The binding efficiency of a series of monotopic (1–2) and heteroditopic (3–4) calix [4] arene- based receptors has been evaluated in chloroform solution toward N-methylpyridinium ion pairs using NMR and UV/vis spectroscopic techniques. These experiments provided evidence that, due to a positive cooperative effect, the presence of a phenylurea H-bond donor group on the upper rim of the calix [4] arene macrocycle increases the recognition ...