Anion Binding by Fluorescent Ureido-Hexahomotrioxacalix[3]arene Receptors: An NMR, Absorption and Emission Spectroscopic Study.

Fluorescent receptors ( 4a - 4c ) based on (thio)ureido-functionalized hexahomotrioxacalix[3]arenes were synthesised and obtained in the partial cone conformation in solution. Naphthyl or pyrenyl fluorogenic units were introduced at the lower rim of the calixarene skeleton via a butyl spacer. The binding of biologically and environmentally relevant anions was studied with NMR, UV-vis absorption, and fluorescence titrations. Fluorescence of the pyrenyl receptor 4c displays both monomer and excimer fluorescence. The thermodynamics of complexation was determined in acetonitrile and was entropy-driven. Computational studies were also performed to bring further insight into the binding process. The data showed that association constants increase with the anion basicity, and AcO - , BzO - and F - were the best bound anions for all receptors. Pyrenylurea 4c is a slightly better receptor than naphthylurea 4a , and both are more efficient than naphthyl thiourea 4b . In addition, ureas 4a and 4c were also tested as ditopic receptors in the recognition of alkylammonium salts.