Self-inclusion complexation

Figure 12 Structures of discrete tetranuclear inclusion self-inclusion complexes formed by noncentrosymmetric koilands 17 with EtOH (a) and 21 with THF (b). For the sake of clarity, hydrogen atoms are not represented.

Berthault and Birlirakis by the use of vicinal proton-proton couplings. In particular, a formation of a stable self-inclusion complex accompanied by a local structural modification of the substituted altrose ring has been observed by the authors. 

Harada and coworkers developed a variety of supramolecular polymers based on the host-guest interaction of cyclodextrin (CD) derivatives with hydrophobic guests, such as the adamantyl and cinnamoyl groups [45,46]. Consequently, various structures were obtained by conjugating cinnamoyl and hydrocinnamoyl groups to a- and/3-CD. When the hydrocinnamoyl group was attached to -CD, the flexibility of the linker resulted in the formation of a self-inclusion complex - in other words. 

Figure J4.12 Supramolecular polymers obtained by Harada and coworkers, (a) Self-inclusion complex (cyclic monomer) formed as a result of the flexibility of the linker between the hydrophobic guest and yS-cyclodextrin (CD) [47] (b) Cyclic dimer and trimer formed when more rigid linkers were applied [48] (c) Supramolecular...

Figure 7.2 The structure of pillar[5]arene 7.1 (a), and the solvent-tuned self-inclusion complex formed by 7.1 (b). Reproduced from ref. 33 with permission from The Royal Society of Chemistry.

By using the click reaction, Stoddart and co-workers connected a viologen unit to the pillar[5]arene moiety to obtain a mono-functionalized pillar[5]arene (7.13 Scheme 7.1). As the concentration increased in the range of 0.1-100 mM in dichloromethane, the assemblies of 7.13 changed from self-inclusion complexes to linear supramolecular polymers. In addition, gels could be formed after a sealed solution of 7.13 at concentrations above 25 mM in dichloromethane had been left to stand for 12 h. 

It was established that 32-34 form in aqueous solutions the self-inclusion complexes, whereas in PEG/PPG solutions the self-inclusion equilibrium is perturbed by host-guest interaction with polymer molecules. Moreover, the aggregation of PEG/PPG chains occurs, therefore the environment around the spin probes is less viscous as compared to the average viscosity in the bulk solution. 

In the study of CDs containing PEG, i.e. poly(ethylene glycol) chain, terminated by azobenzene unit, the thermal and photochemical conformational changes of trans-52 were analyzed in aqueous solution [101]. The results of the thermal study show that at low concentration trans-52 exists at 80°C as a dethreading form 53. At GO C trans-52 exists as a self-inclusion complex 54 where the CD cavity entraps the azobenzene moiety and at 1°C the self-inclusion complex 55 where CD cavity encircles the carboxylic acid unit is present. It was observed that at high concentration trans-52 forms intermolecular complexes 56. 

The results of the photochemical study indicate that the irradiation of trans-52 or of its intermolecular complexes 56 with UV light leads to c/5-52 in the form of a self-inclusion complex 57 in which the CD cavity encloses the azobenzene moiety, regardless to concentration. 

The CD derivatives with a suitable pendant group can form self-inclusion complexes intramolecularly or intermolecularly as shown in Fig. 2 [30-33]. 

Fig. 2. Schematic representation of various types of cyclodextrin self-inclusion complexes.

The difference in the stability of the self-inclusion states produces the difference in the binding abilities, because the guest binding is in competition with binding of the dansyl moiety. In fact, the difference in the stability of the self-inclusion complex between 2 and 3 are estimated to be 1.7-fold from the... 

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