New tripodal and dipodal colorimetric sensors for anions based on tris/bis-urea/thiourea moieties

Abstract

Seven neutral tripodal and dipodal receptors having mesitylene/triethylbenzene as core moiety, urea/thiourea as binding groups and p-nitrobenzene as signalling unit have been reported. The receptors act as selective colorimetric, naked eye sensors for small and spherical F− ion with some interference from tetrahedral ions. Thiourea derivatives form stable 1:1 H-bonded complexes with F− anion and to some extent with anions, whereas for urea derivatives, the recognition is simply based on acid–base reaction between ureidic protons and basic F− ions and is a completely reversible phenomenon. The pre-organisation of thiourea derivatives coupled with their high-intrinsic acidity is supposed to help them in the formation of strong H-bonded complexes with F− anion. The urea-based receptors do not respond at lower concentrations of the anion, but at higher concentrations, they undergo completely reversible deprotonation concomitant with a colorimetric change, with the production of stable [HF2]− anion.