part of the spectrum it leads to a visually discernable change in the color of the solution. Such signal systems are usually called colorimetric or "naked-eye" chemosensors.The action of fluorescent chemosensors is based both on change of the fluorescence intensity and on shift of the emission band of the initial compounds during the formation of a complex with the substrate.Photoswitchable chemosensors are capable of reversible "switching on-switching off" of their sensor characteristics under the influence of light.Among ionophore-receptors capable of combining with positively charged ions, it is possible to include chelating agents, podands, coronands (or crown ethers), cryptands, calixarenes, cyclodextrins, etc. [15].Crown ethers, discovered by Pedersen in 1962, occupy a special position among receptors and are widely used in the design of new chemosensors based on their unique ability to combine with the cations of alkali metals, their fairly high selectivity, and their accessibility. In addition to alkali metals, crown ethers are also effective complexing reagents for the cations of alkaline-earth metal, Pb 2+ , and Tl 2+ , and when nitrogen and sulfur atoms are inserted into their structure they become sensitive to Ag + , Hg 2+ , and Cd 2+ ions. The __________________________________________________________________________________________
