Selective Probing of Potassium Ion in Solution by Intramolecular Excimer Fluorescence of Dibenzo-Crown Ethers

Observation of an excimer fluorescence in solution is proposed for detecting the encapsulation of potassium ion as opposed to other alkali ions by dibenzo-crown ethers. The scheme has been validated by ultraviolet photodissociation (UVPD) spectroscopy of dibenzo-21-crown-7 and dibenzo-24-crown-8 complexes with potassium ion, K⁺  ⋅ DB21C7 and K⁺  ⋅ DB24C8, performed under cold (∼10 K) conditions in the gas phase and by quantum chemical calculations of the geometry and electronic structures of the complexes. Calculations suggest the formation of a closely spaced excimer structure of benzene rings only for the K⁺  ⋅ DB24C8. Interaction of the rings may lead to lifetime broadening in UV absorption, which is experimentally observed in the gas phase, indeed, only for this cold complex. Consistently, intramolecular excimer fluorescence of DB24C8 in solution is observed only for K⁺  ⋅ DB24C8. The excimer fluorescence is not observed with other alkali metal ions. The detection of such intramolecular excimer fluorescence can, therefore, potentially serve as a simple, background-free, selective probe of potassium ion in solution.