Synthesis and spectroscopic characterisation of fluorescent indicators for Na+ and K+

The synthesis and the spectral and cation binding properties of new fluorescent indicators for Na+, dicaesium 5-[5-(2,3,5,6,8,9,11,12-octahydro-1,4,7,10,13-benzopentaoxacyclopentadecin-15-yl)-2-thienyl]isophthalate (Benzos), and for K+, dicaesium 5-[5-(2,3,5,6,8,9,11,12,14,15-decahydro-1,4,7,10,13,16-benzohexaoxacyclooctadecin-18-yl)-2-thienyl]isophthalate (Benzop), are reported. Both indicators consist of a benzocrown ether with an appropriate cavity for the sodium and the potassium cation, respectively, linked to an aryl thiophene fluorophore. In aqueous solution, the indicators have an absorption maximum at 340 nm and an emission maximum at 430 nm. The chelating abilities of Benzos and Benzop with Na+, K+, Li+, Cs+, Mg2+ and Ca2+ were studied by means of their fluorescence properties. Upon cation binding, a change in the fluorescence intensity is observed, whereas the spectral distribution remains unaltered. While Benzos cannot discriminate between Na+ and other monovalent cations, Benzop shows a three-fold selectivity for K+ over Na+.