Highly K + Selective Probes with Fluorescence Emission Wavelengths Higher than 500 nm in Water

Herein, we report on the synthesis of highly K + /Na + selective fluorescent probes in a feasible number of synthetic steps. These K + selective fluorescent probes, so called fluoroionophores, 1 and 2 consists of different highly K + /Na + selective building blocks the alkoxy‐substituted N‐phenylaza‐18‐crown‐6 lariat ethers (ionophores) and “green” (cf. coumarin unit in 1 ) or “red” (cf. nile red unit in 2 ) fluorescent moieties (fluorophores). The fluorescent probes 1 and 2 show K + induced fluorescence enhancement factors of 4.1 for 1 and 1.9 for 2 and dissociation constants for the corresponding K + complexes of 43 mM ( 1 +K + ) and 18 mM ( 2 +K + ) in buffered aqueous solution. The fluorescence signal of 1 and 2 is changed by less than 5 % by pH values in the range of 6.8 to 8.8. Thus, 1 and 2 are capable fluorescent tools to determine extracellular K + levels by fluorescence enhancements at wavelengths higher than 500 nm.