Covalent Assembly of Molecular Ladders

摘要

A current challenge for self-assembly is the synthesis of shape-persistent nanostructures with a high degree of structural control. These artificial macromolecules could ultimately approach the sophistication of biomolecules, allowing atomic-level spatial control over multi-nanometer length scales in two and three dimensions, and the construction of molecular objects of size commensurate with top-down nanofabrication. Much like the current use of DNA as a structural element for nanotechnology, a multitopic oligomer-based approach allows, in principle, the formation of complex nonsymmetrical structures by incorporating instructions for self-assembly into sequence. However, in contrast to small-molecule building blocks, the use of linear oligomers as components for nanostructure construction is relatively undeveloped, with a few exceptions. Here we show that discrete oligomers are viable building blocks for fully covalent shape-persistent molecular grids. We have found that, using dynamic covalent chemistry (DCC), complimentary m-phenylene ethynylene (mPE) oligomers will self-assemble into [n]-rung molecular ladders (1~n, n = 3-6). In the largest case (1~6, Figure 1), the core region is approximately 6.2 x 1.6 nm. In all cases, the desired ladder structure is the only product observed by MALDI mass spectrometry, although gel permeation chromatography (GPC) indicates an increasing fraction of high molecular weight byproducts with increasing n. These ladders represent the first step toward larger grids that will be developed as high precision nanofiltration membranes, or as "smart-matrix" grids to position units for solar energy harvesting.