Two-dimensional assembly of rod amphiphiles into planar networks.

The ability of rod building blocks to self-assemble into supramolecular structures provides unique opportunities for the design of nanoscale materials with predictable properties and functions.1 Extensive efforts thus have been directed toward supramolecular systems for exploration of novel properties and functions that are not easily available without specific assembly of molecular components. Aggregation of rod building blocks in solution can generate various one-dimensional nano-objects including helice,2 ribbons,3 tubules,4and rings,5 depending on the molecular structure. The hierarchical assembly of such one-dimensional structures could result in the formation of three-dimensional (3D) networks through interconnecting the extended micelles.6 On the other hand, anisotropic interactions of nanoparticles result in the spontaneous formation of a free-suspended 2D structure in solution.7 Similarly, surface-layer proteins in bacterial cells are well established to selfassemble into 2D structures with in-plane ordered arrays of pores through anisotropic interactions between the individual subunits.8 However, self-assembly of synthetic molecules into planar nets remains unexplored.9 We present here the spontaneous formation of nanoporous sheets in aqueous solution without a template to guide them, which is based on the self-assembly of dumbbell-shaped rod amphiphiles driven by a fine balance of anisotropic hydrophobic interactions and steric constraints endowed with bulky dendritic wedges. The dumbbell-shaped rod amphiphiles that form these aggregates consist of a stiff rod segment that is grafted by hydrophilic polyether dendrons at one end and hydrophobic branches at the other end (falkyl/PEO ) 0.97) (Figure 1). Transmission electron microscopy (TEM) studies showed that both molecular dumbbells self-assemble into stable 2D aggregates. Figure 2a shows a low magnification micrograph obtained from the 0.01 wt % aqueous solution of 1 cast onto a TEM grid. The negatively stained sample with uranyl acetate shows a single layer of planar nets with protruding cylinders and closed loops against a dark background (Figure 2a). The image at higher magnification shows that the nets consist of interconnected cylindrical components with a uniform cross-section of 16 nm and in-plane ordered packing of pores with a diameter of ∼15 nm (Figure 2b). The Fourier transform shown as an inset to Figure 2b indicates that the pores are arranged into a roughly 2D hexagonal symmetry with a lattice dimension of ∼30 nm. For direct imaging of the 2D objects formed in aqueous solution, cryogenic TEM (cryoTEM) experiments were performed with a 0.01 wt % aqueous solution.10 A large area micrograph shows dark, wrinkled sheets against the vitrified solution background (Figure 2c), demonstrating that the 2D structure forms in bulk solution, not on the substrates during drying process. A zoomed-in image clearly shows that these sheets are based on a network structure with a uniform cylindrical cross-section. The formation of the 2D objects in aqueous solution was further confirmed by fluorescence microscopy with the 0.01 wt % aqueous † Yonsei University. ‡ Pohang University of Science and Technology. Figure 1. Chemical structure of rod amphiphiles 1 and 2.