Calixarenes bearing a tropylium substituent as hosts for organic cations
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Four novel calixarenes bearing one cycloheptatrienyl substituent and two calixarenes with one tropylium substituent at the wider rim were synthesized. Cycloheptatrienyl- and tropylium-calixarenes represent two states of a potentially photoswitchable calixarene host. The complexation of selected hosts with organic cations such as quinolinium, ammonium and tropylium ions was studied. It was found that the complexation of organic cations by the tropylium-substituted host was much stronger in CDCl3 solution than by the related cycloheptatrienyl-host. Aryltropylium ions are bound by tropylium hosts. Accordingly, dimers of the host itself are formed both in CDCl3 solution and in the gas phase. Because of the intramolecular charge transfer tropyliumcalixarenes are qualified as chromogenic hosts. These undergo two acid-base equilibriums depending on the concentration and the solvent.
KeywordsCalixarenes Cycloheptatriene Tropylium Complexation NMR-titration Isomeric complexes
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Financial support by Deutsche Forschungsgemeinschaft is gratefully acknowledged. We thank Dr. B. Ziemer, HU Berlin, for the determination of the crystal structures.
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