In host-guest chemistry, taking precise control over the isomerism in the complexation process can not only simplify and regulate the structures of the final assemblies including mechanical interlocked molecules, but also exert great influence on their performance in carrying out tasks as molecular machines. When a linearly nonsymmetrical guest threads into the cavity of a three-dimensionally nonsymmetrical host, orientational isomers will be formed in theory resulting from the nondegenerate threading directions of the guest. This type of isomerism has been attracting the interest of researchers, and much progress has been made in how to control the threading direction of the guest by rationally designing the structures of both the host and the guest. In this review, the characterization methods of the relative host-guest orientation, the mechanism of the threading process and the construction of assemblies based on the complexation between three-dimensionally nonsymmetrical hosts, including cyclodextrins, calixarenes and triptycene-derived macrocycles, and linearly nonsymmetrical guests are summarized. The prospect and future application are also discussed.
Contents
1 Introduction
2 The general strategies of determining the threading direction and its selectivity of the guest molecule
2.1 The strategies of determining the relative orientation of host and guest
2.2 The strategies of determining the relative amount of orientational isomers
3 Directional threading of guests and construction of assemblies based on cyclodextrins
3.1 Thermodynamically controlled directional threading
3.2 Kinetically controlled directional threading
4 Directional threading of guests and construction of assemblies based on calixarenes
4.1 Directional threading based on urea-derived calix[6]arenes
4.2 Directional threading based on classical calix[6]arenes
5 Directional threading and construction of assemblies based on triptycene-derived macrocyclic hosts
6 Conclusion and outlook