Helical tape

Figure 5.24 Model of hierarchical self-assembly of chiral rodlike monomers.109 (a) Local arrangements (c-f) and corresponding global equilibrium conformations (c -f) for hierarchical selfassembling structures formed in solutions of chiral molecules (a), which have complementary donor and acceptor groups, shown by arrows, via which they interact and align to form tapes (c). Black and the white surfaces of rod (a) are reflected in sides of helical tape (c), which is chosen to curl toward black side (c ). (b) Phase diagram of solution of twisted ribbons that form fibrils. Scaled variables relative helix pitch of isolated ribbons h hh /a. relative side-by-side attraction energy between fibrils eaur/e. Reprinted with permission from Ref. 109. Copyright 2001 by the National Academy of Sciences, U.S.A.

Judicious choices of substituents have led to adducts that possess a wide range of properties, and examples include monolayers on gold surfaces [174] or at air-water interfaces [175-178] and for use as liquid crystals [179]. Expanded components, in which the distance between the faces is increased have also been studied [180,181]. TEM studies have shown the existence of strands, whose diameter is consistent with either stacked rosettes or helical tapes. TEM has also been used to study the products from reactions between bismelamines and biscyanuric acids [182],andbis(diamidopyridines) andbisuracils [183]. 

FIGURE 15.134 Fraction of liquid evaporated before onset of critical heat flux as a function of mass velocity for a plain tube and for a tube with helical tape inserts q" is the flux corresponding to the evaporation of all the injected water (from Moeck et al. [326], with permission). 

The chiral l,3,5-triazepane-2,6-dione 149 and its ring fused analogue 150 have been shown to form H-bonded helical molecular tapes with P chirality on self assembly in the solid state. With 149, this self assembly proceeds through aromatic-aromatic ring interactions resulting in hollow tubular structures <06CC4069>. 

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