Selective Self-Organization of Guest Molecules in Self-Assembled Molecular Boxes

摘要

This article describes the synthesis and binding properties of highly selective noncovalent molecular receptors 1_3·(DEB)_6 and 3_3·(DEB)_6 for different hydroxyl functionalized anthraquinones 2. These receptors are formed by the self-assembly of three calix[4]arene dimelamine derivative molecules (1 or 3) and six diethylbarbiturate (DEB) molecules to give 1_3·(DEB)_6 or 3_3·(DEB)_6. Encapsulation of 2 occurs in a highly organized manner; that is, a noncovalent hydrogen-bonded trimer of 2 is formed within the hydrogen-bonded receptors 1_3·(DEB)_6 and 3_3·(DEB)_6. Both receptors 1_3·(DEB)_6 and 3_3·(DEB)_6 change conformation from staggered to eclipsed upon complexation to afford a better fit for the 2_3 trimer. The receptor selectivity toward different anthraquinone derivatives 2 has been studied using ~1H NMR spectroscopy, X-ray crystallography, UV spectroscopy, and isothermal microcalorimetry (ITC). The π_π stacking between the electron-deficient center ring of the anthraquinone derivatives 2a-c and 2e-g and the relatively electronpoor melamine units of the receptor is the driving force for the encapsulation of the guest molecules. The selectivity of the hydrogen-bonded host for the anthraquinone derivatives is the result of steric interactions between the guest molecules and the calix[4]arene aromatic rings of the host.