Self-assembly of nanostructured materials through irreversible covalent bond formation.

Over the past decades, numerous efforts have been devoted to synthesizing nanostructured materials with specific morphology because their size and shape play an important role in determining their functions. Self-assembly using weak and reversible interactions or bonds has provided synthetic routes toward various nanostructures because it allows a "self-checking" and "self-error-correcting" process under thermodynamic control. By contrast, the use of irreversible covalent bonds, despite the potential to generate more robust structures, has been disfavored in the synthesis of well-defined nanomaterials largely due to the lack of such self-error-correcting mechanisms. To date, the use of irreversible bonds is largely limited to covalent fixation of preorganized building blocks on a template, which, though capable of producing shape-persistent and robust nanostructured materials, often requires a laborious and time-consuming multistep processes. Constructing well-defined nanostructures by self-assembly using irreversible covalent bonds without help of templates or preorganization of components remains a challenge. This Account describes our recent discoveries and progress in self-assembly of nanostructured materials through strong, practically irreversible covalent bond formation and their applications in various areas including drug delivery, anticancer therapy, and heterogeneous catalysis. The key to the success of this approach is the use of rationally designed building blocks possessing multiple in-plane reactive groups at the periphery. These blocks can then successfully grow into flat oligomeric patches through irreversible covalent bond formation without the aid of preorganization or templates. Further growth of the patches with or without curvature generation drives the system to the formation of polymer nanocapsules, two-dimensional (2D) polymer films, and toroidal nanotubular microrings. Remarkably, the final morphology can be specified by a few simple parameters: the reaction medium, bending rigidity of the system, and orientation of the reactive groups. Theoretical studies support the spontaneous formation of such nanostructured materials in terms of energetics and successfully predict or explain their size distributions. Although the lack of self-error-correcting mechanisms results in defect sites in these nanostructures, the high efficiency and relative simplicity of our novel approach demonstrates the potential power of using irreversible covalent bonds to generate a diverse range of shape-persistent and robust nanostructures that is likely to enrich the repertoire of self-assembled nanomaterials.