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

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Abstract
This article describes the synthesis and binding properties of highly selective noncovalent molecular receptors 13·(DEB)6 and 33·(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 13·(DEB)6 or 33·(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 13·(DEB)6 and 33·(DEB)6. Both receptors 13·(DEB)6 and 33·(DEB)6 change conformation from staggered to eclipsed upon complexation to afford a better fit for the 23 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 electron-poor 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.