Synthesis and spectroscopic characterisation of fluorescent indicators for Na+ and K+ based on (di)pyridino crown ethers

Abstract

The synthesis and the spectral and cation binding properties of a fluorescent indicator for Na⁺, caesium 5-{5-[3,6,9,12-tetraoxa-18-azabicyclo[12.3.1]octadeca-1(18),14,16-trien-16-yl]-2-thienyl}isophthalate (SPYR), and of two fluorescent indicators for K⁺, caesium 5-{5-[3,6,9,12,15-pentaoxa-21-azabicyclo[15.3.1]henicosa-1(21),17,19-trien-19-yl]-2-thienyl}isophthalate (PPYR) and dicaesium 5-{5-[21-(5-{1-[3,5-di(caesiooxycarbonyl)phenyl]}-2-thienyl)-3,6,14,17-tetraoxa-23,24-diazatricyclo[17.3.1.1^8,12]tetracosa-1(23),8(24),9,11,19,21-hexaene-10,21-diyl]-2-thienyl}isophthalate (BIPY) are reported. To investigate the effect of an sp²-nitrogen in the aza-crown ether cavity on the complex formation behaviour, a (bi)pyridino crown compound with an appropriate cavity for the sodium and the potassium cation, respectively, is linked to an aryl thiophene fluorophore. New, versatile routes for the synthesis of 4-substituted pyridino crown ethers and 4,4′-substituted sym-dipyridino crown ethers are described. The chelating abilities of the indicators with monovalent cations have been studied in aqueous solution by their fluorescence properties. Both the free and complexed forms of the indicators have an absorption maximum around 345 nm and an emission maximum at 410 nm, displaying only a decrease in the fluorescence intensity upon cation binding. While SPYR cannot discriminate between Na⁺ and other monovalent cations, PPYR and BIPY show a peak selectivity for K⁺ with a dissociation constant of 1.2 ± 0.3 mM and 4.4 ± 0.7 mM, respectively.