Design of Na + ‐Selective Fluorescent Probes: A Systematic Study of the Na + ‐Complex Stability and the Na + /K + Selectivity in Acetonitrile and Water

There is a tremendous demand for highly Na + ‐selective fluoroionophores to monitor the top analyte Na + in life science. Here, we report a systematic route to develop highly Na + /K + selective fluorescent probes. Thus, we synthesized a set of fluoroionophores 1 , 3 , 4 , 5 , 8 and 9 (see Scheme [Scheme 1. Scheme Synthesis of Na+‐responsive fluoroionophores 1, 3, 4, 5, ...]) to investigate the Na + /K + selectivity and Na + ‐ complex stability in CH 3 CN and H 2 O. These Na + ‐probes bear different 15‐crown‐5 moieties to bind Na + stronger than K + . In the set of the diethylaminocoumarin‐substituted fluoroionophores 1 – 5 , the following trend of fluorescence quenching 1 > 3 > 2 > 4 > 5 in CH 3 CN was observed. Therefore, the flexibility of the aza‐15‐crown‐5 moieties in 1 – 4 determines the conjugation of the nitrogen lone pair with the aromatic ring. As a consequence, 1 showed in CH 3 CN the highest Na + ‐induced fluorescence enhancement (FE) by a factor of 46.5 and a weaker K + induced FE of 3.7. The Na + ‐complex stability of 1 – 4 in CH 3 CN is enhanced in the following order of 2 > 4 > 3 > 1 , assuming that the O‐atom of the methoxy group in the ortho ‐position, as shown in 2 , strengthened the Na + ‐complex formation. Furthermore, we found for the N ‐( o ‐methoxyphenyl)aza‐15‐crown‐5 substituted fluoroionophores 2 , 8 and 9 in H 2 O, an enhanced Na + ‐complex stability in the following order 8 > 2 > 9 and an increased Na + /K + selectivity in the reverse order 9 > 2 > 8 . Notably, the Na + ‐induced FE of 8 (FEF=10.9), 2 (FEF=5.0) and 9 (FEF=2.0) showed a similar trend associated with a decreased K + ‐induced FE [ 8 (FEF=2.7)> 2 (FEF=1.5)> 9 (FEF=1.1)]. Here, the Na + ‐complex stability and Na + /K + selectivity is also influenced by the fluorophore moiety. Thus, fluorescent probe 8 ( K d =48 m m ) allows high‐contrast, sensitive, and selective Na + measurements over extracellular K + levels. A higher Na + /K + selectivity showed fluorescent probe 9 , but also a higher K d value of 223 m m . Therefore, 9 is a suitable tool to measure Na + concentrations up to 300 m m at a fluorescence emission of 614 nm.