Synthesis and Metal Binding Properties of Anthraquinone Bridged with Coumarin

Abstract

Molecular switching in macrocyclic polyether systems has been the subject of intense study during the past 30 years. Especially, photochemical switching has been well-studied by Shinkai, Ueno, Takagi, Tabushi, and others. In addition, changes in pH, light and oxidation-reduction chemistry have all been explored as switching mechanisms. The lariat ethers, podands having nitroaromatic sidearms and similar species derived from anthraquinones have been concerned with electrochemical switching. Especially the anthraquinone system is an interesting one for the study of reducible and switchable lariat ethers because its anion is stable for months in water so long as oxygen is excluded. Although podands (noncyclic polyethylene glycols) generally exhibit poor cation binding, when combined with the semiquinone nucleus, very strong cation complexation is observed for the radical anion. The anthraquinone podands can be used for electrochemically switched cation transport in bulk organic membranes and both monoanion and dianion species can be formed and detected. Both oneelectron and two-electron reduction can lead to enhanced cation binding in anthraquinone-derived podands and lariat ethers. Anthraquinones differ significantly from the nitroaromatic systems in their ability to undergo discrete one or two electron reduction and also anthraquinone substituted podands exhibit surprising geometrical effects during the electrochemical switching process. Gokel and co-workers have reported two armed podands based on the anthraquinone ring systems, which showed the cation binding enhancement by cyclic voltammetry. 1,8-Anthraquinone derived crown ether having a total of six oxygen donors in the macrocycling is an exceptionally strong cation binder not only when reduced, but in its neutral form as well. Anthraquinone plays an important role in the various photochemical and colorimetric sensor systems. In addition, coumarins are interesting chromophores by their photochemical and photophysical properties, and have been used to convert crown ethers into fluorescent probes for alkali metal ions. Herein we synthesized a new cation receptor 4 which combined coumarin and anthraquinone with ether units, and studied the binding properties of cation guests. Results and Discussion