Non-planar macrocycles possessing intriguing self-assembling behaviors and ethene/ethyne capture properties

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Design and synthesis of new macrocycles featuring novel host-guest properties and characteristic self-assembly behaviors are always one of the cutting-edge research topics in supramolecular chemistry. Particularly, the self-assembly of macrocycles into higher order nanostructures such as columns and channels has found wide applications in the fields of supramolecular delivery systems and porous materials. However, not all the reported macrocycles seem to be able to stack into columns or channels particularly for those of non-planar arene-based macrocycles such as calix[n]arenes, pillar[n]arenes, corona[n]arenes and so on, mainly because of their non-planar structural conformation or the lack of the robust intermolecular non-covalent forces. Therefore, it is of great challenge to construct new macrocyclic arenes exhibiting unconventional self-assembly behaviors as well as distinct recognition functions.

Figure 1. One-step synthesis of DPA[n]s (n = 3-7).

Herein, the authors present a new family of macrocyclic diphenylamine[n]arenes (DPA[n]s, n = 3-7) through a facile one-pot synthesis strategy (Figure 1). Unlike many other reported cyclic arenes, the resultant non-planar DPA[3] and DPA[4] feature intrinsic π-π stacking interactions, interesting self-assembly behaviors and ethene/ethyne capture properties. Specifically, ideally and strong multiple intermolecular edge-to-face aromatic interactions have been observed in the crystal of DPA[3] for the first time, while the conventional offset π-π stacking geometry dominated the packing of DPA[4] (Figure 2a and 2b). The association constants KE of DPA[3] and DPA[4] are determined to be as high as 4.18 × 103 and 1.53 × 103 M-1, respectively. The different KE values of DPA[3] and DPA[4] are likely to be associated with their different π-π stacking geometries, i.e., the slightly bigger KE of DPA[3] can be attributed to the edge-to-face π-π interaction that is more stable than offset π-π interaction. Consequently, the robust aromatic interactions facilitate their columnar self-assembly behaviors which are systematically investigated both in solution and solid states. Benefitting from the three-dimensional cavity, the inherent intermolecular edge-to-face aromatic interactions and columnar self-assembling behavior, the designed DPA[3] has demonstrated its encapsulation toward ethene and ethyne gases. The authors successfully obtained the single crystal of DPA[3]⊃ethene and DPA[3]⊃ethyne suitable for X-ray diffraction (Figure 2c and 2d). We infer that the formation of host-guest complex might be attributed to the weak CH-π or electrostatic interaction between DPA[3] and the guest. This study provides a new direction for the design and self-assembly of functional non-planar macrocyclic arenes. Our work also sheds light on the fundamental mechanism of multiple intermolecular edge-to-face π-π stacking in macrocycles, that helps to better understand the importance of π-π stacking interaction in supramolecular self-assembly.

Figure 2. Intermolecular interaction and crystal packing of DPA[3] (a) and DPA[4] (b). X-ray crystal structure and 3D packing of DPA[3]⊃ethene (c) and DPA[3]⊃ethyne (d).

Xueliang Shi

Associate professor, East China Normal University

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