Switchable nanoporous sheets by the aqueous self-assembly of aromatic macrobicycles.

The molecular self-assembly of aromatic building blocks is a powerful approach to the construction of dynamic nanostructures that are able to respond to external stimuli by changing their shape and/or macroscopic properties. In this context, it is possible to modulate the interplay between multiple noncovalent interactions through the rational design of molecular modules. The directionality of these interactions plays a critical role in controlling the shape of the selfassembled structures. Laterally grafted rod building blocks lead to one-dimensional nanofibers through unidirectional guiding of the elongated aromatic rods in the self-assembly process. Anisotropic micelles also undergo directional growth to form low-dimensional supramolecular structures in selected solvents. For example, hydrophobic oblate micelles with hydrophilic side faces stack on top of each other to form 1D nanofibers. On the other hand, hydrophobic aromatic rod bundles with hydrophilic up and down grow through side-to-side interactions to form freely suspended 2D structures in bulk solution. A reduction in the strength of the aromatic interactions leads the resulting planar sheets to form 2D networks. However, the elaborate construction of 2D structures requires the rational design of molecular building blocks that are able to grow in two dimensions. One possibility is the sideby-side arrangement of 2D molecular building blocks, such as flat disk-shaped aromatic molecules. Nonetheless, most flat aromatic amphiphiles stack together through face-to-face interactions to form nanofibers. To frustrate continuous aromatic stacking in one dimension, one can introduce flexible chains on the basal plane of the aromatic structures. The basal-plane chains should enforce a lateral arrangement of the flat aromatic segments and their two-dimensional growth rather than growth through conventional 1D aromatic stacking (Figure 1). With this idea in mind, we designed a flat aromatic bicycle with a hydrophilic dendron at the center of the basal plane. Another important issue regarding 2D planar structures is the possibility of the integration of dynamic response characteristics triggered by environmental changes. The combination of principles of 2D planar structures with responsive properties would generate a new class of intelligent nanomaterials. Herein we report the spontaneous formation of 2D porous sheets in aqueous solution through the two-dimensional selfassembly of an aromatic macrobicycle with a hydrophilic dendron attached to its basal plane. Remarkably, unlike conventional nanoporous materials, the resulting porous sheets underwent dynamic motion between open and closed states, as triggered by guest intercalation (Figure 4). The self-assembling molecules that form this aggregate consist of a macrobicyclic aromatic segment and a hydrophilic oligoether dendron grafted at the center of the basal plane (Figure 1). The synthesis of the flat aromatic amphiphiles began with the Sonogashira coupling of a dendron-substituted diiodobenzene derivative with 2,6-dibromoethynylbenzene to provide a tetrabromo building block (see the Supporting Information). Suzuki coupling of the tetrabromo compound with 3-((triisopropylsilyl)ethynyl)phenylboronic acid ester, followed by silyl-group deprotection with tetra-n-butylammonium fluoride, then provided a precursor with four terminal alkyne groups. The final aromatic amphiphiles were synthesized efficiently by intramolecular Glaser-type coupling of the terminal alkyne groups under dilute reaction Figure 1. Molecular structure of aromatic macrobicycles with dendrons attached to their basal plane and schematic representation of the 2D growth of the flat micelles derived by pairwise stacking.