Self-Assembly of Discrete Organic Nanotubes

Not only amphiphiles but also non-amphiphilic or pconjugated molecules, if rationally designed, have been found to self-assemble in liquid media to form discrete tubular architectures with well-defined dimensions. This review covers most of the molecular building blocks that spontaneously produce organic nanotubes (ONTs) through self-assembly. Starting with the comparison of synthetic procedures and physical properties between the ONTs and carbon nanotubes (CNTs), the author discusses the classification of formation mechanism for the self-assembled ONTs. Then, membrane- or sheet-based, nanoring- or nanotoroid-based, stacking-based, and supra-molecular stacking-based pathways of the self-assembly are described in relation to the presence or absence of intermediate structures. Concerning the membrane- or sheet- based mechanism, the molecular design and functionalities of each building block, and the dimensions of obtained ONTs are discussed in terms of the self-assembling features of amino acids, bile acids, carbohydrates, nucleotides, photoresponsive amphiphiles, functional dyes, fused aromatics, carbon allotropes, heterocycles, peptide derivatives, and other related molecules. Naphthalene-diimide, porphyrins, and azobenzene amphiphiles are introduced to feature the ONT formation mediated by nanoring- or nanotoroid structures. Cyclic peptides, polymer cyclic peptides, and peptide helices are then described as building blocks that result in the stacking-based self-assembly of the ONTs. Trimesic acids, pyrimido pyrimidine, ferrocene aromatics, and bent-shaped or cyclic aromatic amphiphiles are also discussed from the viewpoint of supramolecular stacking into the ONTs. Finally, currently important but critical issues for further development of the ONTs toward practical applications are described.