Synthesis, Solid and Solution Studies of Paraquat Dichloride Calixarene Complexes. Molecular Modelling

Abstract

The interaction of the herbicide paraquat dichloride (PQ,substrate) with p-tert butylcalix[6,8]arenes (L, receptor) was investigated in both the solution and solid states. The isolated paraquat calixarene complexes were characterised by UV-visible, 1H NMR, ESI-MS, Luminescence and IR spectroscopies and elemental analysis. The stoichiometry of complexes 1 and 2 was 1:1 (1 herbicide:1 calixarene) and both revealed a biexponential luminescence decay with lifetimes depending on the size and the conformational particularity of the calixarenes. Molecular modelling suggested that both calixarenes interact with the herbicide through cation-π interaction. PQ is included in the p-tert-butylcalix[8]arene cavity, a situation favoured by its pinched conformation in polar solvent while it is partially included in the p-tert-butylcalix[6]arene cavity because of its in-out cone conformation. The theoretical results, in particular using MOPAC procedures, were in agreement with the experimental findings.