Azo-diazacrown ethers as metal ion and anion sensors

Abstract

Six azobenzene-diaza-18-crown-6 derivatives were synthesized and characterized by 1H, 13C-NMR, and mass spectroscopies. The three ester derivatives E1, E2 and EE2 did not show any special sensing property towards any metal ion. Acid and nitro derivatives showed solvatochromic effects and dependent sensing properties in various solvents. The acid derivative A2 displayed inseparable color yellow or green from orange upon addition of transition and heavy metal ions in DMSO which, unfortunately, could not be used as optical chemosensor. In contrast, the nitro derivatives N1 and N2 showed selectivity toward Cr3+ in MeCN by color changing from pink to colorless with a blue shift of 125 and 165 nm, respectively. Both sensors form complex ML2 and ML with Cr3+​ in MeCN. Hg2+ also induced a large blue shift around 160 nm in the UV-Visible spectra of N1 and N2 in DMSO with color change from purple to colorless. The value of log K for ML complex of N2 with Hg2+ is equal to 5.97. The naked eye detection limit towards target ions are 0.09 and 0.27 ppm for N1 and N2 for Cr3+ in MeCN, and 0.90 and 1.40 ppm for N1 and N2 for Hg2+ in DMSO. This low detection limit for Cr3+ could be used to monitor Cr3+ that bring acute toxic to aquatic life. Furthermore, N1 and N2 showed selectivity towards F– and in both MeCN and DMSO: F– could induce a color change of N1 from pink to purple in MeCN and from purple to blue in DMSO as a result of a red shift of 50-60 nm. H2PO4- also induced a color change from pink or purple to colorless with a 130-160 nm blue shift. Thus N1 and N2 could be potentially employed as chromogenic sensors for both metal ions such as Cr3+ and Hg2+ and anions like F– and H2PO4-.