SYNTHESIS AND CHARACTERIZATION OF NOVEL TBAF-PROMOTED DESILYLATION OF FLUOROGENIC 3-CYANO-7-HYDROXY-4-METHYLCOUMARIN DERIVATIVES AS POTENTIAL RAPID FLUORIDE DETECTORS IN POTABLE WATER

Abstract

Over the years, Fluoride has been identified as a contaminant of great global concern on drinking water safety. The need to prevent the population from consuming higher fluoride content in water lies in fluoride sensing. A number of the reported fluoride probes are water-insoluble and suffer from a longer time taken to respond thus, limiting application in real-time fluoride detection. To solve the above problems, new and accurate fluoride detectors with ability to detect very low Fluoride concentrations in water in the presence of other anions have been developed. Three novel sensors 7-O-tert-butyldimethylsylyl-3-cyano-4-methylcoumarin (2), 7-O-tert-butyldiphenylsylyl-3-cyano-4-methylcoumarin (3) and 7-O-tert-butyldimethylsylyl-2-(hydroxyimino)-4-methyl-2H-chromene-3-carbonitrile (4) were synthesized by introducing fluoride responsive tert-butyldimethylsilylchloride (TBDMS) and tert-butyldiphenylsilylchloride (TBDPS) moieties into the 3-cyano-7-hydroxy-4-methylcoumarin (1) derivatives and by modifying the lactone moiety of compound (1) by substituting the lactone carbonyl with a hydroxyimino group. The starting reagent (1) and the synthesized compounds 7-O-tert-butyldimethylsylyl-3-cyano-4-methylcoumarin (2), 7-O-tert-butyldiphenylsylyl-3-cyano-4-methylcoumarin (3) and 7-O-tert-butyldimethylsylyl-2-(hydroxyimino)-4-methyl-2H-chromene-3-carbonitrile (4) were characterised using GC-MS spectrometer (Micromass, Wythenshawe, Waters, Inc. UK), Bruker Avance NEO 500 MHz (TXO cryogenic probe) NMR spectrometer and Agilent 400 MHz -DD2 NMR spectrometers. The NMR spectral data were processed by MestreNova (v14.0.0) software. The absorption and fluorescence characteristics of these synthesized fluoride sensors were studied by use of UV-Vis spectrophotometer and Fluorospectrophotometer. An investigation on the response of the synthesized sensors 2, 7-O-tert-butyldiphenylsylyl-3-cyano-4-methylcoumarin (3) and 7-O-tert-butyldimethylsylyl-2-(hydroxyimino)-4-methyl-2H-chromene-3-carbonitrile (4) to environmentally and biologically relevant anions revealed lower levels of background of fluorescence in the range of 2.1 to 3.9 AFU. However, their fluorescence responses were not considerably affected due to direct competition with the tested anions apart from fluoride. These results undoubtedly show that environmentally relevant groundwater anions found in do not impede with fluoride detection by the synthesized sensors 7-O-tert-butyldimethylsylyl-3-cyano-4-methylcoumarin (2), 7-O-tert-butyldiphenylsylyl-3-cyano-4-methylcoumarin (3) and 7-O-tert-butyldimethylsylyl-2-(hydroxyimino)-4-methyl-2H-chromene-3-carbonitrile (4). The detection limits of these sensors were established to be 4.5-8.5 μM in aqueous NaF solutions with 7-O-tert-butyldimethylsylyl-2-(hydroxyimino)-4-methyl-2H-chromene-3-carbonitrile (4) being the most sensitive. However, they confirmed their capability to determine TBAF concentrations as lower concentrations of between 0.18 μM and 1.9 μM. This is considered much lower than the current WHO guidelines for maximum fluoride concentration in potable water of 789.4 μM (1.5 mgL-1). The second order rate constants for these sensors were between 1.4 x10 M-1min-1 and 1.9 x10 M-1min-1. This demonstrates their ability to rapidly sense the existence of aqueous fluorides in potable water. The quantum yields of these sensors compared against quinine sulphate (standard) were established to be between 0.65 and 0.72. This indicates that all the synthesized sensors had higher efficiency of photon emission than Quinine sulphate which stands at 0.546.