Inclusion of two guest molecules

The formation of a one host-two guests inclusion-complex could proceed by way of two main mechanisms (/) dimerization of the guest, followed by inclusion and (2) stepwise inclusion of two guest molecules. In order to distinguish between these two possibilities, several temperature-jump relaxation kinetic studies have been carried us-... 

The pillar[5]arene moiety in H5.23 formed a stable host-guest complex with 1,4-dicyanobutane (G5.29) [kT=(6.1 0.2)xl0 (run 118)]. In contrast, the crown ether moiety in H5.23 captured paraquat (G5.49) stably [iir=(2.6 0.6)xl0 (run 119)]. When the two guest molecules were added to H5.23 at the same time, the selective inclusion of two guest molecules by pillar[5]arene and crown ether rings took place, which was confirmed by single X-ray crystalline analysis of the complex. 

Nishioka and Fujita 78> have determined the Kd values for a- and P-cyclodextrin complexes with m- and p-substituted phenols at pH 7.0. Taking into account the directionality in inclusion of a guest molecule, they assumed three and two probable orientational isomers for the cyclodextrin complexes with m- and p-substituted phenols respectively (Fig. 6). Then the observed Kd values were divided into two or three terms corresponding to the dissociation of the orientational isomers involved (Eqs. 16, 17) ... 

Figure 10.38 shows modified / - and y-cydodextrins with two identical appended fluorophores that are able to form extimers (Ueno et al., 1997). They have been studied in 10% ethylene glycol aqueous solutions. /J-cyclodextrins with two 2-naphthylsulfonyl moieties linked to the smaller rim (compounds / -CDl, / -CD2, / -CD3), have a cavity that is too small to include both fluorophores one of them is outside the cavity and the other is inside. The latter can be excluded from the cavity upon inclusion of a guest molecule. Therefore, the excimer band in the fluorescence spectrum increases upon guest inclusion. 

The lattice of the host in the form it takes in the clathrate is usually thermodynamically unstable by itself—that is, with the holes empty. It is stabilized by inclusion of the guest molecules, and it is of obvious interest in connection with the nonstoichiometry of clathrates to consider the extent to which the cavities in the host lattice must be filled before the system achieves thermodynamic stability. The cavities in the host lattice may all be identical in size and environment, as in the hydroquinone clathrates, or they may be of more than one kind. The gas hydrates, for example, have two possible structures, in each of which there are two sorts of cavity, van der Waals and Platteeuw (15) have developed a general statistical theory of clathrates containing more than one type of cavity. 

Inclusion compounds allow the realization of copolymerization in the crystal state (1-6). This is a further difference with respect to typical solid state reactions. Both block- and statistical copolymers can be obtained the former involves a two-step process, with subsequent inclusion and polymerization of two different monomers (21) the latter requires the simultaneous inclusion of two guests. This phenomenon has a much wider occurrence than thought at first, especially when a not very selective host such as PHTP is used. Research with this host started with mixtures of 2-methylpentadiene and 4-methylpentadiene, two almost exactly superimposable molecules (22), but was successfully extended to very dissimilar monomers, such as butadiene and 2,3-dimethylbutadiene. 

The example specified as II a in Fig. 19 reveals the pattern found invariably in the methanol, ethanol, and 2-propanol inclusions of /. It is characterized by a loop of H-bonds which always involves two guest molecules opposing each other through a center of symmetry and two carboxyl groups of two symmetry-related molecules of 1 thus having adverse chirality (Fig. 17 a). The loop of H-bonds seems to be formed with... 

Up to now, evidence for the presence of a conformational change during the inclusion process has been presented only for alpha cyclodextrin, and then only in the case of certain guest molecules. Whether the two-step mechanism is generally applicable is not known, because, owing to the nature of kinetic studies, the results are frequently open to more than one interpretation. Nevertheless, sufficient support for the two-step mechanism has appeared to necessitate its consideration in any future kinetic studies. 

Thus the boundaries of the enclosures in organized media may be of two types they may be stiff (i.e, none of the guest molecules can diffuse out and the walls do not bend), as in the case of crystals and some inclusion complexes, or flexible (i.e., some of the guest molecules may exit the cavity and the walls of the cavity are sufficiently mobile to allow considerable internal motion of the enclosed molecules), as in the case of micelles and liquid crystals. In these two extremes, free volume needed for a reaction is intrinsic (built into the reaction cavity) and latent (can be provided on demand). 

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