Colorimetric thiosemicarbazide anion sensors

Abstract

A series of eight simple model thiosemicarbazide receptors containing three N-H donors (including a thiourea moiety) and substituted by a combination of phenyl, 4-nitrophenyl and naphthalene units, have been synthesised and studied for their anion binding abilities towards inorganic anions including OH-, F-, Aco-, H2P04- and CI. The anion binding properties were studied in DMSO, 9: 1 DMSO/H20 and acetonitrile through UV-visible absorption and 1H/13C/15N NMR spectroscopic techniques. Significant changes were observed in the UV-visible absorption spectra of the compounds, which were red shifted when titrated with the basic anions OH~, F , AcO , H2P04- and accompanied by dramatic colour changes clearly visible to the naked eye. These changes were determined to be due to the deprotonation of the central N-H proton (belonging to the thiourea) and not due to hydrogen bonding of these anions to the receptors. This was confirmed by using 1H/15N NMR titration studies with AcO-in DMSO-d6' which indicated the disappearance of the resonance belonging to the thiourea proton, the formation of acetic acid and the observation of a significant chemical shift in the 15N NMR spectrum. These phenomena were taken as direct evidence for a deprotonation process.