A Ratiometric Fluorescent Probe for K + in Water Based on a Phenylaza‐18‐Crown‐6 Lariat Ether

This work presents two molecular fluorescent probes 1 and 2 for the selective determination of physiologically relevant K + levels in water based on a highly K + /Na + selective building block, the o ‐(2‐methoxyethoxy)phenylaza‐18‐crown‐6 lariat ether unit. Fluorescent probe 1 showed a high K + ‐induced fluorescence enhancement (FE) by a factor of 7.7 of the anthracenic emission and a dissociation constant ( K d ) value of 38 m m in water. Further, for 2 +K + , we observed a dual emission behavior at 405 and 505 nm. K + increases the fluorescence intensity of 2 at 405 nm by a factor of approximately 4.6 and K + decreases the fluorescence intensity at 505 nm by a factor of about 4.8. Fluorescent probe 2 +K + exhibited a K d value of approximately 8 m m in Na + ‐free solutions and in combined K + /Na + solution a similar K d value of about 9 m m was found, reflecting the high K + /Na + selectivity of 2 in water. Therefore, 2 is a promising fluorescent tool to measure ratiometrically and selectively physiologically relevant K + levels.