1,3-Alternate Calix[4]arene Bifuntional Fluorescent Receptor Containing Urea and Crown Ether Moieties

Abstract

Scheme 1. Synthetic routes to fluorescent chemosensor 1 Calix[4]arenes are important building blocks in supramolecular chemistry. The calixarene framework provides a unique possibility to organize several binding sites in an array complementary to a potential guest. A variety of calixarene-based receptors for cations, anions and neutral molecules have been synthesized in the past decade. Though anion recognition appears to be a relatively new area of research, both positively charged and neutral receptors for anionic species have been prepared in the last few years. As fluorogenic units, pyrenes are one of the most useful because of their relatively efficient excimer formation and emission. Since the intensity ratio of the excimer to the monomer emission (IE/IM) is sensitive to conformational changes of the pyrene-appended receptors, changes in IE/IM upon metal ion complexation can be an informative parameter in various sensing systems. The simultaneous binding of cationic and anionic guest species by ditopic receptors is a rapidly developing new field for ion pair recognition of environment and biological importance. By taking advantage of calixarene framework, a few neutral bifunctional receptors were developed and showed the simultaneous complexation properties of hydrophilic anions and cations in organic media. Those receptors are, however, only focused on the cone conformers of calix[4]arene. It has been known that 1,3-alternate conformation of calix[4]arene could provide the two excellent binding sites for guest molecules when the proper functionalization could be achieved. We have previously reported 'bifunctional calix[4]arene receptors' containing two urea groups and crown ether moieties at the calix[4]arene fixed in the 1,3-alternate conformation. It was shown the simultaneous binding properties of cation and anion guests with enhanced binding affinity of halide anions in the presence of potassium cation. Here we report a new bifunctional receptor 1 based on 1,3alternate conformer of calix[4]arene, which contains urea linking pyrene fluorophores instead of phenylurea and crown ether moieties at the opposite side of lower rim of calix[4]arene fixed in the 1,3-alternate conformation. Also, we studied binding affinity of various anions in the present of potassium ion.