Resorcinarenes with 2-benzimidazolone bridges: Self-aggregation, self-assembled dimeric capsules, and guest encapsulation

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Abstract
The synthesis and spectroscopic characterization of self-assembled dimeric resorcinarenes 1a-d containing four 2-benzimidazolone (cyclic urea) bridges are reported. The nanometer-size capsules are held together by a cyclic array of complementary hydrogen bonds. Unlike the related imide-bridged resorcinarenes reported by Rebek and coworkers [Heinz, T., Rudkevich, D. M. & Rebek, J., Jr. (1998) Nature (London) 394, 764-766], these strongly bound dimers aggregate in chloroform solutions yielding different self-organized structures, depending on the nature and length of the four carbon chains attached at the bottom of each resorcinarene platform, as revealed by transmission electron microscopy. Thus, phenethyl groups (dimer 1c x 1c) produce long fibers, probably arising from tail to tail contacts and subsequent threading of the resulting linear self-assembled polymers, whereas long alkyl chains (dimers 1a x 1a and 1b x 1b) induce formation of large reverse vesicles of 0.8-2.2 microm diameter through side to side extensive stacking. Presumably, the rigidity of the dimer precludes folding of the aggregate into smaller vesicles. On the contrary, dimer 1d. 1d, containing four nine-carbon chains and a cis-double bond, does not substantially aggregate and gives rise to reasonably resolved (1)H NMR spectra. The compound was shown to be dimeric either by matrix-assisted laser desorption ionization-time-of-flight and vapor pressure osmometry. Encapsulation studies were followed by NMR. Propionic or pivalic acid was included in the capsules, probably as head to head hydrogen-bonded dimers in mesitylene-d(12), a solvent too big to be a guest by its own. Longer dimeric carboxylic acids or larger substrates, like 2-adamantyl azide or cyclohexylcarbodiimide, do not encapsulate, but mixtures of a long and a short carboxylic acid (i.e., propionic-adamantyl or propionic-cyclohexyl) yield pairwise complexes.

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