Nanotubes from Hydrogen Bonded Cyclic Ureas

2 Nanotubes from Hydrogen Bonded Cyclic Ureas 

In addition to rigid bisurea macrocycles, more flexible bisurea-derived macrocycles can also form tubular assemblies in the solid state. For example, Rangana-than and co-workers reported that macrocyclic bisureas prepared from L-cystine was able to form nanotubes of uniform shape and internal diameter [52], The two adjacent rings were bound together by two hydrogen bonds from bisamides and four hydrogen bonds from bisureas. The tubular structure was further stabilized by 

7 Nanotubes from Hydrogen Bonded Wedge- or Sector-like Molecules 

Another approach to nanotubes relies on the hydrogen bonding-induced self-assembly of wedge- or sector-shaped building blocks. The building blocks could first form disk-like assemblies which further stack to form nanotubes or helical 

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Cyclic ureas have also been used for nanotube formation in which the ureas are responsible for establishing the necessary hydrogen bonds to allow the association and orientation of cycloureas. One example of this process concerns the use of diphenyl ether linkers between ureas (Figure 24g). " The system forms a ring that leaves the C=0 and NH from the ureas in the appropriate position to form hydrogen bonds between the different subunits (position perpendicular to the plane of the backbone of the molecule). In the crystal structure, the nanotubes derived from these ureas contain acetic acid molecules in their cavity. 

Fig. 8.11 Hydrogen bonded nanotubes from the stacking of cyclic ureas, a The stiucture of bisurea macrocycles and b their crystal structures. Reprinted with permission from Ref. [47].

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